usb/power-management.txt: convert to ReST and add to driver-api book

This document describe some USB core functions. Add it to the
driver-api book.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 0 insertions, 772 deletions

diff --git a/Documentation/usb/power-management.txt b/Documentation/usb/power-management.txt
deleted file mode 100644
index 00e706997130..000000000000
--- a/Documentation/usb/power-management.txt
+++ /dev/null
@@ -1,772 +0,0 @@
-			Power Management for USB
-
-		 Alan Stern <stern@rowland.harvard.edu>
-
-		       Last-updated: February 2014
-
-
-	Contents:
-	---------
-	* What is Power Management?
-	* What is Remote Wakeup?
-	* When is a USB device idle?
-	* Forms of dynamic PM
-	* The user interface for dynamic PM
-	* Changing the default idle-delay time
-	* Warnings
-	* The driver interface for Power Management
-	* The driver interface for autosuspend and autoresume
-	* Other parts of the driver interface
-	* Mutual exclusion
-	* Interaction between dynamic PM and system PM
-	* xHCI hardware link PM
-	* USB Port Power Control
-	* User Interface for Port Power Control
-	* Suggested Userspace Port Power Policy
-
-
-	What is Power Management?
-	-------------------------
-
-Power Management (PM) is the practice of saving energy by suspending
-parts of a computer system when they aren't being used.  While a
-component is "suspended" it is in a nonfunctional low-power state; it
-might even be turned off completely.  A suspended component can be
-"resumed" (returned to a functional full-power state) when the kernel
-needs to use it.  (There also are forms of PM in which components are
-placed in a less functional but still usable state instead of being
-suspended; an example would be reducing the CPU's clock rate.  This
-document will not discuss those other forms.)
-
-When the parts being suspended include the CPU and most of the rest of
-the system, we speak of it as a "system suspend".  When a particular
-device is turned off while the system as a whole remains running, we
-call it a "dynamic suspend" (also known as a "runtime suspend" or
-"selective suspend").  This document concentrates mostly on how
-dynamic PM is implemented in the USB subsystem, although system PM is
-covered to some extent (see Documentation/power/*.txt for more
-information about system PM).
-
-System PM support is present only if the kernel was built with CONFIG_SUSPEND
-or CONFIG_HIBERNATION enabled.  Dynamic PM support for USB is present whenever
-the kernel was built with CONFIG_PM enabled.
-
-[Historically, dynamic PM support for USB was present only if the
-kernel had been built with CONFIG_USB_SUSPEND enabled (which depended on
-CONFIG_PM_RUNTIME).  Starting with the 3.10 kernel release, dynamic PM support
-for USB was present whenever the kernel was built with CONFIG_PM_RUNTIME
-enabled.  The CONFIG_USB_SUSPEND option had been eliminated.]
-
-
-	What is Remote Wakeup?
-	----------------------
-
-When a device has been suspended, it generally doesn't resume until
-the computer tells it to.  Likewise, if the entire computer has been
-suspended, it generally doesn't resume until the user tells it to, say
-by pressing a power button or opening the cover.
-
-However some devices have the capability of resuming by themselves, or
-asking the kernel to resume them, or even telling the entire computer
-to resume.  This capability goes by several names such as "Wake On
-LAN"; we will refer to it generically as "remote wakeup".  When a
-device is enabled for remote wakeup and it is suspended, it may resume
-itself (or send a request to be resumed) in response to some external
-event.  Examples include a suspended keyboard resuming when a key is
-pressed, or a suspended USB hub resuming when a device is plugged in.
-
-
-	When is a USB device idle?
-	--------------------------
-
-A device is idle whenever the kernel thinks it's not busy doing
-anything important and thus is a candidate for being suspended.  The
-exact definition depends on the device's driver; drivers are allowed
-to declare that a device isn't idle even when there's no actual
-communication taking place.  (For example, a hub isn't considered idle
-unless all the devices plugged into that hub are already suspended.)
-In addition, a device isn't considered idle so long as a program keeps
-its usbfs file open, whether or not any I/O is going on.
-
-If a USB device has no driver, its usbfs file isn't open, and it isn't
-being accessed through sysfs, then it definitely is idle.
-
-
-	Forms of dynamic PM
-	-------------------
-
-Dynamic suspends occur when the kernel decides to suspend an idle
-device.  This is called "autosuspend" for short.  In general, a device
-won't be autosuspended unless it has been idle for some minimum period
-of time, the so-called idle-delay time.
-
-Of course, nothing the kernel does on its own initiative should
-prevent the computer or its devices from working properly.  If a
-device has been autosuspended and a program tries to use it, the
-kernel will automatically resume the device (autoresume).  For the
-same reason, an autosuspended device will usually have remote wakeup
-enabled, if the device supports remote wakeup.
-
-It is worth mentioning that many USB drivers don't support
-autosuspend.  In fact, at the time of this writing (Linux 2.6.23) the
-only drivers which do support it are the hub driver, kaweth, asix,
-usblp, usblcd, and usb-skeleton (which doesn't count).  If a
-non-supporting driver is bound to a device, the device won't be
-autosuspended.  In effect, the kernel pretends the device is never
-idle.
