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Alan Sterncd38c1e2007-10-10 16:24:56 -04001 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
Alan Stern3c886c52007-11-16 11:58:15 -0500281If the device is disconnected or powered down while it is suspended,
282the disconnect method will be called instead of the resume or
283reset_resume method. This is also quite likely to happen when
284waking up from hibernation, as many systems do not maintain suspend
285current to the USB host controllers during hibernation. (It's
286possible to work around the hibernation-forces-disconnect problem by
287using the USB Persist facility.)
288
Alan Sterncd38c1e2007-10-10 16:24:56 -0400289The reset_resume method is used by the USB Persist facility (see
290Documentation/usb/persist.txt) and it can also be used under certain
291circumstances when CONFIG_USB_PERSIST is not enabled. Currently, if a
292device is reset during a resume and the driver does not have a
293reset_resume method, the driver won't receive any notification about
294the resume. Later kernels will call the driver's disconnect method;
2952.6.23 doesn't do this.
296
297USB drivers are bound to interfaces, so their suspend and resume
298methods get called when the interfaces are suspended or resumed. In
299principle one might want to suspend some interfaces on a device (i.e.,
300force the drivers for those interface to stop all activity) without
301suspending the other interfaces. The USB core doesn't allow this; all
302interfaces are suspended when the device itself is suspended and all
303interfaces are resumed when the device is resumed. It isn't possible
304to suspend or resume some but not all of a device's interfaces. The
305closest you can come is to unbind the interfaces' drivers.
306
307
308 The driver interface for autosuspend and autoresume
309 ---------------------------------------------------
310
311To support autosuspend and autoresume, a driver should implement all
312three of the methods listed above. In addition, a driver indicates
313that it supports autosuspend by setting the .supports_autosuspend flag
314in its usb_driver structure. It is then responsible for informing the
315USB core whenever one of its interfaces becomes busy or idle. The
Alan Stern9ac39f22008-11-12 16:19:49 -0500316driver does so by calling these five functions:
Alan Sterncd38c1e2007-10-10 16:24:56 -0400317
318 int usb_autopm_get_interface(struct usb_interface *intf);
319 void usb_autopm_put_interface(struct usb_interface *intf);
320 int usb_autopm_set_interface(struct usb_interface *intf);
Alan Stern9ac39f22008-11-12 16:19:49 -0500321 int usb_autopm_get_interface_async(struct usb_interface *intf);
322 void usb_autopm_put_interface_async(struct usb_interface *intf);
Alan Sterncd38c1e2007-10-10 16:24:56 -0400323
324The functions work by maintaining a counter in the usb_interface
325structure. When intf->pm_usage_count is > 0 then the interface is
326deemed to be busy, and the kernel will not autosuspend the interface's
327device. When intf->pm_usage_count is <= 0 then the interface is
328considered to be idle, and the kernel may autosuspend the device.
329
330(There is a similar pm_usage_count field in struct usb_device,
331associated with the device itself rather than any of its interfaces.
332This field is used only by the USB core.)
333
334The driver owns intf->pm_usage_count; it can modify the value however
Alan Stern9ac39f22008-11-12 16:19:49 -0500335and whenever it likes. A nice aspect of the non-async usb_autopm_*
336routines is that the changes they make are protected by the usb_device
337structure's PM mutex (udev->pm_mutex); however drivers may change
338pm_usage_count without holding the mutex. Drivers using the async
339routines are responsible for their own synchronization and mutual
340exclusion.
Alan Sterncd38c1e2007-10-10 16:24:56 -0400341
342 usb_autopm_get_interface() increments pm_usage_count and
343 attempts an autoresume if the new value is > 0 and the
344 device is suspended.
345
346 usb_autopm_put_interface() decrements pm_usage_count and
347 attempts an autosuspend if the new value is <= 0 and the
348 device isn't suspended.
349
350 usb_autopm_set_interface() leaves pm_usage_count alone.
351 It attempts an autoresume if the value is > 0 and the device
352 is suspended, and it attempts an autosuspend if the value is
353 <= 0 and the device isn't suspended.
354
Alan Stern9ac39f22008-11-12 16:19:49 -0500355 usb_autopm_get_interface_async() and
356 usb_autopm_put_interface_async() do almost the same things as
357 their non-async counterparts. The differences are: they do
358 not acquire the PM mutex, and they use a workqueue to do their
359 jobs. As a result they can be called in an atomic context,
360 such as an URB's completion handler, but when they return the
361 device will not generally not yet be in the desired state.
