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1<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook V4.1//EN">
2
3<book>
4<?dbhtml filename="index.html">
5
6<!-- ****************************************************** -->
7<!-- Header -->
8<!-- ****************************************************** -->
9 <bookinfo>
10 <title>Writing an ALSA Driver</title>
11 <author>
12 <firstname>Takashi</firstname>
13 <surname>Iwai</surname>
14 <affiliation>
15 <address>
16 <email>tiwai@suse.de</email>
17 </address>
18 </affiliation>
19 </author>
20
21 <date>March 6, 2005</date>
22 <edition>0.3.4</edition>
23
24 <abstract>
25 <para>
26 This document describes how to write an ALSA (Advanced Linux
27 Sound Architecture) driver.
28 </para>
29 </abstract>
30
31 <legalnotice>
32 <para>
33 Copyright (c) 2002-2004 Takashi Iwai <email>tiwai@suse.de</email>
34 </para>
35
36 <para>
37 This document is free; you can redistribute it and/or modify it
38 under the terms of the GNU General Public License as published by
39 the Free Software Foundation; either version 2 of the License, or
40 (at your option) any later version.
41 </para>
42
43 <para>
44 This document is distributed in the hope that it will be useful,
45 but <emphasis>WITHOUT ANY WARRANTY</emphasis>; without even the
46 implied warranty of <emphasis>MERCHANTABILITY or FITNESS FOR A
47 PARTICULAR PURPOSE</emphasis>. See the GNU General Public License
48 for more details.
49 </para>
50
51 <para>
52 You should have received a copy of the GNU General Public
53 License along with this program; if not, write to the Free
54 Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
55 MA 02111-1307 USA
56 </para>
57 </legalnotice>
58
59 </bookinfo>
60
61<!-- ****************************************************** -->
62<!-- Preface -->
63<!-- ****************************************************** -->
64 <preface id="preface">
65 <title>Preface</title>
66 <para>
67 This document describes how to write an
68 <ulink url="http://www.alsa-project.org/"><citetitle>
69 ALSA (Advanced Linux Sound Architecture)</citetitle></ulink>
70 driver. The document focuses mainly on the PCI soundcard.
71 In the case of other device types, the API might
72 be different, too. However, at least the ALSA kernel API is
73 consistent, and therefore it would be still a bit help for
74 writing them.
75 </para>
76
77 <para>
78 The target of this document is ones who already have enough
79 skill of C language and have the basic knowledge of linux
80 kernel programming. This document doesn't explain the general
81 topics of linux kernel codes and doesn't cover the detail of
82 implementation of each low-level driver. It describes only how is
83 the standard way to write a PCI sound driver on ALSA.
84 </para>
85
86 <para>
87 If you are already familiar with the older ALSA ver.0.5.x, you
88 can check the drivers such as <filename>es1938.c</filename> or
89 <filename>maestro3.c</filename> which have also almost the same
90 code-base in the ALSA 0.5.x tree, so you can compare the differences.
91 </para>
92
93 <para>
94 This document is still a draft version. Any feedbacks and
95 corrections, please!!
96 </para>
97 </preface>
98
99
100<!-- ****************************************************** -->
101<!-- File Tree Structure -->
102<!-- ****************************************************** -->
103 <chapter id="file-tree">
104 <title>File Tree Structure</title>
105
106 <section id="file-tree-general">
107 <title>General</title>
108 <para>
109 The ALSA drivers are provided in the two ways.
110 </para>
111
112 <para>
113 One is the trees provided as a tarball or via cvs from the
114 ALSA's ftp site, and another is the 2.6 (or later) Linux kernel
115 tree. To synchronize both, the ALSA driver tree is split into
116 two different trees: alsa-kernel and alsa-driver. The former
117 contains purely the source codes for the Linux 2.6 (or later)
118 tree. This tree is designed only for compilation on 2.6 or
119 later environment. The latter, alsa-driver, contains many subtle
120 files for compiling the ALSA driver on the outside of Linux
121 kernel like configure script, the wrapper functions for older,
122 2.2 and 2.4 kernels, to adapt the latest kernel API,
123 and additional drivers which are still in development or in
124 tests. The drivers in alsa-driver tree will be moved to
125 alsa-kernel (eventually 2.6 kernel tree) once when they are
126 finished and confirmed to work fine.
127 </para>
128
129 <para>
130 The file tree structure of ALSA driver is depicted below. Both
131 alsa-kernel and alsa-driver have almost the same file
132 structure, except for <quote>core</quote> directory. It's
133 named as <quote>acore</quote> in alsa-driver tree.
134
135 <example>
136 <title>ALSA File Tree Structure</title>
137 <literallayout>
138 sound
139 /core
140 /oss
141 /seq
142 /oss
143 /instr
144 /ioctl32
145 /include
146 /drivers
147 /mpu401
148 /opl3
149 /i2c
150 /l3
151 /synth
152 /emux
153 /pci
154 /(cards)
155 /isa
156 /(cards)
157 /arm
158 /ppc
159 /sparc
160 /usb
161 /pcmcia /(cards)
162 /oss
163 </literallayout>
164 </example>
165 </para>
166 </section>
167
168 <section id="file-tree-core-directory">
169 <title>core directory</title>
170 <para>
171 This directory contains the middle layer, that is, the heart
172 of ALSA drivers. In this directory, the native ALSA modules are
173 stored. The sub-directories contain different modules and are
174 dependent upon the kernel config.
175 </para>
176
177 <section id="file-tree-core-directory-oss">
178 <title>core/oss</title>
179
180 <para>
181 The codes for PCM and mixer OSS emulation modules are stored
182 in this directory. The rawmidi OSS emulation is included in
183 the ALSA rawmidi code since it's quite small. The sequencer
184 code is stored in core/seq/oss directory (see
185 <link linkend="file-tree-core-directory-seq-oss"><citetitle>
186 below</citetitle></link>).
187 </para>
188 </section>
189
190 <section id="file-tree-core-directory-ioctl32">
191 <title>core/ioctl32</title>
192
193 <para>
194 This directory contains the 32bit-ioctl wrappers for 64bit
195 architectures such like x86-64, ppc64 and sparc64. For 32bit
196 and alpha architectures, these are not compiled.
197 </para>
198 </section>
199
200 <section id="file-tree-core-directory-seq">
201 <title>core/seq</title>
202 <para>
203 This and its sub-directories are for the ALSA
204 sequencer. This directory contains the sequencer core and
205 primary sequencer modules such like snd-seq-midi,
206 snd-seq-virmidi, etc. They are compiled only when
207 <constant>CONFIG_SND_SEQUENCER</constant> is set in the kernel
208 config.
209 </para>
210 </section>
211
212 <section id="file-tree-core-directory-seq-oss">
213 <title>core/seq/oss</title>
214 <para>
215 This contains the OSS sequencer emulation codes.
216 </para>
217 </section>
218
219 <section id="file-tree-core-directory-deq-instr">
220 <title>core/seq/instr</title>
221 <para>
222 This directory contains the modules for the sequencer
223 instrument layer.
224 </para>
225 </section>
226 </section>
227
228 <section id="file-tree-include-directory">
229 <title>include directory</title>
230 <para>
231 This is the place for the public header files of ALSA drivers,
232 which are to be exported to the user-space, or included by
233 several files at different directories. Basically, the private
234 header files should not be placed in this directory, but you may
235 still find files there, due to historical reason :)
236 </para>
237 </section>
238
239 <section id="file-tree-drivers-directory">
240 <title>drivers directory</title>
241 <para>
242 This directory contains the codes shared among different drivers
243 on the different architectures. They are hence supposed not to be
244 architecture-specific.
245 For example, the dummy pcm driver and the serial MIDI
246 driver are found in this directory. In the sub-directories,
247 there are the codes for components which are independent from
248 bus and cpu architectures.
249 </para>
250
251 <section id="file-tree-drivers-directory-mpu401">
252 <title>drivers/mpu401</title>
253 <para>
254 The MPU401 and MPU401-UART modules are stored here.
255 </para>
256 </section>
257
258 <section id="file-tree-drivers-directory-opl3">
259 <title>drivers/opl3 and opl4</title>
260 <para>
261 The OPL3 and OPL4 FM-synth stuff is found here.
262 </para>
263 </section>
264 </section>
265
266 <section id="file-tree-i2c-directory">
267 <title>i2c directory</title>
268 <para>
269 This contains the ALSA i2c components.
270 </para>
271
272 <para>
273 Although there is a standard i2c layer on Linux, ALSA has its
274 own i2c codes for some cards, because the soundcard needs only a
275 simple operation and the standard i2c API is too complicated for
276 such a purpose.
277 </para>
278
279 <section id="file-tree-i2c-directory-l3">
280 <title>i2c/l3</title>
281 <para>
282 This is a sub-directory for ARM L3 i2c.
283 </para>
284 </section>
285 </section>
286
287 <section id="file-tree-synth-directory">
288 <title>synth directory</title>
289 <para>
290 This contains the synth middle-level modules.
291 </para>
292
293 <para>
294 So far, there is only Emu8000/Emu10k1 synth driver under
295 synth/emux sub-directory.
296 </para>
297 </section>
298
299 <section id="file-tree-pci-directory">
300 <title>pci directory</title>
301 <para>
302 This and its sub-directories hold the top-level card modules
303 for PCI soundcards and the codes specific to the PCI BUS.
304 </para>
305
306 <para>
307 The drivers compiled from a single file is stored directly on
308 pci directory, while the drivers with several source files are
309 stored on its own sub-directory (e.g. emu10k1, ice1712).
310 </para>
311 </section>
312
313 <section id="file-tree-isa-directory">
314 <title>isa directory</title>
315 <para>
316 This and its sub-directories hold the top-level card modules
317 for ISA soundcards.
318 </para>
319 </section>
320
321 <section id="file-tree-arm-ppc-sparc-directories">
322 <title>arm, ppc, and sparc directories</title>
323 <para>
324 These are for the top-level card modules which are
325 specific to each given architecture.
326 </para>
327 </section>
328
329 <section id="file-tree-usb-directory">
330 <title>usb directory</title>
331 <para>
332 This contains the USB-audio driver. On the latest version, the
333 USB MIDI driver is integrated together with usb-audio driver.
334 </para>
335 </section>
336
337 <section id="file-tree-pcmcia-directory">
338 <title>pcmcia directory</title>
339 <para>
340 The PCMCIA, especially PCCard drivers will go here. CardBus
341 drivers will be on pci directory, because its API is identical
342 with the standard PCI cards.
343 </para>
344 </section>
345
346 <section id="file-tree-oss-directory">
347 <title>oss directory</title>
348 <para>
349 The OSS/Lite source files are stored here on Linux 2.6 (or
350 later) tree. (In the ALSA driver tarball, it's empty, of course :)
351 </para>
352 </section>
353 </chapter>
354
355
356<!-- ****************************************************** -->
357<!-- Basic Flow for PCI Drivers -->
358<!-- ****************************************************** -->
359 <chapter id="basic-flow">
360 <title>Basic Flow for PCI Drivers</title>
361
362 <section id="basic-flow-outline">
363 <title>Outline</title>
364 <para>
365 The minimum flow of PCI soundcard is like the following:
366
367 <itemizedlist>
368 <listitem><para>define the PCI ID table (see the section
369 <link linkend="pci-resource-entries"><citetitle>PCI Entries
370 </citetitle></link>).</para></listitem>
371 <listitem><para>create <function>probe()</function> callback.</para></listitem>
372 <listitem><para>create <function>remove()</function> callback.</para></listitem>
373 <listitem><para>create pci_driver table which contains the three pointers above.</para></listitem>
374 <listitem><para>create <function>init()</function> function just calling <function>pci_module_init()</function> to register the pci_driver table defined above.</para></listitem>
375 <listitem><para>create <function>exit()</function> function to call <function>pci_unregister_driver()</function> function.</para></listitem>
376 </itemizedlist>
377 </para>
378 </section>
379
380 <section id="basic-flow-example">
381 <title>Full Code Example</title>
382 <para>
383 The code example is shown below. Some parts are kept
384 unimplemented at this moment but will be filled in the
385 succeeding sections. The numbers in comment lines of
386 <function>snd_mychip_probe()</function> function are the
387 markers.
388
389 <example>
390 <title>Basic Flow for PCI Drivers Example</title>
391 <programlisting>
392<![CDATA[
393 #include <sound/driver.h>
394 #include <linux/init.h>
395 #include <linux/pci.h>
396 #include <linux/slab.h>
397 #include <sound/core.h>
398 #include <sound/initval.h>
399
400 /* module parameters (see "Module Parameters") */
401 static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX;
402 static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR;
403 static int enable[SNDRV_CARDS] = SNDRV_DEFAULT_ENABLE_PNP;
404
405 /* definition of the chip-specific record */
406 typedef struct snd_mychip mychip_t;
407 struct snd_mychip {
408 snd_card_t *card;
409 // rest of implementation will be in the section
410 // "PCI Resource Managements"
411 };
412
413 /* chip-specific destructor
414 * (see "PCI Resource Managements")
415 */
416 static int snd_mychip_free(mychip_t *chip)
417 {
418 .... // will be implemented later...
419 }
420
421 /* component-destructor
422 * (see "Management of Cards and Components")
423 */
424 static int snd_mychip_dev_free(snd_device_t *device)
425 {
426 mychip_t *chip = device->device_data;
427 return snd_mychip_free(chip);
428 }
429
430 /* chip-specific constructor
431 * (see "Management of Cards and Components")
432 */
433 static int __devinit snd_mychip_create(snd_card_t *card,
434 struct pci_dev *pci,
435 mychip_t **rchip)
436 {
437 mychip_t *chip;
438 int err;
439 static snd_device_ops_t ops = {
440 .dev_free = snd_mychip_dev_free,
441 };
442
443 *rchip = NULL;
444
445 // check PCI availability here
446 // (see "PCI Resource Managements")
447 ....
448
449 /* allocate a chip-specific data with zero filled */
450 chip = kcalloc(1, sizeof(*chip), GFP_KERNEL);
451 if (chip == NULL)
452 return -ENOMEM;
453
454 chip->card = card;
455
456 // rest of initialization here; will be implemented
457 // later, see "PCI Resource Managements"
458 ....
459
460 if ((err = snd_device_new(card, SNDRV_DEV_LOWLEVEL,
461 chip, &ops)) < 0) {
462 snd_mychip_free(chip);
463 return err;
464 }
465
466 snd_card_set_dev(card, &pci->dev);
467
468 *rchip = chip;
469 return 0;
470 }
471
472 /* constructor -- see "Constructor" sub-section */
473 static int __devinit snd_mychip_probe(struct pci_dev *pci,
474 const struct pci_device_id *pci_id)
475 {
476 static int dev;
477 snd_card_t *card;
478 mychip_t *chip;
479 int err;
480
481 /* (1) */
482 if (dev >= SNDRV_CARDS)
483 return -ENODEV;
484 if (!enable[dev]) {
485 dev++;
486 return -ENOENT;
487 }
488
489 /* (2) */
490 card = snd_card_new(index[dev], id[dev], THIS_MODULE, 0);
491 if (card == NULL)
492 return -ENOMEM;
493
494 /* (3) */
495 if ((err = snd_mychip_create(card, pci, &chip)) < 0) {
496 snd_card_free(card);
497 return err;
498 }
499
500 /* (4) */
501 strcpy(card->driver, "My Chip");
502 strcpy(card->shortname, "My Own Chip 123");
503 sprintf(card->longname, "%s at 0x%lx irq %i",
504 card->shortname, chip->ioport, chip->irq);
505
506 /* (5) */
507 .... // implemented later
508
509 /* (6) */
510 if ((err = snd_card_register(card)) < 0) {
511 snd_card_free(card);
512 return err;
513 }
514
515 /* (7) */
516 pci_set_drvdata(pci, card);
517 dev++;
518 return 0;
519 }
520
521 /* destructor -- see "Destructor" sub-section */
522 static void __devexit snd_mychip_remove(struct pci_dev *pci)
523 {
524 snd_card_free(pci_get_drvdata(pci));
525 pci_set_drvdata(pci, NULL);
526 }
527]]>
528 </programlisting>
529 </example>
530 </para>
531 </section>
532
533 <section id="basic-flow-constructor">
534 <title>Constructor</title>
535 <para>
536 The real constructor of PCI drivers is probe callback. The
537 probe callback and other component-constructors which are called
538 from probe callback should be defined with
539 <parameter>__devinit</parameter> prefix. You
540 cannot use <parameter>__init</parameter> prefix for them,
541 because any PCI device could be a hotplug device.
542 </para>
543
544 <para>
545 In the probe callback, the following scheme is often used.
546 </para>
547
548 <section id="basic-flow-constructor-device-index">
549 <title>1) Check and increment the device index.</title>
550 <para>
551 <informalexample>
552 <programlisting>
553<![CDATA[
554 static int dev;
555 ....
556 if (dev >= SNDRV_CARDS)
557 return -ENODEV;
558 if (!enable[dev]) {
559 dev++;
560 return -ENOENT;
561 }
562]]>
563 </programlisting>
564 </informalexample>
565
566 where enable[dev] is the module option.
567 </para>
568
569 <para>
570 At each time probe callback is called, check the
571 availability of the device. If not available, simply increment
572 the device index and returns. dev will be incremented also
573 later (<link
574 linkend="basic-flow-constructor-set-pci"><citetitle>step
575 7</citetitle></link>).
576 </para>
577 </section>
578
579 <section id="basic-flow-constructor-create-card">
580 <title>2) Create a card instance</title>
581 <para>
582 <informalexample>
583 <programlisting>
584<![CDATA[
585 snd_card_t *card;
586 ....
587 card = snd_card_new(index[dev], id[dev], THIS_MODULE, 0);
588]]>
589 </programlisting>
590 </informalexample>
591 </para>
592
593 <para>
594 The detail will be explained in the section
595 <link linkend="card-management-card-instance"><citetitle>
596 Management of Cards and Components</citetitle></link>.
597 </para>
598 </section>
599
600 <section id="basic-flow-constructor-create-main">
601 <title>3) Create a main component</title>
602 <para>
603 In this part, the PCI resources are allocated.
604
605 <informalexample>
606 <programlisting>
607<![CDATA[
608 mychip_t *chip;
609 ....
610 if ((err = snd_mychip_create(card, pci, &chip)) < 0) {
611 snd_card_free(card);
612 return err;
613 }
614]]>
615 </programlisting>
616 </informalexample>
617
618 The detail will be explained in the section <link
619 linkend="pci-resource"><citetitle>PCI Resource
620 Managements</citetitle></link>.
621 </para>
622 </section>
623
624 <section id="basic-flow-constructor-main-component">
625 <title>4) Set the driver ID and name strings.</title>
626 <para>
627 <informalexample>
628 <programlisting>
629<![CDATA[
630 strcpy(card->driver, "My Chip");
631 strcpy(card->shortname, "My Own Chip 123");
632 sprintf(card->longname, "%s at 0x%lx irq %i",
633 card->shortname, chip->ioport, chip->irq);
634]]>
635 </programlisting>
636 </informalexample>
637
638 The driver field holds the minimal ID string of the
639 chip. This is referred by alsa-lib's configurator, so keep it
640 simple but unique.
641 Even the same driver can have different driver IDs to
642 distinguish the functionality of each chip type.
643 </para>
644
645 <para>
646 The shortname field is a string shown as more verbose
647 name. The longname field contains the information which is
648 shown in <filename>/proc/asound/cards</filename>.
649 </para>
650 </section>
651
652 <section id="basic-flow-constructor-create-other">
653 <title>5) Create other components, such as mixer, MIDI, etc.</title>
654 <para>
655 Here you define the basic components such as
656 <link linkend="pcm-interface"><citetitle>PCM</citetitle></link>,
657 mixer (e.g. <link linkend="api-ac97"><citetitle>AC97</citetitle></link>),
658 MIDI (e.g. <link linkend="midi-interface"><citetitle>MPU-401</citetitle></link>),
659 and other interfaces.
660 Also, if you want a <link linkend="proc-interface"><citetitle>proc
661 file</citetitle></link>, define it here, too.
662 </para>
663 </section>
664
665 <section id="basic-flow-constructor-register-card">
666 <title>6) Register the card instance.</title>
667 <para>
668 <informalexample>
669 <programlisting>
670<![CDATA[
671 if ((err = snd_card_register(card)) < 0) {
672 snd_card_free(card);
673 return err;
674 }
675]]>
676 </programlisting>
677 </informalexample>
678 </para>
679
680 <para>
681 Will be explained in the section <link
682 linkend="card-management-registration"><citetitle>Management
683 of Cards and Components</citetitle></link>, too.
684 </para>
685 </section>
686
687 <section id="basic-flow-constructor-set-pci">
688 <title>7) Set the PCI driver data and return zero.</title>
689 <para>
690 <informalexample>
691 <programlisting>
692<![CDATA[
693 pci_set_drvdata(pci, card);
694 dev++;
695 return 0;
696]]>
697 </programlisting>
698 </informalexample>
699
700 In the above, the card record is stored. This pointer is
701 referred in the remove callback and power-management
702 callbacks, too.
703 </para>
704 </section>
705 </section>
706
707 <section id="basic-flow-destructor">
708 <title>Destructor</title>
709 <para>
710 The destructor, remove callback, simply releases the card
711 instance. Then the ALSA middle layer will release all the
712 attached components automatically.
713 </para>
714
715 <para>
716 It would be typically like the following:
717
718 <informalexample>
719 <programlisting>
720<![CDATA[
721 static void __devexit snd_mychip_remove(struct pci_dev *pci)
722 {
723 snd_card_free(pci_get_drvdata(pci));
724 pci_set_drvdata(pci, NULL);
725 }
726]]>
727 </programlisting>
728 </informalexample>
729
730 The above code assumes that the card pointer is set to the PCI
731 driver data.
732 </para>
733 </section>
734
735 <section id="basic-flow-header-files">
736 <title>Header Files</title>
737 <para>
738 For the above example, at least the following include files
739 are necessary.
740
741 <informalexample>
742 <programlisting>
743<![CDATA[
744 #include <sound/driver.h>
745 #include <linux/init.h>
746 #include <linux/pci.h>
747 #include <linux/slab.h>
748 #include <sound/core.h>
749 #include <sound/initval.h>
750]]>
751 </programlisting>
752 </informalexample>
753
754 where the last one is necessary only when module options are
755 defined in the source file. If the codes are split to several
756 files, the file without module options don't need them.
757 </para>
758
759 <para>
760 In addition to them, you'll need
761 <filename>&lt;linux/interrupt.h&gt;</filename> for the interrupt
762 handling, and <filename>&lt;asm/io.h&gt;</filename> for the i/o
763 access. If you use <function>mdelay()</function> or
764 <function>udelay()</function> functions, you'll need to include
765 <filename>&lt;linux/delay.h&gt;</filename>, too.
766 </para>
767
768 <para>
769 The ALSA interfaces like PCM or control API are defined in other
770 header files as <filename>&lt;sound/xxx.h&gt;</filename>.
771 They have to be included after
772 <filename>&lt;sound/core.h&gt;</filename>.
773 </para>
774
775 </section>
776 </chapter>
777
778
779<!-- ****************************************************** -->
780<!-- Management of Cards and Components -->
781<!-- ****************************************************** -->
782 <chapter id="card-management">
783 <title>Management of Cards and Components</title>
784
785 <section id="card-management-card-instance">
786 <title>Card Instance</title>
787 <para>
788 For each soundcard, a <quote>card</quote> record must be allocated.
789 </para>
790
791 <para>
792 A card record is the headquarters of the soundcard. It manages
793 the list of whole devices (components) on the soundcard, such as
794 PCM, mixers, MIDI, synthesizer, and so on. Also, the card
795 record holds the ID and the name strings of the card, manages
796 the root of proc files, and controls the power-management states
797 and hotplug disconnections. The component list on the card
798 record is used to manage the proper releases of resources at
799 destruction.
800 </para>
801
802 <para>
803 As mentioned above, to create a card instance, call
804 <function>snd_card_new()</function>.
805
806 <informalexample>
807 <programlisting>
808<![CDATA[
809 snd_card_t *card;
810 card = snd_card_new(index, id, module, extra_size);
811]]>
812 </programlisting>
813 </informalexample>
814 </para>
815
816 <para>
817 The function takes four arguments, the card-index number, the
818 id string, the module pointer (usually
819 <constant>THIS_MODULE</constant>),
820 and the size of extra-data space. The last argument is used to
821 allocate card-&gt;private_data for the
822 chip-specific data. Note that this data
823 <emphasis>is</emphasis> allocated by
824 <function>snd_card_new()</function>.
825 </para>
826 </section>
827
828 <section id="card-management-component">
829 <title>Components</title>
830 <para>
831 After the card is created, you can attach the components
832 (devices) to the card instance. On ALSA driver, a component is
833 represented as a <type>snd_device_t</type> object.
834 A component can be a PCM instance, a control interface, a raw
835 MIDI interface, etc. Each of such instances has one component
836 entry.
837 </para>
838
839 <para>
840 A component can be created via
841 <function>snd_device_new()</function> function.
