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Based on kernel version 2.6.33. Page generated on 2010-02-24 15:37 EST.

1				Power Management for USB
2	
3			 Alan Stern <stern[AT]rowland.harvard[DOT]edu>
4	
5				    November 10, 2009
6	
7	
8	
9		What is Power Management?
10		-------------------------
11	
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.)
21	
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).
30	
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.
35	
36	
37		What is Remote Wakeup?
38		----------------------
39	
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.
44	
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.
53	
54	
55		When is a USB device idle?
56		--------------------------
57	
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.
66	
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.
69	
70	
71		Forms of dynamic PM
72		-------------------
73	
74	Dynamic suspends occur when the kernel decides to suspend an idle
75	device.  This is called "autosuspend" for short.  In general, a device
76	won't be autosuspended unless it has been idle for some minimum period
77	of time, the so-called idle-delay time.
78	
79	Of course, nothing the kernel does on its own initiative should
80	prevent the computer or its devices from working properly.  If a
81	device has been autosuspended and a program tries to use it, the
82	kernel will automatically resume the device (autoresume).  For the
83	same reason, an autosuspended device will usually have remote wakeup
84	enabled, if the device supports remote wakeup.
85	
86	It is worth mentioning that many USB drivers don't support
87	autosuspend.  In fact, at the time of this writing (Linux 2.6.23) the
88	only drivers which do support it are the hub driver, kaweth, asix,
89	usblp, usblcd, and usb-skeleton (which doesn't count).  If a
90	non-supporting driver is bound to a device, the device won't be
91	autosuspended.  In effect, the kernel pretends the device is never
92	idle.
93	
94	We can categorize power management events in two broad classes:
95	external and internal.  External events are those triggered by some
96	agent outside the USB stack: system suspend/resume (triggered by
97	userspace), manual dynamic resume (also triggered by userspace), and
98	remote wakeup (triggered by the device).  Internal events are those
99	triggered within the USB stack: autosuspend and autoresume.  Note that
100	all dynamic suspend events are internal; external agents are not
101	allowed to issue dynamic suspends.
102	
103	
104		The user interface for dynamic PM
105		---------------------------------
106	
107	The user interface for controlling dynamic PM is located in the power/
108	subdirectory of each USB device's sysfs directory, that is, in
109	/sys/bus/usb/devices/.../power/ where "..." is the device's ID.  The
110	relevant attribute files are: wakeup, level, and autosuspend.
111	
112		power/wakeup
113	
114			This file is empty if the device does not support
115			remote wakeup.  Otherwise the file contains either the
116			word "enabled" or the word "disabled", and you can
117			write those words to the file.  The setting determines
118			whether or not remote wakeup will be enabled when the
119			device is next suspended.  (If the setting is changed
120			while the device is suspended, the change won't take
121			effect until the following suspend.)
122	
123		power/level
124	
125			This file contains one of two words: "on" or "auto".
126			You can write those words to the file to change the
127			device's setting.
128	
129			"on" means that the device should be resumed and
130			autosuspend is not allowed.  (Of course, system
131			suspends are still allowed.)
132	
133			"auto" is the normal state in which the kernel is
134			allowed to autosuspend and autoresume the device.
135	
136			(In kernels up to 2.6.32, you could also specify
137			"suspend", meaning that the device should remain
138			suspended and autoresume was not allowed.  This
139			setting is no longer supported.)
140	
141		power/autosuspend
142	
143			This file contains an integer value, which is the
144			number of seconds the device should remain idle before
145			the kernel will autosuspend it (the idle-delay time).
146			The default is 2.  0 means to autosuspend as soon as
147			the device becomes idle, and negative values mean
148			never to autosuspend.  You can write a number to the
149			file to change the autosuspend idle-delay time.
150	
151	Writing "-1" to power/autosuspend and writing "on" to power/level do
152	essentially the same thing -- they both prevent the device from being
153	autosuspended.  Yes, this is a redundancy in the API.
154	
155	(In 2.6.21 writing "0" to power/autosuspend would prevent the device
156	from being autosuspended; the behavior was changed in 2.6.22.  The
157	power/autosuspend attribute did not exist prior to 2.6.21, and the
158	power/level attribute did not exist prior to 2.6.22.)
