xref: /linux/Documentation/driver-api/usb/power-management.rst (revision 905e46acd3272d04566fec49afbd7ad9e2ed9ae3)
1.. _usb-power-management:
2
3Power Management for USB
4~~~~~~~~~~~~~~~~~~~~~~~~
5
6:Author: Alan Stern <stern@rowland.harvard.edu>
7:Date: Last-updated: February 2014
8
9..
10	Contents:
11	---------
12	* What is Power Management?
13	* What is Remote Wakeup?
14	* When is a USB device idle?
15	* Forms of dynamic PM
16	* The user interface for dynamic PM
17	* Changing the default idle-delay time
18	* Warnings
19	* The driver interface for Power Management
20	* The driver interface for autosuspend and autoresume
21	* Other parts of the driver interface
22	* Mutual exclusion
23	* Interaction between dynamic PM and system PM
24	* xHCI hardware link PM
25	* USB Port Power Control
26	* User Interface for Port Power Control
27	* Suggested Userspace Port Power Policy
28
29
30What is Power Management?
31-------------------------
32
33Power Management (PM) is the practice of saving energy by suspending
34parts of a computer system when they aren't being used.  While a
35component is ``suspended`` it is in a nonfunctional low-power state; it
36might even be turned off completely.  A suspended component can be
37``resumed`` (returned to a functional full-power state) when the kernel
38needs to use it.  (There also are forms of PM in which components are
39placed in a less functional but still usable state instead of being
40suspended; an example would be reducing the CPU's clock rate.  This
41document will not discuss those other forms.)
42
43When the parts being suspended include the CPU and most of the rest of
44the system, we speak of it as a "system suspend".  When a particular
45device is turned off while the system as a whole remains running, we
46call it a "dynamic suspend" (also known as a "runtime suspend" or
47"selective suspend").  This document concentrates mostly on how
48dynamic PM is implemented in the USB subsystem, although system PM is
49covered to some extent (see ``Documentation/power/*.txt`` for more
50information about system PM).
51
52System PM support is present only if the kernel was built with
53``CONFIG_SUSPEND`` or ``CONFIG_HIBERNATION`` enabled.  Dynamic PM support
54
55for USB is present whenever
56the kernel was built with ``CONFIG_PM`` enabled.
57
58[Historically, dynamic PM support for USB was present only if the
59kernel had been built with ``CONFIG_USB_SUSPEND`` enabled (which depended on
60``CONFIG_PM_RUNTIME``).  Starting with the 3.10 kernel release, dynamic PM
61support for USB was present whenever the kernel was built with
62``CONFIG_PM_RUNTIME`` enabled.  The ``CONFIG_USB_SUSPEND`` option had been
63eliminated.]
64
65
66What is Remote Wakeup?
67----------------------
68
69When a device has been suspended, it generally doesn't resume until
70the computer tells it to.  Likewise, if the entire computer has been
71suspended, it generally doesn't resume until the user tells it to, say
72by pressing a power button or opening the cover.
73
74However some devices have the capability of resuming by themselves, or
75asking the kernel to resume them, or even telling the entire computer
76to resume.  This capability goes by several names such as "Wake On
77LAN"; we will refer to it generically as "remote wakeup".  When a
78device is enabled for remote wakeup and it is suspended, it may resume
79itself (or send a request to be resumed) in response to some external
80event.  Examples include a suspended keyboard resuming when a key is
81pressed, or a suspended USB hub resuming when a device is plugged in.
82
83
84When is a USB device idle?
85--------------------------
86
87A device is idle whenever the kernel thinks it's not busy doing
88anything important and thus is a candidate for being suspended.  The
89exact definition depends on the device's driver; drivers are allowed
90to declare that a device isn't idle even when there's no actual
91communication taking place.  (For example, a hub isn't considered idle
92unless all the devices plugged into that hub are already suspended.)
93In addition, a device isn't considered idle so long as a program keeps
94its usbfs file open, whether or not any I/O is going on.
95
96If a USB device has no driver, its usbfs file isn't open, and it isn't
97being accessed through sysfs, then it definitely is idle.
98
99
100Forms of dynamic PM
101-------------------
102
103Dynamic suspends occur when the kernel decides to suspend an idle
104device.  This is called ``autosuspend`` for short.  In general, a device
105won't be autosuspended unless it has been idle for some minimum period
106of time, the so-called idle-delay time.
