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authorLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 18:20:36 -0400
committerLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 18:20:36 -0400
commit1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (patch)
tree0bba044c4ce775e45a88a51686b5d9f90697ea9d /Documentation/input/input-programming.txt
Linux-2.6.12-rc2v2.6.12-rc2
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
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1$Id: input-programming.txt,v 1.4 2001/05/04 09:47:14 vojtech Exp $
2
3Programming input drivers
4~~~~~~~~~~~~~~~~~~~~~~~~~
5
61. Creating an input device driver
7~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
8
91.0 The simplest example
10~~~~~~~~~~~~~~~~~~~~~~~~
11
12Here comes a very simple example of an input device driver. The device has
13just one button and the button is accessible at i/o port BUTTON_PORT. When
14pressed or released a BUTTON_IRQ happens. The driver could look like:
15
16#include <linux/input.h>
17#include <linux/module.h>
18#include <linux/init.h>
19
20#include <asm/irq.h>
21#include <asm/io.h>
22
23static void button_interrupt(int irq, void *dummy, struct pt_regs *fp)
24{
25 input_report_key(&button_dev, BTN_1, inb(BUTTON_PORT) & 1);
26 input_sync(&button_dev);
27}
28
29static int __init button_init(void)
30{
31 if (request_irq(BUTTON_IRQ, button_interrupt, 0, "button", NULL)) {
32 printk(KERN_ERR "button.c: Can't allocate irq %d\n", button_irq);
33 return -EBUSY;
34 }
35
36 button_dev.evbit[0] = BIT(EV_KEY);
37 button_dev.keybit[LONG(BTN_0)] = BIT(BTN_0);
38
39 input_register_device(&button_dev);
40}
41
42static void __exit button_exit(void)
43{
44 input_unregister_device(&button_dev);
45 free_irq(BUTTON_IRQ, button_interrupt);
46}
47
48module_init(button_init);
49module_exit(button_exit);
50
511.1 What the example does
52~~~~~~~~~~~~~~~~~~~~~~~~~
53
54First it has to include the <linux/input.h> file, which interfaces to the
55input subsystem. This provides all the definitions needed.
56
57In the _init function, which is called either upon module load or when
58booting the kernel, it grabs the required resources (it should also check
59for the presence of the device).
60
61Then it sets the input bitfields. This way the device driver tells the other
62parts of the input systems what it is - what events can be generated or
63accepted by this input device. Our example device can only generate EV_KEY type
64events, and from those only BTN_0 event code. Thus we only set these two
65bits. We could have used
66
67 set_bit(EV_KEY, button_dev.evbit);
68 set_bit(BTN_0, button_dev.keybit);
69
70as well, but with more than single bits the first approach tends to be
71shorter.
72
73Then the example driver registers the input device structure by calling
74
75 input_register_device(&button_dev);
76
77This adds the button_dev structure to linked lists of the input driver and
78calls device handler modules _connect functions to tell them a new input
79device has appeared. Because the _connect functions may call kmalloc(,
80GFP_KERNEL), which can sleep, input_register_device() must not be called
81from an interrupt or with a spinlock held.
82
83While in use, the only used function of the driver is
84
85 button_interrupt()
86
87which upon every interrupt from the button checks its state and reports it
88via the
89
90 input_report_key()
91
92call to the input system. There is no need to check whether the interrupt
93routine isn't reporting two same value events (press, press for example) to
94the input system, because the input_report_* functions check that
95themselves.
96
97Then there is the
98
99 input_sync()
100
101call to tell those who receive the events that we've sent a complete report.
102This doesn't seem important in the one button case, but is quite important
103for for example mouse movement, where you don't want the X and Y values
104to be interpreted separately, because that'd result in a different movement.
105
1061.2 dev->open() and dev->close()
107~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
108
109In case the driver has to repeatedly poll the device, because it doesn't
110have an interrupt coming from it and the polling is too expensive to be done
111all the time, or if the device uses a valuable resource (eg. interrupt), it
112can use the open and close callback to know when it can stop polling or
113release the interrupt and when it must resume polling or grab the interrupt
114again. To do that, we would add this to our example driver:
115
116int button_used = 0;
117
118static int button_open(struct input_dev *dev)
119{
120 if (button_used++)
121 return 0;
122
123 if (request_irq(BUTTON_IRQ, button_interrupt, 0, "button", NULL)) {
124 printk(KERN_ERR "button.c: Can't allocate irq %d\n", button_irq);
125 button_used--;
126 return -EBUSY;
127 }
128
129 return 0;
130}
131
132static void button_close(struct input_dev *dev)
133{
134 if (!--button_used)
135 free_irq(IRQ_AMIGA_VERTB, button_interrupt);
136}
137
138static int __init button_init(void)
139{
140 ...
141 button_dev.open = button_open;
142 button_dev.close = button_close;
143 ...
144}
145
146Note the button_used variable - we have to track how many times the open
147function was called to know when exactly our device stops being used.