842
843 <informalexample>
844 <programlisting>
845<![CDATA[
846 snd_device_new(card, SNDRV_DEV_XXX, chip, &ops);
847]]>
848 </programlisting>
849 </informalexample>
850 </para>
851
852 <para>
853 This takes the card pointer, the device-level
854 (<constant>SNDRV_DEV_XXX</constant>), the data pointer, and the
855 callback pointers (<parameter>&amp;ops</parameter>). The
856 device-level defines the type of components and the order of
857 registration and de-registration. For most of components, the
858 device-level is already defined. For a user-defined component,
859 you can use <constant>SNDRV_DEV_LOWLEVEL</constant>.
860 </para>
861
862 <para>
863 This function itself doesn't allocate the data space. The data
864 must be allocated manually beforehand, and its pointer is passed
865 as the argument. This pointer is used as the identifier
866 (<parameter>chip</parameter> in the above example) for the
867 instance.
868 </para>
869
870 <para>
871 Each ALSA pre-defined component such as ac97 or pcm calls
872 <function>snd_device_new()</function> inside its
873 constructor. The destructor for each component is defined in the
874 callback pointers. Hence, you don't need to take care of
875 calling a destructor for such a component.
876 </para>
877
878 <para>
879 If you would like to create your own component, you need to
880 set the destructor function to dev_free callback in
881 <parameter>ops</parameter>, so that it can be released
882 automatically via <function>snd_card_free()</function>. The
883 example will be shown later as an implementation of a
884 chip-specific data.
885 </para>
886 </section>
887
888 <section id="card-management-chip-specific">
889 <title>Chip-Specific Data</title>
890 <para>
891 The chip-specific information, e.g. the i/o port address, its
892 resource pointer, or the irq number, is stored in the
893 chip-specific record.
894 Usually, the chip-specific record is typedef'ed as
895 <type>xxx_t</type> like the following:
896
897 <informalexample>
898 <programlisting>
899<![CDATA[
900 typedef struct snd_mychip mychip_t;
901 struct snd_mychip {
902 ....
903 };
904]]>
905 </programlisting>
906 </informalexample>
907 </para>
908
909 <para>
910 In general, there are two ways to allocate the chip record.
911 </para>
912
913 <section id="card-management-chip-specific-snd-card-new">
914 <title>1. Allocating via <function>snd_card_new()</function>.</title>
915 <para>
916 As mentioned above, you can pass the extra-data-length to the 4th argument of <function>snd_card_new()</function>, i.e.
917
918 <informalexample>
919 <programlisting>
920<![CDATA[
921 card = snd_card_new(index[dev], id[dev], THIS_MODULE, sizeof(mychip_t));
922]]>
923 </programlisting>
924 </informalexample>
925
926 whether <type>mychip_t</type> is the type of the chip record.
927 </para>
928
929 <para>
930 In return, the allocated record can be accessed as
931
932 <informalexample>
933 <programlisting>
934<![CDATA[
935 mychip_t *chip = (mychip_t *)card->private_data;
936]]>
937 </programlisting>
938 </informalexample>
939
940 With this method, you don't have to allocate twice.
941 The record is released together with the card instance.
942 </para>
943 </section>
944
945 <section id="card-management-chip-specific-allocate-extra">
946 <title>2. Allocating an extra device.</title>
947
948 <para>
949 After allocating a card instance via
950 <function>snd_card_new()</function> (with
951 <constant>NULL</constant> on the 4th arg), call
952 <function>kcalloc()</function>.
953
954 <informalexample>
955 <programlisting>
956<![CDATA[
957 snd_card_t *card;
958 mychip_t *chip;
959 card = snd_card_new(index[dev], id[dev], THIS_MODULE, NULL);
960 .....
961 chip = kcalloc(1, sizeof(*chip), GFP_KERNEL);
962]]>
963 </programlisting>
964 </informalexample>
965 </para>
966
967 <para>
968 The chip record should have the field to hold the card
969 pointer at least,
970
971 <informalexample>
972 <programlisting>
973<![CDATA[
974 struct snd_mychip {
975 snd_card_t *card;
976 ....
977 };
978]]>
979 </programlisting>
980 </informalexample>
981 </para>
982
983 <para>
984 Then, set the card pointer in the returned chip instance.
985
986 <informalexample>
987 <programlisting>
988<![CDATA[
989 chip->card = card;
990]]>
991 </programlisting>
992 </informalexample>
993 </para>
994
995 <para>
996 Next, initialize the fields, and register this chip
997 record as a low-level device with a specified
998 <parameter>ops</parameter>,
999
1000 <informalexample>
1001 <programlisting>
1002<![CDATA[
1003 static snd_device_ops_t ops = {
1004 .dev_free = snd_mychip_dev_free,
1005 };
1006 ....
1007 snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
1008]]>
1009 </programlisting>
1010 </informalexample>
1011
1012 <function>snd_mychip_dev_free()</function> is the
1013 device-destructor function, which will call the real
1014 destructor.
1015 </para>
1016
1017 <para>
1018 <informalexample>
1019 <programlisting>
1020<![CDATA[
1021 static int snd_mychip_dev_free(snd_device_t *device)
1022 {
1023 mychip_t *chip = device->device_data;
1024 return snd_mychip_free(chip);
1025 }
1026]]>
1027 </programlisting>
1028 </informalexample>
1029
1030 where <function>snd_mychip_free()</function> is the real destructor.
1031 </para>
1032 </section>
1033 </section>
1034
1035 <section id="card-management-registration">
1036 <title>Registration and Release</title>
1037 <para>
1038 After all components are assigned, register the card instance
1039 by calling <function>snd_card_register()</function>. The access
1040 to the device files are enabled at this point. That is, before
1041 <function>snd_card_register()</function> is called, the
1042 components are safely inaccessible from external side. If this
1043 call fails, exit the probe function after releasing the card via
1044 <function>snd_card_free()</function>.
1045 </para>
1046
1047 <para>
1048 For releasing the card instance, you can call simply
1049 <function>snd_card_free()</function>. As already mentioned, all
1050 components are released automatically by this call.
1051 </para>
1052
1053 <para>
1054 As further notes, the destructors (both
1055 <function>snd_mychip_dev_free</function> and
1056 <function>snd_mychip_free</function>) cannot be defined with
1057 <parameter>__devexit</parameter> prefix, because they may be
1058 called from the constructor, too, at the false path.
1059 </para>
1060
1061 <para>
1062 For a device which allows hotplugging, you can use
1063 <function>snd_card_free_in_thread</function>. This one will
1064 postpone the destruction and wait in a kernel-thread until all
1065 devices are closed.
1066 </para>
1067
1068 </section>
1069
1070 </chapter>
1071
1072
1073<!-- ****************************************************** -->
1074<!-- PCI Resource Managements -->
1075<!-- ****************************************************** -->
1076 <chapter id="pci-resource">
1077 <title>PCI Resource Managements</title>
1078
1079 <section id="pci-resource-example">
1080 <title>Full Code Example</title>
1081 <para>
1082 In this section, we'll finish the chip-specific constructor,
1083 destructor and PCI entries. The example code is shown first,
1084 below.
1085
1086 <example>
1087 <title>PCI Resource Managements Example</title>
1088 <programlisting>
1089<![CDATA[
1090 struct snd_mychip {
1091 snd_card_t *card;
1092 struct pci_dev *pci;
1093
1094 unsigned long port;
1095 int irq;
1096 };
1097
1098 static int snd_mychip_free(mychip_t *chip)
1099 {
1100 /* disable hardware here if any */
1101 .... // (not implemented in this document)
1102
1103 /* release the irq */
1104 if (chip->irq >= 0)
1105 free_irq(chip->irq, (void *)chip);
1106 /* release the i/o ports & memory */
1107 pci_release_regions(chip->pci);
1108 /* disable the PCI entry */
1109 pci_disable_device(chip->pci);
1110 /* release the data */
1111 kfree(chip);
1112 return 0;
1113 }
1114
1115 /* chip-specific constructor */
1116 static int __devinit snd_mychip_create(snd_card_t *card,
1117 struct pci_dev *pci,
1118 mychip_t **rchip)
1119 {
1120 mychip_t *chip;
1121 int err;
1122 static snd_device_ops_t ops = {
1123 .dev_free = snd_mychip_dev_free,
1124 };
1125
1126 *rchip = NULL;
1127
1128 /* initialize the PCI entry */
1129 if ((err = pci_enable_device(pci)) < 0)
1130 return err;
1131 /* check PCI availability (28bit DMA) */
1132 if (pci_set_dma_mask(pci, 0x0fffffff) < 0 ||
1133 pci_set_consistent_dma_mask(pci, 0x0fffffff) < 0) {
1134 printk(KERN_ERR "error to set 28bit mask DMA\n");
1135 pci_disable_device(pci);
1136 return -ENXIO;
1137 }
1138
1139 chip = kcalloc(1, sizeof(*chip), GFP_KERNEL);
1140 if (chip == NULL) {
1141 pci_disable_device(pci);
1142 return -ENOMEM;
1143 }
1144
1145 /* initialize the stuff */
1146 chip->card = card;
1147 chip->pci = pci;
1148 chip->irq = -1;
1149
1150 /* (1) PCI resource allocation */
1151 if ((err = pci_request_regions(pci, "My Chip")) < 0) {
1152 kfree(chip);
1153 pci_disable_device(pci);
1154 return err;
1155 }
1156 chip->port = pci_resource_start(pci, 0);
1157 if (request_irq(pci->irq, snd_mychip_interrupt,
1158 SA_INTERRUPT|SA_SHIRQ, "My Chip",
1159 (void *)chip)) {
1160 printk(KERN_ERR "cannot grab irq %d\n", pci->irq);
1161 snd_mychip_free(chip);
1162 return -EBUSY;
1163 }
1164 chip->irq = pci->irq;
1165
1166 /* (2) initialization of the chip hardware */
1167 .... // (not implemented in this document)
1168
1169 if ((err = snd_device_new(card, SNDRV_DEV_LOWLEVEL,
1170 chip, &ops)) < 0) {
1171 snd_mychip_free(chip);
1172 return err;
1173 }
1174
1175 snd_card_set_dev(card, &pci->dev);
1176
1177 *rchip = chip;
1178 return 0;
1179 }
1180
1181 /* PCI IDs */
1182 static struct pci_device_id snd_mychip_ids[] = {
1183 { PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR,
1184 PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, },
1185 ....
1186 { 0, }
1187 };
1188 MODULE_DEVICE_TABLE(pci, snd_mychip_ids);
1189
1190 /* pci_driver definition */
1191 static struct pci_driver driver = {
1192 .name = "My Own Chip",
1193 .id_table = snd_mychip_ids,
1194 .probe = snd_mychip_probe,
1195 .remove = __devexit_p(snd_mychip_remove),
1196 };
1197
1198 /* initialization of the module */
1199 static int __init alsa_card_mychip_init(void)
1200 {
1201 return pci_module_init(&driver);
1202 }
1203
1204 /* clean up the module */
1205 static void __exit alsa_card_mychip_exit(void)
1206 {
1207 pci_unregister_driver(&driver);
1208 }
1209
1210 module_init(alsa_card_mychip_init)
1211 module_exit(alsa_card_mychip_exit)
1212
1213 EXPORT_NO_SYMBOLS; /* for old kernels only */
1214]]>
1215 </programlisting>
1216 </example>
1217 </para>
1218 </section>
1219
1220 <section id="pci-resource-some-haftas">
1221 <title>Some Hafta's</title>
1222 <para>
1223 The allocation of PCI resources is done in the
1224 <function>probe()</function> function, and usually an extra
1225 <function>xxx_create()</function> function is written for this
1226 purpose.
1227 </para>
1228
1229 <para>
1230 In the case of PCI devices, you have to call at first
1231 <function>pci_enable_device()</function> function before
1232 allocating resources. Also, you need to set the proper PCI DMA
1233 mask to limit the accessed i/o range. In some cases, you might
1234 need to call <function>pci_set_master()</function> function,
1235 too.
1236 </para>
1237
1238 <para>
1239 Suppose the 28bit mask, and the code to be added would be like:
1240
1241 <informalexample>
1242 <programlisting>
1243<![CDATA[
1244 if ((err = pci_enable_device(pci)) < 0)
1245 return err;
1246 if (pci_set_dma_mask(pci, 0x0fffffff) < 0 ||
1247 pci_set_consistent_dma_mask(pci, 0x0fffffff) < 0) {
1248 printk(KERN_ERR "error to set 28bit mask DMA\n");
1249 pci_disable_device(pci);
1250 return -ENXIO;
1251 }
1252
1253]]>
1254 </programlisting>
1255 </informalexample>
1256 </para>
1257 </section>
1258
1259 <section id="pci-resource-resource-allocation">
1260 <title>Resource Allocation</title>
1261 <para>
1262 The allocation of I/O ports and irqs are done via standard kernel
1263 functions. Unlike ALSA ver.0.5.x., there are no helpers for
1264 that. And these resources must be released in the destructor
1265 function (see below). Also, on ALSA 0.9.x, you don't need to
1266 allocate (pseudo-)DMA for PCI like ALSA 0.5.x.
1267 </para>
1268
1269 <para>
1270 Now assume that this PCI device has an I/O port with 8 bytes
1271 and an interrupt. Then <type>mychip_t</type> will have the
1272 following fields:
1273
1274 <informalexample>
1275 <programlisting>
1276<![CDATA[
1277 struct snd_mychip {
1278 snd_card_t *card;
1279
1280 unsigned long port;
1281 int irq;
1282 };
1283]]>
1284 </programlisting>
1285 </informalexample>
1286 </para>
1287
1288 <para>
1289 For an i/o port (and also a memory region), you need to have
1290 the resource pointer for the standard resource management. For
1291 an irq, you have to keep only the irq number (integer). But you
1292 need to initialize this number as -1 before actual allocation,
1293 since irq 0 is valid. The port address and its resource pointer
1294 can be initialized as null by
1295 <function>kcalloc()</function> automatically, so you
1296 don't have to take care of resetting them.
1297 </para>
1298
1299 <para>
1300 The allocation of an i/o port is done like this:
1301
1302 <informalexample>
1303 <programlisting>
1304<![CDATA[
1305 if ((err = pci_request_regions(pci, "My Chip")) < 0) {
1306 kfree(chip);
1307 pci_disable_device(pci);
1308 return err;
1309 }
1310 chip->port = pci_resource_start(pci, 0);
1311]]>
1312 </programlisting>
1313 </informalexample>
1314 </para>
1315
1316 <para>
1317 <!-- obsolete -->
1318 It will reserve the i/o port region of 8 bytes of the given
1319 PCI device. The returned value, chip-&gt;res_port, is allocated
1320 via <function>kmalloc()</function> by
1321 <function>request_region()</function>. The pointer must be
1322 released via <function>kfree()</function>, but there is some
1323 problem regarding this. This issue will be explained more below.
1324 </para>
1325
1326 <para>
1327 The allocation of an interrupt source is done like this:
1328
1329 <informalexample>
1330 <programlisting>
1331<![CDATA[
1332 if (request_irq(pci->irq, snd_mychip_interrupt,
1333 SA_INTERRUPT|SA_SHIRQ, "My Chip",
1334 (void *)chip)) {
1335 printk(KERN_ERR "cannot grab irq %d\n", pci->irq);
1336 snd_mychip_free(chip);
1337 return -EBUSY;
1338 }
1339 chip->irq = pci->irq;
1340]]>
1341 </programlisting>
1342 </informalexample>
1343
1344 where <function>snd_mychip_interrupt()</function> is the
1345 interrupt handler defined <link
1346 linkend="pcm-interface-interrupt-handler"><citetitle>later</citetitle></link>.
1347 Note that chip-&gt;irq should be defined
1348 only when <function>request_irq()</function> succeeded.
1349 </para>
1350
1351 <para>
1352 On the PCI bus, the interrupts can be shared. Thus,
1353 <constant>SA_SHIRQ</constant> is given as the interrupt flag of
1354 <function>request_irq()</function>.
1355 </para>
1356
1357 <para>
1358 The last argument of <function>request_irq()</function> is the
1359 data pointer passed to the interrupt handler. Usually, the
1360 chip-specific record is used for that, but you can use what you
1361 like, too.
1362 </para>
1363
1364 <para>
1365 I won't define the detail of the interrupt handler at this
1366 point, but at least its appearance can be explained now. The
1367 interrupt handler looks usually like the following:
1368
1369 <informalexample>
1370 <programlisting>
1371<![CDATA[
1372 static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id,
1373 struct pt_regs *regs)
1374 {
1375 mychip_t *chip = dev_id;
1376 ....
1377 return IRQ_HANDLED;
1378 }
1379]]>
1380 </programlisting>
1381 </informalexample>
1382 </para>
1383
1384 <para>
1385 Now let's write the corresponding destructor for the resources
1386 above. The role of destructor is simple: disable the hardware
1387 (if already activated) and release the resources. So far, we
1388 have no hardware part, so the disabling is not written here.
1389 </para>
1390
1391 <para>
1392 For releasing the resources, <quote>check-and-release</quote>
1393 method is a safer way. For the interrupt, do like this:
1394
1395 <informalexample>
1396 <programlisting>
1397<![CDATA[
1398 if (chip->irq >= 0)
1399 free_irq(chip->irq, (void *)chip);
1400]]>
1401 </programlisting>
1402 </informalexample>
1403
1404 Since the irq number can start from 0, you should initialize
1405 chip-&gt;irq with a negative value (e.g. -1), so that you can
1406 check the validity of the irq number as above.
1407 </para>
1408
1409 <para>
1410 When you requested I/O ports or memory regions via
1411 <function>pci_request_region()</function> or
1412 <function>pci_request_regions()</function> like this example,
1413 release the resource(s) using the corresponding function,
1414 <function>pci_release_region()</function> or
1415 <function>pci_release_regions()</function>.
1416
1417 <informalexample>
1418 <programlisting>
1419<![CDATA[
1420 pci_release_regions(chip->pci);
1421]]>
1422 </programlisting>
1423 </informalexample>
1424 </para>
1425
1426 <para>
1427 When you requested manually via <function>request_region()</function>
1428 or <function>request_mem_region</function>, you can release it via
1429 <function>release_resource()</function>. Suppose that you keep
1430 the resource pointer returned from <function>request_region()</function>
1431 in chip-&gt;res_port, the release procedure looks like below:
1432
1433 <informalexample>
1434 <programlisting>
1435<![CDATA[
1436 if (chip->res_port) {
1437 release_resource(chip->res_port);
1438 kfree_nocheck(chip->res_port);
1439 }
1440]]>
1441 </programlisting>
1442 </informalexample>
1443
1444 As you can see, the resource pointer is also to be freed
1445 via <function>kfree_nocheck()</function> after
1446 <function>release_resource()</function> is called. You
1447 cannot use <function>kfree()</function> here, because on ALSA,
1448 <function>kfree()</function> may be a wrapper to its own
1449 allocator with the memory debugging. Since the resource pointer
1450 is allocated externally outside the ALSA, it must be released
1451 via the native
1452 <function>kfree()</function>.
1453 <function>kfree_nocheck()</function> is used for that; it calls
1454 the native <function>kfree()</function> without wrapper.
1455 </para>
1456
1457 <para>
1458 Don't forget to call <function>pci_disable_device()</function>
1459 before all finished.
1460 </para>
1461
1462 <para>
1463 And finally, release the chip-specific record.
1464
1465 <informalexample>
1466 <programlisting>
1467<![CDATA[
1468 kfree(chip);
1469]]>
1470 </programlisting>
1471 </informalexample>
1472 </para>
1473
1474 <para>
1475 Again, remember that you cannot
1476 set <parameter>__devexit</parameter> prefix for this destructor.
1477 </para>
1478
1479 <para>
1480 We didn't implement the hardware-disabling part in the above.
1481 If you need to do this, please note that the destructor may be
1482 called even before the initialization of the chip is completed.
1483 It would be better to have a flag to skip the hardware-disabling
1484 if the hardware was not initialized yet.
1485 </para>
1486
1487 <para>
1488 When the chip-data is assigned to the card using
1489 <function>snd_device_new()</function> with
1490 <constant>SNDRV_DEV_LOWLELVEL</constant> , its destructor is
1491 called at the last. That is, it is assured that all other
1492 components like PCMs and controls have been already released.
1493 You don't have to call stopping PCMs, etc. explicitly, but just
1494 stop the hardware in the low-level.
1495 </para>
1496
1497 <para>
1498 The management of a memory-mapped region is almost as same as
1499 the management of an i/o port. You'll need three fields like
1500 the following:
1501
1502 <informalexample>
1503 <programlisting>
1504<![CDATA[
1505 struct snd_mychip {
1506 ....
1507 unsigned long iobase_phys;
1508 void __iomem *iobase_virt;
1509 };
1510]]>
1511 </programlisting>
1512 </informalexample>
1513
1514 and the allocation would be like below:
1515
1516 <informalexample>
1517 <programlisting>
1518<![CDATA[
1519 if ((err = pci_request_regions(pci, "My Chip")) < 0) {
1520 kfree(chip);
1521 return err;
1522 }
1523 chip->iobase_phys = pci_resource_start(pci, 0);
1524 chip->iobase_virt = ioremap_nocache(chip->iobase_phys,
1525 pci_resource_len(pci, 0));
1526]]>
1527 </programlisting>
1528 </informalexample>
1529
1530 and the corresponding destructor would be:
1531
1532 <informalexample>
1533 <programlisting>
1534<![CDATA[
1535 static int snd_mychip_free(mychip_t *chip)
1536 {
1537 ....
1538 if (chip->iobase_virt)
1539 iounmap(chip->iobase_virt);
1540 ....
1541 pci_release_regions(chip->pci);
1542 ....
1543 }
1544]]>
1545 </programlisting>
1546 </informalexample>
1547 </para>
1548
1549 </section>
1550
1551 <section id="pci-resource-device-struct">
1552 <title>Registration of Device Struct</title>
1553 <para>
1554 At some point, typically after calling <function>snd_device_new()</function>,
1555 you need to register the <structname>struct device</structname> of the chip
1556 you're handling for udev and co. ALSA provides a macro for compatibility with
1557 older kernels. Simply call like the following:
1558 <informalexample>
1559 <programlisting>
1560<![CDATA[
1561 snd_card_set_dev(card, &pci->dev);
1562]]>
1563 </programlisting>
1564 </informalexample>
1565 so that it stores the PCI's device pointer to the card. This will be
1566 referred by ALSA core functions later when the devices are registered.
1567 </para>
1568 <para>
1569 In the case of non-PCI, pass the proper device struct pointer of the BUS
1570 instead. (In the case of legacy ISA without PnP, you don't have to do
1571 anything.)
1572 </para>
1573 </section>
1574
1575 <section id="pci-resource-entries">
1576 <title>PCI Entries</title>
1577 <para>
1578 So far, so good. Let's finish the rest of missing PCI
1579 stuffs. At first, we need a
1580 <structname>pci_device_id</structname> table for this
1581 chipset. It's a table of PCI vendor/device ID number, and some
1582 masks.
1583 </para>
1584
1585 <para>
1586 For example,
1587
1588 <informalexample>
1589 <programlisting>
1590<![CDATA[
1591 static struct pci_device_id snd_mychip_ids[] = {
1592 { PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR,
1593 PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, },
1594 ....
1595 { 0, }
1596 };
1597 MODULE_DEVICE_TABLE(pci, snd_mychip_ids);
1598]]>
1599 </programlisting>
1600 </informalexample>
1601 </para>
1602
1603 <para>
1604 The first and second fields of
1605 <structname>pci_device_id</structname> struct are the vendor and
1606 device IDs. If you have nothing special to filter the matching
1607 devices, you can use the rest of fields like above. The last
1608 field of <structname>pci_device_id</structname> struct is a
1609 private data for this entry. You can specify any value here, for
1610 example, to tell the type of different operations per each
1611 device IDs. Such an example is found in intel8x0 driver.
1612 </para>
1613
1614 <para>
1615 The last entry of this list is the terminator. You must
1616 specify this all-zero entry.
1617 </para>
1618
1619 <para>
1620 Then, prepare the <structname>pci_driver</structname> record:
1621
1622 <informalexample>
1623 <programlisting>
1624<![CDATA[
1625 static struct pci_driver driver = {
1626 .name = "My Own Chip",
1627 .id_table = snd_mychip_ids,
1628 .probe = snd_mychip_probe,
1629 .remove = __devexit_p(snd_mychip_remove),
1630 };
1631]]>
1632 </programlisting>
1633 </informalexample>
1634 </para>
1635
1636 <para>
1637 The <structfield>probe</structfield> and
1638 <structfield>remove</structfield> functions are what we already
1639 defined in
1640 the previous sections. The <structfield>remove</structfield> should
1641 be defined with
1642 <function>__devexit_p()</function> macro, so that it's not
1643 defined for built-in (and non-hot-pluggable) case. The
1644 <structfield>name</structfield>
1645 field is the name string of this device. Note that you must not
1646 use a slash <quote>/</quote> in this string.