362
Alan Sterncd38c1e2007-10-10 16:24:56 -0400363There also are a couple of utility routines drivers can use:
364
Geoff Levand81ab5b82008-09-20 14:41:47 -0700365 usb_autopm_enable() sets pm_usage_cnt to 0 and then calls
Alan Sterncd38c1e2007-10-10 16:24:56 -0400366 usb_autopm_set_interface(), which will attempt an autosuspend.
367
Geoff Levand81ab5b82008-09-20 14:41:47 -0700368 usb_autopm_disable() sets pm_usage_cnt to 1 and then calls
369 usb_autopm_set_interface(), which will attempt an autoresume.
370
Alan Sterncd38c1e2007-10-10 16:24:56 -0400371The conventional usage pattern is that a driver calls
372usb_autopm_get_interface() in its open routine and
373usb_autopm_put_interface() in its close or release routine. But
374other patterns are possible.
375
376The autosuspend attempts mentioned above will often fail for one
377reason or another. For example, the power/level attribute might be
378set to "on", or another interface in the same device might not be
379idle. This is perfectly normal. If the reason for failure was that
380the device hasn't been idle for long enough, a delayed workqueue
381routine is automatically set up to carry out the operation when the
382autosuspend idle-delay has expired.
383
384Autoresume attempts also can fail. This will happen if power/level is
385set to "suspend" or if the device doesn't manage to resume properly.
386Unlike autosuspend, there's no delay for an autoresume.
387
388
389 Other parts of the driver interface
390 -----------------------------------
391
392Sometimes a driver needs to make sure that remote wakeup is enabled
393during autosuspend. For example, there's not much point
394autosuspending a keyboard if the user can't cause the keyboard to do a
395remote wakeup by typing on it. If the driver sets
396intf->needs_remote_wakeup to 1, the kernel won't autosuspend the
397device if remote wakeup isn't available or has been disabled through
398the power/wakeup attribute. (If the device is already autosuspended,
399though, setting this flag won't cause the kernel to autoresume it.
400Normally a driver would set this flag in its probe method, at which
401time the device is guaranteed not to be autosuspended.)
402
403The usb_autopm_* routines have to run in a sleepable process context;
404they must not be called from an interrupt handler or while holding a
405spinlock. In fact, the entire autosuspend mechanism is not well geared
406toward interrupt-driven operation. However there is one thing a
407driver can do in an interrupt handler:
408
409 usb_mark_last_busy(struct usb_device *udev);
410
411This sets udev->last_busy to the current time. udev->last_busy is the
412field used for idle-delay calculations; updating it will cause any
413pending autosuspend to be moved back. The usb_autopm_* routines will
414also set the last_busy field to the current time.
415
416Calling urb_mark_last_busy() from within an URB completion handler is
417subject to races: The kernel may have just finished deciding the
418device has been idle for long enough but not yet gotten around to
419calling the driver's suspend method. The driver would have to be
420responsible for synchronizing its suspend method with its URB
421completion handler and causing the autosuspend to fail with -EBUSY if
422an URB had completed too recently.
423
424External suspend calls should never be allowed to fail in this way,
425only autosuspend calls. The driver can tell them apart by checking
426udev->auto_pm; this flag will be set to 1 for internal PM events
427(autosuspend or autoresume) and 0 for external PM events.
428
429Many of the ingredients in the autosuspend framework are oriented
430towards interfaces: The usb_interface structure contains the
431pm_usage_cnt field, and the usb_autopm_* routines take an interface
432pointer as their argument. But somewhat confusingly, a few of the
433pieces (usb_mark_last_busy() and udev->auto_pm) use the usb_device
434structure instead. Drivers need to keep this straight; they can call
435interface_to_usbdev() to find the device structure for a given
436interface.
437
438
439 Locking requirements
440 --------------------
441
442All three suspend/resume methods are always called while holding the
443usb_device's PM mutex. For external events -- but not necessarily for
444autosuspend or autoresume -- the device semaphore (udev->dev.sem) will
445also be held. This implies that external suspend/resume events are
446mutually exclusive with calls to probe, disconnect, pre_reset, and
447post_reset; the USB core guarantees that this is true of internal
448suspend/resume events as well.