148
149The open() callback should return a 0 in case of success or any nonzero value
150in case of failure. The close() callback (which is void) must always succeed.
151
1521.3 Basic event types
153~~~~~~~~~~~~~~~~~~~~~
154
155The most simple event type is EV_KEY, which is used for keys and buttons.
156It's reported to the input system via:
157
158 input_report_key(struct input_dev *dev, int code, int value)
159
160See linux/input.h for the allowable values of code (from 0 to KEY_MAX).
161Value is interpreted as a truth value, ie any nonzero value means key
162pressed, zero value means key released. The input code generates events only
163in case the value is different from before.
164
165In addition to EV_KEY, there are two more basic event types: EV_REL and
166EV_ABS. They are used for relative and absolute values supplied by the
167device. A relative value may be for example a mouse movement in the X axis.
168The mouse reports it as a relative difference from the last position,
169because it doesn't have any absolute coordinate system to work in. Absolute
170events are namely for joysticks and digitizers - devices that do work in an
171absolute coordinate systems.
172
173Having the device report EV_REL buttons is as simple as with EV_KEY, simply
174set the corresponding bits and call the
175
176 input_report_rel(struct input_dev *dev, int code, int value)
177
178function. Events are generated only for nonzero value.
179
180However EV_ABS requires a little special care. Before calling
181input_register_device, you have to fill additional fields in the input_dev
182struct for each absolute axis your device has. If our button device had also
183the ABS_X axis:
184
185 button_dev.absmin[ABS_X] = 0;
186 button_dev.absmax[ABS_X] = 255;
187 button_dev.absfuzz[ABS_X] = 4;
188 button_dev.absflat[ABS_X] = 8;
189
190This setting would be appropriate for a joystick X axis, with the minimum of
1910, maximum of 255 (which the joystick *must* be able to reach, no problem if
192it sometimes reports more, but it must be able to always reach the min and
193max values), with noise in the data up to +- 4, and with a center flat
194position of size 8.
195
196If you don't need absfuzz and absflat, you can set them to zero, which mean
197that the thing is precise and always returns to exactly the center position
198(if it has any).
199
2001.4 The void *private field
201~~~~~~~~~~~~~~~~~~~~~~~~~~~
202
203This field in the input structure can be used to point to any private data
204structures in the input device driver, in case the driver handles more than
205one device. You'll need it in the open and close callbacks.
206
2071.5 NBITS(), LONG(), BIT()
208~~~~~~~~~~~~~~~~~~~~~~~~~~
209
210These three macros from input.h help some bitfield computations:
211
212 NBITS(x) - returns the length of a bitfield array in longs for x bits
213 LONG(x) - returns the index in the array in longs for bit x
214 BIT(x) - returns the index in a long for bit x
215
2161.6 The number, id* and name fields
217~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
218
219The dev->number is assigned by the input system to the input device when it
220is registered. It has no use except for identifying the device to the user
221in system messages.
222
223The dev->name should be set before registering the input device by the input
224device driver. It's a string like 'Generic button device' containing a
225user friendly name of the device.
226
227The id* fields contain the bus ID (PCI, USB, ...), vendor ID and device ID
228of the device. The bus IDs are defined in input.h. The vendor and device ids
229are defined in pci_ids.h, usb_ids.h and similar include files. These fields
230should be set by the input device driver before registering it.
231
232The idtype field can be used for specific information for the input device
233driver.
234
235The id and name fields can be passed to userland via the evdev interface.
236
2371.7 The keycode, keycodemax, keycodesize fields
238~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
239
240These two fields will be used for any input devices that report their data
241as scancodes. If not all scancodes can be known by autodetection, they may
242need to be set by userland utilities. The keycode array then is an array
243used to map from scancodes to input system keycodes. The keycode max will
244contain the size of the array and keycodesize the size of each entry in it
245(in bytes).
246
2471.8 Key autorepeat
248~~~~~~~~~~~~~~~~~~
249
250... is simple. It is handled by the input.c module. Hardware autorepeat is
251not used, because it's not present in many devices and even where it is
252present, it is broken sometimes (at keyboards: Toshiba notebooks). To enable
253autorepeat for your device, just set EV_REP in dev->evbit. All will be
254handled by the input system.
255
2561.9 Other event types, handling output events
257~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
258
259The other event types up to now are:
260
261EV_LED - used for the keyboard LEDs.
262EV_SND - used for keyboard beeps.
263
264They are very similar to for example key events, but they go in the other
265direction - from the system to the input device driver. If your input device
266driver can handle these events, it has to set the respective bits in evbit,
267*and* also the callback routine:
268
269 button_dev.event = button_event;
270
271int button_event(struct input_dev *dev, unsigned int type, unsigned int code, int value);
272{
273 if (type == EV_SND && code == SND_BELL) {
274 outb(value, BUTTON_BELL);
275 return 0;
276 }
277 return -1;
278}
279
280This callback routine can be called from an interrupt or a BH (although that
281isn't a rule), and thus must not sleep, and must not take too long to finish.