1647 </para>
1648
1649 <para>
1650 And at last, the module entries:
1651
1652 <informalexample>
1653 <programlisting>
1654<![CDATA[
1655 static int __init alsa_card_mychip_init(void)
1656 {
1657 return pci_module_init(&driver);
1658 }
1659
1660 static void __exit alsa_card_mychip_exit(void)
1661 {
1662 pci_unregister_driver(&driver);
1663 }
1664
1665 module_init(alsa_card_mychip_init)
1666 module_exit(alsa_card_mychip_exit)
1667]]>
1668 </programlisting>
1669 </informalexample>
1670 </para>
1671
1672 <para>
1673 Note that these module entries are tagged with
1674 <parameter>__init</parameter> and
1675 <parameter>__exit</parameter> prefixes, not
1676 <parameter>__devinit</parameter> nor
1677 <parameter>__devexit</parameter>.
1678 </para>
1679
1680 <para>
1681 Oh, one thing was forgotten. If you have no exported symbols,
1682 you need to declare it on 2.2 or 2.4 kernels (on 2.6 kernels
1683 it's not necessary, though).
1684
1685 <informalexample>
1686 <programlisting>
1687<![CDATA[
1688 EXPORT_NO_SYMBOLS;
1689]]>
1690 </programlisting>
1691 </informalexample>
1692
1693 That's all!
1694 </para>
1695 </section>
1696 </chapter>
1697
1698
1699<!-- ****************************************************** -->
1700<!-- PCM Interface -->
1701<!-- ****************************************************** -->
1702 <chapter id="pcm-interface">
1703 <title>PCM Interface</title>
1704
1705 <section id="pcm-interface-general">
1706 <title>General</title>
1707 <para>
1708 The PCM middle layer of ALSA is quite powerful and it is only
1709 necessary for each driver to implement the low-level functions
1710 to access its hardware.
1711 </para>
1712
1713 <para>
1714 For accessing to the PCM layer, you need to include
1715 <filename>&lt;sound/pcm.h&gt;</filename> above all. In addition,
1716 <filename>&lt;sound/pcm_params.h&gt;</filename> might be needed
1717 if you access to some functions related with hw_param.
1718 </para>
1719
1720 <para>
1721 Each card device can have up to four pcm instances. A pcm
1722 instance corresponds to a pcm device file. The limitation of
1723 number of instances comes only from the available bit size of
1724 the linux's device number. Once when 64bit device number is
1725 used, we'll have more available pcm instances.
1726 </para>
1727
1728 <para>
1729 A pcm instance consists of pcm playback and capture streams,
1730 and each pcm stream consists of one or more pcm substreams. Some
1731 soundcard supports the multiple-playback function. For example,
1732 emu10k1 has a PCM playback of 32 stereo substreams. In this case, at
1733 each open, a free substream is (usually) automatically chosen
1734 and opened. Meanwhile, when only one substream exists and it was
1735 already opened, the succeeding open will result in the blocking
1736 or the error with <constant>EAGAIN</constant> according to the
1737 file open mode. But you don't have to know the detail in your
1738 driver. The PCM middle layer will take all such jobs.
1739 </para>
1740 </section>
1741
1742 <section id="pcm-interface-example">
1743 <title>Full Code Example</title>
1744 <para>
1745 The example code below does not include any hardware access
1746 routines but shows only the skeleton, how to build up the PCM
1747 interfaces.
1748
1749 <example>
1750 <title>PCM Example Code</title>
1751 <programlisting>
1752<![CDATA[
1753 #include <sound/pcm.h>
1754 ....
1755
1756 /* hardware definition */
1757 static snd_pcm_hardware_t snd_mychip_playback_hw = {
1758 .info = (SNDRV_PCM_INFO_MMAP |
1759 SNDRV_PCM_INFO_INTERLEAVED |
1760 SNDRV_PCM_INFO_BLOCK_TRANSFER |
1761 SNDRV_PCM_INFO_MMAP_VALID),
1762 .formats = SNDRV_PCM_FMTBIT_S16_LE,
1763 .rates = SNDRV_PCM_RATE_8000_48000,
1764 .rate_min = 8000,
1765 .rate_max = 48000,
1766 .channels_min = 2,
1767 .channels_max = 2,
1768 .buffer_bytes_max = 32768,
1769 .period_bytes_min = 4096,
1770 .period_bytes_max = 32768,
1771 .periods_min = 1,
1772 .periods_max = 1024,
1773 };
1774
1775 /* hardware definition */
1776 static snd_pcm_hardware_t snd_mychip_capture_hw = {
1777 .info = (SNDRV_PCM_INFO_MMAP |
1778 SNDRV_PCM_INFO_INTERLEAVED |
1779 SNDRV_PCM_INFO_BLOCK_TRANSFER |
1780 SNDRV_PCM_INFO_MMAP_VALID),
1781 .formats = SNDRV_PCM_FMTBIT_S16_LE,
1782 .rates = SNDRV_PCM_RATE_8000_48000,
1783 .rate_min = 8000,
1784 .rate_max = 48000,
1785 .channels_min = 2,
1786 .channels_max = 2,
1787 .buffer_bytes_max = 32768,
1788 .period_bytes_min = 4096,
1789 .period_bytes_max = 32768,
1790 .periods_min = 1,
1791 .periods_max = 1024,
1792 };
1793
1794 /* open callback */
1795 static int snd_mychip_playback_open(snd_pcm_substream_t *substream)
1796 {
1797 mychip_t *chip = snd_pcm_substream_chip(substream);
1798 snd_pcm_runtime_t *runtime = substream->runtime;
1799
1800 runtime->hw = snd_mychip_playback_hw;
1801 // more hardware-initialization will be done here
1802 return 0;
1803 }
1804
1805 /* close callback */
1806 static int snd_mychip_playback_close(snd_pcm_substream_t *substream)
1807 {
1808 mychip_t *chip = snd_pcm_substream_chip(substream);
1809 // the hardware-specific codes will be here
1810 return 0;
1811
1812 }
1813
1814 /* open callback */
1815 static int snd_mychip_capture_open(snd_pcm_substream_t *substream)
1816 {
1817 mychip_t *chip = snd_pcm_substream_chip(substream);
1818 snd_pcm_runtime_t *runtime = substream->runtime;
1819
1820 runtime->hw = snd_mychip_capture_hw;
1821 // more hardware-initialization will be done here
1822 return 0;
1823 }
1824
1825 /* close callback */
1826 static int snd_mychip_capture_close(snd_pcm_substream_t *substream)
1827 {
1828 mychip_t *chip = snd_pcm_substream_chip(substream);
1829 // the hardware-specific codes will be here
1830 return 0;
1831
1832 }
1833
1834 /* hw_params callback */
1835 static int snd_mychip_pcm_hw_params(snd_pcm_substream_t *substream,
1836 snd_pcm_hw_params_t * hw_params)
1837 {
1838 return snd_pcm_lib_malloc_pages(substream,
1839 params_buffer_bytes(hw_params));
1840 }
1841
1842 /* hw_free callback */
1843 static int snd_mychip_pcm_hw_free(snd_pcm_substream_t *substream)
1844 {
1845 return snd_pcm_lib_free_pages(substream);
1846 }
1847
1848 /* prepare callback */
1849 static int snd_mychip_pcm_prepare(snd_pcm_substream_t *substream)
1850 {
1851 mychip_t *chip = snd_pcm_substream_chip(substream);
1852 snd_pcm_runtime_t *runtime = substream->runtime;
1853
1854 /* set up the hardware with the current configuration
1855 * for example...
1856 */
1857 mychip_set_sample_format(chip, runtime->format);
1858 mychip_set_sample_rate(chip, runtime->rate);
1859 mychip_set_channels(chip, runtime->channels);
1860 mychip_set_dma_setup(chip, runtime->dma_area,
1861 chip->buffer_size,
1862 chip->period_size);
1863 return 0;
1864 }
1865
1866 /* trigger callback */
1867 static int snd_mychip_pcm_trigger(snd_pcm_substream_t *substream,
1868 int cmd)
1869 {
1870 switch (cmd) {
1871 case SNDRV_PCM_TRIGGER_START:
1872 // do something to start the PCM engine
1873 break;
1874 case SNDRV_PCM_TRIGGER_STOP:
1875 // do something to stop the PCM engine
1876 break;
1877 default:
1878 return -EINVAL;
1879 }
1880 }
1881
1882 /* pointer callback */
1883 static snd_pcm_uframes_t
1884 snd_mychip_pcm_pointer(snd_pcm_substream_t *substream)
1885 {
1886 mychip_t *chip = snd_pcm_substream_chip(substream);
1887 unsigned int current_ptr;
1888
1889 /* get the current hardware pointer */
1890 current_ptr = mychip_get_hw_pointer(chip);
1891 return current_ptr;
1892 }
1893
1894 /* operators */
1895 static snd_pcm_ops_t snd_mychip_playback_ops = {
1896 .open = snd_mychip_playback_open,
1897 .close = snd_mychip_playback_close,
1898 .ioctl = snd_pcm_lib_ioctl,
1899 .hw_params = snd_mychip_pcm_hw_params,
1900 .hw_free = snd_mychip_pcm_hw_free,
1901 .prepare = snd_mychip_pcm_prepare,
1902 .trigger = snd_mychip_pcm_trigger,
1903 .pointer = snd_mychip_pcm_pointer,
1904 };
1905
1906 /* operators */
1907 static snd_pcm_ops_t snd_mychip_capture_ops = {
1908 .open = snd_mychip_capture_open,
1909 .close = snd_mychip_capture_close,
1910 .ioctl = snd_pcm_lib_ioctl,
1911 .hw_params = snd_mychip_pcm_hw_params,
1912 .hw_free = snd_mychip_pcm_hw_free,
1913 .prepare = snd_mychip_pcm_prepare,
1914 .trigger = snd_mychip_pcm_trigger,
1915 .pointer = snd_mychip_pcm_pointer,
1916 };
1917
1918 /*
1919 * definitions of capture are omitted here...
1920 */
1921
1922 /* create a pcm device */
1923 static int __devinit snd_mychip_new_pcm(mychip_t *chip)
1924 {
1925 snd_pcm_t *pcm;
1926 int err;
1927
1928 if ((err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1,
1929 &pcm)) < 0)
1930 return err;
1931 pcm->private_data = chip;
1932 strcpy(pcm->name, "My Chip");
1933 chip->pcm = pcm;
1934 /* set operators */
1935 snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
1936 &snd_mychip_playback_ops);
1937 snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
1938 &snd_mychip_capture_ops);
1939 /* pre-allocation of buffers */
1940 /* NOTE: this may fail */
1941 snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
1942 snd_dma_pci_data(chip->pci),
1943 64*1024, 64*1024);
1944 return 0;
1945 }
1946]]>
1947 </programlisting>
1948 </example>
1949 </para>
1950 </section>
1951
1952 <section id="pcm-interface-constructor">
1953 <title>Constructor</title>
1954 <para>
1955 A pcm instance is allocated by <function>snd_pcm_new()</function>
1956 function. It would be better to create a constructor for pcm,
1957 namely,
1958
1959 <informalexample>
1960 <programlisting>
1961<![CDATA[
1962 static int __devinit snd_mychip_new_pcm(mychip_t *chip)
1963 {
1964 snd_pcm_t *pcm;
1965 int err;
1966
1967 if ((err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1,
1968 &pcm)) < 0)
1969 return err;
1970 pcm->private_data = chip;
1971 strcpy(pcm->name, "My Chip");
1972 chip->pcm = pcm;
1973 ....
1974 return 0;
1975 }
1976]]>
1977 </programlisting>
1978 </informalexample>
1979 </para>
1980
1981 <para>
1982 The <function>snd_pcm_new()</function> function takes the four
1983 arguments. The first argument is the card pointer to which this
1984 pcm is assigned, and the second is the ID string.
1985 </para>
1986
1987 <para>
1988 The third argument (<parameter>index</parameter>, 0 in the
1989 above) is the index of this new pcm. It begins from zero. When
1990 you will create more than one pcm instances, specify the
1991 different numbers in this argument. For example,
1992 <parameter>index</parameter> = 1 for the second PCM device.
1993 </para>
1994
1995 <para>
1996 The fourth and fifth arguments are the number of substreams
1997 for playback and capture, respectively. Here both 1 are given in
1998 the above example. When no playback or no capture is available,
1999 pass 0 to the corresponding argument.
2000 </para>
2001
2002 <para>
2003 If a chip supports multiple playbacks or captures, you can
2004 specify more numbers, but they must be handled properly in
2005 open/close, etc. callbacks. When you need to know which
2006 substream you are referring to, then it can be obtained from
2007 <type>snd_pcm_substream_t</type> data passed to each callback
2008 as follows:
2009
2010 <informalexample>
2011 <programlisting>
2012<![CDATA[
2013 snd_pcm_substream_t *substream;
2014 int index = substream->number;
2015]]>
2016 </programlisting>
2017 </informalexample>
2018 </para>
2019
2020 <para>
2021 After the pcm is created, you need to set operators for each
2022 pcm stream.
2023
2024 <informalexample>
2025 <programlisting>
2026<![CDATA[
2027 snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
2028 &snd_mychip_playback_ops);
2029 snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
2030 &snd_mychip_capture_ops);
2031]]>
2032 </programlisting>
2033 </informalexample>
2034 </para>
2035
2036 <para>
2037 The operators are defined typically like this:
2038
2039 <informalexample>
2040 <programlisting>
2041<![CDATA[
2042 static snd_pcm_ops_t snd_mychip_playback_ops = {
2043 .open = snd_mychip_pcm_open,
2044 .close = snd_mychip_pcm_close,
2045 .ioctl = snd_pcm_lib_ioctl,
2046 .hw_params = snd_mychip_pcm_hw_params,
2047 .hw_free = snd_mychip_pcm_hw_free,
2048 .prepare = snd_mychip_pcm_prepare,
2049 .trigger = snd_mychip_pcm_trigger,
2050 .pointer = snd_mychip_pcm_pointer,
2051 };
2052]]>
2053 </programlisting>
2054 </informalexample>
2055
2056 Each of callbacks is explained in the subsection
2057 <link linkend="pcm-interface-operators"><citetitle>
2058 Operators</citetitle></link>.
2059 </para>
2060
2061 <para>
2062 After setting the operators, most likely you'd like to
2063 pre-allocate the buffer. For the pre-allocation, simply call
2064 the following:
2065
2066 <informalexample>
2067 <programlisting>
2068<![CDATA[
2069 snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
2070 snd_dma_pci_data(chip->pci),
2071 64*1024, 64*1024);
2072]]>
2073 </programlisting>
2074 </informalexample>
2075
2076 It will allocate up to 64kB buffer as default. The details of
2077 buffer management will be described in the later section <link
2078 linkend="buffer-and-memory"><citetitle>Buffer and Memory
2079 Management</citetitle></link>.
2080 </para>
2081
2082 <para>
2083 Additionally, you can set some extra information for this pcm
2084 in pcm-&gt;info_flags.
2085 The available values are defined as
2086 <constant>SNDRV_PCM_INFO_XXX</constant> in
2087 <filename>&lt;sound/asound.h&gt;</filename>, which is used for
2088 the hardware definition (described later). When your soundchip
2089 supports only half-duplex, specify like this:
2090
2091 <informalexample>
2092 <programlisting>
2093<![CDATA[
2094 pcm->info_flags = SNDRV_PCM_INFO_HALF_DUPLEX;
2095]]>
2096 </programlisting>
2097 </informalexample>
2098 </para>
2099 </section>
2100
2101 <section id="pcm-interface-destructor">
2102 <title>... And the Destructor?</title>
2103 <para>
2104 The destructor for a pcm instance is not always
2105 necessary. Since the pcm device will be released by the middle
2106 layer code automatically, you don't have to call destructor
2107 explicitly.
2108 </para>
2109
2110 <para>
2111 The destructor would be necessary when you created some
2112 special records internally and need to release them. In such a
2113 case, set the destructor function to
2114 pcm-&gt;private_free:
2115
2116 <example>
2117 <title>PCM Instance with a Destructor</title>
2118 <programlisting>
2119<![CDATA[
2120 static void mychip_pcm_free(snd_pcm_t *pcm)
2121 {
2122 mychip_t *chip = snd_pcm_chip(pcm);
2123 /* free your own data */
2124 kfree(chip->my_private_pcm_data);
2125 // do what you like else
2126 ....
2127 }
2128
2129 static int __devinit snd_mychip_new_pcm(mychip_t *chip)
2130 {
2131 snd_pcm_t *pcm;
2132 ....
2133 /* allocate your own data */
2134 chip->my_private_pcm_data = kmalloc(...);
2135 /* set the destructor */
2136 pcm->private_data = chip;
2137 pcm->private_free = mychip_pcm_free;
2138 ....
2139 }
2140]]>
2141 </programlisting>
2142 </example>
2143 </para>
2144 </section>
2145
2146 <section id="pcm-interface-runtime">
2147 <title>Runtime Pointer - The Chest of PCM Information</title>
2148 <para>
2149 When the PCM substream is opened, a PCM runtime instance is
2150 allocated and assigned to the substream. This pointer is
2151 accessible via <constant>substream-&gt;runtime</constant>.
2152 This runtime pointer holds the various information; it holds
2153 the copy of hw_params and sw_params configurations, the buffer
2154 pointers, mmap records, spinlocks, etc. Almost everyhing you
2155 need for controlling the PCM can be found there.
2156 </para>
2157
2158 <para>
2159 The definition of runtime instance is found in
2160 <filename>&lt;sound/pcm.h&gt;</filename>. Here is the
2161 copy from the file.
2162 <informalexample>
2163 <programlisting>
2164<![CDATA[
2165struct _snd_pcm_runtime {
2166 /* -- Status -- */
2167 snd_pcm_substream_t *trigger_master;
2168 snd_timestamp_t trigger_tstamp; /* trigger timestamp */
2169 int overrange;
2170 snd_pcm_uframes_t avail_max;
2171 snd_pcm_uframes_t hw_ptr_base; /* Position at buffer restart */
2172 snd_pcm_uframes_t hw_ptr_interrupt; /* Position at interrupt time*/
2173
2174 /* -- HW params -- */
2175 snd_pcm_access_t access; /* access mode */
2176 snd_pcm_format_t format; /* SNDRV_PCM_FORMAT_* */
2177 snd_pcm_subformat_t subformat; /* subformat */
2178 unsigned int rate; /* rate in Hz */
2179 unsigned int channels; /* channels */
2180 snd_pcm_uframes_t period_size; /* period size */
2181 unsigned int periods; /* periods */
2182 snd_pcm_uframes_t buffer_size; /* buffer size */
2183 unsigned int tick_time; /* tick time */
2184 snd_pcm_uframes_t min_align; /* Min alignment for the format */
2185 size_t byte_align;
2186 unsigned int frame_bits;
2187 unsigned int sample_bits;
2188 unsigned int info;
2189 unsigned int rate_num;
2190 unsigned int rate_den;
2191
2192 /* -- SW params -- */
2193 int tstamp_timespec; /* use timeval (0) or timespec (1) */
2194 snd_pcm_tstamp_t tstamp_mode; /* mmap timestamp is updated */
2195 unsigned int period_step;
2196 unsigned int sleep_min; /* min ticks to sleep */
2197 snd_pcm_uframes_t xfer_align; /* xfer size need to be a multiple */
2198 snd_pcm_uframes_t start_threshold;
2199 snd_pcm_uframes_t stop_threshold;
2200 snd_pcm_uframes_t silence_threshold; /* Silence filling happens when
2201 noise is nearest than this */
2202 snd_pcm_uframes_t silence_size; /* Silence filling size */
2203 snd_pcm_uframes_t boundary; /* pointers wrap point */
2204
2205 snd_pcm_uframes_t silenced_start;
2206 snd_pcm_uframes_t silenced_size;
2207
2208 snd_pcm_sync_id_t sync; /* hardware synchronization ID */
2209
2210 /* -- mmap -- */
2211 volatile snd_pcm_mmap_status_t *status;
2212 volatile snd_pcm_mmap_control_t *control;
2213 atomic_t mmap_count;
2214
2215 /* -- locking / scheduling -- */
2216 spinlock_t lock;
2217 wait_queue_head_t sleep;
2218 struct timer_list tick_timer;
2219 struct fasync_struct *fasync;
2220
2221 /* -- private section -- */
2222 void *private_data;
2223 void (*private_free)(snd_pcm_runtime_t *runtime);
2224
2225 /* -- hardware description -- */
2226 snd_pcm_hardware_t hw;
2227 snd_pcm_hw_constraints_t hw_constraints;
2228
2229 /* -- interrupt callbacks -- */
2230 void (*transfer_ack_begin)(snd_pcm_substream_t *substream);
2231 void (*transfer_ack_end)(snd_pcm_substream_t *substream);
2232
2233 /* -- timer -- */
2234 unsigned int timer_resolution; /* timer resolution */
2235
2236 /* -- DMA -- */
2237 unsigned char *dma_area; /* DMA area */
2238 dma_addr_t dma_addr; /* physical bus address (not accessible from main CPU) */
2239 size_t dma_bytes; /* size of DMA area */
2240
2241 struct snd_dma_buffer *dma_buffer_p; /* allocated buffer */
2242
2243#if defined(CONFIG_SND_PCM_OSS) || defined(CONFIG_SND_PCM_OSS_MODULE)
2244 /* -- OSS things -- */
2245 snd_pcm_oss_runtime_t oss;
2246#endif
2247};
2248]]>
2249 </programlisting>
2250 </informalexample>
2251 </para>
2252
2253 <para>
2254 For the operators (callbacks) of each sound driver, most of
2255 these records are supposed to be read-only. Only the PCM
2256 middle-layer changes / updates these info. The exceptions are
2257 the hardware description (hw), interrupt callbacks
2258 (transfer_ack_xxx), DMA buffer information, and the private
2259 data. Besides, if you use the standard buffer allocation
2260 method via <function>snd_pcm_lib_malloc_pages()</function>,
2261 you don't need to set the DMA buffer information by yourself.
2262 </para>
2263
2264 <para>
2265 In the sections below, important records are explained.
2266 </para>
2267
2268 <section id="pcm-interface-runtime-hw">
2269 <title>Hardware Description</title>
2270 <para>
2271 The hardware descriptor (<type>snd_pcm_hardware_t</type>)
2272 contains the definitions of the fundamental hardware
2273 configuration. Above all, you'll need to define this in
2274 <link linkend="pcm-interface-operators-open-callback"><citetitle>
2275 the open callback</citetitle></link>.
2276 Note that the runtime instance holds the copy of the
2277 descriptor, not the pointer to the existing descriptor. That
2278 is, in the open callback, you can modify the copied descriptor
2279 (<constant>runtime-&gt;hw</constant>) as you need. For example, if the maximum
2280 number of channels is 1 only on some chip models, you can
2281 still use the same hardware descriptor and change the
2282 channels_max later:
2283 <informalexample>
2284 <programlisting>
2285<![CDATA[
2286 snd_pcm_runtime_t *runtime = substream->runtime;
2287 ...
2288 runtime->hw = snd_mychip_playback_hw; /* common definition */
2289 if (chip->model == VERY_OLD_ONE)
2290 runtime->hw.channels_max = 1;
2291]]>
2292 </programlisting>
2293 </informalexample>
2294 </para>
2295
2296 <para>
2297 Typically, you'll have a hardware descriptor like below:
2298 <informalexample>
2299 <programlisting>
2300<![CDATA[
2301 static snd_pcm_hardware_t snd_mychip_playback_hw = {
2302 .info = (SNDRV_PCM_INFO_MMAP |
2303 SNDRV_PCM_INFO_INTERLEAVED |
2304 SNDRV_PCM_INFO_BLOCK_TRANSFER |
2305 SNDRV_PCM_INFO_MMAP_VALID),
2306 .formats = SNDRV_PCM_FMTBIT_S16_LE,
2307 .rates = SNDRV_PCM_RATE_8000_48000,
2308 .rate_min = 8000,
2309 .rate_max = 48000,
2310 .channels_min = 2,
2311 .channels_max = 2,
2312 .buffer_bytes_max = 32768,
2313 .period_bytes_min = 4096,
2314 .period_bytes_max = 32768,
2315 .periods_min = 1,
2316 .periods_max = 1024,
2317 };
2318]]>
2319 </programlisting>
2320 </informalexample>
2321 </para>
2322
2323 <para>
2324 <itemizedlist>
2325 <listitem><para>
2326 The <structfield>info</structfield> field contains the type and
2327 capabilities of this pcm. The bit flags are defined in
2328 <filename>&lt;sound/asound.h&gt;</filename> as
2329 <constant>SNDRV_PCM_INFO_XXX</constant>. Here, at least, you
2330 have to specify whether the mmap is supported and which
2331 interleaved format is supported.
2332 When the mmap is supported, add
2333 <constant>SNDRV_PCM_INFO_MMAP</constant> flag here. When the
2334 hardware supports the interleaved or the non-interleaved
2335 format, <constant>SNDRV_PCM_INFO_INTERLEAVED</constant> or
2336 <constant>SNDRV_PCM_INFO_NONINTERLEAVED</constant> flag must
2337 be set, respectively. If both are supported, you can set both,
2338 too.
2339 </para>
2340
2341 <para>
2342 In the above example, <constant>MMAP_VALID</constant> and
2343 <constant>BLOCK_TRANSFER</constant> are specified for OSS mmap
2344 mode. Usually both are set. Of course,
2345 <constant>MMAP_VALID</constant> is set only if the mmap is
2346 really supported.
