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authorAlan Stern <stern@rowland.harvard.edu>2007-10-10 16:24:56 -0400
committerGreg Kroah-Hartman <gregkh@suse.de>2007-10-12 17:55:34 -0400
commitcd38c1e1ae5273c28a12baacaf17c1faa062661f (patch)
tree941e5e2623725947ef40a68c04da371e9170fc20 /Documentation/usb
parent271f9e68f3450ac8d1ff3bda36581f1ec0d0cc1f (diff)
USB: documentation for USB power management
This patch (as998) adds documentation on how USB power management works and how to use it. Signed-off-by: Alan Stern <stern@rowland.harvard.edu> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
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1 Power Management for USB
2
3 Alan Stern <stern@rowland.harvard.edu>
4
5 October 5, 2007
6
7
8
9 What is Power Management?
10 -------------------------
11
12Power Management (PM) is the practice of saving energy by suspending
13parts of a computer system when they aren't being used. While a
14component is "suspended" it is in a nonfunctional low-power state; it
15might even be turned off completely. A suspended component can be
16"resumed" (returned to a functional full-power state) when the kernel
17needs to use it. (There also are forms of PM in which components are
18placed in a less functional but still usable state instead of being
19suspended; an example would be reducing the CPU's clock rate. This
20document will not discuss those other forms.)
21
22When the parts being suspended include the CPU and most of the rest of
23the system, we speak of it as a "system suspend". When a particular
24device is turned off while the system as a whole remains running, we
25call it a "dynamic suspend" (also known as a "runtime suspend" or
26"selective suspend"). This document concentrates mostly on how
27dynamic PM is implemented in the USB subsystem, although system PM is
28covered to some extent (see Documentation/power/*.txt for more
29information about system PM).
30
31Note: Dynamic PM support for USB is present only if the kernel was
32built with CONFIG_USB_SUSPEND enabled. System PM support is present
33only if the kernel was built with CONFIG_SUSPEND or CONFIG_HIBERNATION
34enabled.
35
36
37 What is Remote Wakeup?
38 ----------------------
39
40When a device has been suspended, it generally doesn't resume until
41the computer tells it to. Likewise, if the entire computer has been
42suspended, it generally doesn't resume until the user tells it to, say
43by pressing a power button or opening the cover.
44
45However some devices have the capability of resuming by themselves, or
46asking the kernel to resume them, or even telling the entire computer
47to resume. This capability goes by several names such as "Wake On
48LAN"; we will refer to it generically as "remote wakeup". When a
49device is enabled for remote wakeup and it is suspended, it may resume
50itself (or send a request to be resumed) in response to some external
51event. Examples include a suspended keyboard resuming when a key is
52pressed, or a suspended USB hub resuming when a device is plugged in.
53
54
55 When is a USB device idle?
56 --------------------------
57
58A device is idle whenever the kernel thinks it's not busy doing
59anything important and thus is a candidate for being suspended. The
60exact definition depends on the device's driver; drivers are allowed
61to declare that a device isn't idle even when there's no actual
62communication taking place. (For example, a hub isn't considered idle
63unless all the devices plugged into that hub are already suspended.)
64In addition, a device isn't considered idle so long as a program keeps
65its usbfs file open, whether or not any I/O is going on.
66
67If a USB device has no driver, its usbfs file isn't open, and it isn't
68being accessed through sysfs, then it definitely is idle.
69
70
71 Forms of dynamic PM
72 -------------------
73
74Dynamic suspends can occur in two ways: manual and automatic.
75"Manual" means that the user has told the kernel to suspend a device,
76whereas "automatic" means that the kernel has decided all by itself to
77suspend a device. Automatic suspend is called "autosuspend" for
78short. In general, a device won't be autosuspended unless it has been
79idle for some minimum period of time, the so-called idle-delay time.
80
81Of course, nothing the kernel does on its own initiative should
82prevent the computer or its devices from working properly. If a
83device has been autosuspended and a program tries to use it, the
84kernel will automatically resume the device (autoresume). For the
85same reason, an autosuspended device will usually have remote wakeup
86enabled, if the device supports remote wakeup.
87
88It is worth mentioning that many USB drivers don't support
89autosuspend. In fact, at the time of this writing (Linux 2.6.23) the
90only drivers which do support it are the hub driver, kaweth, asix,
91usblp, usblcd, and usb-skeleton (which doesn't count). If a
92non-supporting driver is bound to a device, the device won't be
93autosuspended. In effect, the kernel pretends the device is never
94idle.