107
108Of course, nothing the kernel does on its own initiative should
109prevent the computer or its devices from working properly.  If a
110device has been autosuspended and a program tries to use it, the
111kernel will automatically resume the device (autoresume).  For the
112same reason, an autosuspended device will usually have remote wakeup
113enabled, if the device supports remote wakeup.
114
115It is worth mentioning that many USB drivers don't support
116autosuspend.  In fact, at the time of this writing (Linux 2.6.23) the
117only drivers which do support it are the hub driver, kaweth, asix,
118usblp, usblcd, and usb-skeleton (which doesn't count).  If a
119non-supporting driver is bound to a device, the device won't be
120autosuspended.  In effect, the kernel pretends the device is never
121idle.
122
123We can categorize power management events in two broad classes:
124external and internal.  External events are those triggered by some
125agent outside the USB stack: system suspend/resume (triggered by
126userspace), manual dynamic resume (also triggered by userspace), and
127remote wakeup (triggered by the device).  Internal events are those
128triggered within the USB stack: autosuspend and autoresume.  Note that
129all dynamic suspend events are internal; external agents are not
130allowed to issue dynamic suspends.
131
132
133The user interface for dynamic PM
134---------------------------------
135
136The user interface for controlling dynamic PM is located in the ``power/``
137subdirectory of each USB device's sysfs directory, that is, in
138``/sys/bus/usb/devices/.../power/`` where "..." is the device's ID.  The
139relevant attribute files are: wakeup, control, and
140``autosuspend_delay_ms``.  (There may also be a file named ``level``; this
141file was deprecated as of the 2.6.35 kernel and replaced by the
142``control`` file.  In 2.6.38 the ``autosuspend`` file will be deprecated
143and replaced by the ``autosuspend_delay_ms`` file.  The only difference
144is that the newer file expresses the delay in milliseconds whereas the
145older file uses seconds.  Confusingly, both files are present in 2.6.37
146but only ``autosuspend`` works.)
147
148	``power/wakeup``
149
150		This file is empty if the device does not support
151		remote wakeup.  Otherwise the file contains either the
152		word ``enabled`` or the word ``disabled``, and you can
153		write those words to the file.  The setting determines
154		whether or not remote wakeup will be enabled when the
155		device is next suspended.  (If the setting is changed
156		while the device is suspended, the change won't take
157		effect until the following suspend.)
158
159	``power/control``
160
161		This file contains one of two words: ``on`` or ``auto``.
162		You can write those words to the file to change the
163		device's setting.
164
165		- ``on`` means that the device should be resumed and
166		  autosuspend is not allowed.  (Of course, system
167		  suspends are still allowed.)
168
169		- ``auto`` is the normal state in which the kernel is
170		  allowed to autosuspend and autoresume the device.
171
172		(In kernels up to 2.6.32, you could also specify
173		``suspend``, meaning that the device should remain
174		suspended and autoresume was not allowed.  This
175		setting is no longer supported.)
176
177	``power/autosuspend_delay_ms``
178
179		This file contains an integer value, which is the
180		number of milliseconds the device should remain idle
181		before the kernel will autosuspend it (the idle-delay
182		time).  The default is 2000.  0 means to autosuspend
183		as soon as the device becomes idle, and negative
184		values mean never to autosuspend.  You can write a
185		number to the file to change the autosuspend
186		idle-delay time.
187
188Writing ``-1`` to ``power/autosuspend_delay_ms`` and writing ``on`` to
189``power/control`` do essentially the same thing -- they both prevent the
190device from being autosuspended.  Yes, this is a redundancy in the
191API.
192
193(In 2.6.21 writing ``0`` to ``power/autosuspend`` would prevent the device
194from being autosuspended; the behavior was changed in 2.6.22.  The
195``power/autosuspend`` attribute did not exist prior to 2.6.21, and the
196``power/level`` attribute did not exist prior to 2.6.22.  ``power/control``
197was added in 2.6.34, and ``power/autosuspend_delay_ms`` was added in
1982.6.37 but did not become functional until 2.6.38.)