2347 </para>
2348
2349 <para>
2350 The other possible flags are
2351 <constant>SNDRV_PCM_INFO_PAUSE</constant> and
2352 <constant>SNDRV_PCM_INFO_RESUME</constant>. The
2353 <constant>PAUSE</constant> bit means that the pcm supports the
2354 <quote>pause</quote> operation, while the
2355 <constant>RESUME</constant> bit means that the pcm supports
2356 the <quote>suspend/resume</quote> operation. If these flags
2357 are set, the <structfield>trigger</structfield> callback below
2358 must handle the corresponding commands.
2359 </para>
2360
2361 <para>
2362 When the PCM substreams can be synchronized (typically,
2363 synchorinized start/stop of a playback and a capture streams),
2364 you can give <constant>SNDRV_PCM_INFO_SYNC_START</constant>,
2365 too. In this case, you'll need to check the linked-list of
2366 PCM substreams in the trigger callback. This will be
2367 described in the later section.
2368 </para>
2369 </listitem>
2370
2371 <listitem>
2372 <para>
2373 <structfield>formats</structfield> field contains the bit-flags
2374 of supported formats (<constant>SNDRV_PCM_FMTBIT_XXX</constant>).
2375 If the hardware supports more than one format, give all or'ed
2376 bits. In the example above, the signed 16bit little-endian
2377 format is specified.
2378 </para>
2379 </listitem>
2380
2381 <listitem>
2382 <para>
2383 <structfield>rates</structfield> field contains the bit-flags of
2384 supported rates (<constant>SNDRV_PCM_RATE_XXX</constant>).
2385 When the chip supports continuous rates, pass
2386 <constant>CONTINUOUS</constant> bit additionally.
2387 The pre-defined rate bits are provided only for typical
2388 rates. If your chip supports unconventional rates, you need to add
2389 <constant>KNOT</constant> bit and set up the hardware
2390 constraint manually (explained later).
2391 </para>
2392 </listitem>
2393
2394 <listitem>
2395 <para>
2396 <structfield>rate_min</structfield> and
2397 <structfield>rate_max</structfield> define the minimal and
2398 maximal sample rate. This should correspond somehow to
2399 <structfield>rates</structfield> bits.
2400 </para>
2401 </listitem>
2402
2403 <listitem>
2404 <para>
2405 <structfield>channel_min</structfield> and
2406 <structfield>channel_max</structfield>
2407 define, as you might already expected, the minimal and maximal
2408 number of channels.
2409 </para>
2410 </listitem>
2411
2412 <listitem>
2413 <para>
2414 <structfield>buffer_bytes_max</structfield> defines the
2415 maximal buffer size in bytes. There is no
2416 <structfield>buffer_bytes_min</structfield> field, since
2417 it can be calculated from the minimal period size and the
2418 minimal number of periods.
2419 Meanwhile, <structfield>period_bytes_min</structfield> and
2420 define the minimal and maximal size of the period in bytes.
2421 <structfield>periods_max</structfield> and
2422 <structfield>periods_min</structfield> define the maximal and
2423 minimal number of periods in the buffer.
2424 </para>
2425
2426 <para>
2427 The <quote>period</quote> is a term, that corresponds to
2428 fragment in the OSS world. The period defines the size at
2429 which the PCM interrupt is generated. This size strongly
2430 depends on the hardware.
2431 Generally, the smaller period size will give you more
2432 interrupts, that is, more controls.
2433 In the case of capture, this size defines the input latency.
2434 On the other hand, the whole buffer size defines the
2435 output latency for the playback direction.
2436 </para>
2437 </listitem>
2438
2439 <listitem>
2440 <para>
2441 There is also a field <structfield>fifo_size</structfield>.
2442 This specifies the size of the hardware FIFO, but it's not
2443 used currently in the driver nor in the alsa-lib. So, you
2444 can ignore this field.
2445 </para>
2446 </listitem>
2447 </itemizedlist>
2448 </para>
2449 </section>
2450
2451 <section id="pcm-interface-runtime-config">
2452 <title>PCM Configurations</title>
2453 <para>
2454 Ok, let's go back again to the PCM runtime records.
2455 The most frequently referred records in the runtime instance are
2456 the PCM configurations.
2457 The PCM configurations are stored on runtime instance
2458 after the application sends <type>hw_params</type> data via
2459 alsa-lib. There are many fields copied from hw_params and
2460 sw_params structs. For example,
2461 <structfield>format</structfield> holds the format type
2462 chosen by the application. This field contains the enum value
2463 <constant>SNDRV_PCM_FORMAT_XXX</constant>.
2464 </para>
2465
2466 <para>
2467 One thing to be noted is that the configured buffer and period
2468 sizes are stored in <quote>frames</quote> in the runtime
2469 In the ALSA world, 1 frame = channels * samples-size.
2470 For conversion between frames and bytes, you can use the
2471 helper functions, <function>frames_to_bytes()</function> and
2472 <function>bytes_to_frames()</function>.
2473 <informalexample>
2474 <programlisting>
2475<![CDATA[
2476 period_bytes = frames_to_bytes(runtime, runtime->period_size);
2477]]>
2478 </programlisting>
2479 </informalexample>
2480 </para>
2481
2482 <para>
2483 Also, many software parameters (sw_params) are
2484 stored in frames, too. Please check the type of the field.
2485 <type>snd_pcm_uframes_t</type> is for the frames as unsigned
2486 integer while <type>snd_pcm_sframes_t</type> is for the frames
2487 as signed integer.
2488 </para>
2489 </section>
2490
2491 <section id="pcm-interface-runtime-dma">
2492 <title>DMA Buffer Information</title>
2493 <para>
2494 The DMA buffer is defined by the following four fields,
2495 <structfield>dma_area</structfield>,
2496 <structfield>dma_addr</structfield>,
2497 <structfield>dma_bytes</structfield> and
2498 <structfield>dma_private</structfield>.
2499 The <structfield>dma_area</structfield> holds the buffer
2500 pointer (the logical address). You can call
2501 <function>memcpy</function> from/to
2502 this pointer. Meanwhile, <structfield>dma_addr</structfield>
2503 holds the physical address of the buffer. This field is
2504 specified only when the buffer is a linear buffer.
2505 <structfield>dma_bytes</structfield> holds the size of buffer
2506 in bytes. <structfield>dma_private</structfield> is used for
2507 the ALSA DMA allocator.
2508 </para>
2509
2510 <para>
2511 If you use a standard ALSA function,
2512 <function>snd_pcm_lib_malloc_pages()</function>, for
2513 allocating the buffer, these fields are set by the ALSA middle
2514 layer, and you should <emphasis>not</emphasis> change them by
2515 yourself. You can read them but not write them.
2516 On the other hand, if you want to allocate the buffer by
2517 yourself, you'll need to manage it in hw_params callback.
2518 At least, <structfield>dma_bytes</structfield> is mandatory.
2519 <structfield>dma_area</structfield> is necessary when the
2520 buffer is mmapped. If your driver doesn't support mmap, this
2521 field is not necessary. <structfield>dma_addr</structfield>
2522 is also not mandatory. You can use
2523 <structfield>dma_private</structfield> as you like, too.
2524 </para>
2525 </section>
2526
2527 <section id="pcm-interface-runtime-status">
2528 <title>Running Status</title>
2529 <para>
2530 The running status can be referred via <constant>runtime-&gt;status</constant>.
2531 This is the pointer to <type>snd_pcm_mmap_status_t</type>
2532 record. For example, you can get the current DMA hardware
2533 pointer via <constant>runtime-&gt;status-&gt;hw_ptr</constant>.
2534 </para>
2535
2536 <para>
2537 The DMA application pointer can be referred via
2538 <constant>runtime-&gt;control</constant>, which points
2539 <type>snd_pcm_mmap_control_t</type> record.
2540 However, accessing directly to this value is not recommended.
2541 </para>
2542 </section>
2543
2544 <section id="pcm-interface-runtime-private">
2545 <title>Private Data</title>
2546 <para>
2547 You can allocate a record for the substream and store it in
2548 <constant>runtime-&gt;private_data</constant>. Usually, this
2549 done in
2550 <link linkend="pcm-interface-operators-open-callback"><citetitle>
2551 the open callback</citetitle></link>.
2552 Don't mix this with <constant>pcm-&gt;private_data</constant>.
2553 The <constant>pcm-&gt;private_data</constant> usually points the
2554 chip instance assigned statically at the creation of PCM, while the
2555 <constant>runtime-&gt;private_data</constant> points a dynamic
2556 data created at the PCM open callback.
2557
2558 <informalexample>
2559 <programlisting>
2560<![CDATA[
2561 static int snd_xxx_open(snd_pcm_substream_t *substream)
2562 {
2563 my_pcm_data_t *data;
2564 ....
2565 data = kmalloc(sizeof(*data), GFP_KERNEL);
2566 substream->runtime->private_data = data;
2567 ....
2568 }
2569]]>
2570 </programlisting>
2571 </informalexample>
2572 </para>
2573
2574 <para>
2575 The allocated object must be released in
2576 <link linkend="pcm-interface-operators-open-callback"><citetitle>
2577 the close callback</citetitle></link>.
2578 </para>
2579 </section>
2580
2581 <section id="pcm-interface-runtime-intr">
2582 <title>Interrupt Callbacks</title>
2583 <para>
2584 The field <structfield>transfer_ack_begin</structfield> and
2585 <structfield>transfer_ack_end</structfield> are called at
2586 the beginning and the end of
2587 <function>snd_pcm_period_elapsed()</function>, respectively.
2588 </para>
2589 </section>
2590
2591 </section>
2592
2593 <section id="pcm-interface-operators">
2594 <title>Operators</title>
2595 <para>
2596 OK, now let me explain the detail of each pcm callback
2597 (<parameter>ops</parameter>). In general, every callback must
2598 return 0 if successful, or a negative number with the error
2599 number such as <constant>-EINVAL</constant> at any
2600 error.
2601 </para>
2602
2603 <para>
2604 The callback function takes at least the argument with
2605 <type>snd_pcm_substream_t</type> pointer. For retrieving the
2606 chip record from the given substream instance, you can use the
2607 following macro.
2608
2609 <informalexample>
2610 <programlisting>
2611<![CDATA[
2612 int xxx() {
2613 mychip_t *chip = snd_pcm_substream_chip(substream);
2614 ....
2615 }
2616]]>
2617 </programlisting>
2618 </informalexample>
2619
2620 The macro reads <constant>substream-&gt;private_data</constant>,
2621 which is a copy of <constant>pcm-&gt;private_data</constant>.
2622 You can override the former if you need to assign different data
2623 records per PCM substream. For example, cmi8330 driver assigns
2624 different private_data for playback and capture directions,
2625 because it uses two different codecs (SB- and AD-compatible) for
2626 different directions.
2627 </para>
2628
2629 <section id="pcm-interface-operators-open-callback">
2630 <title>open callback</title>
2631 <para>
2632 <informalexample>
2633 <programlisting>
2634<![CDATA[
2635 static int snd_xxx_open(snd_pcm_substream_t *substream);
2636]]>
2637 </programlisting>
2638 </informalexample>
2639
2640 This is called when a pcm substream is opened.
2641 </para>
2642
2643 <para>
2644 At least, here you have to initialize the runtime-&gt;hw
2645 record. Typically, this is done by like this:
2646
2647 <informalexample>
2648 <programlisting>
2649<![CDATA[
2650 static int snd_xxx_open(snd_pcm_substream_t *substream)
2651 {
2652 mychip_t *chip = snd_pcm_substream_chip(substream);
2653 snd_pcm_runtime_t *runtime = substream->runtime;
2654
2655 runtime->hw = snd_mychip_playback_hw;
2656 return 0;
2657 }
2658]]>
2659 </programlisting>
2660 </informalexample>
2661
2662 where <parameter>snd_mychip_playback_hw</parameter> is the
2663 pre-defined hardware description.
2664 </para>
2665
2666 <para>
2667 You can allocate a private data in this callback, as described
2668 in <link linkend="pcm-interface-runtime-private"><citetitle>
2669 Private Data</citetitle></link> section.
2670 </para>
2671
2672 <para>
2673 If the hardware configuration needs more constraints, set the
2674 hardware constraints here, too.
2675 See <link linkend="pcm-interface-constraints"><citetitle>
2676 Constraints</citetitle></link> for more details.
2677 </para>
2678 </section>
2679
2680 <section id="pcm-interface-operators-close-callback">
2681 <title>close callback</title>
2682 <para>
2683 <informalexample>
2684 <programlisting>
2685<![CDATA[
2686 static int snd_xxx_close(snd_pcm_substream_t *substream);
2687]]>
2688 </programlisting>
2689 </informalexample>
2690
2691 Obviously, this is called when a pcm substream is closed.
2692 </para>
2693
2694 <para>
2695 Any private instance for a pcm substream allocated in the
2696 open callback will be released here.
2697
2698 <informalexample>
2699 <programlisting>
2700<![CDATA[
2701 static int snd_xxx_close(snd_pcm_substream_t *substream)
2702 {
2703 ....
2704 kfree(substream->runtime->private_data);
2705 ....
2706 }
2707]]>
2708 </programlisting>
2709 </informalexample>
2710 </para>
2711 </section>
2712
2713 <section id="pcm-interface-operators-ioctl-callback">
2714 <title>ioctl callback</title>
2715 <para>
2716 This is used for any special action to pcm ioctls. But
2717 usually you can pass a generic ioctl callback,
2718 <function>snd_pcm_lib_ioctl</function>.
2719 </para>
2720 </section>
2721
2722 <section id="pcm-interface-operators-hw-params-callback">
2723 <title>hw_params callback</title>
2724 <para>
2725 <informalexample>
2726 <programlisting>
2727<![CDATA[
2728 static int snd_xxx_hw_params(snd_pcm_substream_t * substream,
2729 snd_pcm_hw_params_t * hw_params);
2730]]>
2731 </programlisting>
2732 </informalexample>
2733
2734 This and <structfield>hw_free</structfield> callbacks exist
2735 only on ALSA 0.9.x.
2736 </para>
2737
2738 <para>
2739 This is called when the hardware parameter
2740 (<structfield>hw_params</structfield>) is set
2741 up by the application,
2742 that is, once when the buffer size, the period size, the
2743 format, etc. are defined for the pcm substream.
2744 </para>
2745
2746 <para>
2747 Many hardware set-up should be done in this callback,
2748 including the allocation of buffers.
2749 </para>
2750
2751 <para>
2752 Parameters to be initialized are retrieved by
2753 <function>params_xxx()</function> macros. For allocating a
2754 buffer, you can call a helper function,
2755
2756 <informalexample>
2757 <programlisting>
2758<![CDATA[
2759 snd_pcm_lib_malloc_pages(substream, params_buffer_bytes(hw_params));
2760]]>
2761 </programlisting>
2762 </informalexample>
2763
2764 <function>snd_pcm_lib_malloc_pages()</function> is available
2765 only when the DMA buffers have been pre-allocated.
2766 See the section <link
2767 linkend="buffer-and-memory-buffer-types"><citetitle>
2768 Buffer Types</citetitle></link> for more details.
2769 </para>
2770
2771 <para>
2772 Note that this and <structfield>prepare</structfield> callbacks
2773 may be called multiple times per initialization.
2774 For example, the OSS emulation may
2775 call these callbacks at each change via its ioctl.
2776 </para>
2777
2778 <para>
2779 Thus, you need to take care not to allocate the same buffers
2780 many times, which will lead to memory leak! Calling the
2781 helper function above many times is OK. It will release the
2782 previous buffer automatically when it was already allocated.
2783 </para>
2784
2785 <para>
2786 Another note is that this callback is non-atomic
2787 (schedulable). This is important, because the
2788 <structfield>trigger</structfield> callback
2789 is atomic (non-schedulable). That is, mutex or any
2790 schedule-related functions are not available in
2791 <structfield>trigger</structfield> callback.
2792 Please see the subsection
2793 <link linkend="pcm-interface-atomicity"><citetitle>
2794 Atomicity</citetitle></link> for details.
2795 </para>
2796 </section>
2797
2798 <section id="pcm-interface-operators-hw-free-callback">
2799 <title>hw_free callback</title>
2800 <para>
2801 <informalexample>
2802 <programlisting>
2803<![CDATA[
2804 static int snd_xxx_hw_free(snd_pcm_substream_t * substream);
2805]]>
2806 </programlisting>
2807 </informalexample>
2808 </para>
2809
2810 <para>
2811 This is called to release the resources allocated via
2812 <structfield>hw_params</structfield>. For example, releasing the
2813 buffer via
2814 <function>snd_pcm_lib_malloc_pages()</function> is done by
2815 calling the following:
2816
2817 <informalexample>
2818 <programlisting>
2819<![CDATA[
2820 snd_pcm_lib_free_pages(substream);
2821]]>
2822 </programlisting>
2823 </informalexample>
2824 </para>
2825
2826 <para>
2827 This function is always called before the close callback is called.
2828 Also, the callback may be called multiple times, too.
2829 Keep track whether the resource was already released.
2830 </para>
2831 </section>
2832
2833 <section id="pcm-interface-operators-prepare-callback">
2834 <title>prepare callback</title>
2835 <para>
2836 <informalexample>
2837 <programlisting>
2838<![CDATA[
2839 static int snd_xxx_prepare(snd_pcm_substream_t * substream);
2840]]>
2841 </programlisting>
2842 </informalexample>
2843 </para>
2844
2845 <para>
2846 This callback is called when the pcm is
2847 <quote>prepared</quote>. You can set the format type, sample
2848 rate, etc. here. The difference from
2849 <structfield>hw_params</structfield> is that the
2850 <structfield>prepare</structfield> callback will be called at each
2851 time
2852 <function>snd_pcm_prepare()</function> is called, i.e. when
2853 recovered after underruns, etc.
2854 </para>
2855
2856 <para>
2857 Note that this callback became non-atomic since the recent version.
2858 You can use schedule-related fucntions safely in this callback now.
2859 </para>
2860
2861 <para>
2862 In this and the following callbacks, you can refer to the
2863 values via the runtime record,
2864 substream-&gt;runtime.
2865 For example, to get the current
2866 rate, format or channels, access to
2867 runtime-&gt;rate,
2868 runtime-&gt;format or
2869 runtime-&gt;channels, respectively.
2870 The physical address of the allocated buffer is set to
2871 runtime-&gt;dma_area. The buffer and period sizes are
2872 in runtime-&gt;buffer_size and runtime-&gt;period_size,
2873 respectively.
2874 </para>
2875
2876 <para>
2877 Be careful that this callback will be called many times at
2878 each set up, too.
2879 </para>
2880 </section>
2881
2882 <section id="pcm-interface-operators-trigger-callback">
2883 <title>trigger callback</title>
2884 <para>
2885 <informalexample>
2886 <programlisting>
2887<![CDATA[
2888 static int snd_xxx_trigger(snd_pcm_substream_t * substream, int cmd);
2889]]>
2890 </programlisting>
2891 </informalexample>
2892
2893 This is called when the pcm is started, stopped or paused.
2894 </para>
2895
2896 <para>
2897 Which action is specified in the second argument,
2898 <constant>SNDRV_PCM_TRIGGER_XXX</constant> in
2899 <filename>&lt;sound/pcm.h&gt;</filename>. At least,
2900 <constant>START</constant> and <constant>STOP</constant>
2901 commands must be defined in this callback.
2902
2903 <informalexample>
2904 <programlisting>
2905<![CDATA[
2906 switch (cmd) {
2907 case SNDRV_PCM_TRIGGER_START:
2908 // do something to start the PCM engine
2909 break;
2910 case SNDRV_PCM_TRIGGER_STOP:
2911 // do something to stop the PCM engine
2912 break;
2913 default:
2914 return -EINVAL;
2915 }
2916]]>
2917 </programlisting>
2918 </informalexample>
2919 </para>
2920
2921 <para>
2922 When the pcm supports the pause operation (given in info
2923 field of the hardware table), <constant>PAUSE_PUSE</constant>
2924 and <constant>PAUSE_RELEASE</constant> commands must be
2925 handled here, too. The former is the command to pause the pcm,
2926 and the latter to restart the pcm again.
2927 </para>
2928
2929 <para>
2930 When the pcm supports the suspend/resume operation
2931 (i.e. <constant>SNDRV_PCM_INFO_RESUME</constant> flag is set),
2932 <constant>SUSPEND</constant> and <constant>RESUME</constant>
2933 commands must be handled, too.
2934 These commands are issued when the power-management status is
2935 changed. Obviously, the <constant>SUSPEND</constant> and
2936 <constant>RESUME</constant>
2937 do suspend and resume of the pcm substream, and usually, they
2938 are identical with <constant>STOP</constant> and
2939 <constant>START</constant> commands, respectively.
2940 </para>
2941
2942 <para>
2943 As mentioned, this callback is atomic. You cannot call
2944 the function going to sleep.
2945 The trigger callback should be as minimal as possible,
2946 just really triggering the DMA. The other stuff should be
2947 initialized hw_params and prepare callbacks properly
2948 beforehand.
2949 </para>
2950 </section>
2951
2952 <section id="pcm-interface-operators-pointer-callback">
2953 <title>pointer callback</title>
2954 <para>
2955 <informalexample>
2956 <programlisting>
2957<![CDATA[
2958 static snd_pcm_uframes_t snd_xxx_pointer(snd_pcm_substream_t * substream)
2959]]>
2960 </programlisting>
2961 </informalexample>
2962
2963 This callback is called when the PCM middle layer inquires
2964 the current hardware position on the buffer. The position must
2965 be returned in frames (which was in bytes on ALSA 0.5.x),
2966 ranged from 0 to buffer_size - 1.
2967 </para>
2968
2969 <para>
2970 This is called usually from the buffer-update routine in the
2971 pcm middle layer, which is invoked when
2972 <function>snd_pcm_period_elapsed()</function> is called in the
2973 interrupt routine. Then the pcm middle layer updates the
2974 position and calculates the available space, and wakes up the
2975 sleeping poll threads, etc.
2976 </para>
2977
2978 <para>
2979 This callback is also atomic.
2980 </para>
2981 </section>
2982
2983 <section id="pcm-interface-operators-copy-silence">
2984 <title>copy and silence callbacks</title>
2985 <para>
2986 These callbacks are not mandatory, and can be omitted in
2987 most cases. These callbacks are used when the hardware buffer
2988 cannot be on the normal memory space. Some chips have their
2989 own buffer on the hardware which is not mappable. In such a
2990 case, you have to transfer the data manually from the memory
2991 buffer to the hardware buffer. Or, if the buffer is
2992 non-contiguous on both physical and virtual memory spaces,
2993 these callbacks must be defined, too.
2994 </para>
2995
2996 <para>
2997 If these two callbacks are defined, copy and set-silence
2998 operations are done by them. The detailed will be described in
2999 the later section <link
3000 linkend="buffer-and-memory"><citetitle>Buffer and Memory
3001 Management</citetitle></link>.
3002 </para>
3003 </section>
3004
3005 <section id="pcm-interface-operators-ack">
3006 <title>ack callback</title>
3007 <para>
3008 This callback is also not mandatory. This callback is called
3009 when the appl_ptr is updated in read or write operations.
3010 Some drivers like emu10k1-fx and cs46xx need to track the
3011 current appl_ptr for the internal buffer, and this callback
3012 is useful only for such a purpose.
3013 </para>
3014 <para>
3015 This callback is atomic.
3016 </para>
3017 </section>
3018
3019 <section id="pcm-interface-operators-page-callback">
3020 <title>page callback</title>
3021
3022 <para>
3023 This callback is also not mandatory. This callback is used
3024 mainly for the non-contiguous buffer. The mmap calls this
3025 callback to get the page address. Some examples will be
3026 explained in the later section <link
3027 linkend="buffer-and-memory"><citetitle>Buffer and Memory
3028 Management</citetitle></link>, too.
3029 </para>
3030 </section>
3031 </section>
3032
3033 <section id="pcm-interface-interrupt-handler">
3034 <title>Interrupt Handler</title>
3035 <para>
3036 The rest of pcm stuff is the PCM interrupt handler. The
3037 role of PCM interrupt handler in the sound driver is to update
3038 the buffer position and to tell the PCM middle layer when the
3039 buffer position goes across the prescribed period size. To
3040 inform this, call <function>snd_pcm_period_elapsed()</function>
3041 function.
3042 </para>
3043
3044 <para>
3045 There are several types of sound chips to generate the interrupts.
3046 </para>
3047
3048 <section id="pcm-interface-interrupt-handler-boundary">
3049 <title>Interrupts at the period (fragment) boundary</title>
3050 <para>
3051 This is the most frequently found type: the hardware
3052 generates an interrupt at each period boundary.
3053 In this case, you can call
3054 <function>snd_pcm_period_elapsed()</function> at each
3055 interrupt.
3056 </para>
3057
3058 <para>
3059 <function>snd_pcm_period_elapsed()</function> takes the
3060 substream pointer as its argument. Thus, you need to keep the
3061 substream pointer accessible from the chip instance. For
3062 example, define substream field in the chip record to hold the
3063 current running substream pointer, and set the pointer value
3064 at open callback (and reset at close callback).
