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 must be careful to balance their overall changes to the usage
374counter.  Unbalanced "get"s will remain in effect when a driver is
375unbound from its interface, preventing the device from going into
376runtime suspend should the interface be bound to a driver again.  On
377the other hand, drivers are allowed to achieve this balance by calling
378the ``usb_autopm_*`` functions even after their ``disconnect`` routine
379has returned -- say from within a work-queue routine -- provided they
380retain an active reference to the interface (via ``usb_get_intf`` and
381``usb_put_intf``).
382
383Drivers using the async routines are responsible for their own
384synchronization and mutual exclusion.
385
386	:c:func:`usb_autopm_get_interface` increments the usage counter and
387	does an autoresume if the device is suspended.  If the
388	autoresume fails, the counter is decremented back.
389
390	:c:func:`usb_autopm_put_interface` decrements the usage counter and
391	attempts an autosuspend if the new value is = 0.
392
393	:c:func:`usb_autopm_get_interface_async` and
394	:c:func:`usb_autopm_put_interface_async` do almost the same things as
395	their non-async counterparts.  The big difference is that they
396	use a workqueue to do the resume or suspend part of their
397	jobs.  As a result they can be called in an atomic context,
398	such as an URB's completion handler, but when they return the
399	device will generally not yet be in the desired state.
400
401	:c:func:`usb_autopm_get_interface_no_resume` and
402	:c:func:`usb_autopm_put_interface_no_suspend` merely increment or
403	decrement the usage counter; they do not attempt to carry out
404	an autoresume or an autosuspend.  Hence they can be called in
405	an atomic context.
406
407The simplest usage pattern is that a driver calls
408:c:func:`usb_autopm_get_interface` in its open routine and
409:c:func:`usb_autopm_put_interface` in its close or release routine.  But other
410patterns are possible.
411
412The autosuspend attempts mentioned above will often fail for one
413reason or another.  For example, the ``power/control`` attribute might be
414set to ``on``, or another interface in the same device might not be
415idle.  This is perfectly normal.  If the reason for failure was that
416the device hasn't been idle for long enough, a timer is scheduled to
417carry out the operation automatically when the autosuspend idle-delay
418has expired.
419
420Autoresume attempts also can fail, although failure would mean that
421the device is no longer present or operating properly.  Unlike
422autosuspend, there's no idle-delay for an autoresume.
423
424
425Other parts of the driver interface
426-----------------------------------
427
428Drivers can enable autosuspend for their devices by calling::
429
430	usb_enable_autosuspend(struct usb_device *udev);
431
432in their :c:func:`probe` routine, if they know that the device is capable of
433suspending and resuming correctly.  This is exactly equivalent to
434writing ``auto`` to the device's ``power/control`` attribute.  Likewise,
435drivers can disable autosuspend by calling::
436
437	usb_disable_autosuspend(struct usb_device *udev);
438
439This is exactly the same as writing ``on`` to the ``power/control`` attribute.
440
441Sometimes a driver needs to make sure that remote wakeup is enabled
442during autosuspend.  For example, there's not much point
443autosuspending a keyboard if the user can't cause the keyboard to do a
444remote wakeup by typing on it.  If the driver sets
445``intf->needs_remote_wakeup`` to 1, the kernel won't autosuspend the
446device if remote wakeup isn't available.  (If the device is already
447autosuspended, though, setting this flag won't cause the kernel to
448autoresume it.  Normally a driver would set this flag in its ``probe``
449method, at which time the device is guaranteed not to be
450autosuspended.)
451
452If a driver does its I/O asynchronously in interrupt context, it
453should call :c:func:`usb_autopm_get_interface_async` before starting output and
454:c:func:`usb_autopm_put_interface_async` when the output queue drains.  When
455it receives an input event, it should call::
456
457	usb_mark_last_busy(struct usb_device *udev);
458
459in the event handler.  This tells the PM core that the device was just
460busy and therefore the next autosuspend idle-delay expiration should
461be pushed back.  Many of the usb_autopm_* routines also make this call,
462so drivers need to worry only when interrupt-driven input arrives.
463
464Asynchronous operation is always subject to races.  For example, a
465driver may call the :c:func:`usb_autopm_get_interface_async` routine at a time
466when the core has just finished deciding the device has been idle for
467long enough but not yet gotten around to calling the driver's ``suspend``
468method.  The ``suspend`` method must be responsible for synchronizing with
469the I/O request routine and the URB completion handler; it should
470cause autosuspends to fail with -EBUSY if the driver needs to use the
471device.
472
473External suspend calls should never be allowed to fail in this way,
474only autosuspend calls.  The driver can tell them apart by applying
475the :c:func:`PMSG_IS_AUTO` macro to the message argument to the ``suspend``
476method; it will return True for internal PM events (autosuspend) and
477False for external PM events.