199
200
201Changing the default idle-delay time
202------------------------------------
203
204The default autosuspend idle-delay time (in seconds) is controlled by
205a module parameter in usbcore.  You can specify the value when usbcore
206is loaded.  For example, to set it to 5 seconds instead of 2 you would
207do::
208
209	modprobe usbcore autosuspend=5
210
211Equivalently, you could add to a configuration file in /etc/modprobe.d
212a line saying::
213
214	options usbcore autosuspend=5
215
216Some distributions load the usbcore module very early during the boot
217process, by means of a program or script running from an initramfs
218image.  To alter the parameter value you would have to rebuild that
219image.
220
221If usbcore is compiled into the kernel rather than built as a loadable
222module, you can add::
223
224	usbcore.autosuspend=5
225
226to the kernel's boot command line.
227
228Finally, the parameter value can be changed while the system is
229running.  If you do::
230
231	echo 5 >/sys/module/usbcore/parameters/autosuspend
232
233then each new USB device will have its autosuspend idle-delay
234initialized to 5.  (The idle-delay values for already existing devices
235will not be affected.)
236
237Setting the initial default idle-delay to -1 will prevent any
238autosuspend of any USB device.  This has the benefit of allowing you
239then to enable autosuspend for selected devices.
240
241
242Warnings
243--------
244
245The USB specification states that all USB devices must support power
246management.  Nevertheless, the sad fact is that many devices do not
247support it very well.  You can suspend them all right, but when you
248try to resume them they disconnect themselves from the USB bus or
249they stop working entirely.  This seems to be especially prevalent
250among printers and scanners, but plenty of other types of device have
251the same deficiency.
252
253For this reason, by default the kernel disables autosuspend (the
254``power/control`` attribute is initialized to ``on``) for all devices other
255than hubs.  Hubs, at least, appear to be reasonably well-behaved in
256this regard.
257
258(In 2.6.21 and 2.6.22 this wasn't the case.  Autosuspend was enabled
259by default for almost all USB devices.  A number of people experienced
260problems as a result.)
261
262This means that non-hub devices won't be autosuspended unless the user
263or a program explicitly enables it.  As of this writing there aren't
264any widespread programs which will do this; we hope that in the near
265future device managers such as HAL will take on this added
266responsibility.  In the meantime you can always carry out the
267necessary operations by hand or add them to a udev script.  You can
268also change the idle-delay time; 2 seconds is not the best choice for
269every device.
270
271If a driver knows that its device has proper suspend/resume support,
272it can enable autosuspend all by itself.  For example, the video
273driver for a laptop's webcam might do this (in recent kernels they
274do), since these devices are rarely used and so should normally be
275autosuspended.
276
277Sometimes it turns out that even when a device does work okay with
278autosuspend there are still problems.  For example, the usbhid driver,
279which manages keyboards and mice, has autosuspend support.  Tests with
280a number of keyboards show that typing on a suspended keyboard, while
281causing the keyboard to do a remote wakeup all right, will nonetheless
282frequently result in lost keystrokes.  Tests with mice show that some
283of them will issue a remote-wakeup request in response to button
284presses but not to motion, and some in response to neither.
285
286The kernel will not prevent you from enabling autosuspend on devices
287that can't handle it.  It is even possible in theory to damage a
288device by suspending it at the wrong time.  (Highly unlikely, but
289possible.)  Take care.
290
291
292The driver interface for Power Management
293-----------------------------------------
294
295The requirements for a USB driver to support external power management
296are pretty modest; the driver need only define::
297
298	.suspend
299	.resume
300	.reset_resume
301
302methods in its :c:type:`usb_driver` structure, and the ``reset_resume`` method
303is optional.  The methods' jobs are quite simple:
304
305      - The ``suspend`` method is called to warn the driver that the
306	device is going to be suspended.  If the driver returns a
307	negative error code, the suspend will be aborted.  Normally
308	the driver will return 0, in which case it must cancel all
309	outstanding URBs (:c:func:`usb_kill_urb`) and not submit any more.
310
311      - The ``resume`` method is called to tell the driver that the
312	device has been resumed and the driver can return to normal
313	operation.  URBs may once more be submitted.
314
315      - The ``reset_resume`` method is called to tell the driver that
316	the device has been resumed and it also has been reset.