3065 </para>
3066
3067 <para>
3068 If you aquire a spinlock in the interrupt handler, and the
3069 lock is used in other pcm callbacks, too, then you have to
3070 release the lock before calling
3071 <function>snd_pcm_period_elapsed()</function>, because
3072 <function>snd_pcm_period_elapsed()</function> calls other pcm
3073 callbacks inside.
3074 </para>
3075
3076 <para>
3077 A typical coding would be like:
3078
3079 <example>
3080 <title>Interrupt Handler Case #1</title>
3081 <programlisting>
3082<![CDATA[
3083 static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id,
3084 struct pt_regs *regs)
3085 {
3086 mychip_t *chip = dev_id;
3087 spin_lock(&chip->lock);
3088 ....
3089 if (pcm_irq_invoked(chip)) {
3090 /* call updater, unlock before it */
3091 spin_unlock(&chip->lock);
3092 snd_pcm_period_elapsed(chip->substream);
3093 spin_lock(&chip->lock);
3094 // acknowledge the interrupt if necessary
3095 }
3096 ....
3097 spin_unlock(&chip->lock);
3098 return IRQ_HANDLED;
3099 }
3100]]>
3101 </programlisting>
3102 </example>
3103 </para>
3104 </section>
3105
3106 <section id="pcm-interface-interrupt-handler-timer">
3107 <title>High-frequent timer interrupts</title>
3108 <para>
3109 This is the case when the hardware doesn't generate interrupts
3110 at the period boundary but do timer-interrupts at the fixed
3111 timer rate (e.g. es1968 or ymfpci drivers).
3112 In this case, you need to check the current hardware
3113 position and accumulates the processed sample length at each
3114 interrupt. When the accumulated size overcomes the period
3115 size, call
3116 <function>snd_pcm_period_elapsed()</function> and reset the
3117 accumulator.
3118 </para>
3119
3120 <para>
3121 A typical coding would be like the following.
3122
3123 <example>
3124 <title>Interrupt Handler Case #2</title>
3125 <programlisting>
3126<![CDATA[
3127 static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id,
3128 struct pt_regs *regs)
3129 {
3130 mychip_t *chip = dev_id;
3131 spin_lock(&chip->lock);
3132 ....
3133 if (pcm_irq_invoked(chip)) {
3134 unsigned int last_ptr, size;
3135 /* get the current hardware pointer (in frames) */
3136 last_ptr = get_hw_ptr(chip);
3137 /* calculate the processed frames since the
3138 * last update
3139 */
3140 if (last_ptr < chip->last_ptr)
3141 size = runtime->buffer_size + last_ptr
3142 - chip->last_ptr;
3143 else
3144 size = last_ptr - chip->last_ptr;
3145 /* remember the last updated point */
3146 chip->last_ptr = last_ptr;
3147 /* accumulate the size */
3148 chip->size += size;
3149 /* over the period boundary? */
3150 if (chip->size >= runtime->period_size) {
3151 /* reset the accumulator */
3152 chip->size %= runtime->period_size;
3153 /* call updater */
3154 spin_unlock(&chip->lock);
3155 snd_pcm_period_elapsed(substream);
3156 spin_lock(&chip->lock);
3157 }
3158 // acknowledge the interrupt if necessary
3159 }
3160 ....
3161 spin_unlock(&chip->lock);
3162 return IRQ_HANDLED;
3163 }
3164]]>
3165 </programlisting>
3166 </example>
3167 </para>
3168 </section>
3169
3170 <section id="pcm-interface-interrupt-handler-both">
3171 <title>On calling <function>snd_pcm_period_elapsed()</function></title>
3172 <para>
3173 In both cases, even if more than one period are elapsed, you
3174 don't have to call
3175 <function>snd_pcm_period_elapsed()</function> many times. Call
3176 only once. And the pcm layer will check the current hardware
3177 pointer and update to the latest status.
3178 </para>
3179 </section>
3180 </section>
3181
3182 <section id="pcm-interface-atomicity">
3183 <title>Atomicity</title>
3184 <para>
3185 One of the most important (and thus difficult to debug) problem
3186 on the kernel programming is the race condition.
3187 On linux kernel, usually it's solved via spin-locks or
3188 semaphores. In general, if the race condition may
3189 happen in the interrupt handler, it's handled as atomic, and you
3190 have to use spinlock for protecting the critical session. If it
3191 never happens in the interrupt and it may take relatively long
3192 time, you should use semaphore.
3193 </para>
3194
3195 <para>
3196 As already seen, some pcm callbacks are atomic and some are
3197 not. For example, <parameter>hw_params</parameter> callback is
3198 non-atomic, while <parameter>trigger</parameter> callback is
3199 atomic. This means, the latter is called already in a spinlock
3200 held by the PCM middle layer. Please take this atomicity into
3201 account when you use a spinlock or a semaphore in the callbacks.
3202 </para>
3203
3204 <para>
3205 In the atomic callbacks, you cannot use functions which may call
3206 <function>schedule</function> or go to
3207 <function>sleep</function>. The semaphore and mutex do sleep,
3208 and hence they cannot be used inside the atomic callbacks
3209 (e.g. <parameter>trigger</parameter> callback).
3210 For taking a certain delay in such a callback, please use
3211 <function>udelay()</function> or <function>mdelay()</function>.
3212 </para>
3213
3214 <para>
3215 All three atomic callbacks (trigger, pointer, and ack) are
3216 called with local interrupts disabled.
3217 </para>
3218
3219 </section>
3220 <section id="pcm-interface-constraints">
3221 <title>Constraints</title>
3222 <para>
3223 If your chip supports unconventional sample rates, or only the
3224 limited samples, you need to set a constraint for the
3225 condition.
3226 </para>
3227
3228 <para>
3229 For example, in order to restrict the sample rates in the some
3230 supported values, use
3231 <function>snd_pcm_hw_constraint_list()</function>.
3232 You need to call this function in the open callback.
3233
3234 <example>
3235 <title>Example of Hardware Constraints</title>
3236 <programlisting>
3237<![CDATA[
3238 static unsigned int rates[] =
3239 {4000, 10000, 22050, 44100};
3240 static snd_pcm_hw_constraint_list_t constraints_rates = {
3241 .count = ARRAY_SIZE(rates),
3242 .list = rates,
3243 .mask = 0,
3244 };
3245
3246 static int snd_mychip_pcm_open(snd_pcm_substream_t *substream)
3247 {
3248 int err;
3249 ....
3250 err = snd_pcm_hw_constraint_list(substream->runtime, 0,
3251 SNDRV_PCM_HW_PARAM_RATE,
3252 &constraints_rates);
3253 if (err < 0)
3254 return err;
3255 ....
3256 }
3257]]>
3258 </programlisting>
3259 </example>
3260 </para>
3261
3262 <para>
3263 There are many different constraints.
3264 Look in <filename>sound/pcm.h</filename> for a complete list.
3265 You can even define your own constraint rules.
3266 For example, let's suppose my_chip can manage a substream of 1 channel
3267 if and only if the format is S16_LE, otherwise it supports any format
3268 specified in the <type>snd_pcm_hardware_t</type> stucture (or in any
3269 other constraint_list). You can build a rule like this:
3270
3271 <example>
3272 <title>Example of Hardware Constraints for Channels</title>
3273 <programlisting>
3274<![CDATA[
3275 static int hw_rule_format_by_channels(snd_pcm_hw_params_t *params,
3276 snd_pcm_hw_rule_t *rule)
3277 {
3278 snd_interval_t *c = hw_param_interval(params, SNDRV_PCM_HW_PARAM_CHANNELS);
3279 snd_mask_t *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT);
3280 snd_mask_t fmt;
3281
3282 snd_mask_any(&fmt); /* Init the struct */
3283 if (c->min < 2) {
3284 fmt.bits[0] &= SNDRV_PCM_FMTBIT_S16_LE;
3285 return snd_mask_refine(f, &fmt);
3286 }
3287 return 0;
3288 }
3289]]>
3290 </programlisting>
3291 </example>
3292 </para>
3293
3294 <para>
3295 Then you need to call this function to add your rule:
3296
3297 <informalexample>
3298 <programlisting>
3299<![CDATA[
3300 snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_CHANNELS,
3301 hw_rule_channels_by_format, 0, SNDRV_PCM_HW_PARAM_FORMAT,
3302 -1);
3303]]>
3304 </programlisting>
3305 </informalexample>
3306 </para>
3307
3308 <para>
3309 The rule function is called when an application sets the number of
3310 channels. But an application can set the format before the number of
3311 channels. Thus you also need to define the inverse rule:
3312
3313 <example>
3314 <title>Example of Hardware Constraints for Channels</title>
3315 <programlisting>
3316<![CDATA[
3317 static int hw_rule_channels_by_format(snd_pcm_hw_params_t *params,
3318 snd_pcm_hw_rule_t *rule)
3319 {
3320 snd_interval_t *c = hw_param_interval(params, SNDRV_PCM_HW_PARAM_CHANNELS);
3321 snd_mask_t *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT);
3322 snd_interval_t ch;
3323
3324 snd_interval_any(&ch);
3325 if (f->bits[0] == SNDRV_PCM_FMTBIT_S16_LE) {
3326 ch.min = ch.max = 1;
3327 ch.integer = 1;
3328 return snd_interval_refine(c, &ch);
3329 }
3330 return 0;
3331 }
3332]]>
3333 </programlisting>
3334 </example>
3335 </para>
3336
3337 <para>
3338 ...and in the open callback:
3339 <informalexample>
3340 <programlisting>
3341<![CDATA[
3342 snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_FORMAT,
3343 hw_rule_format_by_channels, 0, SNDRV_PCM_HW_PARAM_CHANNELS,
3344 -1);
3345]]>
3346 </programlisting>
3347 </informalexample>
3348 </para>
3349
3350 <para>
3351 I won't explain more details here, rather I
3352 would like to say, <quote>Luke, use the source.</quote>
3353 </para>
3354 </section>
3355
3356 </chapter>
3357
3358
3359<!-- ****************************************************** -->
3360<!-- Control Interface -->
3361<!-- ****************************************************** -->
3362 <chapter id="control-interface">
3363 <title>Control Interface</title>
3364
3365 <section id="control-interface-general">
3366 <title>General</title>
3367 <para>
3368 The control interface is used widely for many switches,
3369 sliders, etc. which are accessed from the user-space. Its most
3370 important use is the mixer interface. In other words, on ALSA
3371 0.9.x, all the mixer stuff is implemented on the control kernel
3372 API (while there was an independent mixer kernel API on 0.5.x).
3373 </para>
3374
3375 <para>
3376 ALSA has a well-defined AC97 control module. If your chip
3377 supports only the AC97 and nothing else, you can skip this
3378 section.
3379 </para>
3380
3381 <para>
3382 The control API is defined in
3383 <filename>&lt;sound/control.h&gt;</filename>.
3384 Include this file if you add your own controls.
3385 </para>
3386 </section>
3387
3388 <section id="control-interface-definition">
3389 <title>Definition of Controls</title>
3390 <para>
3391 For creating a new control, you need to define the three
3392 callbacks: <structfield>info</structfield>,
3393 <structfield>get</structfield> and
3394 <structfield>put</structfield>. Then, define a
3395 <type>snd_kcontrol_new_t</type> record, such as:
3396
3397 <example>
3398 <title>Definition of a Control</title>
3399 <programlisting>
3400<![CDATA[
3401 static snd_kcontrol_new_t my_control __devinitdata = {
3402 .iface = SNDRV_CTL_ELEM_IFACE_MIXER,
3403 .name = "PCM Playback Switch",
3404 .index = 0,
3405 .access = SNDRV_CTL_ELEM_ACCESS_READWRITE,
3406 .private_values = 0xffff,
3407 .info = my_control_info,
3408 .get = my_control_get,
3409 .put = my_control_put
3410 };
3411]]>
3412 </programlisting>
3413 </example>
3414 </para>
3415
3416 <para>
3417 Most likely the control is created via
3418 <function>snd_ctl_new1()</function>, and in such a case, you can
3419 add <parameter>__devinitdata</parameter> prefix to the
3420 definition like above.
3421 </para>
3422
3423 <para>
3424 The <structfield>iface</structfield> field specifies the type of
3425 the control,
3426 <constant>SNDRV_CTL_ELEM_IFACE_XXX</constant>. There are
3427 <constant>MIXER</constant>, <constant>PCM</constant>,
3428 <constant>CARD</constant>, etc.
3429 </para>
3430
3431 <para>
3432 The <structfield>name</structfield> is the name identifier
3433 string. On ALSA 0.9.x, the control name is very important,
3434 because its role is classified from its name. There are
3435 pre-defined standard control names. The details are described in
3436 the subsection
3437 <link linkend="control-interface-control-names"><citetitle>
3438 Control Names</citetitle></link>.
3439 </para>
3440
3441 <para>
3442 The <structfield>index</structfield> field holds the index number
3443 of this control. If there are several different controls with
3444 the same name, they can be distinguished by the index
3445 number. This is the case when
3446 several codecs exist on the card. If the index is zero, you can
3447 omit the definition above.
3448 </para>
3449
3450 <para>
3451 The <structfield>access</structfield> field contains the access
3452 type of this control. Give the combination of bit masks,
3453 <constant>SNDRV_CTL_ELEM_ACCESS_XXX</constant>, there.
3454 The detailed will be explained in the subsection
3455 <link linkend="control-interface-access-flags"><citetitle>
3456 Access Flags</citetitle></link>.
3457 </para>
3458
3459 <para>
3460 The <structfield>private_values</structfield> field contains
3461 an arbitrary long integer value for this record. When using
3462 generic <structfield>info</structfield>,
3463 <structfield>get</structfield> and
3464 <structfield>put</structfield> callbacks, you can pass a value
3465 through this field. If several small numbers are necessary, you can
3466 combine them in bitwise. Or, it's possible to give a pointer
3467 (casted to unsigned long) of some record to this field, too.
3468 </para>
3469
3470 <para>
3471 The other three are
3472 <link linkend="control-interface-callbacks"><citetitle>
3473 callback functions</citetitle></link>.
3474 </para>
3475 </section>
3476
3477 <section id="control-interface-control-names">
3478 <title>Control Names</title>
3479 <para>
3480 There are some standards for defining the control names. A
3481 control is usually defined from the three parts as
3482 <quote>SOURCE DIRECTION FUNCTION</quote>.
3483 </para>
3484
3485 <para>
3486 The first, <constant>SOURCE</constant>, specifies the source
3487 of the control, and is a string such as <quote>Master</quote>,
3488 <quote>PCM</quote>, <quote>CD</quote> or
3489 <quote>Line</quote>. There are many pre-defined sources.
3490 </para>
3491
3492 <para>
3493 The second, <constant>DIRECTION</constant>, is one of the
3494 following strings according to the direction of the control:
3495 <quote>Playback</quote>, <quote>Capture</quote>, <quote>Bypass
3496 Playback</quote> and <quote>Bypass Capture</quote>. Or, it can
3497 be omitted, meaning both playback and capture directions.
3498 </para>
3499
3500 <para>
3501 The third, <constant>FUNCTION</constant>, is one of the
3502 following strings according to the function of the control:
3503 <quote>Switch</quote>, <quote>Volume</quote> and
3504 <quote>Route</quote>.
3505 </para>
3506
3507 <para>
3508 The example of control names are, thus, <quote>Master Capture
3509 Switch</quote> or <quote>PCM Playback Volume</quote>.
3510 </para>
3511
3512 <para>
3513 There are some exceptions:
3514 </para>
3515
3516 <section id="control-interface-control-names-global">
3517 <title>Global capture and playback</title>
3518 <para>
3519 <quote>Capture Source</quote>, <quote>Capture Switch</quote>
3520 and <quote>Capture Volume</quote> are used for the global
3521 capture (input) source, switch and volume. Similarly,
3522 <quote>Playback Switch</quote> and <quote>Playback
3523 Volume</quote> are used for the global output gain switch and
3524 volume.
3525 </para>
3526 </section>
3527
3528 <section id="control-interface-control-names-tone">
3529 <title>Tone-controls</title>
3530 <para>
3531 tone-control switch and volumes are specified like
3532 <quote>Tone Control - XXX</quote>, e.g. <quote>Tone Control -
3533 Switch</quote>, <quote>Tone Control - Bass</quote>,
3534 <quote>Tone Control - Center</quote>.
3535 </para>
3536 </section>
3537
3538 <section id="control-interface-control-names-3d">
3539 <title>3D controls</title>
3540 <para>
3541 3D-control switches and volumes are specified like <quote>3D
3542 Control - XXX</quote>, e.g. <quote>3D Control -
3543 Switch</quote>, <quote>3D Control - Center</quote>, <quote>3D
3544 Control - Space</quote>.
3545 </para>
3546 </section>
3547
3548 <section id="control-interface-control-names-mic">
3549 <title>Mic boost</title>
3550 <para>
3551 Mic-boost switch is set as <quote>Mic Boost</quote> or
3552 <quote>Mic Boost (6dB)</quote>.
3553 </para>
3554
3555 <para>
3556 More precise information can be found in
3557 <filename>Documentation/sound/alsa/ControlNames.txt</filename>.
3558 </para>
3559 </section>
3560 </section>
3561
3562 <section id="control-interface-access-flags">
3563 <title>Access Flags</title>
3564
3565 <para>
3566 The access flag is the bit-flags which specifies the access type
3567 of the given control. The default access type is
3568 <constant>SNDRV_CTL_ELEM_ACCESS_READWRITE</constant>,
3569 which means both read and write are allowed to this control.
3570 When the access flag is omitted (i.e. = 0), it is
3571 regarded as <constant>READWRITE</constant> access as default.
3572 </para>
3573
3574 <para>
3575 When the control is read-only, pass
3576 <constant>SNDRV_CTL_ELEM_ACCESS_READ</constant> instead.
3577 In this case, you don't have to define
3578 <structfield>put</structfield> callback.
3579 Similarly, when the control is write-only (although it's a rare
3580 case), you can use <constant>WRITE</constant> flag instead, and
3581 you don't need <structfield>get</structfield> callback.
3582 </para>
3583
3584 <para>
3585 If the control value changes frequently (e.g. the VU meter),
3586 <constant>VOLATILE</constant> flag should be given. This means
3587 that the control may be changed without
3588 <link linkend="control-interface-change-notification"><citetitle>
3589 notification</citetitle></link>. Applications should poll such
3590 a control constantly.
3591 </para>
3592
3593 <para>
3594 When the control is inactive, set
3595 <constant>INACTIVE</constant> flag, too.
3596 There are <constant>LOCK</constant> and
3597 <constant>OWNER</constant> flags for changing the write
3598 permissions.
3599 </para>
3600
3601 </section>
3602
3603 <section id="control-interface-callbacks">
3604 <title>Callbacks</title>
3605
3606 <section id="control-interface-callbacks-info">
3607 <title>info callback</title>
3608 <para>
3609 The <structfield>info</structfield> callback is used to get
3610 the detailed information of this control. This must store the
3611 values of the given <type>snd_ctl_elem_info_t</type>
3612 object. For example, for a boolean control with a single
3613 element will be:
3614
3615 <example>
3616 <title>Example of info callback</title>
3617 <programlisting>
3618<![CDATA[
3619 static int snd_myctl_info(snd_kcontrol_t *kcontrol,
3620 snd_ctl_elem_info_t *uinfo)
3621 {
3622 uinfo->type = SNDRV_CTL_ELEM_TYPE_BOOLEAN;
3623 uinfo->count = 1;
3624 uinfo->value.integer.min = 0;
3625 uinfo->value.integer.max = 1;
3626 return 0;
3627 }
3628]]>
3629 </programlisting>
3630 </example>
3631 </para>
3632
3633 <para>
3634 The <structfield>type</structfield> field specifies the type
3635 of the control. There are <constant>BOOLEAN</constant>,
3636 <constant>INTEGER</constant>, <constant>ENUMERATED</constant>,
3637 <constant>BYTES</constant>, <constant>IEC958</constant> and
3638 <constant>INTEGER64</constant>. The
3639 <structfield>count</structfield> field specifies the
3640 number of elements in this control. For example, a stereo
3641 volume would have count = 2. The
3642 <structfield>value</structfield> field is a union, and
3643 the values stored are depending on the type. The boolean and
3644 integer are identical.
3645 </para>
3646
3647 <para>
3648 The enumerated type is a bit different from others. You'll
3649 need to set the string for the currently given item index.
3650
3651 <informalexample>
3652 <programlisting>
3653<![CDATA[
3654 static int snd_myctl_info(snd_kcontrol_t *kcontrol,
3655 snd_ctl_elem_info_t *uinfo)
3656 {
3657 static char *texts[4] = {
3658 "First", "Second", "Third", "Fourth"
3659 };
3660 uinfo->type = SNDRV_CTL_ELEM_TYPE_ENUMERATED;
3661 uinfo->count = 1;
3662 uinfo->value.enumerated.items = 4;
3663 if (uinfo->value.enumerated.item > 3)
3664 uinfo->value.enumerated.item = 3;
3665 strcpy(uinfo->value.enumerated.name,
3666 texts[uinfo->value.enumerated.item]);
3667 return 0;
3668 }
3669]]>
3670 </programlisting>
3671 </informalexample>
3672 </para>
3673 </section>
3674
3675 <section id="control-interface-callbacks-get">
3676 <title>get callback</title>
3677
3678 <para>
3679 This callback is used to read the current value of the
3680 control and to return to the user-space.
3681 </para>
3682
3683 <para>
3684 For example,
3685
3686 <example>
3687 <title>Example of get callback</title>
3688 <programlisting>
3689<![CDATA[
3690 static int snd_myctl_get(snd_kcontrol_t *kcontrol,
3691 snd_ctl_elem_value_t *ucontrol)
3692 {
3693 mychip_t *chip = snd_kcontrol_chip(kcontrol);
3694 ucontrol->value.integer.value[0] = get_some_value(chip);
3695 return 0;
3696 }
3697]]>
3698 </programlisting>
3699 </example>
3700 </para>
3701
3702 <para>
3703 Here, the chip instance is retrieved via
3704 <function>snd_kcontrol_chip()</function> macro. This macro
3705 converts from kcontrol-&gt;private_data to the type defined by
3706 <type>chip_t</type>. The
3707 kcontrol-&gt;private_data field is
3708 given as the argument of <function>snd_ctl_new()</function>
3709 (see the later subsection
3710 <link linkend="control-interface-constructor"><citetitle>Constructor</citetitle></link>).
3711 </para>
3712
3713 <para>
3714 The <structfield>value</structfield> field is depending on
3715 the type of control as well as on info callback. For example,
3716 the sb driver uses this field to store the register offset,
3717 the bit-shift and the bit-mask. The
3718 <structfield>private_value</structfield> is set like
3719 <informalexample>
3720 <programlisting>
3721<![CDATA[
3722 .private_value = reg | (shift << 16) | (mask << 24)
3723]]>
3724 </programlisting>
3725 </informalexample>
3726 and is retrieved in callbacks like
3727 <informalexample>
3728 <programlisting>
3729<![CDATA[
3730 static int snd_sbmixer_get_single(snd_kcontrol_t *kcontrol,
3731 snd_ctl_elem_value_t *ucontrol)
3732 {
3733 int reg = kcontrol->private_value & 0xff;
3734 int shift = (kcontrol->private_value >> 16) & 0xff;
3735 int mask = (kcontrol->private_value >> 24) & 0xff;
3736 ....
3737 }
3738]]>
3739 </programlisting>
3740 </informalexample>
3741 </para>
3742
3743 <para>
3744 In <structfield>get</structfield> callback, you have to fill all the elements if the
3745 control has more than one elements,
3746 i.e. <structfield>count</structfield> &gt; 1.
3747 In the example above, we filled only one element
3748 (<structfield>value.integer.value[0]</structfield>) since it's
3749 assumed as <structfield>count</structfield> = 1.
3750 </para>
3751 </section>
3752
3753 <section id="control-interface-callbacks-put">
3754 <title>put callback</title>
3755
3756 <para>
3757 This callback is used to write a value from the user-space.
3758 </para>
3759
3760 <para>
3761 For example,
3762
3763 <example>
3764 <title>Example of put callback</title>
3765 <programlisting>
3766<![CDATA[
3767 static int snd_myctl_put(snd_kcontrol_t *kcontrol,
3768 snd_ctl_elem_value_t *ucontrol)
3769 {
3770 mychip_t *chip = snd_kcontrol_chip(kcontrol);
3771 int changed = 0;
3772 if (chip->current_value !=
3773 ucontrol->value.integer.value[0]) {
3774 change_current_value(chip,
3775 ucontrol->value.integer.value[0]);
3776 changed = 1;
3777 }
3778 return changed;
3779 }
3780]]>
3781 </programlisting>
3782 </example>
3783
3784 As seen above, you have to return 1 if the value is
3785 changed. If the value is not changed, return 0 instead.
3786 If any fatal error happens, return a negative error code as
3787 usual.
3788 </para>
3789
3790 <para>
3791 Like <structfield>get</structfield> callback,
3792 when the control has more than one elements,
3793 all elemehts must be evaluated in this callback, too.
