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