317	The driver should redo any necessary device initialization,
318	since the device has probably lost most or all of its state
319	(although the interfaces will be in the same altsettings as
320	before the suspend).
321
322If the device is disconnected or powered down while it is suspended,
323the ``disconnect`` method will be called instead of the ``resume`` or
324``reset_resume`` method.  This is also quite likely to happen when
325waking up from hibernation, as many systems do not maintain suspend
326current to the USB host controllers during hibernation.  (It's
327possible to work around the hibernation-forces-disconnect problem by
328using the USB Persist facility.)
329
330The ``reset_resume`` method is used by the USB Persist facility (see
331:ref:`usb-persist`) and it can also be used under certain
332circumstances when ``CONFIG_USB_PERSIST`` is not enabled.  Currently, if a
333device is reset during a resume and the driver does not have a
334``reset_resume`` method, the driver won't receive any notification about
335the resume.  Later kernels will call the driver's ``disconnect`` method;
3362.6.23 doesn't do this.
337
338USB drivers are bound to interfaces, so their ``suspend`` and ``resume``
339methods get called when the interfaces are suspended or resumed.  In
340principle one might want to suspend some interfaces on a device (i.e.,
341force the drivers for those interface to stop all activity) without
342suspending the other interfaces.  The USB core doesn't allow this; all
343interfaces are suspended when the device itself is suspended and all
344interfaces are resumed when the device is resumed.  It isn't possible
345to suspend or resume some but not all of a device's interfaces.  The
346closest you can come is to unbind the interfaces' drivers.
347
348
349The driver interface for autosuspend and autoresume
350---------------------------------------------------
351
352To support autosuspend and autoresume, a driver should implement all
353three of the methods listed above.  In addition, a driver indicates
354that it supports autosuspend by setting the ``.supports_autosuspend`` flag
355in its usb_driver structure.  It is then responsible for informing the
356USB core whenever one of its interfaces becomes busy or idle.  The
357driver does so by calling these six functions::
358
359	int  usb_autopm_get_interface(struct usb_interface *intf);
360	void usb_autopm_put_interface(struct usb_interface *intf);
361	int  usb_autopm_get_interface_async(struct usb_interface *intf);
362	void usb_autopm_put_interface_async(struct usb_interface *intf);
363	void usb_autopm_get_interface_no_resume(struct usb_interface *intf);
364	void usb_autopm_put_interface_no_suspend(struct usb_interface *intf);
365
366The functions work by maintaining a usage counter in the
367usb_interface's embedded device structure.  When the counter is > 0
368then the interface is deemed to be busy, and the kernel will not
369autosuspend the interface's device.  When the usage counter is = 0
370then the interface is considered to be idle, and the kernel may
371autosuspend the device.
372
373Drivers need not be concerned about balancing changes to the usage
374counter; the USB core will undo any remaining "get"s when a driver
375is unbound from its interface.  As a corollary, drivers must not call
376any of the ``usb_autopm_*`` functions after their ``disconnect``
377routine has returned.
378
379Drivers using the async routines are responsible for their own
380synchronization and mutual exclusion.
381
382	:c:func:`usb_autopm_get_interface` increments the usage counter and
383	does an autoresume if the device is suspended.  If the
384	autoresume fails, the counter is decremented back.
385
386	:c:func:`usb_autopm_put_interface` decrements the usage counter and
387	attempts an autosuspend if the new value is = 0.
388
389	:c:func:`usb_autopm_get_interface_async` and
390	:c:func:`usb_autopm_put_interface_async` do almost the same things as
391	their non-async counterparts.  The big difference is that they
392	use a workqueue to do the resume or suspend part of their
393	jobs.  As a result they can be called in an atomic context,
394	such as an URB's completion handler, but when they return the
395	device will generally not yet be in the desired state.
396
397	:c:func:`usb_autopm_get_interface_no_resume` and
398	:c:func:`usb_autopm_put_interface_no_suspend` merely increment or
399	decrement the usage counter; they do not attempt to carry out
400	an autoresume or an autosuspend.  Hence they can be called in
401	an atomic context.
402
403The simplest usage pattern is that a driver calls
404:c:func:`usb_autopm_get_interface` in its open routine and
405:c:func:`usb_autopm_put_interface` in its close or release routine.  But other
406patterns are possible.