3794 </para>
3795 </section>
3796
3797 <section id="control-interface-callbacks-all">
3798 <title>Callbacks are not atomic</title>
3799 <para>
3800 All these three callbacks are basically not atomic.
3801 </para>
3802 </section>
3803 </section>
3804
3805 <section id="control-interface-constructor">
3806 <title>Constructor</title>
3807 <para>
3808 When everything is ready, finally we can create a new
3809 control. For creating a control, there are two functions to be
3810 called, <function>snd_ctl_new1()</function> and
3811 <function>snd_ctl_add()</function>.
3812 </para>
3813
3814 <para>
3815 In the simplest way, you can do like this:
3816
3817 <informalexample>
3818 <programlisting>
3819<![CDATA[
3820 if ((err = snd_ctl_add(card, snd_ctl_new1(&my_control, chip))) < 0)
3821 return err;
3822]]>
3823 </programlisting>
3824 </informalexample>
3825
3826 where <parameter>my_control</parameter> is the
3827 <type>snd_kcontrol_new_t</type> object defined above, and chip
3828 is the object pointer to be passed to
3829 kcontrol-&gt;private_data
3830 which can be referred in callbacks.
3831 </para>
3832
3833 <para>
3834 <function>snd_ctl_new1()</function> allocates a new
3835 <type>snd_kcontrol_t</type> instance (that's why the definition
3836 of <parameter>my_control</parameter> can be with
3837 <parameter>__devinitdata</parameter>
3838 prefix), and <function>snd_ctl_add</function> assigns the given
3839 control component to the card.
3840 </para>
3841 </section>
3842
3843 <section id="control-interface-change-notification">
3844 <title>Change Notification</title>
3845 <para>
3846 If you need to change and update a control in the interrupt
3847 routine, you can call <function>snd_ctl_notify()</function>. For
3848 example,
3849
3850 <informalexample>
3851 <programlisting>
3852<![CDATA[
3853 snd_ctl_notify(card, SNDRV_CTL_EVENT_MASK_VALUE, id_pointer);
3854]]>
3855 </programlisting>
3856 </informalexample>
3857
3858 This function takes the card pointer, the event-mask, and the
3859 control id pointer for the notification. The event-mask
3860 specifies the types of notification, for example, in the above
3861 example, the change of control values is notified.
3862 The id pointer is the pointer of <type>snd_ctl_elem_id_t</type>
3863 to be notified.
3864 You can find some examples in <filename>es1938.c</filename> or
3865 <filename>es1968.c</filename> for hardware volume interrupts.
3866 </para>
3867 </section>
3868
3869 </chapter>
3870
3871
3872<!-- ****************************************************** -->
3873<!-- API for AC97 Codec -->
3874<!-- ****************************************************** -->
3875 <chapter id="api-ac97">
3876 <title>API for AC97 Codec</title>
3877
3878 <section>
3879 <title>General</title>
3880 <para>
3881 The ALSA AC97 codec layer is a well-defined one, and you don't
3882 have to write many codes to control it. Only low-level control
3883 routines are necessary. The AC97 codec API is defined in
3884 <filename>&lt;sound/ac97_codec.h&gt;</filename>.
3885 </para>
3886 </section>
3887
3888 <section id="api-ac97-example">
3889 <title>Full Code Example</title>
3890 <para>
3891 <example>
3892 <title>Example of AC97 Interface</title>
3893 <programlisting>
3894<![CDATA[
3895 struct snd_mychip {
3896 ....
3897 ac97_t *ac97;
3898 ....
3899 };
3900
3901 static unsigned short snd_mychip_ac97_read(ac97_t *ac97,
3902 unsigned short reg)
3903 {
3904 mychip_t *chip = ac97->private_data;
3905 ....
3906 // read a register value here from the codec
3907 return the_register_value;
3908 }
3909
3910 static void snd_mychip_ac97_write(ac97_t *ac97,
3911 unsigned short reg, unsigned short val)
3912 {
3913 mychip_t *chip = ac97->private_data;
3914 ....
3915 // write the given register value to the codec
3916 }
3917
3918 static int snd_mychip_ac97(mychip_t *chip)
3919 {
3920 ac97_bus_t *bus;
3921 ac97_template_t ac97;
3922 int err;
3923 static ac97_bus_ops_t ops = {
3924 .write = snd_mychip_ac97_write,
3925 .read = snd_mychip_ac97_read,
3926 };
3927
3928 if ((err = snd_ac97_bus(chip->card, 0, &ops, NULL, &bus)) < 0)
3929 return err;
3930 memset(&ac97, 0, sizeof(ac97));
3931 ac97.private_data = chip;
3932 return snd_ac97_mixer(bus, &ac97, &chip->ac97);
3933 }
3934
3935]]>
3936 </programlisting>
3937 </example>
3938 </para>
3939 </section>
3940
3941 <section id="api-ac97-constructor">
3942 <title>Constructor</title>
3943 <para>
3944 For creating an ac97 instance, first call <function>snd_ac97_bus</function>
3945 with an <type>ac97_bus_ops_t</type> record with callback functions.
3946
3947 <informalexample>
3948 <programlisting>
3949<![CDATA[
3950 ac97_bus_t *bus;
3951 static ac97_bus_ops_t ops = {
3952 .write = snd_mychip_ac97_write,
3953 .read = snd_mychip_ac97_read,
3954 };
3955
3956 snd_ac97_bus(card, 0, &ops, NULL, &pbus);
3957]]>
3958 </programlisting>
3959 </informalexample>
3960
3961 The bus record is shared among all belonging ac97 instances.
3962 </para>
3963
3964 <para>
3965 And then call <function>snd_ac97_mixer()</function> with an <type>ac97_template_t</type>
3966 record together with the bus pointer created above.
3967
3968 <informalexample>
3969 <programlisting>
3970<![CDATA[
3971 ac97_template_t ac97;
3972 int err;
3973
3974 memset(&ac97, 0, sizeof(ac97));
3975 ac97.private_data = chip;
3976 snd_ac97_mixer(bus, &ac97, &chip->ac97);
3977]]>
3978 </programlisting>
3979 </informalexample>
3980
3981 where chip-&gt;ac97 is the pointer of a newly created
3982 <type>ac97_t</type> instance.
3983 In this case, the chip pointer is set as the private data, so that
3984 the read/write callback functions can refer to this chip instance.
3985 This instance is not necessarily stored in the chip
3986 record. When you need to change the register values from the
3987 driver, or need the suspend/resume of ac97 codecs, keep this
3988 pointer to pass to the corresponding functions.
3989 </para>
3990 </section>
3991
3992 <section id="api-ac97-callbacks">
3993 <title>Callbacks</title>
3994 <para>
3995 The standard callbacks are <structfield>read</structfield> and
3996 <structfield>write</structfield>. Obviously they
3997 correspond to the functions for read and write accesses to the
3998 hardware low-level codes.
3999 </para>
4000
4001 <para>
4002 The <structfield>read</structfield> callback returns the
4003 register value specified in the argument.
4004
4005 <informalexample>
4006 <programlisting>
4007<![CDATA[
4008 static unsigned short snd_mychip_ac97_read(ac97_t *ac97,
4009 unsigned short reg)
4010 {
4011 mychip_t *chip = ac97->private_data;
4012 ....
4013 return the_register_value;
4014 }
4015]]>
4016 </programlisting>
4017 </informalexample>
4018
4019 Here, the chip can be cast from ac97-&gt;private_data.
4020 </para>
4021
4022 <para>
4023 Meanwhile, the <structfield>write</structfield> callback is
4024 used to set the register value.
4025
4026 <informalexample>
4027 <programlisting>
4028<![CDATA[
4029 static void snd_mychip_ac97_write(ac97_t *ac97,
4030 unsigned short reg, unsigned short val)
4031]]>
4032 </programlisting>
4033 </informalexample>
4034 </para>
4035
4036 <para>
4037 These callbacks are non-atomic like the callbacks of control API.
4038 </para>
4039
4040 <para>
4041 There are also other callbacks:
4042 <structfield>reset</structfield>,
4043 <structfield>wait</structfield> and
4044 <structfield>init</structfield>.
4045 </para>
4046
4047 <para>
4048 The <structfield>reset</structfield> callback is used to reset
4049 the codec. If the chip requires a special way of reset, you can
4050 define this callback.
4051 </para>
4052
4053 <para>
4054 The <structfield>wait</structfield> callback is used for a
4055 certain wait at the standard initialization of the codec. If the
4056 chip requires the extra wait-time, define this callback.
4057 </para>
4058
4059 <para>
4060 The <structfield>init</structfield> callback is used for
4061 additional initialization of the codec.
4062 </para>
4063 </section>
4064
4065 <section id="api-ac97-updating-registers">
4066 <title>Updating Registers in The Driver</title>
4067 <para>
4068 If you need to access to the codec from the driver, you can
4069 call the following functions:
4070 <function>snd_ac97_write()</function>,
4071 <function>snd_ac97_read()</function>,
4072 <function>snd_ac97_update()</function> and
4073 <function>snd_ac97_update_bits()</function>.
4074 </para>
4075
4076 <para>
4077 Both <function>snd_ac97_write()</function> and
4078 <function>snd_ac97_update()</function> functions are used to
4079 set a value to the given register
4080 (<constant>AC97_XXX</constant>). The difference between them is
4081 that <function>snd_ac97_update()</function> doesn't write a
4082 value if the given value has been already set, while
4083 <function>snd_ac97_write()</function> always rewrites the
4084 value.
4085
4086 <informalexample>
4087 <programlisting>
4088<![CDATA[
4089 snd_ac97_write(ac97, AC97_MASTER, 0x8080);
4090 snd_ac97_update(ac97, AC97_MASTER, 0x8080);
4091]]>
4092 </programlisting>
4093 </informalexample>
4094 </para>
4095
4096 <para>
4097 <function>snd_ac97_read()</function> is used to read the value
4098 of the given register. For example,
4099
4100 <informalexample>
4101 <programlisting>
4102<![CDATA[
4103 value = snd_ac97_read(ac97, AC97_MASTER);
4104]]>
4105 </programlisting>
4106 </informalexample>
4107 </para>
4108
4109 <para>
4110 <function>snd_ac97_update_bits()</function> is used to update
4111 some bits of the given register.
4112
4113 <informalexample>
4114 <programlisting>
4115<![CDATA[
4116 snd_ac97_update_bits(ac97, reg, mask, value);
4117]]>
4118 </programlisting>
4119 </informalexample>
4120 </para>
4121
4122 <para>
4123 Also, there is a function to change the sample rate (of a
4124 certain register such as
4125 <constant>AC97_PCM_FRONT_DAC_RATE</constant>) when VRA or
4126 DRA is supported by the codec:
4127 <function>snd_ac97_set_rate()</function>.
4128
4129 <informalexample>
4130 <programlisting>
4131<![CDATA[
4132 snd_ac97_set_rate(ac97, AC97_PCM_FRONT_DAC_RATE, 44100);
4133]]>
4134 </programlisting>
4135 </informalexample>
4136 </para>
4137
4138 <para>
4139 The following registers are available for setting the rate:
4140 <constant>AC97_PCM_MIC_ADC_RATE</constant>,
4141 <constant>AC97_PCM_FRONT_DAC_RATE</constant>,
4142 <constant>AC97_PCM_LR_ADC_RATE</constant>,
4143 <constant>AC97_SPDIF</constant>. When the
4144 <constant>AC97_SPDIF</constant> is specified, the register is
4145 not really changed but the corresponding IEC958 status bits will
4146 be updated.
4147 </para>
4148 </section>
4149
4150 <section id="api-ac97-clock-adjustment">
4151 <title>Clock Adjustment</title>
4152 <para>
4153 On some chip, the clock of the codec isn't 48000 but using a
4154 PCI clock (to save a quartz!). In this case, change the field
4155 bus-&gt;clock to the corresponding
4156 value. For example, intel8x0
4157 and es1968 drivers have the auto-measurement function of the
4158 clock.
4159 </para>
4160 </section>
4161
4162 <section id="api-ac97-proc-files">
4163 <title>Proc Files</title>
4164 <para>
4165 The ALSA AC97 interface will create a proc file such as
4166 <filename>/proc/asound/card0/codec97#0/ac97#0-0</filename> and
4167 <filename>ac97#0-0+regs</filename>. You can refer to these files to
4168 see the current status and registers of the codec.
4169 </para>
4170 </section>
4171
4172 <section id="api-ac97-multiple-codecs">
4173 <title>Multiple Codecs</title>
4174 <para>
4175 When there are several codecs on the same card, you need to
4176 call <function>snd_ac97_new()</function> multiple times with
4177 ac97.num=1 or greater. The <structfield>num</structfield> field
4178 specifies the codec
4179 number.
4180 </para>
4181
4182 <para>
4183 If you have set up multiple codecs, you need to either write
4184 different callbacks for each codec or check
4185 ac97-&gt;num in the
4186 callback routines.
4187 </para>
4188 </section>
4189
4190 </chapter>
4191
4192
4193<!-- ****************************************************** -->
4194<!-- MIDI (MPU401-UART) Interface -->
4195<!-- ****************************************************** -->
4196 <chapter id="midi-interface">
4197 <title>MIDI (MPU401-UART) Interface</title>
4198
4199 <section id="midi-interface-general">
4200 <title>General</title>
4201 <para>
4202 Many soundcards have built-in MIDI (MPU401-UART)
4203 interfaces. When the soundcard supports the standard MPU401-UART
4204 interface, most likely you can use the ALSA MPU401-UART API. The
4205 MPU401-UART API is defined in
4206 <filename>&lt;sound/mpu401.h&gt;</filename>.
4207 </para>
4208
4209 <para>
4210 Some soundchips have similar but a little bit different
4211 implementation of mpu401 stuff. For example, emu10k1 has its own
4212 mpu401 routines.
4213 </para>
4214 </section>
4215
4216 <section id="midi-interface-constructor">
4217 <title>Constructor</title>
4218 <para>
4219 For creating a rawmidi object, call
4220 <function>snd_mpu401_uart_new()</function>.
4221
4222 <informalexample>
4223 <programlisting>
4224<![CDATA[
4225 snd_rawmidi_t *rmidi;
4226 snd_mpu401_uart_new(card, 0, MPU401_HW_MPU401, port, integrated,
4227 irq, irq_flags, &rmidi);
4228]]>
4229 </programlisting>
4230 </informalexample>
4231 </para>
4232
4233 <para>
4234 The first argument is the card pointer, and the second is the
4235 index of this component. You can create up to 8 rawmidi
4236 devices.
4237 </para>
4238
4239 <para>
4240 The third argument is the type of the hardware,
4241 <constant>MPU401_HW_XXX</constant>. If it's not a special one,
4242 you can use <constant>MPU401_HW_MPU401</constant>.
4243 </para>
4244
4245 <para>
4246 The 4th argument is the i/o port address. Many
4247 backward-compatible MPU401 has an i/o port such as 0x330. Or, it
4248 might be a part of its own PCI i/o region. It depends on the
4249 chip design.
4250 </para>
4251
4252 <para>
4253 When the i/o port address above is a part of the PCI i/o
4254 region, the MPU401 i/o port might have been already allocated
4255 (reserved) by the driver itself. In such a case, pass non-zero
4256 to the 5th argument
4257 (<parameter>integrated</parameter>). Otherwise, pass 0 to it,
4258 and
4259 the mpu401-uart layer will allocate the i/o ports by itself.
4260 </para>
4261
4262 <para>
4263 Usually, the port address corresponds to the command port and
4264 port + 1 corresponds to the data port. If not, you may change
4265 the <structfield>cport</structfield> field of
4266 <type>mpu401_t</type> manually
4267 afterward. However, <type>mpu401_t</type> pointer is not
4268 returned explicitly by
4269 <function>snd_mpu401_uart_new()</function>. You need to cast
4270 rmidi-&gt;private_data to
4271 <type>mpu401_t</type> explicitly,
4272
4273 <informalexample>
4274 <programlisting>
4275<![CDATA[
4276 mpu401_t *mpu;
4277 mpu = rmidi->private_data;
4278]]>
4279 </programlisting>
4280 </informalexample>
4281
4282 and reset the cport as you like:
4283
4284 <informalexample>
4285 <programlisting>
4286<![CDATA[
4287 mpu->cport = my_own_control_port;
4288]]>
4289 </programlisting>
4290 </informalexample>
4291 </para>
4292
4293 <para>
4294 The 6th argument specifies the irq number for UART. If the irq
4295 is already allocated, pass 0 to the 7th argument
4296 (<parameter>irq_flags</parameter>). Otherwise, pass the flags
4297 for irq allocation
4298 (<constant>SA_XXX</constant> bits) to it, and the irq will be
4299 reserved by the mpu401-uart layer. If the card doesn't generates
4300 UART interrupts, pass -1 as the irq number. Then a timer
4301 interrupt will be invoked for polling.
4302 </para>
4303 </section>
4304
4305 <section id="midi-interface-interrupt-handler">
4306 <title>Interrupt Handler</title>
4307 <para>
4308 When the interrupt is allocated in
4309 <function>snd_mpu401_uart_new()</function>, the private
4310 interrupt handler is used, hence you don't have to do nothing
4311 else than creating the mpu401 stuff. Otherwise, you have to call
4312 <function>snd_mpu401_uart_interrupt()</function> explicitly when
4313 a UART interrupt is invoked and checked in your own interrupt
4314 handler.
4315 </para>
4316
4317 <para>
4318 In this case, you need to pass the private_data of the
4319 returned rawmidi object from
4320 <function>snd_mpu401_uart_new()</function> as the second
4321 argument of <function>snd_mpu401_uart_interrupt()</function>.
4322
4323 <informalexample>
4324 <programlisting>
4325<![CDATA[
4326 snd_mpu401_uart_interrupt(irq, rmidi->private_data, regs);
4327]]>
4328 </programlisting>
4329 </informalexample>
4330 </para>
4331 </section>
4332
4333 </chapter>
4334
4335
4336<!-- ****************************************************** -->
4337<!-- RawMIDI Interface -->
4338<!-- ****************************************************** -->
4339 <chapter id="rawmidi-interface">
4340 <title>RawMIDI Interface</title>
4341
4342 <section id="rawmidi-interface-overview">
4343 <title>Overview</title>
4344
4345 <para>
4346 The raw MIDI interface is used for hardware MIDI ports that can
4347 be accessed as a byte stream. It is not used for synthesizer
4348 chips that do not directly understand MIDI.
4349 </para>
4350
4351 <para>
4352 ALSA handles file and buffer management. All you have to do is
4353 to write some code to move data between the buffer and the
4354 hardware.
4355 </para>
4356
4357 <para>
4358 The rawmidi API is defined in
4359 <filename>&lt;sound/rawmidi.h&gt;</filename>.
4360 </para>
4361 </section>
4362
4363 <section id="rawmidi-interface-constructor">
4364 <title>Constructor</title>
4365
4366 <para>
4367 To create a rawmidi device, call the
4368 <function>snd_rawmidi_new</function> function:
4369 <informalexample>
4370 <programlisting>
4371<![CDATA[
4372 snd_rawmidi_t *rmidi;
4373 err = snd_rawmidi_new(chip->card, "MyMIDI", 0, outs, ins, &rmidi);
4374 if (err < 0)
4375 return err;
4376 rmidi->private_data = chip;
4377 strcpy(rmidi->name, "My MIDI");
4378 rmidi->info_flags = SNDRV_RAWMIDI_INFO_OUTPUT |
4379 SNDRV_RAWMIDI_INFO_INPUT |
4380 SNDRV_RAWMIDI_INFO_DUPLEX;
4381]]>
4382 </programlisting>
4383 </informalexample>
4384 </para>
4385
4386 <para>
4387 The first argument is the card pointer, the second argument is
4388 the ID string.
4389 </para>
4390
4391 <para>
4392 The third argument is the index of this component. You can
4393 create up to 8 rawmidi devices.
4394 </para>
4395
4396 <para>
4397 The fourth and fifth arguments are the number of output and
4398 input substreams, respectively, of this device. (A substream is
4399 the equivalent of a MIDI port.)
4400 </para>
4401
4402 <para>
4403 Set the <structfield>info_flags</structfield> field to specify
4404 the capabilities of the device.
4405 Set <constant>SNDRV_RAWMIDI_INFO_OUTPUT</constant> if there is
4406 at least one output port,
4407 <constant>SNDRV_RAWMIDI_INFO_INPUT</constant> if there is at
4408 least one input port,
4409 and <constant>SNDRV_RAWMIDI_INFO_DUPLEX</constant> if the device
4410 can handle output and input at the same time.
4411 </para>
4412
4413 <para>
4414 After the rawmidi device is created, you need to set the
4415 operators (callbacks) for each substream. There are helper
4416 functions to set the operators for all substream of a device:
4417 <informalexample>
4418 <programlisting>
4419<![CDATA[
4420 snd_rawmidi_set_ops(rmidi, SNDRV_RAWMIDI_STREAM_OUTPUT, &snd_mymidi_output_ops);
4421 snd_rawmidi_set_ops(rmidi, SNDRV_RAWMIDI_STREAM_INPUT, &snd_mymidi_input_ops);
4422]]>
4423 </programlisting>
4424 </informalexample>
4425 </para>
4426
4427 <para>
4428 The operators are usually defined like this:
4429 <informalexample>
4430 <programlisting>
4431<![CDATA[
4432 static snd_rawmidi_ops_t snd_mymidi_output_ops = {
4433 .open = snd_mymidi_output_open,
4434 .close = snd_mymidi_output_close,
4435 .trigger = snd_mymidi_output_trigger,
4436 };
4437]]>
4438 </programlisting>
4439 </informalexample>
4440 These callbacks are explained in the <link
4441 linkend="rawmidi-interface-callbacks"><citetitle>Callbacks</citetitle></link>
4442 section.
4443 </para>
4444
4445 <para>
4446 If there is more than one substream, you should give each one a
4447 unique name:
4448 <informalexample>
4449 <programlisting>
4450<![CDATA[
4451 struct list_head *list;
4452 snd_rawmidi_substream_t *substream;
4453 list_for_each(list, &rmidi->streams[SNDRV_RAWMIDI_STREAM_OUTPUT].substreams) {
4454 substream = list_entry(list, snd_rawmidi_substream_t, list);
4455 sprintf(substream->name, "My MIDI Port %d", substream->number + 1);
4456 }
4457 /* same for SNDRV_RAWMIDI_STREAM_INPUT */
4458]]>
4459 </programlisting>
4460 </informalexample>
4461 </para>
4462 </section>
4463
4464 <section id="rawmidi-interface-callbacks">
4465 <title>Callbacks</title>
4466
4467 <para>
4468 In all callbacks, the private data that you've set for the
4469 rawmidi device can be accessed as
4470 substream-&gt;rmidi-&gt;private_data.
4471 <!-- <code> isn't available before DocBook 4.3 -->
4472 </para>
4473
4474 <para>
4475 If there is more than one port, your callbacks can determine the
4476 port index from the snd_rawmidi_substream_t data passed to each
4477 callback:
4478 <informalexample>
4479 <programlisting>
4480<![CDATA[
4481 snd_rawmidi_substream_t *substream;
4482 int index = substream->number;
4483]]>
4484 </programlisting>
4485 </informalexample>
4486 </para>
4487
4488 <section id="rawmidi-interface-op-open">
4489 <title><function>open</function> callback</title>
4490
4491 <informalexample>
4492 <programlisting>
4493<![CDATA[
4494 static int snd_xxx_open(snd_rawmidi_substream_t *substream);
4495]]>
4496 </programlisting>
4497 </informalexample>
4498
4499 <para>
4500 This is called when a substream is opened.
4501 You can initialize the hardware here, but you should not yet
4502 start transmitting/receiving data.
4503 </para>
4504 </section>
4505
4506 <section id="rawmidi-interface-op-close">
4507 <title><function>close</function> callback</title>
4508
4509 <informalexample>
4510 <programlisting>
4511<![CDATA[
4512 static int snd_xxx_close(snd_rawmidi_substream_t *substream);
4513]]>
4514 </programlisting>
4515 </informalexample>
4516
4517 <para>
4518 Guess what.
4519 </para>
4520
4521 <para>
4522 The <function>open</function> and <function>close</function>
4523 callbacks of a rawmidi device are serialized with a mutex,
4524 and can sleep.
4525 </para>
4526 </section>
4527
4528 <section id="rawmidi-interface-op-trigger-out">
4529 <title><function>trigger</function> callback for output
4530 substreams</title>
4531
4532 <informalexample>
4533 <programlisting>
4534<![CDATA[
4535 static void snd_xxx_output_trigger(snd_rawmidi_substream_t *substream, int up);
4536]]>
4537 </programlisting>
4538 </informalexample>
4539
4540 <para>
4541 This is called with a nonzero <parameter>up</parameter>
4542 parameter when there is some data in the substream buffer that
4543 must be transmitted.
4544 </para>
4545
4546 <para>
4547 To read data from the buffer, call
4548 <function>snd_rawmidi_transmit_peek</function>. It will
4549 return the number of bytes that have been read; this will be
4550 less than the number of bytes requested when there is no more
4551 data in the buffer.