159	
160	
161		Changing the default idle-delay time
162		------------------------------------
163	
164	The default autosuspend idle-delay time is controlled by a module
165	parameter in usbcore.  You can specify the value when usbcore is
166	loaded.  For example, to set it to 5 seconds instead of 2 you would
167	do:
168	
169		modprobe usbcore autosuspend=5
170	
171	Equivalently, you could add to /etc/modprobe.conf a line saying:
172	
173		options usbcore autosuspend=5
174	
175	Some distributions load the usbcore module very early during the boot
176	process, by means of a program or script running from an initramfs
177	image.  To alter the parameter value you would have to rebuild that
178	image.
179	
180	If usbcore is compiled into the kernel rather than built as a loadable
181	module, you can add
182	
183		usbcore.autosuspend=5
184	
185	to the kernel's boot command line.
186	
187	Finally, the parameter value can be changed while the system is
188	running.  If you do:
189	
190		echo 5 >/sys/module/usbcore/parameters/autosuspend
191	
192	then each new USB device will have its autosuspend idle-delay
193	initialized to 5.  (The idle-delay values for already existing devices
194	will not be affected.)
195	
196	Setting the initial default idle-delay to -1 will prevent any
197	autosuspend of any USB device.  This is a simple alternative to
198	disabling CONFIG_USB_SUSPEND and rebuilding the kernel, and it has the
199	added benefit of allowing you to enable autosuspend for selected
200	devices.
201	
202	
203		Warnings
204		--------
205	
206	The USB specification states that all USB devices must support power
207	management.  Nevertheless, the sad fact is that many devices do not
208	support it very well.  You can suspend them all right, but when you
209	try to resume them they disconnect themselves from the USB bus or
210	they stop working entirely.  This seems to be especially prevalent
211	among printers and scanners, but plenty of other types of device have
212	the same deficiency.
213	
214	For this reason, by default the kernel disables autosuspend (the
215	power/level attribute is initialized to "on") for all devices other
216	than hubs.  Hubs, at least, appear to be reasonably well-behaved in
217	this regard.
218	
219	(In 2.6.21 and 2.6.22 this wasn't the case.  Autosuspend was enabled
220	by default for almost all USB devices.  A number of people experienced
221	problems as a result.)
222	
223	This means that non-hub devices won't be autosuspended unless the user
224	or a program explicitly enables it.  As of this writing there aren't
225	any widespread programs which will do this; we hope that in the near
226	future device managers such as HAL will take on this added
227	responsibility.  In the meantime you can always carry out the
228	necessary operations by hand or add them to a udev script.  You can
229	also change the idle-delay time; 2 seconds is not the best choice for
230	every device.
231	
232	Sometimes it turns out that even when a device does work okay with
233	autosuspend there are still problems.  For example, there are
234	experimental patches adding autosuspend support to the usbhid driver,
235	which manages keyboards and mice, among other things.  Tests with a
236	number of keyboards showed that typing on a suspended keyboard, while
237	causing the keyboard to do a remote wakeup all right, would
238	nonetheless frequently result in lost keystrokes.  Tests with mice
239	showed that some of them would issue a remote-wakeup request in
240	response to button presses but not to motion, and some in response to
241	neither.
242	
243	The kernel will not prevent you from enabling autosuspend on devices
244	that can't handle it.  It is even possible in theory to damage a
245	device by suspending it at the wrong time -- for example, suspending a
246	USB hard disk might cause it to spin down without parking the heads.
247	(Highly unlikely, but possible.)  Take care.
248	
249	
250		The driver interface for Power Management
251		-----------------------------------------
252	
253	The requirements for a USB driver to support external power management
254	are pretty modest; the driver need only define
255	
256		.suspend
257		.resume
258		.reset_resume
259	
260	methods in its usb_driver structure, and the reset_resume method is
261	optional.  The methods' jobs are quite simple:
262	
263		The suspend method is called to warn the driver that the
264		device is going to be suspended.  If the driver returns a
265		negative error code, the suspend will be aborted.  Normally
266		the driver will return 0, in which case it must cancel all
267		outstanding URBs (usb_kill_urb()) and not submit any more.