407
408The autosuspend attempts mentioned above will often fail for one
409reason or another.  For example, the ``power/control`` attribute might be
410set to ``on``, or another interface in the same device might not be
411idle.  This is perfectly normal.  If the reason for failure was that
412the device hasn't been idle for long enough, a timer is scheduled to
413carry out the operation automatically when the autosuspend idle-delay
414has expired.
415
416Autoresume attempts also can fail, although failure would mean that
417the device is no longer present or operating properly.  Unlike
418autosuspend, there's no idle-delay for an autoresume.
419
420
421Other parts of the driver interface
422-----------------------------------
423
424Drivers can enable autosuspend for their devices by calling::
425
426	usb_enable_autosuspend(struct usb_device *udev);
427
428in their :c:func:`probe` routine, if they know that the device is capable of
429suspending and resuming correctly.  This is exactly equivalent to
430writing ``auto`` to the device's ``power/control`` attribute.  Likewise,
431drivers can disable autosuspend by calling::
432
433	usb_disable_autosuspend(struct usb_device *udev);
434
435This is exactly the same as writing ``on`` to the ``power/control`` attribute.
436
437Sometimes a driver needs to make sure that remote wakeup is enabled
438during autosuspend.  For example, there's not much point
439autosuspending a keyboard if the user can't cause the keyboard to do a
440remote wakeup by typing on it.  If the driver sets
441``intf->needs_remote_wakeup`` to 1, the kernel won't autosuspend the
442device if remote wakeup isn't available.  (If the device is already
443autosuspended, though, setting this flag won't cause the kernel to
444autoresume it.  Normally a driver would set this flag in its ``probe``
445method, at which time the device is guaranteed not to be
446autosuspended.)
447
448If a driver does its I/O asynchronously in interrupt context, it
449should call :c:func:`usb_autopm_get_interface_async` before starting output and
450:c:func:`usb_autopm_put_interface_async` when the output queue drains.  When
451it receives an input event, it should call::
452
453	usb_mark_last_busy(struct usb_device *udev);
454
455in the event handler.  This tells the PM core that the device was just
456busy and therefore the next autosuspend idle-delay expiration should
457be pushed back.  Many of the usb_autopm_* routines also make this call,
458so drivers need to worry only when interrupt-driven input arrives.
459
460Asynchronous operation is always subject to races.  For example, a
461driver may call the :c:func:`usb_autopm_get_interface_async` routine at a time
462when the core has just finished deciding the device has been idle for
463long enough but not yet gotten around to calling the driver's ``suspend``
464method.  The ``suspend`` method must be responsible for synchronizing with
465the I/O request routine and the URB completion handler; it should
466cause autosuspends to fail with -EBUSY if the driver needs to use the
467device.
468
469External suspend calls should never be allowed to fail in this way,
470only autosuspend calls.  The driver can tell them apart by applying
471the :c:func:`PMSG_IS_AUTO` macro to the message argument to the ``suspend``
472method; it will return True for internal PM events (autosuspend) and
473False for external PM events.
474
475
476Mutual exclusion
477----------------
478
479For external events -- but not necessarily for autosuspend or
480autoresume -- the device semaphore (udev->dev.sem) will be held when a
481``suspend`` or ``resume`` method is called.  This implies that external
482suspend/resume events are mutually exclusive with calls to ``probe``,
483``disconnect``, ``pre_reset``, and ``post_reset``; the USB core guarantees that
484this is true of autosuspend/autoresume events as well.
485
486If a driver wants to block all suspend/resume calls during some
487critical section, the best way is to lock the device and call
488:c:func:`usb_autopm_get_interface` (and do the reverse at the end of the
489critical section).  Holding the device semaphore will block all
490external PM calls, and the :c:func:`usb_autopm_get_interface` will prevent any
491internal PM calls, even if it fails.  (Exercise: Why?)
492
493
494Interaction between dynamic PM and system PM
495--------------------------------------------
496
497Dynamic power management and system power management can interact in
498a couple of ways.
499
500Firstly, a device may already be autosuspended when a system suspend
501occurs.  Since system suspends are supposed to be as transparent as
502possible, the device should remain suspended following the system
503resume.  But this theory may not work out well in practice; over time
504the kernel's behavior in this regard has changed.  As of 2.6.37 the
505policy is to resume all devices during a system resume and let them
506handle their own runtime suspends afterward.