4552 After the data has been transmitted successfully, call
4553 <function>snd_rawmidi_transmit_ack</function> to remove the
4554 data from the substream buffer:
4555 <informalexample>
4556 <programlisting>
4557<![CDATA[
4558 unsigned char data;
4559 while (snd_rawmidi_transmit_peek(substream, &data, 1) == 1) {
4560 if (mychip_try_to_transmit(data))
4561 snd_rawmidi_transmit_ack(substream, 1);
4562 else
4563 break; /* hardware FIFO full */
4564 }
4565]]>
4566 </programlisting>
4567 </informalexample>
4568 </para>
4569
4570 <para>
4571 If you know beforehand that the hardware will accept data, you
4572 can use the <function>snd_rawmidi_transmit</function> function
4573 which reads some data and removes it from the buffer at once:
4574 <informalexample>
4575 <programlisting>
4576<![CDATA[
4577 while (mychip_transmit_possible()) {
4578 unsigned char data;
4579 if (snd_rawmidi_transmit(substream, &data, 1) != 1)
4580 break; /* no more data */
4581 mychip_transmit(data);
4582 }
4583]]>
4584 </programlisting>
4585 </informalexample>
4586 </para>
4587
4588 <para>
4589 If you know beforehand how many bytes you can accept, you can
4590 use a buffer size greater than one with the
4591 <function>snd_rawmidi_transmit*</function> functions.
4592 </para>
4593
4594 <para>
4595 The <function>trigger</function> callback must not sleep. If
4596 the hardware FIFO is full before the substream buffer has been
4597 emptied, you have to continue transmitting data later, either
4598 in an interrupt handler, or with a timer if the hardware
4599 doesn't have a MIDI transmit interrupt.
4600 </para>
4601
4602 <para>
4603 The <function>trigger</function> callback is called with a
4604 zero <parameter>up</parameter> parameter when the transmission
4605 of data should be aborted.
4606 </para>
4607 </section>
4608
4609 <section id="rawmidi-interface-op-trigger-in">
4610 <title><function>trigger</function> callback for input
4611 substreams</title>
4612
4613 <informalexample>
4614 <programlisting>
4615<![CDATA[
4616 static void snd_xxx_input_trigger(snd_rawmidi_substream_t *substream, int up);
4617]]>
4618 </programlisting>
4619 </informalexample>
4620
4621 <para>
4622 This is called with a nonzero <parameter>up</parameter>
4623 parameter to enable receiving data, or with a zero
4624 <parameter>up</parameter> parameter do disable receiving data.
4625 </para>
4626
4627 <para>
4628 The <function>trigger</function> callback must not sleep; the
4629 actual reading of data from the device is usually done in an
4630 interrupt handler.
4631 </para>
4632
4633 <para>
4634 When data reception is enabled, your interrupt handler should
4635 call <function>snd_rawmidi_receive</function> for all received
4636 data:
4637 <informalexample>
4638 <programlisting>
4639<![CDATA[
4640 void snd_mychip_midi_interrupt(...)
4641 {
4642 while (mychip_midi_available()) {
4643 unsigned char data;
4644 data = mychip_midi_read();
4645 snd_rawmidi_receive(substream, &data, 1);
4646 }
4647 }
4648]]>
4649 </programlisting>
4650 </informalexample>
4651 </para>
4652 </section>
4653
4654 <section id="rawmidi-interface-op-drain">
4655 <title><function>drain</function> callback</title>
4656
4657 <informalexample>
4658 <programlisting>
4659<![CDATA[
4660 static void snd_xxx_drain(snd_rawmidi_substream_t *substream);
4661]]>
4662 </programlisting>
4663 </informalexample>
4664
4665 <para>
4666 This is only used with output substreams. This function should wait
4667 until all data read from the substream buffer has been transmitted.
4668 This ensures that the device can be closed and the driver unloaded
4669 without losing data.
4670 </para>
4671
4672 <para>
4673 This callback is optional. If you do not set
4674 <structfield>drain</structfield> in the snd_rawmidi_ops_t
4675 structure, ALSA will simply wait for 50&nbsp;milliseconds
4676 instead.
4677 </para>
4678 </section>
4679 </section>
4680
4681 </chapter>
4682
4683
4684<!-- ****************************************************** -->
4685<!-- Miscellaneous Devices -->
4686<!-- ****************************************************** -->
4687 <chapter id="misc-devices">
4688 <title>Miscellaneous Devices</title>
4689
4690 <section id="misc-devices-opl3">
4691 <title>FM OPL3</title>
4692 <para>
4693 The FM OPL3 is still used on many chips (mainly for backward
4694 compatibility). ALSA has a nice OPL3 FM control layer, too. The
4695 OPL3 API is defined in
4696 <filename>&lt;sound/opl3.h&gt;</filename>.
4697 </para>
4698
4699 <para>
4700 FM registers can be directly accessed through direct-FM API,
4701 defined in <filename>&lt;sound/asound_fm.h&gt;</filename>. In
4702 ALSA native mode, FM registers are accessed through
4703 Hardware-Dependant Device direct-FM extension API, whereas in
4704 OSS compatible mode, FM registers can be accessed with OSS
4705 direct-FM compatible API on <filename>/dev/dmfmX</filename> device.
4706 </para>
4707
4708 <para>
4709 For creating the OPL3 component, you have two functions to
4710 call. The first one is a constructor for <type>opl3_t</type>
4711 instance.
4712
4713 <informalexample>
4714 <programlisting>
4715<![CDATA[
4716 opl3_t *opl3;
4717 snd_opl3_create(card, lport, rport, OPL3_HW_OPL3_XXX,
4718 integrated, &opl3);
4719]]>
4720 </programlisting>
4721 </informalexample>
4722 </para>
4723
4724 <para>
4725 The first argument is the card pointer, the second one is the
4726 left port address, and the third is the right port address. In
4727 most cases, the right port is placed at the left port + 2.
4728 </para>
4729
4730 <para>
4731 The fourth argument is the hardware type.
4732 </para>
4733
4734 <para>
4735 When the left and right ports have been already allocated by
4736 the card driver, pass non-zero to the fifth argument
4737 (<parameter>integrated</parameter>). Otherwise, opl3 module will
4738 allocate the specified ports by itself.
4739 </para>
4740
4741 <para>
4742 When the accessing to the hardware requires special method
4743 instead of the standard I/O access, you can create opl3 instance
4744 separately with <function>snd_opl3_new()</function>.
4745
4746 <informalexample>
4747 <programlisting>
4748<![CDATA[
4749 opl3_t *opl3;
4750 snd_opl3_new(card, OPL3_HW_OPL3_XXX, &opl3);
4751]]>
4752 </programlisting>
4753 </informalexample>
4754 </para>
4755
4756 <para>
4757 Then set <structfield>command</structfield>,
4758 <structfield>private_data</structfield> and
4759 <structfield>private_free</structfield> for the private
4760 access function, the private data and the destructor.
4761 The l_port and r_port are not necessarily set. Only the
4762 command must be set properly. You can retrieve the data
4763 from opl3-&gt;private_data field.
4764 </para>
4765
4766 <para>
4767 After creating the opl3 instance via <function>snd_opl3_new()</function>,
4768 call <function>snd_opl3_init()</function> to initialize the chip to the
4769 proper state. Note that <function>snd_opl3_create()</function> always
4770 calls it internally.
4771 </para>
4772
4773 <para>
4774 If the opl3 instance is created successfully, then create a
4775 hwdep device for this opl3.
4776
4777 <informalexample>
4778 <programlisting>
4779<![CDATA[
4780 snd_hwdep_t *opl3hwdep;
4781 snd_opl3_hwdep_new(opl3, 0, 1, &opl3hwdep);
4782]]>
4783 </programlisting>
4784 </informalexample>
4785 </para>
4786
4787 <para>
4788 The first argument is the <type>opl3_t</type> instance you
4789 created, and the second is the index number, usually 0.
4790 </para>
4791
4792 <para>
4793 The third argument is the index-offset for the sequencer
4794 client assigned to the OPL3 port. When there is an MPU401-UART,
4795 give 1 for here (UART always takes 0).
4796 </para>
4797 </section>
4798
4799 <section id="misc-devices-hardware-dependent">
4800 <title>Hardware-Dependent Devices</title>
4801 <para>
4802 Some chips need the access from the user-space for special
4803 controls or for loading the micro code. In such a case, you can
4804 create a hwdep (hardware-dependent) device. The hwdep API is
4805 defined in <filename>&lt;sound/hwdep.h&gt;</filename>. You can
4806 find examples in opl3 driver or
4807 <filename>isa/sb/sb16_csp.c</filename>.
4808 </para>
4809
4810 <para>
4811 Creation of the <type>hwdep</type> instance is done via
4812 <function>snd_hwdep_new()</function>.
4813
4814 <informalexample>
4815 <programlisting>
4816<![CDATA[
4817 snd_hwdep_t *hw;
4818 snd_hwdep_new(card, "My HWDEP", 0, &hw);
4819]]>
4820 </programlisting>
4821 </informalexample>
4822
4823 where the third argument is the index number.
4824 </para>
4825
4826 <para>
4827 You can then pass any pointer value to the
4828 <parameter>private_data</parameter>.
4829 If you assign a private data, you should define the
4830 destructor, too. The destructor function is set to
4831 <structfield>private_free</structfield> field.
4832
4833 <informalexample>
4834 <programlisting>
4835<![CDATA[
4836 mydata_t *p = kmalloc(sizeof(*p), GFP_KERNEL);
4837 hw->private_data = p;
4838 hw->private_free = mydata_free;
4839]]>
4840 </programlisting>
4841 </informalexample>
4842
4843 and the implementation of destructor would be:
4844
4845 <informalexample>
4846 <programlisting>
4847<![CDATA[
4848 static void mydata_free(snd_hwdep_t *hw)
4849 {
4850 mydata_t *p = hw->private_data;
4851 kfree(p);
4852 }
4853]]>
4854 </programlisting>
4855 </informalexample>
4856 </para>
4857
4858 <para>
4859 The arbitrary file operations can be defined for this
4860 instance. The file operators are defined in
4861 <parameter>ops</parameter> table. For example, assume that
4862 this chip needs an ioctl.
4863
4864 <informalexample>
4865 <programlisting>
4866<![CDATA[
4867 hw->ops.open = mydata_open;
4868 hw->ops.ioctl = mydata_ioctl;
4869 hw->ops.release = mydata_release;
4870]]>
4871 </programlisting>
4872 </informalexample>
4873
4874 And implement the callback functions as you like.
4875 </para>
4876 </section>
4877
4878 <section id="misc-devices-IEC958">
4879 <title>IEC958 (S/PDIF)</title>
4880 <para>
4881 Usually the controls for IEC958 devices are implemented via
4882 control interface. There is a macro to compose a name string for
4883 IEC958 controls, <function>SNDRV_CTL_NAME_IEC958()</function>
4884 defined in <filename>&lt;include/asound.h&gt;</filename>.
4885 </para>
4886
4887 <para>
4888 There are some standard controls for IEC958 status bits. These
4889 controls use the type <type>SNDRV_CTL_ELEM_TYPE_IEC958</type>,
4890 and the size of element is fixed as 4 bytes array
4891 (value.iec958.status[x]). For <structfield>info</structfield>
4892 callback, you don't specify
4893 the value field for this type (the count field must be set,
4894 though).
4895 </para>
4896
4897 <para>
4898 <quote>IEC958 Playback Con Mask</quote> is used to return the
4899 bit-mask for the IEC958 status bits of consumer mode. Similarly,
4900 <quote>IEC958 Playback Pro Mask</quote> returns the bitmask for
4901 professional mode. They are read-only controls, and are defined
4902 as MIXER controls (iface =
4903 <constant>SNDRV_CTL_ELEM_IFACE_MIXER</constant>).
4904 </para>
4905
4906 <para>
4907 Meanwhile, <quote>IEC958 Playback Default</quote> control is
4908 defined for getting and setting the current default IEC958
4909 bits. Note that this one is usually defined as a PCM control
4910 (iface = <constant>SNDRV_CTL_ELEM_IFACE_PCM</constant>),
4911 although in some places it's defined as a MIXER control.
4912 </para>
4913
4914 <para>
4915 In addition, you can define the control switches to
4916 enable/disable or to set the raw bit mode. The implementation
4917 will depend on the chip, but the control should be named as
4918 <quote>IEC958 xxx</quote>, preferably using
4919 <function>SNDRV_CTL_NAME_IEC958()</function> macro.
4920 </para>
4921
4922 <para>
4923 You can find several cases, for example,
4924 <filename>pci/emu10k1</filename>,
4925 <filename>pci/ice1712</filename>, or
4926 <filename>pci/cmipci.c</filename>.
4927 </para>
4928 </section>
4929
4930 </chapter>
4931
4932
4933<!-- ****************************************************** -->
4934<!-- Buffer and Memory Management -->
4935<!-- ****************************************************** -->
4936 <chapter id="buffer-and-memory">
4937 <title>Buffer and Memory Management</title>
4938
4939 <section id="buffer-and-memory-buffer-types">
4940 <title>Buffer Types</title>
4941 <para>
4942 ALSA provides several different buffer allocation functions
4943 depending on the bus and the architecture. All these have a
4944 consistent API. The allocation of physically-contiguous pages is
4945 done via
4946 <function>snd_malloc_xxx_pages()</function> function, where xxx
4947 is the bus type.
4948 </para>
4949
4950 <para>
4951 The allocation of pages with fallback is
4952 <function>snd_malloc_xxx_pages_fallback()</function>. This
4953 function tries to allocate the specified pages but if the pages
4954 are not available, it tries to reduce the page sizes until the
4955 enough space is found.
4956 </para>
4957
4958 <para>
4959 For releasing the space, call
4960 <function>snd_free_xxx_pages()</function> function.
4961 </para>
4962
4963 <para>
4964 Usually, ALSA drivers try to allocate and reserve
4965 a large contiguous physical space
4966 at the time the module is loaded for the later use.
4967 This is called <quote>pre-allocation</quote>.
4968 As already written, you can call the following function at the
4969 construction of pcm instance (in the case of PCI bus).
4970
4971 <informalexample>
4972 <programlisting>
4973<![CDATA[
4974 snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
4975 snd_dma_pci_data(pci), size, max);
4976]]>
4977 </programlisting>
4978 </informalexample>
4979
4980 where <parameter>size</parameter> is the byte size to be
4981 pre-allocated and the <parameter>max</parameter> is the maximal
4982 size to be changed via <filename>prealloc</filename> proc file.
4983 The allocator will try to get as large area as possible
4984 within the given size.
4985 </para>
4986
4987 <para>
4988 The second argument (type) and the third argument (device pointer)
4989 are dependent on the bus.
4990 In the case of ISA bus, pass <function>snd_dma_isa_data()</function>
4991 as the third argument with <constant>SNDRV_DMA_TYPE_DEV</constant> type.
4992 For the continuous buffer unrelated to the bus can be pre-allocated
4993 with <constant>SNDRV_DMA_TYPE_CONTINUOUS</constant> type and the
4994 <function>snd_dma_continuous_data(GFP_KERNEL)</function> device pointer,
4995 whereh <constant>GFP_KERNEL</constant> is the kernel allocation flag to
4996 use. For the SBUS, <constant>SNDRV_DMA_TYPE_SBUS</constant> and
4997 <function>snd_dma_sbus_data(sbus_dev)</function> are used instead.
4998 For the PCI scatter-gather buffers, use
4999 <constant>SNDRV_DMA_TYPE_DEV_SG</constant> with
5000 <function>snd_dma_pci_data(pci)</function>
5001 (see the section
5002 <link linkend="buffer-and-memory-non-contiguous"><citetitle>Non-Contiguous Buffers
5003 </citetitle></link>).
5004 </para>
5005
5006 <para>
5007 Once when the buffer is pre-allocated, you can use the
5008 allocator in the <structfield>hw_params</structfield> callback
5009
5010 <informalexample>
5011 <programlisting>
5012<![CDATA[
5013 snd_pcm_lib_malloc_pages(substream, size);
5014]]>
5015 </programlisting>
5016 </informalexample>
5017
5018 Note that you have to pre-allocate to use this function.
5019 </para>
5020 </section>
5021
5022 <section id="buffer-and-memory-external-hardware">
5023 <title>External Hardware Buffers</title>
5024 <para>
5025 Some chips have their own hardware buffers and the DMA
5026 transfer from the host memory is not available. In such a case,
5027 you need to either 1) copy/set the audio data directly to the
5028 external hardware buffer, or 2) make an intermediate buffer and
5029 copy/set the data from it to the external hardware buffer in
5030 interrupts (or in tasklets, preferably).
5031 </para>
5032
5033 <para>
5034 The first case works fine if the external hardware buffer is enough
5035 large. This method doesn't need any extra buffers and thus is
5036 more effective. You need to define the
5037 <structfield>copy</structfield> and
5038 <structfield>silence</structfield> callbacks for
5039 the data transfer. However, there is a drawback: it cannot
5040 be mmapped. The examples are GUS's GF1 PCM or emu8000's
5041 wavetable PCM.
5042 </para>
5043
5044 <para>
5045 The second case allows the mmap of the buffer, although you have
5046 to handle an interrupt or a tasklet for transferring the data
5047 from the intermediate buffer to the hardware buffer. You can find an
5048 example in vxpocket driver.
5049 </para>
5050
5051 <para>
5052 Another case is that the chip uses a PCI memory-map
5053 region for the buffer instead of the host memory. In this case,
5054 mmap is available only on certain architectures like intel. In
5055 non-mmap mode, the data cannot be transferred as the normal
5056 way. Thus you need to define <structfield>copy</structfield> and
5057 <structfield>silence</structfield> callbacks as well
5058 as in the cases above. The examples are found in
5059 <filename>rme32.c</filename> and <filename>rme96.c</filename>.
5060 </para>
5061
5062 <para>
5063 The implementation of <structfield>copy</structfield> and
5064 <structfield>silence</structfield> callbacks depends upon
5065 whether the hardware supports interleaved or non-interleaved
5066 samples. The <structfield>copy</structfield> callback is
5067 defined like below, a bit
5068 differently depending whether the direction is playback or
5069 capture:
5070
5071 <informalexample>
5072 <programlisting>
5073<![CDATA[
5074 static int playback_copy(snd_pcm_substream_t *substream, int channel,
5075 snd_pcm_uframes_t pos, void *src, snd_pcm_uframes_t count);
5076 static int capture_copy(snd_pcm_substream_t *substream, int channel,
5077 snd_pcm_uframes_t pos, void *dst, snd_pcm_uframes_t count);
5078]]>
5079 </programlisting>
5080 </informalexample>
5081 </para>
5082
5083 <para>
5084 In the case of interleaved samples, the second argument
5085 (<parameter>channel</parameter>) is not used. The third argument
5086 (<parameter>pos</parameter>) points the
5087 current position offset in frames.
5088 </para>
5089
5090 <para>
5091 The meaning of the fourth argument is different between
5092 playback and capture. For playback, it holds the source data
5093 pointer, and for capture, it's the destination data pointer.
5094 </para>
5095
5096 <para>
5097 The last argument is the number of frames to be copied.
5098 </para>
5099
5100 <para>
5101 What you have to do in this callback is again different
5102 between playback and capture directions. In the case of
5103 playback, you do: copy the given amount of data
5104 (<parameter>count</parameter>) at the specified pointer
5105 (<parameter>src</parameter>) to the specified offset
5106 (<parameter>pos</parameter>) on the hardware buffer. When
5107 coded like memcpy-like way, the copy would be like:
5108
5109 <informalexample>
5110 <programlisting>
5111<![CDATA[
5112 my_memcpy(my_buffer + frames_to_bytes(runtime, pos), src,
5113 frames_to_bytes(runtime, count));
5114]]>
5115 </programlisting>
5116 </informalexample>
5117 </para>
5118
5119 <para>
5120 For the capture direction, you do: copy the given amount of
5121 data (<parameter>count</parameter>) at the specified offset
5122 (<parameter>pos</parameter>) on the hardware buffer to the
5123 specified pointer (<parameter>dst</parameter>).
5124
5125 <informalexample>
5126 <programlisting>
5127<![CDATA[
5128 my_memcpy(dst, my_buffer + frames_to_bytes(runtime, pos),
5129 frames_to_bytes(runtime, count));
5130]]>
5131 </programlisting>
5132 </informalexample>
5133
5134 Note that both of the position and the data amount are given
5135 in frames.
5136 </para>
5137
5138 <para>
5139 In the case of non-interleaved samples, the implementation
5140 will be a bit more complicated.
5141 </para>
5142
5143 <para>
5144 You need to check the channel argument, and if it's -1, copy
5145 the whole channels. Otherwise, you have to copy only the
5146 specified channel. Please check
5147 <filename>isa/gus/gus_pcm.c</filename> as an example.
5148 </para>
5149
5150 <para>
5151 The <structfield>silence</structfield> callback is also
5152 implemented in a similar way.
5153
5154 <informalexample>
5155 <programlisting>
5156<![CDATA[
5157 static int silence(snd_pcm_substream_t *substream, int channel,
5158 snd_pcm_uframes_t pos, snd_pcm_uframes_t count);
5159]]>
5160 </programlisting>
5161 </informalexample>
5162 </para>
5163
5164 <para>
5165 The meanings of arguments are identical with the
5166 <structfield>copy</structfield>
5167 callback, although there is no <parameter>src/dst</parameter>
5168 argument. In the case of interleaved samples, the channel
5169 argument has no meaning, as well as on
5170 <structfield>copy</structfield> callback.
5171 </para>
5172
5173 <para>
5174 The role of <structfield>silence</structfield> callback is to
5175 set the given amount
5176 (<parameter>count</parameter>) of silence data at the
5177 specified offset (<parameter>pos</parameter>) on the hardware
5178 buffer. Suppose that the data format is signed (that is, the
5179 silent-data is 0), and the implementation using a memset-like
5180 function would be like:
5181
5182 <informalexample>
5183 <programlisting>
5184<![CDATA[
5185 my_memcpy(my_buffer + frames_to_bytes(runtime, pos), 0,
5186 frames_to_bytes(runtime, count));
5187]]>
5188 </programlisting>
5189 </informalexample>
5190 </para>
5191
5192 <para>
5193 In the case of non-interleaved samples, again, the
5194 implementation becomes a bit more complicated. See, for example,
5195 <filename>isa/gus/gus_pcm.c</filename>.
5196 </para>
5197 </section>
5198
5199 <section id="buffer-and-memory-non-contiguous">
5200 <title>Non-Contiguous Buffers</title>
5201 <para>
5202 If your hardware supports the page table like emu10k1 or the
5203 buffer descriptors like via82xx, you can use the scatter-gather
5204 (SG) DMA. ALSA provides an interface for handling SG-buffers.
5205 The API is provided in <filename>&lt;sound/pcm.h&gt;</filename>.
5206 </para>
5207
5208 <para>
5209 For creating the SG-buffer handler, call
5210 <function>snd_pcm_lib_preallocate_pages()</function> or
5211 <function>snd_pcm_lib_preallocate_pages_for_all()</function>
5212 with <constant>SNDRV_DMA_TYPE_DEV_SG</constant>
5213 in the PCM constructor like other PCI pre-allocator.
5214 You need to pass the <function>snd_dma_pci_data(pci)</function>,
5215 where pci is the struct <structname>pci_dev</structname> pointer
5216 of the chip as well.
5217 The <type>snd_sg_buf_t</type> instance is created as
5218 substream-&gt;dma_private. You can cast
5219 the pointer like:
5220
5221 <informalexample>
5222 <programlisting>
5223<![CDATA[
5224 snd_pcm_sgbuf_t *sgbuf = (snd_pcm_sgbuf_t*)substream->dma_private;
5225]]>
5226 </programlisting>
5227 </informalexample>
5228 </para>
5229
5230 <para>
5231 Then call <function>snd_pcm_lib_malloc_pages()</function>
5232 in <structfield>hw_params</structfield> callback
5233 as well as in the case of normal PCI buffer.
5234 The SG-buffer handler will allocate the non-contiguous kernel
5235 pages of the given size and map them onto the virtually contiguous
5236 memory. The virtual pointer is addressed in runtime-&gt;dma_area.
5237 The physical address (runtime-&gt;dma_addr) is set to zero,
5238 because the buffer is physically non-contigous.
5239 The physical address table is set up in sgbuf-&gt;table.
5240 You can get the physical address at a certain offset via
5241 <function>snd_pcm_sgbuf_get_addr()</function>.
5242 </para>
5243
5244 <para>
5245 When a SG-handler is used, you need to set
5246 <function>snd_pcm_sgbuf_ops_page</function> as
5247 the <structfield>page</structfield> callback.
5248 (See <link linkend="pcm-interface-operators-page-callback">
5249 <citetitle>page callback section</citetitle></link>.)
5250 </para>
5251
5252 <para>
5253 For releasing the data, call
5254 <function>snd_pcm_lib_free_pages()</function> in the
5255 <structfield>hw_free</structfield> callback as usual.