-
-We can categorize power management events in two broad classes:
-external and internal.  External events are those triggered by some
-agent outside the USB stack: system suspend/resume (triggered by
-userspace), manual dynamic resume (also triggered by userspace), and
-remote wakeup (triggered by the device).  Internal events are those
-triggered within the USB stack: autosuspend and autoresume.  Note that
-all dynamic suspend events are internal; external agents are not
-allowed to issue dynamic suspends.
-
-
-	The user interface for dynamic PM
-	---------------------------------
-
-The user interface for controlling dynamic PM is located in the power/
-subdirectory of each USB device's sysfs directory, that is, in
-/sys/bus/usb/devices/.../power/ where "..." is the device's ID.  The
-relevant attribute files are: wakeup, control, and
-autosuspend_delay_ms.  (There may also be a file named "level"; this
-file was deprecated as of the 2.6.35 kernel and replaced by the
-"control" file.  In 2.6.38 the "autosuspend" file will be deprecated
-and replaced by the "autosuspend_delay_ms" file.  The only difference
-is that the newer file expresses the delay in milliseconds whereas the
-older file uses seconds.  Confusingly, both files are present in 2.6.37
-but only "autosuspend" works.)
-
-	power/wakeup
-
-		This file is empty if the device does not support
-		remote wakeup.  Otherwise the file contains either the
-		word "enabled" or the word "disabled", and you can
-		write those words to the file.  The setting determines
-		whether or not remote wakeup will be enabled when the
-		device is next suspended.  (If the setting is changed
-		while the device is suspended, the change won't take
-		effect until the following suspend.)
-
-	power/control
-
-		This file contains one of two words: "on" or "auto".
-		You can write those words to the file to change the
-		device's setting.
-
-		"on" means that the device should be resumed and
-		autosuspend is not allowed.  (Of course, system
-		suspends are still allowed.)
-
-		"auto" is the normal state in which the kernel is
-		allowed to autosuspend and autoresume the device.
-
-		(In kernels up to 2.6.32, you could also specify
-		"suspend", meaning that the device should remain
-		suspended and autoresume was not allowed.  This
-		setting is no longer supported.)
-
-	power/autosuspend_delay_ms
-
-		This file contains an integer value, which is the
-		number of milliseconds the device should remain idle
-		before the kernel will autosuspend it (the idle-delay
-		time).  The default is 2000.  0 means to autosuspend
-		as soon as the device becomes idle, and negative
-		values mean never to autosuspend.  You can write a
-		number to the file to change the autosuspend
-		idle-delay time.
-
-Writing "-1" to power/autosuspend_delay_ms and writing "on" to
-power/control do essentially the same thing -- they both prevent the
-device from being autosuspended.  Yes, this is a redundancy in the
-API.
-
-(In 2.6.21 writing "0" to power/autosuspend would prevent the device
-from being autosuspended; the behavior was changed in 2.6.22.  The
-power/autosuspend attribute did not exist prior to 2.6.21, and the
-power/level attribute did not exist prior to 2.6.22.  power/control
-was added in 2.6.34, and power/autosuspend_delay_ms was added in
-2.6.37 but did not become functional until 2.6.38.)
-
-
-	Changing the default idle-delay time
-	------------------------------------
-
-The default autosuspend idle-delay time (in seconds) is controlled by
-a module parameter in usbcore.  You can specify the value when usbcore
-is loaded.  For example, to set it to 5 seconds instead of 2 you would
-do:
-
-	modprobe usbcore autosuspend=5
-
-Equivalently, you could add to a configuration file in /etc/modprobe.d
-a line saying:
-
-	options usbcore autosuspend=5
-
-Some distributions load the usbcore module very early during the boot
-process, by means of a program or script running from an initramfs
-image.  To alter the parameter value you would have to rebuild that
-image.
-
-If usbcore is compiled into the kernel rather than built as a loadable
-module, you can add
-
-	usbcore.autosuspend=5
-
-to the kernel's boot command line.
-
-Finally, the parameter value can be changed while the system is
-running.  If you do:
-
-	echo 5 >/sys/module/usbcore/parameters/autosuspend
-
-then each new USB device will have its autosuspend idle-delay
-initialized to 5.  (The idle-delay values for already existing devices
-will not be affected.)
-
-Setting the initial default idle-delay to -1 will prevent any
-autosuspend of any USB device.  This has the benefit of allowing you
-then to enable autosuspend for selected devices.
-
-
-	Warnings
-	--------
-
-The USB specification states that all USB devices must support power
-management.  Nevertheless, the sad fact is that many devices do not
-support it very well.  You can suspend them all right, but when you
-try to resume them they disconnect themselves from the USB bus or
-they stop working entirely.  This seems to be especially prevalent
-among printers and scanners, but plenty of other types of device have
-the same deficiency.