507
508Secondly, a dynamic power-management event may occur as a system
509suspend is underway.  The window for this is short, since system
510suspends don't take long (a few seconds usually), but it can happen.
511For example, a suspended device may send a remote-wakeup signal while
512the system is suspending.  The remote wakeup may succeed, which would
513cause the system suspend to abort.  If the remote wakeup doesn't
514succeed, it may still remain active and thus cause the system to
515resume as soon as the system suspend is complete.  Or the remote
516wakeup may fail and get lost.  Which outcome occurs depends on timing
517and on the hardware and firmware design.
518
519
520xHCI hardware link PM
521---------------------
522
523xHCI host controller provides hardware link power management to usb2.0
524(xHCI 1.0 feature) and usb3.0 devices which support link PM. By
525enabling hardware LPM, the host can automatically put the device into
526lower power state(L1 for usb2.0 devices, or U1/U2 for usb3.0 devices),
527which state device can enter and resume very quickly.
528
529The user interface for controlling hardware LPM is located in the
530``power/`` subdirectory of each USB device's sysfs directory, that is, in
531``/sys/bus/usb/devices/.../power/`` where "..." is the device's ID. The
532relevant attribute files are ``usb2_hardware_lpm`` and ``usb3_hardware_lpm``.
533
534	``power/usb2_hardware_lpm``
535
536		When a USB2 device which support LPM is plugged to a
537		xHCI host root hub which support software LPM, the
538		host will run a software LPM test for it; if the device
539		enters L1 state and resume successfully and the host
540		supports USB2 hardware LPM, this file will show up and
541		driver will enable hardware LPM	for the device. You
542		can write y/Y/1 or n/N/0 to the file to	enable/disable
543		USB2 hardware LPM manually. This is for	test purpose mainly.
544
545	``power/usb3_hardware_lpm_u1``
546	``power/usb3_hardware_lpm_u2``
547
548		When a USB 3.0 lpm-capable device is plugged in to a
549		xHCI host which supports link PM, it will check if U1
550		and U2 exit latencies have been set in the BOS
551		descriptor; if the check is passed and the host
552		supports USB3 hardware LPM, USB3 hardware LPM will be
553		enabled for the device and these files will be created.
554		The files hold a string value (enable or disable)
555		indicating whether or not USB3 hardware LPM U1 or U2
556		is enabled for the device.
557
558USB Port Power Control
559----------------------
560
561In addition to suspending endpoint devices and enabling hardware
562controlled link power management, the USB subsystem also has the
563capability to disable power to ports under some conditions.  Power is
564controlled through ``Set/ClearPortFeature(PORT_POWER)`` requests to a hub.
565In the case of a root or platform-internal hub the host controller
566driver translates ``PORT_POWER`` requests into platform firmware (ACPI)
567method calls to set the port power state. For more background see the
568Linux Plumbers Conference 2012 slides [#f1]_ and video [#f2]_:
569
570Upon receiving a ``ClearPortFeature(PORT_POWER)`` request a USB port is
571logically off, and may trigger the actual loss of VBUS to the port [#f3]_.
572VBUS may be maintained in the case where a hub gangs multiple ports into
573a shared power well causing power to remain until all ports in the gang
574are turned off.  VBUS may also be maintained by hub ports configured for
575a charging application.  In any event a logically off port will lose
576connection with its device, not respond to hotplug events, and not
577respond to remote wakeup events.
578
579.. warning::
580
581   turning off a port may result in the inability to hot add a device.
582   Please see "User Interface for Port Power Control" for details.
583
584As far as the effect on the device itself it is similar to what a device
585goes through during system suspend, i.e. the power session is lost.  Any
586USB device or driver that misbehaves with system suspend will be
587similarly affected by a port power cycle event.  For this reason the
588implementation shares the same device recovery path (and honors the same
589quirks) as the system resume path for the hub.