478
479
480Mutual exclusion
481----------------
482
483For external events -- but not necessarily for autosuspend or
484autoresume -- the device semaphore (udev->dev.sem) will be held when a
485``suspend`` or ``resume`` method is called.  This implies that external
486suspend/resume events are mutually exclusive with calls to ``probe``,
487``disconnect``, ``pre_reset``, and ``post_reset``; the USB core guarantees that
488this is true of autosuspend/autoresume events as well.
489
490If a driver wants to block all suspend/resume calls during some
491critical section, the best way is to lock the device and call
492:c:func:`usb_autopm_get_interface` (and do the reverse at the end of the
493critical section).  Holding the device semaphore will block all
494external PM calls, and the :c:func:`usb_autopm_get_interface` will prevent any
495internal PM calls, even if it fails.  (Exercise: Why?)
496
497
498Interaction between dynamic PM and system PM
499--------------------------------------------
500
501Dynamic power management and system power management can interact in
502a couple of ways.
503
504Firstly, a device may already be autosuspended when a system suspend
505occurs.  Since system suspends are supposed to be as transparent as
506possible, the device should remain suspended following the system
507resume.  But this theory may not work out well in practice; over time
508the kernel's behavior in this regard has changed.  As of 2.6.37 the
509policy is to resume all devices during a system resume and let them
510handle their own runtime suspends afterward.
511
512Secondly, a dynamic power-management event may occur as a system
513suspend is underway.  The window for this is short, since system
514suspends don't take long (a few seconds usually), but it can happen.
515For example, a suspended device may send a remote-wakeup signal while
516the system is suspending.  The remote wakeup may succeed, which would
517cause the system suspend to abort.  If the remote wakeup doesn't
518succeed, it may still remain active and thus cause the system to
519resume as soon as the system suspend is complete.  Or the remote
520wakeup may fail and get lost.  Which outcome occurs depends on timing
521and on the hardware and firmware design.
522
523
524xHCI hardware link PM
525---------------------
526
527xHCI host controller provides hardware link power management to usb2.0
528(xHCI 1.0 feature) and usb3.0 devices which support link PM. By
529enabling hardware LPM, the host can automatically put the device into
530lower power state(L1 for usb2.0 devices, or U1/U2 for usb3.0 devices),
531which state device can enter and resume very quickly.
532
533The user interface for controlling hardware LPM is located in the
534``power/`` subdirectory of each USB device's sysfs directory, that is, in
535``/sys/bus/usb/devices/.../power/`` where "..." is the device's ID. The
536relevant attribute files are ``usb2_hardware_lpm`` and ``usb3_hardware_lpm``.
537
538	``power/usb2_hardware_lpm``
539
540		When a USB2 device which support LPM is plugged to a
541		xHCI host root hub which support software LPM, the
542		host will run a software LPM test for it; if the device
543		enters L1 state and resume successfully and the host
544		supports USB2 hardware LPM, this file will show up and
545		driver will enable hardware LPM	for the device. You
546		can write y/Y/1 or n/N/0 to the file to	enable/disable
547		USB2 hardware LPM manually. This is for	test purpose mainly.
548
549	``power/usb3_hardware_lpm_u1``
550	``power/usb3_hardware_lpm_u2``
551
552		When a USB 3.0 lpm-capable device is plugged in to a
553		xHCI host which supports link PM, it will check if U1
554		and U2 exit latencies have been set in the BOS
555		descriptor; if the check is passed and the host
556		supports USB3 hardware LPM, USB3 hardware LPM will be
557		enabled for the device and these files will be created.
558		The files hold a string value (enable or disable)
559		indicating whether or not USB3 hardware LPM U1 or U2
560		is enabled for the device.
561
562USB Port Power Control
563----------------------
564
565In addition to suspending endpoint devices and enabling hardware
566controlled link power management, the USB subsystem also has the
567capability to disable power to ports under some conditions.  Power is
568controlled through ``Set/ClearPortFeature(PORT_POWER)`` requests to a hub.
569In the case of a root or platform-internal hub the host controller
570driver translates ``PORT_POWER`` requests into platform firmware (ACPI)
571method calls to set the port power state. For more background see the
572Linux Plumbers Conference 2012 slides [#f1]_ and video [#f2]_:
573
574Upon receiving a ``ClearPortFeature(PORT_POWER)`` request a USB port is
575logically off, and may trigger the actual loss of VBUS to the port [#f3]_.