268	
269		The resume method is called to tell the driver that the
270		device has been resumed and the driver can return to normal
271		operation.  URBs may once more be submitted.
272	
273		The reset_resume method is called to tell the driver that
274		the device has been resumed and it also has been reset.
275		The driver should redo any necessary device initialization,
276		since the device has probably lost most or all of its state
277		(although the interfaces will be in the same altsettings as
278		before the suspend).
279	
280	If the device is disconnected or powered down while it is suspended,
281	the disconnect method will be called instead of the resume or
282	reset_resume method.  This is also quite likely to happen when
283	waking up from hibernation, as many systems do not maintain suspend
284	current to the USB host controllers during hibernation.  (It's
285	possible to work around the hibernation-forces-disconnect problem by
286	using the USB Persist facility.)
287	
288	The reset_resume method is used by the USB Persist facility (see
289	Documentation/usb/persist.txt) and it can also be used under certain
290	circumstances when CONFIG_USB_PERSIST is not enabled.  Currently, if a
291	device is reset during a resume and the driver does not have a
292	reset_resume method, the driver won't receive any notification about
293	the resume.  Later kernels will call the driver's disconnect method;
294	2.6.23 doesn't do this.
295	
296	USB drivers are bound to interfaces, so their suspend and resume
297	methods get called when the interfaces are suspended or resumed.  In
298	principle one might want to suspend some interfaces on a device (i.e.,
299	force the drivers for those interface to stop all activity) without
300	suspending the other interfaces.  The USB core doesn't allow this; all
301	interfaces are suspended when the device itself is suspended and all
302	interfaces are resumed when the device is resumed.  It isn't possible
303	to suspend or resume some but not all of a device's interfaces.  The
304	closest you can come is to unbind the interfaces' drivers.
305	
306	
307		The driver interface for autosuspend and autoresume
308		---------------------------------------------------
309	
310	To support autosuspend and autoresume, a driver should implement all
311	three of the methods listed above.  In addition, a driver indicates
312	that it supports autosuspend by setting the .supports_autosuspend flag
313	in its usb_driver structure.  It is then responsible for informing the
314	USB core whenever one of its interfaces becomes busy or idle.  The
315	driver does so by calling these six functions:
316	
317		int  usb_autopm_get_interface(struct usb_interface *intf);
318		void usb_autopm_put_interface(struct usb_interface *intf);
319		int  usb_autopm_get_interface_async(struct usb_interface *intf);
320		void usb_autopm_put_interface_async(struct usb_interface *intf);
321		void usb_autopm_get_interface_no_resume(struct usb_interface *intf);
322		void usb_autopm_put_interface_no_suspend(struct usb_interface *intf);
323	
324	The functions work by maintaining a counter in the usb_interface
325	structure.  When intf->pm_usage_count is > 0 then the interface is
326	deemed to be busy, and the kernel will not autosuspend the interface's
327	device.  When intf->pm_usage_count is <= 0 then the interface is
328	considered to be idle, and the kernel may autosuspend the device.
329	
330	(There is a similar pm_usage_count field in struct usb_device,
331	associated with the device itself rather than any of its interfaces.
332	This field is used only by the USB core.)
333	
334	Drivers must not modify intf->pm_usage_count directly; its value
335	should be changed only be using the functions listed above.  Drivers
336	are responsible for insuring that the overall change to pm_usage_count
337	during their lifetime balances out to 0 (it may be necessary for the
338	disconnect method to call usb_autopm_put_interface() one or more times
339	to fulfill this requirement).  The first two routines use the PM mutex
340	in struct usb_device for mutual exclusion; drivers using the async
341	routines are responsible for their own synchronization and mutual
342	exclusion.
343	
344		usb_autopm_get_interface() increments pm_usage_count and
345		attempts an autoresume if the new value is > 0 and the
346		device is suspended.
347	
348		usb_autopm_put_interface() decrements pm_usage_count and
349		attempts an autosuspend if the new value is <= 0 and the
350		device isn't suspended.