590
591.. [#f1]
592
593  http://dl.dropbox.com/u/96820575/sarah-sharp-lpt-port-power-off2-mini.pdf
594
595.. [#f2]
596
597  http://linuxplumbers.ubicast.tv/videos/usb-port-power-off-kerneluserspace-api/
598
599.. [#f3]
600
601  USB 3.1 Section 10.12
602
603  wakeup note: if a device is configured to send wakeup events the port
604  power control implementation will block poweroff attempts on that
605  port.
606
607
608User Interface for Port Power Control
609-------------------------------------
610
611The port power control mechanism uses the PM runtime system.  Poweroff is
612requested by clearing the ``power/pm_qos_no_power_off`` flag of the port device
613(defaults to 1).  If the port is disconnected it will immediately receive a
614``ClearPortFeature(PORT_POWER)`` request.  Otherwise, it will honor the pm
615runtime rules and require the attached child device and all descendants to be
616suspended. This mechanism is dependent on the hub advertising port power
617switching in its hub descriptor (wHubCharacteristics logical power switching
618mode field).
619
620Note, some interface devices/drivers do not support autosuspend.  Userspace may
621need to unbind the interface drivers before the :c:type:`usb_device` will
622suspend.  An unbound interface device is suspended by default.  When unbinding,
623be careful to unbind interface drivers, not the driver of the parent usb
624device.  Also, leave hub interface drivers bound.  If the driver for the usb
625device (not interface) is unbound the kernel is no longer able to resume the
626device.  If a hub interface driver is unbound, control of its child ports is
627lost and all attached child-devices will disconnect.  A good rule of thumb is
628that if the 'driver/module' link for a device points to
629``/sys/module/usbcore`` then unbinding it will interfere with port power
630control.
631
632Example of the relevant files for port power control.  Note, in this example
633these files are relative to a usb hub device (prefix)::
634
635     prefix=/sys/devices/pci0000:00/0000:00:14.0/usb3/3-1
636
637                      attached child device +
638                  hub port device +         |
639     hub interface device +       |         |
640                          v       v         v
641                  $prefix/3-1:1.0/3-1-port1/device
642
643     $prefix/3-1:1.0/3-1-port1/power/pm_qos_no_power_off
644     $prefix/3-1:1.0/3-1-port1/device/power/control
645     $prefix/3-1:1.0/3-1-port1/device/3-1.1:<intf0>/driver/unbind
646     $prefix/3-1:1.0/3-1-port1/device/3-1.1:<intf1>/driver/unbind
647     ...
648     $prefix/3-1:1.0/3-1-port1/device/3-1.1:<intfN>/driver/unbind
649
650In addition to these files some ports may have a 'peer' link to a port on
651another hub.  The expectation is that all superspeed ports have a
652hi-speed peer::
653
654  $prefix/3-1:1.0/3-1-port1/peer -> ../../../../usb2/2-1/2-1:1.0/2-1-port1
655  ../../../../usb2/2-1/2-1:1.0/2-1-port1/peer -> ../../../../usb3/3-1/3-1:1.0/3-1-port1
656
657Distinct from 'companion ports', or 'ehci/xhci shared switchover ports'
658peer ports are simply the hi-speed and superspeed interface pins that
659are combined into a single usb3 connector.  Peer ports share the same
660ancestor XHCI device.
661
662While a superspeed port is powered off a device may downgrade its
663connection and attempt to connect to the hi-speed pins.  The
664implementation takes steps to prevent this:
665
6661. Port suspend is sequenced to guarantee that hi-speed ports are powered-off
667   before their superspeed peer is permitted to power-off.  The implication is
668   that the setting ``pm_qos_no_power_off`` to zero on a superspeed port may
669   not cause the port to power-off until its highspeed peer has gone to its
670   runtime suspend state.  Userspace must take care to order the suspensions
671   if it wants to guarantee that a superspeed port will power-off.
672
6732. Port resume is sequenced to force a superspeed port to power-on prior to its
674   highspeed peer.
675
6763. Port resume always triggers an attached child device to resume.  After a
677   power session is lost the device may have been removed, or need reset.
678   Resuming the child device when the parent port regains power resolves those
679   states and clamps the maximum port power cycle frequency at the rate the
680   child device can suspend (autosuspend-delay) and resume (reset-resume
681   latency).