95
96We can categorize power management events in two broad classes:
97external and internal. External events are those triggered by some
98agent outside the USB stack: system suspend/resume (triggered by
99userspace), manual dynamic suspend/resume (also triggered by
100userspace), and remote wakeup (triggered by the device). Internal
101events are those triggered within the USB stack: autosuspend and
102autoresume.
103
104
105 The user interface for dynamic PM
106 ---------------------------------
107
108The user interface for controlling dynamic PM is located in the power/
109subdirectory of each USB device's sysfs directory, that is, in
110/sys/bus/usb/devices/.../power/ where "..." is the device's ID. The
111relevant attribute files are: wakeup, level, and autosuspend.
112
113 power/wakeup
114
115 This file is empty if the device does not support
116 remote wakeup. Otherwise the file contains either the
117 word "enabled" or the word "disabled", and you can
118 write those words to the file. The setting determines
119 whether or not remote wakeup will be enabled when the
120 device is next suspended. (If the setting is changed
121 while the device is suspended, the change won't take
122 effect until the following suspend.)
123
124 power/level
125
126 This file contains one of three words: "on", "auto",
127 or "suspend". You can write those words to the file
128 to change the device's setting.
129
130 "on" means that the device should be resumed and
131 autosuspend is not allowed. (Of course, system
132 suspends are still allowed.)
133
134 "auto" is the normal state in which the kernel is
135 allowed to autosuspend and autoresume the device.
136
137 "suspend" means that the device should remain
138 suspended, and autoresume is not allowed. (But remote
139 wakeup may still be allowed, since it is controlled
140 separately by the power/wakeup attribute.)
141
142 power/autosuspend
143
144 This file contains an integer value, which is the
145 number of seconds the device should remain idle before
146 the kernel will autosuspend it (the idle-delay time).
147 The default is 2. 0 means to autosuspend as soon as
148 the device becomes idle, and -1 means never to
149 autosuspend. You can write a number to the file to
150 change the autosuspend idle-delay time.
151
152Writing "-1" to power/autosuspend and writing "on" to power/level do
153essentially the same thing -- they both prevent the device from being
154autosuspended. Yes, this is a redundancy in the API.
155
156(In 2.6.21 writing "0" to power/autosuspend would prevent the device
157from being autosuspended; the behavior was changed in 2.6.22. The
158power/autosuspend attribute did not exist prior to 2.6.21, and the
159power/level attribute did not exist prior to 2.6.22.)
160
161
162 Changing the default idle-delay time
163 ------------------------------------
164
165The default autosuspend idle-delay time is controlled by a module
166parameter in usbcore. You can specify the value when usbcore is
167loaded. For example, to set it to 5 seconds instead of 2 you would
168do:
169
170 modprobe usbcore autosuspend=5
171
172Equivalently, you could add to /etc/modprobe.conf a line saying:
173
174 options usbcore autosuspend=5
175
176Some distributions load the usbcore module very early during the boot
177process, by means of a program or script running from an initramfs
178image. To alter the parameter value you would have to rebuild that
179image.
180
181If usbcore is compiled into the kernel rather than built as a loadable
182module, you can add
183
184 usbcore.autosuspend=5
185
186to the kernel's boot command line.
187
188Finally, the parameter value can be changed while the system is
189running. If you do:
190
191 echo 5 >/sys/module/usbcore/parameters/autosuspend
192
193then each new USB device will have its autosuspend idle-delay
194initialized to 5. (The idle-delay values for already existing devices
195will not be affected.)
196
197Setting the initial default idle-delay to -1 will prevent any
198autosuspend of any USB device. This is a simple alternative to
199disabling CONFIG_USB_SUSPEND and rebuilding the kernel, and it has the
200added benefit of allowing you to enable autosuspend for selected
201devices.
202
203
204 Warnings
205 --------
206
207The USB specification states that all USB devices must support power
208management. Nevertheless, the sad fact is that many devices do not
209support it very well. You can suspend them all right, but when you
210try to resume them they disconnect themselves from the USB bus or
211they stop working entirely. This seems to be especially prevalent
212among printers and scanners, but plenty of other types of device have
213the same deficiency.