351	
352		usb_autopm_get_interface_async() and
353		usb_autopm_put_interface_async() do almost the same things as
354		their non-async counterparts.  The differences are: they do
355		not acquire the PM mutex, and they use a workqueue to do their
356		jobs.  As a result they can be called in an atomic context,
357		such as an URB's completion handler, but when they return the
358		device will not generally not yet be in the desired state.
359	
360		usb_autopm_get_interface_no_resume() and
361		usb_autopm_put_interface_no_suspend() merely increment or
362		decrement the pm_usage_count value; they do not attempt to
363		carry out an autoresume or an autosuspend.  Hence they can be
364		called in an atomic context.
365	
366	The conventional usage pattern is that a driver calls
367	usb_autopm_get_interface() in its open routine and
368	usb_autopm_put_interface() in its close or release routine.  But
369	other patterns are possible.
370	
371	The autosuspend attempts mentioned above will often fail for one
372	reason or another.  For example, the power/level attribute might be
373	set to "on", or another interface in the same device might not be
374	idle.  This is perfectly normal.  If the reason for failure was that
375	the device hasn't been idle for long enough, a delayed workqueue
376	routine is automatically set up to carry out the operation when the
377	autosuspend idle-delay has expired.
378	
379	Autoresume attempts also can fail, although failure would mean that
380	the device is no longer present or operating properly.  Unlike
381	autosuspend, there's no delay for an autoresume.
382	
383	
384		Other parts of the driver interface
385		-----------------------------------
386	
387	Sometimes a driver needs to make sure that remote wakeup is enabled
388	during autosuspend.  For example, there's not much point
389	autosuspending a keyboard if the user can't cause the keyboard to do a
390	remote wakeup by typing on it.  If the driver sets
391	intf->needs_remote_wakeup to 1, the kernel won't autosuspend the
392	device if remote wakeup isn't available or has been disabled through
393	the power/wakeup attribute.  (If the device is already autosuspended,
394	though, setting this flag won't cause the kernel to autoresume it.
395	Normally a driver would set this flag in its probe method, at which
396	time the device is guaranteed not to be autosuspended.)
397	
398	The synchronous usb_autopm_* routines have to run in a sleepable
399	process context; they must not be called from an interrupt handler or
400	while holding a spinlock.  In fact, the entire autosuspend mechanism
401	is not well geared toward interrupt-driven operation.  However there
402	is one thing a driver can do in an interrupt handler:
403	
404		usb_mark_last_busy(struct usb_device *udev);
405	
406	This sets udev->last_busy to the current time.  udev->last_busy is the
407	field used for idle-delay calculations; updating it will cause any
408	pending autosuspend to be moved back.  The usb_autopm_* routines will
409	also set the last_busy field to the current time.
410	
411	Calling urb_mark_last_busy() from within an URB completion handler is
412	subject to races: The kernel may have just finished deciding the
413	device has been idle for long enough but not yet gotten around to
414	calling the driver's suspend method.  The driver would have to be
415	responsible for synchronizing its suspend method with its URB
416	completion handler and causing the autosuspend to fail with -EBUSY if
417	an URB had completed too recently.
418	
419	External suspend calls should never be allowed to fail in this way,
420	only autosuspend calls.  The driver can tell them apart by checking
421	the PM_EVENT_AUTO bit in the message.event argument to the suspend
422	method; this bit will be set for internal PM events (autosuspend) and
423	clear for external PM events.
424	
425	Many of the ingredients in the autosuspend framework are oriented
426	towards interfaces: The usb_interface structure contains the
427	pm_usage_cnt field, and the usb_autopm_* routines take an interface
428	pointer as their argument.  But somewhat confusingly, a few of the
429	pieces (i.e., usb_mark_last_busy()) use the usb_device structure
430	instead.  Drivers need to keep this straight; they can call
431	interface_to_usbdev() to find the device structure for a given
432	interface.