5256 </para>
5257 </section>
5258
5259 <section id="buffer-and-memory-vmalloced">
5260 <title>Vmalloc'ed Buffers</title>
5261 <para>
5262 It's possible to use a buffer allocated via
5263 <function>vmalloc</function>, for example, for an intermediate
5264 buffer. Since the allocated pages are not contiguous, you need
5265 to set the <structfield>page</structfield> callback to obtain
5266 the physical address at every offset.
5267 </para>
5268
5269 <para>
5270 The implementation of <structfield>page</structfield> callback
5271 would be like this:
5272
5273 <informalexample>
5274 <programlisting>
5275<![CDATA[
5276 #include <linux/vmalloc.h>
5277
5278 /* get the physical page pointer on the given offset */
5279 static struct page *mychip_page(snd_pcm_substream_t *substream,
5280 unsigned long offset)
5281 {
5282 void *pageptr = substream->runtime->dma_area + offset;
5283 return vmalloc_to_page(pageptr);
5284 }
5285]]>
5286 </programlisting>
5287 </informalexample>
5288 </para>
5289 </section>
5290
5291 </chapter>
5292
5293
5294<!-- ****************************************************** -->
5295<!-- Proc Interface -->
5296<!-- ****************************************************** -->
5297 <chapter id="proc-interface">
5298 <title>Proc Interface</title>
5299 <para>
5300 ALSA provides an easy interface for procfs. The proc files are
5301 very useful for debugging. I recommend you set up proc files if
5302 you write a driver and want to get a running status or register
5303 dumps. The API is found in
5304 <filename>&lt;sound/info.h&gt;</filename>.
5305 </para>
5306
5307 <para>
5308 For creating a proc file, call
5309 <function>snd_card_proc_new()</function>.
5310
5311 <informalexample>
5312 <programlisting>
5313<![CDATA[
5314 snd_info_entry_t *entry;
5315 int err = snd_card_proc_new(card, "my-file", &entry);
5316]]>
5317 </programlisting>
5318 </informalexample>
5319
5320 where the second argument specifies the proc-file name to be
5321 created. The above example will create a file
5322 <filename>my-file</filename> under the card directory,
5323 e.g. <filename>/proc/asound/card0/my-file</filename>.
5324 </para>
5325
5326 <para>
5327 Like other components, the proc entry created via
5328 <function>snd_card_proc_new()</function> will be registered and
5329 released automatically in the card registration and release
5330 functions.
5331 </para>
5332
5333 <para>
5334 When the creation is successful, the function stores a new
5335 instance at the pointer given in the third argument.
5336 It is initialized as a text proc file for read only. For using
5337 this proc file as a read-only text file as it is, set the read
5338 callback with a private data via
5339 <function>snd_info_set_text_ops()</function>.
5340
5341 <informalexample>
5342 <programlisting>
5343<![CDATA[
5344 snd_info_set_text_ops(entry, chip, read_size, my_proc_read);
5345]]>
5346 </programlisting>
5347 </informalexample>
5348
5349 where the second argument (<parameter>chip</parameter>) is the
5350 private data to be used in the callbacks. The third parameter
5351 specifies the read buffer size and the fourth
5352 (<parameter>my_proc_read</parameter>) is the callback function, which
5353 is defined like
5354
5355 <informalexample>
5356 <programlisting>
5357<![CDATA[
5358 static void my_proc_read(snd_info_entry_t *entry,
5359 snd_info_buffer_t *buffer);
5360]]>
5361 </programlisting>
5362 </informalexample>
5363
5364 </para>
5365
5366 <para>
5367 In the read callback, use <function>snd_iprintf()</function> for
5368 output strings, which works just like normal
5369 <function>printf()</function>. For example,
5370
5371 <informalexample>
5372 <programlisting>
5373<![CDATA[
5374 static void my_proc_read(snd_info_entry_t *entry,
5375 snd_info_buffer_t *buffer)
5376 {
5377 chip_t *chip = entry->private_data;
5378
5379 snd_iprintf(buffer, "This is my chip!\n");
5380 snd_iprintf(buffer, "Port = %ld\n", chip->port);
5381 }
5382]]>
5383 </programlisting>
5384 </informalexample>
5385 </para>
5386
5387 <para>
5388 The file permission can be changed afterwards. As default, it's
5389 set as read only for all users. If you want to add the write
5390 permission to the user (root as default), set like below:
5391
5392 <informalexample>
5393 <programlisting>
5394<![CDATA[
5395 entry->mode = S_IFREG | S_IRUGO | S_IWUSR;
5396]]>
5397 </programlisting>
5398 </informalexample>
5399
5400 and set the write buffer size and the callback
5401
5402 <informalexample>
5403 <programlisting>
5404<![CDATA[
5405 entry->c.text.write_size = 256;
5406 entry->c.text.write = my_proc_write;
5407]]>
5408 </programlisting>
5409 </informalexample>
5410 </para>
5411
5412 <para>
5413 The buffer size for read is set to 1024 implicitly by
5414 <function>snd_info_set_text_ops()</function>. It should suffice
5415 in most cases (the size will be aligned to
5416 <constant>PAGE_SIZE</constant> anyway), but if you need to handle
5417 very large text files, you can set it explicitly, too.
5418
5419 <informalexample>
5420 <programlisting>
5421<![CDATA[
5422 entry->c.text.read_size = 65536;
5423]]>
5424 </programlisting>
5425 </informalexample>
5426 </para>
5427
5428 <para>
5429 For the write callback, you can use
5430 <function>snd_info_get_line()</function> to get a text line, and
5431 <function>snd_info_get_str()</function> to retrieve a string from
5432 the line. Some examples are found in
5433 <filename>core/oss/mixer_oss.c</filename>, core/oss/and
5434 <filename>pcm_oss.c</filename>.
5435 </para>
5436
5437 <para>
5438 For a raw-data proc-file, set the attributes like the following:
5439
5440 <informalexample>
5441 <programlisting>
5442<![CDATA[
5443 static struct snd_info_entry_ops my_file_io_ops = {
5444 .read = my_file_io_read,
5445 };
5446
5447 entry->content = SNDRV_INFO_CONTENT_DATA;
5448 entry->private_data = chip;
5449 entry->c.ops = &my_file_io_ops;
5450 entry->size = 4096;
5451 entry->mode = S_IFREG | S_IRUGO;
5452]]>
5453 </programlisting>
5454 </informalexample>
5455 </para>
5456
5457 <para>
5458 The callback is much more complicated than the text-file
5459 version. You need to use a low-level i/o functions such as
5460 <function>copy_from/to_user()</function> to transfer the
5461 data.
5462
5463 <informalexample>
5464 <programlisting>
5465<![CDATA[
5466 static long my_file_io_read(snd_info_entry_t *entry,
5467 void *file_private_data,
5468 struct file *file,
5469 char *buf,
5470 unsigned long count,
5471 unsigned long pos)
5472 {
5473 long size = count;
5474 if (pos + size > local_max_size)
5475 size = local_max_size - pos;
5476 if (copy_to_user(buf, local_data + pos, size))
5477 return -EFAULT;
5478 return size;
5479 }
5480]]>
5481 </programlisting>
5482 </informalexample>
5483 </para>
5484
5485 </chapter>
5486
5487
5488<!-- ****************************************************** -->
5489<!-- Power Management -->
5490<!-- ****************************************************** -->
5491 <chapter id="power-management">
5492 <title>Power Management</title>
5493 <para>
5494 If the chip is supposed to work with with suspend/resume
5495 functions, you need to add the power-management codes to the
5496 driver. The additional codes for the power-management should be
5497 <function>ifdef</function>'ed with
5498 <constant>CONFIG_PM</constant>.
5499 </para>
5500
5501 <para>
5502 ALSA provides the common power-management layer. Each card driver
5503 needs to have only low-level suspend and resume callbacks.
5504
5505 <informalexample>
5506 <programlisting>
5507<![CDATA[
5508 #ifdef CONFIG_PM
5509 static int snd_my_suspend(snd_card_t *card, pm_message_t state)
5510 {
5511 .... // do things for suspsend
5512 return 0;
5513 }
5514 static int snd_my_resume(snd_card_t *card)
5515 {
5516 .... // do things for suspsend
5517 return 0;
5518 }
5519 #endif
5520]]>
5521 </programlisting>
5522 </informalexample>
5523 </para>
5524
5525 <para>
5526 The scheme of the real suspend job is as following.
5527
5528 <orderedlist>
5529 <listitem><para>Retrieve the chip data from pm_private_data field.</para></listitem>
5530 <listitem><para>Call <function>snd_pcm_suspend_all()</function> to suspend the running PCM streams.</para></listitem>
5531 <listitem><para>Save the register values if necessary.</para></listitem>
5532 <listitem><para>Stop the hardware if necessary.</para></listitem>
5533 <listitem><para>Disable the PCI device by calling <function>pci_disable_device()</function>.</para></listitem>
5534 </orderedlist>
5535 </para>
5536
5537 <para>
5538 A typical code would be like:
5539
5540 <informalexample>
5541 <programlisting>
5542<![CDATA[
5543 static int mychip_suspend(snd_card_t *card, pm_message_t state)
5544 {
5545 /* (1) */
5546 mychip_t *chip = card->pm_private_data;
5547 /* (2) */
5548 snd_pcm_suspend_all(chip->pcm);
5549 /* (3) */
5550 snd_mychip_save_registers(chip);
5551 /* (4) */
5552 snd_mychip_stop_hardware(chip);
5553 /* (5) */
5554 pci_disable_device(chip->pci);
5555 return 0;
5556 }
5557]]>
5558 </programlisting>
5559 </informalexample>
5560 </para>
5561
5562 <para>
5563 The scheme of the real resume job is as following.
5564
5565 <orderedlist>
5566 <listitem><para>Retrieve the chip data from pm_private_data field.</para></listitem>
5567 <listitem><para>Enable the pci device again by calling
5568 <function>pci_enable_device()</function>.</para></listitem>
5569 <listitem><para>Re-initialize the chip.</para></listitem>
5570 <listitem><para>Restore the saved registers if necessary.</para></listitem>
5571 <listitem><para>Resume the mixer, e.g. calling
5572 <function>snd_ac97_resume()</function>.</para></listitem>
5573 <listitem><para>Restart the hardware (if any).</para></listitem>
5574 </orderedlist>
5575 </para>
5576
5577 <para>
5578 A typical code would be like:
5579
5580 <informalexample>
5581 <programlisting>
5582<![CDATA[
5583 static void mychip_resume(mychip_t *chip)
5584 {
5585 /* (1) */
5586 mychip_t *chip = card->pm_private_data;
5587 /* (2) */
5588 pci_enable_device(chip->pci);
5589 /* (3) */
5590 snd_mychip_reinit_chip(chip);
5591 /* (4) */
5592 snd_mychip_restore_registers(chip);
5593 /* (5) */
5594 snd_ac97_resume(chip->ac97);
5595 /* (6) */
5596 snd_mychip_restart_chip(chip);
5597 return 0;
5598 }
5599]]>
5600 </programlisting>
5601 </informalexample>
5602 </para>
5603
5604 <para>
5605 OK, we have all callbacks now. Let's set up them now. In the
5606 initialization of the card, add the following:
5607
5608 <informalexample>
5609 <programlisting>
5610<![CDATA[
5611 static int __devinit snd_mychip_probe(struct pci_dev *pci,
5612 const struct pci_device_id *pci_id)
5613 {
5614 ....
5615 snd_card_t *card;
5616 mychip_t *chip;
5617 ....
5618 snd_card_set_pm_callback(card, snd_my_suspend, snd_my_resume, chip);
5619 ....
5620 }
5621]]>
5622 </programlisting>
5623 </informalexample>
5624
5625 Here you don't have to put ifdef CONFIG_PM around, since it's already
5626 checked in the header and expanded to empty if not needed.
5627 </para>
5628
5629 <para>
5630 If you need a space for saving the registers, you'll need to
5631 allocate the buffer for it here, too, since it would be fatal
5632 if you cannot allocate a memory in the suspend phase.
5633 The allocated buffer should be released in the corresponding
5634 destructor.
5635 </para>
5636
5637 <para>
5638 And next, set suspend/resume callbacks to the pci_driver,
5639 This can be done by passing a macro SND_PCI_PM_CALLBACKS
5640 in the pci_driver struct. This macro is expanded to the correct
5641 (global) callbacks if CONFIG_PM is set.
5642
5643 <informalexample>
5644 <programlisting>
5645<![CDATA[
5646 static struct pci_driver driver = {
5647 .name = "My Chip",
5648 .id_table = snd_my_ids,
5649 .probe = snd_my_probe,
5650 .remove = __devexit_p(snd_my_remove),
5651 SND_PCI_PM_CALLBACKS
5652 };
5653]]>
5654 </programlisting>
5655 </informalexample>
5656 </para>
5657
5658 </chapter>
5659
5660
5661<!-- ****************************************************** -->
5662<!-- Module Parameters -->
5663<!-- ****************************************************** -->
5664 <chapter id="module-parameters">
5665 <title>Module Parameters</title>
5666 <para>
5667 There are standard module options for ALSA. At least, each
5668 module should have <parameter>index</parameter>,
5669 <parameter>id</parameter> and <parameter>enable</parameter>
5670 options.
5671 </para>
5672
5673 <para>
5674 If the module supports multiple cards (usually up to
5675 8 = <constant>SNDRV_CARDS</constant> cards), they should be
5676 arrays. The default initial values are defined already as
5677 constants for ease of programming:
5678
5679 <informalexample>
5680 <programlisting>
5681<![CDATA[
5682 static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX;
5683 static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR;
5684 static int enable[SNDRV_CARDS] = SNDRV_DEFAULT_ENABLE_PNP;
5685]]>
5686 </programlisting>
5687 </informalexample>
5688 </para>
5689
5690 <para>
5691 If the module supports only a single card, they could be single
5692 variables, instead. <parameter>enable</parameter> option is not
5693 always necessary in this case, but it wouldn't be so bad to have a
5694 dummy option for compatibility.
5695 </para>
5696
5697 <para>
5698 The module parameters must be declared with the standard
5699 <function>module_param()()</function>,
5700 <function>module_param_array()()</function> and
5701 <function>MODULE_PARM_DESC()</function> macros.
5702 </para>
5703
5704 <para>
5705 The typical coding would be like below:
5706
5707 <informalexample>
5708 <programlisting>
5709<![CDATA[
5710 #define CARD_NAME "My Chip"
5711
5712 module_param_array(index, int, NULL, 0444);
5713 MODULE_PARM_DESC(index, "Index value for " CARD_NAME " soundcard.");
5714 module_param_array(id, charp, NULL, 0444);
5715 MODULE_PARM_DESC(id, "ID string for " CARD_NAME " soundcard.");
5716 module_param_array(enable, bool, NULL, 0444);
5717 MODULE_PARM_DESC(enable, "Enable " CARD_NAME " soundcard.");
5718]]>
5719 </programlisting>
5720 </informalexample>
5721 </para>
5722
5723 <para>
5724 Also, don't forget to define the module description, classes,
5725 license and devices. Especially, the recent modprobe requires to
5726 define the module license as GPL, etc., otherwise the system is
5727 shown as <quote>tainted</quote>.
5728
5729 <informalexample>
5730 <programlisting>
5731<![CDATA[
5732 MODULE_DESCRIPTION("My Chip");
5733 MODULE_LICENSE("GPL");
5734 MODULE_SUPPORTED_DEVICE("{{Vendor,My Chip Name}}");
5735]]>
5736 </programlisting>
5737 </informalexample>
5738 </para>
5739
5740 </chapter>
5741
5742
5743<!-- ****************************************************** -->
5744<!-- How To Put Your Driver -->
5745<!-- ****************************************************** -->
5746 <chapter id="how-to-put-your-driver">
5747 <title>How To Put Your Driver Into ALSA Tree</title>
5748 <section>
5749 <title>General</title>
5750 <para>
5751 So far, you've learned how to write the driver codes.
5752 And you might have a question now: how to put my own
5753 driver into the ALSA driver tree?
5754 Here (finally :) the standard procedure is described briefly.
5755 </para>
5756
5757 <para>
5758 Suppose that you'll create a new PCI driver for the card
5759 <quote>xyz</quote>. The card module name would be
5760 snd-xyz. The new driver is usually put into alsa-driver
5761 tree, <filename>alsa-driver/pci</filename> directory in
5762 the case of PCI cards.
5763 Then the driver is evaluated, audited and tested
5764 by developers and users. After a certain time, the driver
5765 will go to alsa-kernel tree (to the corresponding directory,
5766 such as <filename>alsa-kernel/pci</filename>) and eventually
5767 integrated into Linux 2.6 tree (the directory would be
5768 <filename>linux/sound/pci</filename>).
5769 </para>
5770
5771 <para>
5772 In the following sections, the driver code is supposed
5773 to be put into alsa-driver tree. The two cases are assumed:
5774 a driver consisting of a single source file and one consisting
5775 of several source files.
5776 </para>
5777 </section>
5778
5779 <section>
5780 <title>Driver with A Single Source File</title>
5781 <para>
5782 <orderedlist>
5783 <listitem>
5784 <para>
5785 Modify alsa-driver/pci/Makefile
5786 </para>
5787
5788 <para>
5789 Suppose you have a file xyz.c. Add the following
5790 two lines
5791 <informalexample>
5792 <programlisting>
5793<![CDATA[
5794 snd-xyz-objs := xyz.o
5795 obj-$(CONFIG_SND_XYZ) += snd-xyz.o
5796]]>
5797 </programlisting>
5798 </informalexample>
5799 </para>
5800 </listitem>
5801
5802 <listitem>
5803 <para>
5804 Create the Kconfig entry
5805 </para>
5806
5807 <para>
5808 Add the new entry of Kconfig for your xyz driver.
5809 <informalexample>
5810 <programlisting>
5811<![CDATA[
5812 config SND_XYZ
5813 tristate "Foobar XYZ"
5814 depends on SND
5815 select SND_PCM
5816 help
5817 Say Y here to include support for Foobar XYZ soundcard.
5818
5819 To compile this driver as a module, choose M here: the module
5820 will be called snd-xyz.
5821]]>
5822 </programlisting>
5823 </informalexample>
5824
5825 the line, select SND_PCM, specifies that the driver xyz supports
5826 PCM. In addition to SND_PCM, the following components are
5827 supported for select command:
5828 SND_RAWMIDI, SND_TIMER, SND_HWDEP, SND_MPU401_UART,
5829 SND_OPL3_LIB, SND_OPL4_LIB, SND_VX_LIB, SND_AC97_CODEC.
5830 Add the select command for each supported component.
5831 </para>
5832
5833 <para>
5834 Note that some selections imply the lowlevel selections.
5835 For example, PCM includes TIMER, MPU401_UART includes RAWMIDI,
5836 AC97_CODEC includes PCM, and OPL3_LIB includes HWDEP.
5837 You don't need to give the lowlevel selections again.
5838 </para>
5839
5840 <para>
5841 For the details of Kconfig script, refer to the kbuild
5842 documentation.
5843 </para>
5844
5845 </listitem>
5846
5847 <listitem>
5848 <para>
5849 Run cvscompile script to re-generate the configure script and
5850 build the whole stuff again.
5851 </para>
5852 </listitem>
5853 </orderedlist>
5854 </para>
5855 </section>
5856
5857 <section>
5858 <title>Drivers with Several Source Files</title>
5859 <para>
5860 Suppose that the driver snd-xyz have several source files.
5861 They are located in the new subdirectory,
5862 pci/xyz.
5863
5864 <orderedlist>
5865 <listitem>
5866 <para>
5867 Add a new directory (<filename>xyz</filename>) in
5868 <filename>alsa-driver/pci/Makefile</filename> like below
5869
5870 <informalexample>
5871 <programlisting>
5872<![CDATA[
5873 obj-$(CONFIG_SND) += xyz/
5874]]>
5875 </programlisting>
5876 </informalexample>
5877 </para>
5878 </listitem>
5879
5880 <listitem>
5881 <para>
5882 Under the directory <filename>xyz</filename>, create a Makefile
5883
5884 <example>
5885 <title>Sample Makefile for a driver xyz</title>
5886 <programlisting>
5887<![CDATA[
5888 ifndef SND_TOPDIR
5889 SND_TOPDIR=../..
5890 endif
5891
5892 include $(SND_TOPDIR)/toplevel.config
5893 include $(SND_TOPDIR)/Makefile.conf
5894
5895 snd-xyz-objs := xyz.o abc.o def.o
5896
5897 obj-$(CONFIG_SND_XYZ) += snd-xyz.o
5898
5899 include $(SND_TOPDIR)/Rules.make
5900]]>
5901 </programlisting>
5902 </example>
5903 </para>
5904 </listitem>
5905
5906 <listitem>
5907 <para>
5908 Create the Kconfig entry
5909 </para>
5910
5911 <para>
5912 This procedure is as same as in the last section.
5913 </para>
5914 </listitem>
5915
5916 <listitem>
5917 <para>
5918 Run cvscompile script to re-generate the configure script and
5919 build the whole stuff again.
5920 </para>
5921 </listitem>
5922 </orderedlist>
5923 </para>
5924 </section>
5925
5926 </chapter>
5927
5928<!-- ****************************************************** -->
5929<!-- Useful Functions -->
5930<!-- ****************************************************** -->
5931 <chapter id="useful-functions">
5932 <title>Useful Functions</title>
5933
5934 <section id="useful-functions-snd-printk">
5935 <title><function>snd_printk()</function> and friends</title>
5936 <para>
5937 ALSA provides a verbose version of
5938 <function>printk()</function> function. If a kernel config
5939 <constant>CONFIG_SND_VERBOSE_PRINTK</constant> is set, this
5940 function prints the given message together with the file name
5941 and the line of the caller. The <constant>KERN_XXX</constant>
5942 prefix is processed as
5943 well as the original <function>printk()</function> does, so it's
5944 recommended to add this prefix, e.g.
5945
5946 <informalexample>
5947 <programlisting>
5948<![CDATA[
5949 snd_printk(KERN_ERR "Oh my, sorry, it's extremely bad!\n");
5950]]>
5951 </programlisting>
5952 </informalexample>
5953 </para>
5954
5955 <para>
5956 There are also <function>printk()</function>'s for
5957 debugging. <function>snd_printd()</function> can be used for
5958 general debugging purposes. If
5959 <constant>CONFIG_SND_DEBUG</constant> is set, this function is
5960 compiled, and works just like
5961 <function>snd_printk()</function>. If the ALSA is compiled
5962 without the debugging flag, it's ignored.
5963 </para>
5964
5965 <para>
5966 <function>snd_printdd()</function> is compiled in only when
5967 <constant>CONFIG_SND_DEBUG_DETECT</constant> is set. Please note
5968 that <constant>DEBUG_DETECT</constant> is not set as default
5969 even if you configure the alsa-driver with
5970 <option>--with-debug=full</option> option. You need to give
5971 explicitly <option>--with-debug=detect</option> option instead.
5972 </para>
5973 </section>
5974
5975 <section id="useful-functions-snd-assert">
5976 <title><function>snd_assert()</function></title>
5977 <para>
5978 <function>snd_assert()</function> macro is similar with the
5979 normal <function>assert()</function> macro. For example,
5980
5981 <informalexample>
5982 <programlisting>
5983<![CDATA[
5984 snd_assert(pointer != NULL, return -EINVAL);
5985]]>
5986 </programlisting>
5987 </informalexample>
5988 </para>
5989
5990 <para>
5991 The first argument is the expression to evaluate, and the
5992 second argument is the action if it fails. When
5993 <constant>CONFIG_SND_DEBUG</constant>, is set, it will show an
5994 error message such as <computeroutput>BUG? (xxx) (called from
5995 yyy)</computeroutput>. When no debug flag is set, this is
5996 ignored.
5997 </para>
5998 </section>
5999
6000 <section id="useful-functions-snd-runtime-check">
6001 <title><function>snd_runtime_check()</function></title>
6002 <para>
6003 This macro is quite similar with
6004 <function>snd_assert()</function>. Unlike
6005 <function>snd_assert()</function>, the expression is always
6006 evaluated regardless of
6007 <constant>CONFIG_SND_DEBUG</constant>. When
6008 <constant>CONFIG_SND_DEBUG</constant> is set, the macro will
6009 show a message like <computeroutput>ERROR (xx) (called from
6010 yyy)</computeroutput>.
6011 </para>
6012 </section>
6013
6014 <section id="useful-functions-snd-bug">
6015 <title><function>snd_BUG()</function></title>
6016 <para>
6017 It calls <function>snd_assert(0,)</function> -- that is, just
6018 prints the error message at the point. It's useful to show that
6019 a fatal error happens there.
6020 </para>
6021 </section>
6022 </chapter>
6023
6024
6025<!-- ****************************************************** -->
6026<!-- Acknowledgments -->
6027<!-- ****************************************************** -->
6028 <chapter id="acknowledments">
6029 <title>Acknowledgments</title>
6030 <para>
6031 I would like to thank Phil Kerr for his help for improvement and
6032 corrections of this document.
6033 </para>
6034 <para>
6035 Kevin Conder reformatted the original plain-text to the
6036 DocBook format.
6037 </para>
6038 <para>
6039 Giuliano Pochini corrected typos and contributed the example codes
6040 in the hardware constraints section.
6041 </para>
6042 </chapter>
6043
6044
6045</book>
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