214
215For this reason, by default the kernel disables autosuspend (the
216power/level attribute is initialized to "on") for all devices other
217than hubs. Hubs, at least, appear to be reasonably well-behaved in
218this regard.
219
220(In 2.6.21 and 2.6.22 this wasn't the case. Autosuspend was enabled
221by default for almost all USB devices. A number of people experienced
222problems as a result.)
223
224This means that non-hub devices won't be autosuspended unless the user
225or a program explicitly enables it. As of this writing there aren't
226any widespread programs which will do this; we hope that in the near
227future device managers such as HAL will take on this added
228responsibility. In the meantime you can always carry out the
229necessary operations by hand or add them to a udev script. You can
230also change the idle-delay time; 2 seconds is not the best choice for
231every device.
232
233Sometimes it turns out that even when a device does work okay with
234autosuspend there are still problems. For example, there are
235experimental patches adding autosuspend support to the usbhid driver,
236which manages keyboards and mice, among other things. Tests with a
237number of keyboards showed that typing on a suspended keyboard, while
238causing the keyboard to do a remote wakeup all right, would
239nonetheless frequently result in lost keystrokes. Tests with mice
240showed that some of them would issue a remote-wakeup request in
241response to button presses but not to motion, and some in response to
242neither.
243
244The kernel will not prevent you from enabling autosuspend on devices
245that can't handle it. It is even possible in theory to damage a
246device by suspending it at the wrong time -- for example, suspending a
247USB hard disk might cause it to spin down without parking the heads.
248(Highly unlikely, but possible.) Take care.
249
250
251 The driver interface for Power Management
252 -----------------------------------------
253
254The requirements for a USB driver to support external power management
255are pretty modest; the driver need only define
256
257 .suspend
258 .resume
259 .reset_resume
260
261methods in its usb_driver structure, and the reset_resume method is
262optional. The methods' jobs are quite simple:
263
264 The suspend method is called to warn the driver that the
265 device is going to be suspended. If the driver returns a
266 negative error code, the suspend will be aborted. Normally
267 the driver will return 0, in which case it must cancel all
268 outstanding URBs (usb_kill_urb()) and not submit any more.
269
270 The resume method is called to tell the driver that the
271 device has been resumed and the driver can return to normal
272 operation. URBs may once more be submitted.
273
274 The reset_resume method is called to tell the driver that
275 the device has been resumed and it also has been reset.
276 The driver should redo any necessary device initialization,
277 since the device has probably lost most or all of its state
278 (although the interfaces will be in the same altsettings as
279 before the suspend).
280
281The reset_resume method is used by the USB Persist facility (see
282Documentation/usb/persist.txt) and it can also be used under certain
283circumstances when CONFIG_USB_PERSIST is not enabled. Currently, if a
284device is reset during a resume and the driver does not have a
285reset_resume method, the driver won't receive any notification about
286the resume. Later kernels will call the driver's disconnect method;
2872.6.23 doesn't do this.
288
289USB drivers are bound to interfaces, so their suspend and resume
290methods get called when the interfaces are suspended or resumed. In
291principle one might want to suspend some interfaces on a device (i.e.,
292force the drivers for those interface to stop all activity) without
293suspending the other interfaces. The USB core doesn't allow this; all
294interfaces are suspended when the device itself is suspended and all
295interfaces are resumed when the device is resumed. It isn't possible
296to suspend or resume some but not all of a device's interfaces. The
297closest you can come is to unbind the interfaces' drivers.
298
299
300 The driver interface for autosuspend and autoresume
301 ---------------------------------------------------
302
303To support autosuspend and autoresume, a driver should implement all
304three of the methods listed above. In addition, a driver indicates
305that it supports autosuspend by setting the .supports_autosuspend flag
306in its usb_driver structure. It is then responsible for informing the
307USB core whenever one of its interfaces becomes busy or idle. The
308driver does so by calling these three functions:
309
310 int usb_autopm_get_interface(struct usb_interface *intf);
311 void usb_autopm_put_interface(struct usb_interface *intf);
312 int usb_autopm_set_interface(struct usb_interface *intf);
313
314The functions work by maintaining a counter in the usb_interface
315structure. When intf->pm_usage_count is > 0 then the interface is
316deemed to be busy, and the kernel will not autosuspend the interface's
317device. When intf->pm_usage_count is <= 0 then the interface is
318considered to be idle, and the kernel may autosuspend the device.