576VBUS may be maintained in the case where a hub gangs multiple ports into
577a shared power well causing power to remain until all ports in the gang
578are turned off.  VBUS may also be maintained by hub ports configured for
579a charging application.  In any event a logically off port will lose
580connection with its device, not respond to hotplug events, and not
581respond to remote wakeup events.
582
583.. warning::
584
585   turning off a port may result in the inability to hot add a device.
586   Please see "User Interface for Port Power Control" for details.
587
588As far as the effect on the device itself it is similar to what a device
589goes through during system suspend, i.e. the power session is lost.  Any
590USB device or driver that misbehaves with system suspend will be
591similarly affected by a port power cycle event.  For this reason the
592implementation shares the same device recovery path (and honors the same
593quirks) as the system resume path for the hub.
594
595.. [#f1]
596
597  http://dl.dropbox.com/u/96820575/sarah-sharp-lpt-port-power-off2-mini.pdf
598
599.. [#f2]
600
601  http://linuxplumbers.ubicast.tv/videos/usb-port-power-off-kerneluserspace-api/
602
603.. [#f3]
604
605  USB 3.1 Section 10.12
606
607  wakeup note: if a device is configured to send wakeup events the port
608  power control implementation will block poweroff attempts on that
609  port.
610
611
612User Interface for Port Power Control
613-------------------------------------
614
615The port power control mechanism uses the PM runtime system.  Poweroff is
616requested by clearing the ``power/pm_qos_no_power_off`` flag of the port device
617(defaults to 1).  If the port is disconnected it will immediately receive a
618``ClearPortFeature(PORT_POWER)`` request.  Otherwise, it will honor the pm
619runtime rules and require the attached child device and all descendants to be
620suspended. This mechanism is dependent on the hub advertising port power
621switching in its hub descriptor (wHubCharacteristics logical power switching
622mode field).
623
624Note, some interface devices/drivers do not support autosuspend.  Userspace may
625need to unbind the interface drivers before the :c:type:`usb_device` will
626suspend.  An unbound interface device is suspended by default.  When unbinding,
627be careful to unbind interface drivers, not the driver of the parent usb
628device.  Also, leave hub interface drivers bound.  If the driver for the usb
629device (not interface) is unbound the kernel is no longer able to resume the
630device.  If a hub interface driver is unbound, control of its child ports is
631lost and all attached child-devices will disconnect.  A good rule of thumb is
632that if the 'driver/module' link for a device points to
633``/sys/module/usbcore`` then unbinding it will interfere with port power
634control.
635
636Example of the relevant files for port power control.  Note, in this example
637these files are relative to a usb hub device (prefix)::
638
639     prefix=/sys/devices/pci0000:00/0000:00:14.0/usb3/3-1
640
641                      attached child device +
642                  hub port device +         |
643     hub interface device +       |         |
644                          v       v         v
645                  $prefix/3-1:1.0/3-1-port1/device
646
647     $prefix/3-1:1.0/3-1-port1/power/pm_qos_no_power_off
648     $prefix/3-1:1.0/3-1-port1/device/power/control
649     $prefix/3-1:1.0/3-1-port1/device/3-1.1:<intf0>/driver/unbind
650     $prefix/3-1:1.0/3-1-port1/device/3-1.1:<intf1>/driver/unbind
651     ...
652     $prefix/3-1:1.0/3-1-port1/device/3-1.1:<intfN>/driver/unbind
653
654In addition to these files some ports may have a 'peer' link to a port on
655another hub.  The expectation is that all superspeed ports have a
656hi-speed peer::
657
658  $prefix/3-1:1.0/3-1-port1/peer -> ../../../../usb2/2-1/2-1:1.0/2-1-port1
659  ../../../../usb2/2-1/2-1:1.0/2-1-port1/peer -> ../../../../usb3/3-1/3-1:1.0/3-1-port1
660
661Distinct from 'companion ports', or 'ehci/xhci shared switchover ports'
662peer ports are simply the hi-speed and superspeed interface pins that
663are combined into a single usb3 connector.  Peer ports share the same
664ancestor XHCI device.
665
666While a superspeed port is powered off a device may downgrade its
667connection and attempt to connect to the hi-speed pins.  The
668implementation takes steps to prevent this:
669
6701. Port suspend is sequenced to guarantee that hi-speed ports are powered-off
671   before their superspeed peer is permitted to power-off.  The implication is
672   that the setting ``pm_qos_no_power_off`` to zero on a superspeed port may
673   not cause the port to power-off until its highspeed peer has gone to its
674   runtime suspend state.  Userspace must take care to order the suspensions
675   if it wants to guarantee that a superspeed port will power-off.
676
6772. Port resume is sequenced to force a superspeed port to power-on prior to its
678   highspeed peer.