449
450If a driver wants to block all suspend/resume calls during some
Oliver Neukumf0fa7462008-07-04 10:10:53 +0200451critical section, it can simply acquire udev->pm_mutex. Note that
452calls to resume may be triggered indirectly. Block IO due to memory
453allocations can make the vm subsystem resume a device. Thus while
454holding this lock you must not allocate memory with GFP_KERNEL or
455GFP_NOFS.
456
Alan Sterncd38c1e2007-10-10 16:24:56 -0400457Alternatively, if the critical section might call some of the
458usb_autopm_* routines, the driver can avoid deadlock by doing:
459
460 down(&udev->dev.sem);
461 rc = usb_autopm_get_interface(intf);
462
463and at the end of the critical section:
464
465 if (!rc)
466 usb_autopm_put_interface(intf);
467 up(&udev->dev.sem);
468
469Holding the device semaphore will block all external PM calls, and the
470usb_autopm_get_interface() will prevent any internal PM calls, even if
471it fails. (Exercise: Why?)
472
473The rules for locking order are:
474
475 Never acquire any device semaphore while holding any PM mutex.
476
477 Never acquire udev->pm_mutex while holding the PM mutex for
478 a device that isn't a descendant of udev.
479
480In other words, PM mutexes should only be acquired going up the device
481tree, and they should be acquired only after locking all the device
482semaphores you need to hold. These rules don't matter to drivers very
483much; they usually affect just the USB core.
484
485Still, drivers do need to be careful. For example, many drivers use a
486private mutex to synchronize their normal I/O activities with their
487disconnect method. Now if the driver supports autosuspend then it
488must call usb_autopm_put_interface() from somewhere -- maybe from its
489close method. It should make the call while holding the private mutex,
490since a driver shouldn't call any of the usb_autopm_* functions for an
491interface from which it has been unbound.
492
493But the usb_autpm_* routines always acquire the device's PM mutex, and
494consequently the locking order has to be: private mutex first, PM
495mutex second. Since the suspend method is always called with the PM
496mutex held, it mustn't try to acquire the private mutex. It has to
497synchronize with the driver's I/O activities in some other way.
498
499
500 Interaction between dynamic PM and system PM
501 --------------------------------------------
502
503Dynamic power management and system power management can interact in
504a couple of ways.
505
506Firstly, a device may already be manually suspended or autosuspended
507when a system suspend occurs. Since system suspends are supposed to
508be as transparent as possible, the device should remain suspended
509following the system resume. The 2.6.23 kernel obeys this principle
510for manually suspended devices but not for autosuspended devices; they
511do get resumed when the system wakes up. (Presumably they will be
512autosuspended again after their idle-delay time expires.) In later
513kernels this behavior will be fixed.
514
515(There is an exception. If a device would undergo a reset-resume
516instead of a normal resume, and the device is enabled for remote
517wakeup, then the reset-resume takes place even if the device was
518already suspended when the system suspend began. The justification is
519that a reset-resume is a kind of remote-wakeup event. Or to put it
520another way, a device which needs a reset won't be able to generate
521normal remote-wakeup signals, so it ought to be resumed immediately.)
522
523Secondly, a dynamic power-management event may occur as a system
524suspend is underway. The window for this is short, since system
525suspends don't take long (a few seconds usually), but it can happen.
526For example, a suspended device may send a remote-wakeup signal while
527the system is suspending. The remote wakeup may succeed, which would
528cause the system suspend to abort. If the remote wakeup doesn't
529succeed, it may still remain active and thus cause the system to
530resume as soon as the system suspend is complete. Or the remote
531wakeup may fail and get lost. Which outcome occurs depends on timing
532and on the hardware and firmware design.
533
534More interestingly, a device might undergo a manual resume or
535autoresume during system suspend. With current kernels this shouldn't
536happen, because manual resumes must be initiated by userspace and
537autoresumes happen in response to I/O requests, but all user processes
538and I/O should be quiescent during a system suspend -- thanks to the
539freezer. However there are plans to do away with the freezer, which
540would mean these things would become possible. If and when this comes
541about, the USB core will carefully arrange matters so that either type
542of resume will block until the entire system has resumed.