319
320(There is a similar pm_usage_count field in struct usb_device,
321associated with the device itself rather than any of its interfaces.
322This field is used only by the USB core.)
323
324The driver owns intf->pm_usage_count; it can modify the value however
325and whenever it likes. A nice aspect of the usb_autopm_* routines is
326that the changes they make are protected by the usb_device structure's
327PM mutex (udev->pm_mutex); however drivers may change pm_usage_count
328without holding the mutex.
329
330 usb_autopm_get_interface() increments pm_usage_count and
331 attempts an autoresume if the new value is > 0 and the
332 device is suspended.
333
334 usb_autopm_put_interface() decrements pm_usage_count and
335 attempts an autosuspend if the new value is <= 0 and the
336 device isn't suspended.
337
338 usb_autopm_set_interface() leaves pm_usage_count alone.
339 It attempts an autoresume if the value is > 0 and the device
340 is suspended, and it attempts an autosuspend if the value is
341 <= 0 and the device isn't suspended.
342
343There also are a couple of utility routines drivers can use:
344
345 usb_autopm_enable() sets pm_usage_cnt to 1 and then calls
346 usb_autopm_set_interface(), which will attempt an autoresume.
347
348 usb_autopm_disable() sets pm_usage_cnt to 0 and then calls
349 usb_autopm_set_interface(), which will attempt an autosuspend.
350
351The conventional usage pattern is that a driver calls
352usb_autopm_get_interface() in its open routine and
353usb_autopm_put_interface() in its close or release routine. But
354other patterns are possible.
355
356The autosuspend attempts mentioned above will often fail for one
357reason or another. For example, the power/level attribute might be
358set to "on", or another interface in the same device might not be
359idle. This is perfectly normal. If the reason for failure was that
360the device hasn't been idle for long enough, a delayed workqueue
361routine is automatically set up to carry out the operation when the
362autosuspend idle-delay has expired.
363
364Autoresume attempts also can fail. This will happen if power/level is
365set to "suspend" or if the device doesn't manage to resume properly.
366Unlike autosuspend, there's no delay for an autoresume.
367
368
369 Other parts of the driver interface
370 -----------------------------------
371
372Sometimes a driver needs to make sure that remote wakeup is enabled
373during autosuspend. For example, there's not much point
374autosuspending a keyboard if the user can't cause the keyboard to do a
375remote wakeup by typing on it. If the driver sets
376intf->needs_remote_wakeup to 1, the kernel won't autosuspend the
377device if remote wakeup isn't available or has been disabled through
378the power/wakeup attribute. (If the device is already autosuspended,
379though, setting this flag won't cause the kernel to autoresume it.
380Normally a driver would set this flag in its probe method, at which
381time the device is guaranteed not to be autosuspended.)
382
383The usb_autopm_* routines have to run in a sleepable process context;
384they must not be called from an interrupt handler or while holding a
385spinlock. In fact, the entire autosuspend mechanism is not well geared
386toward interrupt-driven operation. However there is one thing a
387driver can do in an interrupt handler:
388
389 usb_mark_last_busy(struct usb_device *udev);
390
391This sets udev->last_busy to the current time. udev->last_busy is the
392field used for idle-delay calculations; updating it will cause any
393pending autosuspend to be moved back. The usb_autopm_* routines will
394also set the last_busy field to the current time.
395
396Calling urb_mark_last_busy() from within an URB completion handler is
397subject to races: The kernel may have just finished deciding the
398device has been idle for long enough but not yet gotten around to
399calling the driver's suspend method. The driver would have to be
400responsible for synchronizing its suspend method with its URB
401completion handler and causing the autosuspend to fail with -EBUSY if
402an URB had completed too recently.
403
404External suspend calls should never be allowed to fail in this way,
405only autosuspend calls. The driver can tell them apart by checking
406udev->auto_pm; this flag will be set to 1 for internal PM events
407(autosuspend or autoresume) and 0 for external PM events.
408
409Many of the ingredients in the autosuspend framework are oriented
410towards interfaces: The usb_interface structure contains the
411pm_usage_cnt field, and the usb_autopm_* routines take an interface
412pointer as their argument. But somewhat confusingly, a few of the
413pieces (usb_mark_last_busy() and udev->auto_pm) use the usb_device
414structure instead. Drivers need to keep this straight; they can call
415interface_to_usbdev() to find the device structure for a given
416interface.