679
6803. Port resume always triggers an attached child device to resume.  After a
681   power session is lost the device may have been removed, or need reset.
682   Resuming the child device when the parent port regains power resolves those
683   states and clamps the maximum port power cycle frequency at the rate the
684   child device can suspend (autosuspend-delay) and resume (reset-resume
685   latency).
686
687Sysfs files relevant for port power control:
688
689	``<hubdev-portX>/power/pm_qos_no_power_off``:
690		This writable flag controls the state of an idle port.
691		Once all children and descendants have suspended the
692		port may suspend/poweroff provided that
693		pm_qos_no_power_off is '0'.  If pm_qos_no_power_off is
694		'1' the port will remain active/powered regardless of
695		the stats of descendants.  Defaults to 1.
696
697	``<hubdev-portX>/power/runtime_status``:
698		This file reflects whether the port is 'active' (power is on)
699		or 'suspended' (logically off).  There is no indication to
700		userspace whether VBUS is still supplied.
701
702	``<hubdev-portX>/connect_type``:
703		An advisory read-only flag to userspace indicating the
704		location and connection type of the port.  It returns
705		one of four values 'hotplug', 'hardwired', 'not used',
706		and 'unknown'.  All values, besides unknown, are set by
707		platform firmware.
708
709		``hotplug`` indicates an externally connectable/visible
710		port on the platform.  Typically userspace would choose
711		to keep such a port powered to handle new device
712		connection events.
713
714		``hardwired`` refers to a port that is not visible but
715		connectable. Examples are internal ports for USB
716		bluetooth that can be disconnected via an external
717		switch or a port with a hardwired USB camera.  It is
718		expected to be safe to allow these ports to suspend
719		provided pm_qos_no_power_off is coordinated with any
720		switch that gates connections.  Userspace must arrange
721		for the device to be connected prior to the port
722		powering off, or to activate the port prior to enabling
723		connection via a switch.
724
725		``not used`` refers to an internal port that is expected
726		to never have a device connected to it.  These may be
727		empty internal ports, or ports that are not physically
728		exposed on a platform.  Considered safe to be
729		powered-off at all times.
730
731		``unknown`` means platform firmware does not provide
732		information for this port.  Most commonly refers to
733		external hub ports which should be considered 'hotplug'
734		for policy decisions.
735
736		.. note::
737
738			- since we are relying on the BIOS to get this ACPI
739			  information correct, the USB port descriptions may
740			  be missing or wrong.
741
742			- Take care in clearing ``pm_qos_no_power_off``. Once
743			  power is off this port will
744			  not respond to new connect events.
745
746	Once a child device is attached additional constraints are
747	applied before the port is allowed to poweroff.
748
749	``<child>/power/control``:
750		Must be ``auto``, and the port will not
751		power down until ``<child>/power/runtime_status``
752		reflects the 'suspended' state.  Default
753		value is controlled by child device driver.
754
755	``<child>/power/persist``:
756		This defaults to ``1`` for most devices and indicates if
757		kernel can persist the device's configuration across a
758		power session loss (suspend / port-power event).  When
759		this value is ``0`` (quirky devices), port poweroff is
760		disabled.
761
762	``<child>/driver/unbind``:
763		Wakeup capable devices will block port poweroff.  At
764		this time the only mechanism to clear the usb-internal
765		wakeup-capability for an interface device is to unbind
766		its driver.
767
768Summary of poweroff pre-requisite settings relative to a port device::
769
770	echo 0 > power/pm_qos_no_power_off
771	echo 0 > peer/power/pm_qos_no_power_off # if it exists
772	echo auto > power/control # this is the default value
773	echo auto > <child>/power/control
774	echo 1 > <child>/power/persist # this is the default value
775
776Suggested Userspace Port Power Policy
777-------------------------------------
778
779As noted above userspace needs to be careful and deliberate about what
780ports are enabled for poweroff.
781
782The default configuration is that all ports start with
783``power/pm_qos_no_power_off`` set to ``1`` causing ports to always remain
784active.
785
786Given confidence in the platform firmware's description of the ports
787(ACPI _PLD record for a port populates 'connect_type') userspace can
788clear pm_qos_no_power_off for all 'not used' ports.  The same can be
789done for 'hardwired' ports provided poweroff is coordinated with any
790connection switch for the port.
791
792A more aggressive userspace policy is to enable USB port power off for
793all ports (set ``<hubdev-portX>/power/pm_qos_no_power_off`` to ``0``) when
794some external factor indicates the user has stopped interacting with the
795system.  For example, a distro may want to enable power off all USB
796ports when the screen blanks, and re-power them when the screen becomes
797active.  Smart phones and tablets may want to power off USB ports when
798the user pushes the power button.
799