682
683Sysfs files relevant for port power control:
684
685	``<hubdev-portX>/power/pm_qos_no_power_off``:
686		This writable flag controls the state of an idle port.
687		Once all children and descendants have suspended the
688		port may suspend/poweroff provided that
689		pm_qos_no_power_off is '0'.  If pm_qos_no_power_off is
690		'1' the port will remain active/powered regardless of
691		the stats of descendants.  Defaults to 1.
692
693	``<hubdev-portX>/power/runtime_status``:
694		This file reflects whether the port is 'active' (power is on)
695		or 'suspended' (logically off).  There is no indication to
696		userspace whether VBUS is still supplied.
697
698	``<hubdev-portX>/connect_type``:
699		An advisory read-only flag to userspace indicating the
700		location and connection type of the port.  It returns
701		one of four values 'hotplug', 'hardwired', 'not used',
702		and 'unknown'.  All values, besides unknown, are set by
703		platform firmware.
704
705		``hotplug`` indicates an externally connectable/visible
706		port on the platform.  Typically userspace would choose
707		to keep such a port powered to handle new device
708		connection events.
709
710		``hardwired`` refers to a port that is not visible but
711		connectable. Examples are internal ports for USB
712		bluetooth that can be disconnected via an external
713		switch or a port with a hardwired USB camera.  It is
714		expected to be safe to allow these ports to suspend
715		provided pm_qos_no_power_off is coordinated with any
716		switch that gates connections.  Userspace must arrange
717		for the device to be connected prior to the port
718		powering off, or to activate the port prior to enabling
719		connection via a switch.
720
721		``not used`` refers to an internal port that is expected
722		to never have a device connected to it.  These may be
723		empty internal ports, or ports that are not physically
724		exposed on a platform.  Considered safe to be
725		powered-off at all times.
726
727		``unknown`` means platform firmware does not provide
728		information for this port.  Most commonly refers to
729		external hub ports which should be considered 'hotplug'
730		for policy decisions.
731
732		.. note::
733
734			- since we are relying on the BIOS to get this ACPI
735			  information correct, the USB port descriptions may
736			  be missing or wrong.
737
738			- Take care in clearing ``pm_qos_no_power_off``. Once
739			  power is off this port will
740			  not respond to new connect events.
741
742	Once a child device is attached additional constraints are
743	applied before the port is allowed to poweroff.
744
745	``<child>/power/control``:
746		Must be ``auto``, and the port will not
747		power down until ``<child>/power/runtime_status``
748		reflects the 'suspended' state.  Default
749		value is controlled by child device driver.
750
751	``<child>/power/persist``:
752		This defaults to ``1`` for most devices and indicates if
753		kernel can persist the device's configuration across a
754		power session loss (suspend / port-power event).  When
755		this value is ``0`` (quirky devices), port poweroff is
756		disabled.
757
758	``<child>/driver/unbind``:
759		Wakeup capable devices will block port poweroff.  At
760		this time the only mechanism to clear the usb-internal
761		wakeup-capability for an interface device is to unbind
762		its driver.
763
764Summary of poweroff pre-requisite settings relative to a port device::
765
766	echo 0 > power/pm_qos_no_power_off
767	echo 0 > peer/power/pm_qos_no_power_off # if it exists
768	echo auto > power/control # this is the default value
769	echo auto > <child>/power/control
770	echo 1 > <child>/power/persist # this is the default value
771
772Suggested Userspace Port Power Policy
773-------------------------------------
774
775As noted above userspace needs to be careful and deliberate about what
776ports are enabled for poweroff.
777
778The default configuration is that all ports start with
779``power/pm_qos_no_power_off`` set to ``1`` causing ports to always remain
780active.
781
782Given confidence in the platform firmware's description of the ports
783(ACPI _PLD record for a port populates 'connect_type') userspace can
784clear pm_qos_no_power_off for all 'not used' ports.  The same can be
785done for 'hardwired' ports provided poweroff is coordinated with any
786connection switch for the port.
787
788A more aggressive userspace policy is to enable USB port power off for
789all ports (set ``<hubdev-portX>/power/pm_qos_no_power_off`` to ``0``) when
790some external factor indicates the user has stopped interacting with the
791system.  For example, a distro may want to enable power off all USB
792ports when the screen blanks, and re-power them when the screen becomes
793active.  Smart phones and tablets may want to power off USB ports when
794the user pushes the power button.
795