417
418
419 Locking requirements
420 --------------------
421
422All three suspend/resume methods are always called while holding the
423usb_device's PM mutex. For external events -- but not necessarily for
424autosuspend or autoresume -- the device semaphore (udev->dev.sem) will
425also be held. This implies that external suspend/resume events are
426mutually exclusive with calls to probe, disconnect, pre_reset, and
427post_reset; the USB core guarantees that this is true of internal
428suspend/resume events as well.
429
430If a driver wants to block all suspend/resume calls during some
431critical section, it can simply acquire udev->pm_mutex.
432Alternatively, if the critical section might call some of the
433usb_autopm_* routines, the driver can avoid deadlock by doing:
434
435 down(&udev->dev.sem);
436 rc = usb_autopm_get_interface(intf);
437
438and at the end of the critical section:
439
440 if (!rc)
441 usb_autopm_put_interface(intf);
442 up(&udev->dev.sem);
443
444Holding the device semaphore will block all external PM calls, and the
445usb_autopm_get_interface() will prevent any internal PM calls, even if
446it fails. (Exercise: Why?)
447
448The rules for locking order are:
449
450 Never acquire any device semaphore while holding any PM mutex.
451
452 Never acquire udev->pm_mutex while holding the PM mutex for
453 a device that isn't a descendant of udev.
454
455In other words, PM mutexes should only be acquired going up the device
456tree, and they should be acquired only after locking all the device
457semaphores you need to hold. These rules don't matter to drivers very
458much; they usually affect just the USB core.
459
460Still, drivers do need to be careful. For example, many drivers use a
461private mutex to synchronize their normal I/O activities with their
462disconnect method. Now if the driver supports autosuspend then it
463must call usb_autopm_put_interface() from somewhere -- maybe from its
464close method. It should make the call while holding the private mutex,
465since a driver shouldn't call any of the usb_autopm_* functions for an
466interface from which it has been unbound.
467
468But the usb_autpm_* routines always acquire the device's PM mutex, and
469consequently the locking order has to be: private mutex first, PM
470mutex second. Since the suspend method is always called with the PM
471mutex held, it mustn't try to acquire the private mutex. It has to
472synchronize with the driver's I/O activities in some other way.
473
474
475 Interaction between dynamic PM and system PM
476 --------------------------------------------
477
478Dynamic power management and system power management can interact in
479a couple of ways.
480
481Firstly, a device may already be manually suspended or autosuspended
482when a system suspend occurs. Since system suspends are supposed to
483be as transparent as possible, the device should remain suspended
484following the system resume. The 2.6.23 kernel obeys this principle
485for manually suspended devices but not for autosuspended devices; they
486do get resumed when the system wakes up. (Presumably they will be
487autosuspended again after their idle-delay time expires.) In later
488kernels this behavior will be fixed.
489
490(There is an exception. If a device would undergo a reset-resume
491instead of a normal resume, and the device is enabled for remote
492wakeup, then the reset-resume takes place even if the device was
493already suspended when the system suspend began. The justification is
494that a reset-resume is a kind of remote-wakeup event. Or to put it
495another way, a device which needs a reset won't be able to generate
496normal remote-wakeup signals, so it ought to be resumed immediately.)
497
498Secondly, a dynamic power-management event may occur as a system
499suspend is underway. The window for this is short, since system
500suspends don't take long (a few seconds usually), but it can happen.
501For example, a suspended device may send a remote-wakeup signal while
502the system is suspending. The remote wakeup may succeed, which would
503cause the system suspend to abort. If the remote wakeup doesn't
504succeed, it may still remain active and thus cause the system to
505resume as soon as the system suspend is complete. Or the remote
506wakeup may fail and get lost. Which outcome occurs depends on timing
507and on the hardware and firmware design.
508
509More interestingly, a device might undergo a manual resume or
510autoresume during system suspend. With current kernels this shouldn't
511happen, because manual resumes must be initiated by userspace and
512autoresumes happen in response to I/O requests, but all user processes
513and I/O should be quiescent during a system suspend -- thanks to the
514freezer. However there are plans to do away with the freezer, which
515would mean these things would become possible. If and when this comes
516about, the USB core will carefully arrange matters so that either type
517of resume will block until the entire system has resumed.