433	
434	
435		Locking requirements
436		--------------------
437	
438	All three suspend/resume methods are always called while holding the
439	usb_device's PM mutex.  For external events -- but not necessarily for
440	autosuspend or autoresume -- the device semaphore (udev->dev.sem) will
441	also be held.  This implies that external suspend/resume events are
442	mutually exclusive with calls to probe, disconnect, pre_reset, and
443	post_reset; the USB core guarantees that this is true of internal
444	suspend/resume events as well.
445	
446	If a driver wants to block all suspend/resume calls during some
447	critical section, it can simply acquire udev->pm_mutex. Note that
448	calls to resume may be triggered indirectly. Block IO due to memory
449	allocations can make the vm subsystem resume a device. Thus while
450	holding this lock you must not allocate memory with GFP_KERNEL or
451	GFP_NOFS.
452	
453	Alternatively, if the critical section might call some of the
454	usb_autopm_* routines, the driver can avoid deadlock by doing:
455	
456		down(&udev->dev.sem);
457		rc = usb_autopm_get_interface(intf);
458	
459	and at the end of the critical section:
460	
461		if (!rc)
462			usb_autopm_put_interface(intf);
463		up(&udev->dev.sem);
464	
465	Holding the device semaphore will block all external PM calls, and the
466	usb_autopm_get_interface() will prevent any internal PM calls, even if
467	it fails.  (Exercise: Why?)
468	
469	The rules for locking order are:
470	
471		Never acquire any device semaphore while holding any PM mutex.
472	
473		Never acquire udev->pm_mutex while holding the PM mutex for
474		a device that isn't a descendant of udev.
475	
476	In other words, PM mutexes should only be acquired going up the device
477	tree, and they should be acquired only after locking all the device
478	semaphores you need to hold.  These rules don't matter to drivers very
479	much; they usually affect just the USB core.
480	
481	Still, drivers do need to be careful.  For example, many drivers use a
482	private mutex to synchronize their normal I/O activities with their
483	disconnect method.  Now if the driver supports autosuspend then it
484	must call usb_autopm_put_interface() from somewhere -- maybe from its
485	close method.  It should make the call while holding the private mutex,
486	since a driver shouldn't call any of the usb_autopm_* functions for an
487	interface from which it has been unbound.
488	
489	But the usb_autpm_* routines always acquire the device's PM mutex, and
490	consequently the locking order has to be: private mutex first, PM
491	mutex second.  Since the suspend method is always called with the PM
492	mutex held, it mustn't try to acquire the private mutex.  It has to
493	synchronize with the driver's I/O activities in some other way.
494	
495	
496		Interaction between dynamic PM and system PM
497		--------------------------------------------
498	
499	Dynamic power management and system power management can interact in
500	a couple of ways.
501	
502	Firstly, a device may already be manually suspended or autosuspended
503	when a system suspend occurs.  Since system suspends are supposed to
504	be as transparent as possible, the device should remain suspended
505	following the system resume.  The 2.6.23 kernel obeys this principle
506	for manually suspended devices but not for autosuspended devices; they
507	do get resumed when the system wakes up.  (Presumably they will be
508	autosuspended again after their idle-delay time expires.)  In later
509	kernels this behavior will be fixed.
510	
511	(There is an exception.  If a device would undergo a reset-resume
512	instead of a normal resume, and the device is enabled for remote
513	wakeup, then the reset-resume takes place even if the device was
514	already suspended when the system suspend began.  The justification is
515	that a reset-resume is a kind of remote-wakeup event.  Or to put it
516	another way, a device which needs a reset won't be able to generate
517	normal remote-wakeup signals, so it ought to be resumed immediately.)
518	
519	Secondly, a dynamic power-management event may occur as a system
520	suspend is underway.  The window for this is short, since system
521	suspends don't take long (a few seconds usually), but it can happen.
522	For example, a suspended device may send a remote-wakeup signal while
523	the system is suspending.  The remote wakeup may succeed, which would
524	cause the system suspend to abort.  If the remote wakeup doesn't
525	succeed, it may still remain active and thus cause the system to
526	resume as soon as the system suspend is complete.  Or the remote
527	wakeup may fail and get lost.  Which outcome occurs depends on timing
528	and on the hardware and firmware design.
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