-
-For this reason, by default the kernel disables autosuspend (the
-power/control attribute is initialized to "on") for all devices other
-than hubs.  Hubs, at least, appear to be reasonably well-behaved in
-this regard.
-
-(In 2.6.21 and 2.6.22 this wasn't the case.  Autosuspend was enabled
-by default for almost all USB devices.  A number of people experienced
-problems as a result.)
-
-This means that non-hub devices won't be autosuspended unless the user
-or a program explicitly enables it.  As of this writing there aren't
-any widespread programs which will do this; we hope that in the near
-future device managers such as HAL will take on this added
-responsibility.  In the meantime you can always carry out the
-necessary operations by hand or add them to a udev script.  You can
-also change the idle-delay time; 2 seconds is not the best choice for
-every device.
-
-If a driver knows that its device has proper suspend/resume support,
-it can enable autosuspend all by itself.  For example, the video
-driver for a laptop's webcam might do this (in recent kernels they
-do), since these devices are rarely used and so should normally be
-autosuspended.
-
-Sometimes it turns out that even when a device does work okay with
-autosuspend there are still problems.  For example, the usbhid driver,
-which manages keyboards and mice, has autosuspend support.  Tests with
-a number of keyboards show that typing on a suspended keyboard, while
-causing the keyboard to do a remote wakeup all right, will nonetheless
-frequently result in lost keystrokes.  Tests with mice show that some
-of them will issue a remote-wakeup request in response to button
-presses but not to motion, and some in response to neither.
-
-The kernel will not prevent you from enabling autosuspend on devices
-that can't handle it.  It is even possible in theory to damage a
-device by suspending it at the wrong time.  (Highly unlikely, but
-possible.)  Take care.
-
-
-	The driver interface for Power Management
-	-----------------------------------------
-
-The requirements for a USB driver to support external power management
-are pretty modest; the driver need only define
-
-	.suspend
-	.resume
-	.reset_resume
-
-methods in its usb_driver structure, and the reset_resume method is
-optional.  The methods' jobs are quite simple:
-
-	The suspend method is called to warn the driver that the
-	device is going to be suspended.  If the driver returns a
-	negative error code, the suspend will be aborted.  Normally
-	the driver will return 0, in which case it must cancel all
-	outstanding URBs (usb_kill_urb()) and not submit any more.
-
-	The resume method is called to tell the driver that the
-	device has been resumed and the driver can return to normal
-	operation.  URBs may once more be submitted.
-
-	The reset_resume method is called to tell the driver that
-	the device has been resumed and it also has been reset.
-	The driver should redo any necessary device initialization,
-	since the device has probably lost most or all of its state
-	(although the interfaces will be in the same altsettings as
-	before the suspend).
-
-If the device is disconnected or powered down while it is suspended,
-the disconnect method will be called instead of the resume or
-reset_resume method.  This is also quite likely to happen when
-waking up from hibernation, as many systems do not maintain suspend
-current to the USB host controllers during hibernation.  (It's
-possible to work around the hibernation-forces-disconnect problem by
-using the USB Persist facility.)
-
-The reset_resume method is used by the USB Persist facility (see
-Documentation/usb/persist.txt) and it can also be used under certain
-circumstances when CONFIG_USB_PERSIST is not enabled.  Currently, if a
-device is reset during a resume and the driver does not have a
-reset_resume method, the driver won't receive any notification about
-the resume.  Later kernels will call the driver's disconnect method;
-2.6.23 doesn't do this.
-
-USB drivers are bound to interfaces, so their suspend and resume
-methods get called when the interfaces are suspended or resumed.  In
-principle one might want to suspend some interfaces on a device (i.e.,
-force the drivers for those interface to stop all activity) without
-suspending the other interfaces.  The USB core doesn't allow this; all
-interfaces are suspended when the device itself is suspended and all
-interfaces are resumed when the device is resumed.  It isn't possible
-to suspend or resume some but not all of a device's interfaces.  The
-closest you can come is to unbind the interfaces' drivers.
-
-
-	The driver interface for autosuspend and autoresume
-	---------------------------------------------------
-
-To support autosuspend and autoresume, a driver should implement all
-three of the methods listed above.  In addition, a driver indicates
-that it supports autosuspend by setting the .supports_autosuspend flag
-in its usb_driver structure.  It is then responsible for informing the
-USB core whenever one of its interfaces becomes busy or idle.  The
-driver does so by calling these six functions:
-
-	int  usb_autopm_get_interface(struct usb_interface *intf);
-	void usb_autopm_put_interface(struct usb_interface *intf);
-	int  usb_autopm_get_interface_async(struct usb_interface *intf);
-	void usb_autopm_put_interface_async(struct usb_interface *intf);
-	void usb_autopm_get_interface_no_resume(struct usb_interface *intf);
-	void usb_autopm_put_interface_no_suspend(struct usb_interface *intf);
-
-The functions work by maintaining a usage counter in the
-usb_interface's embedded device structure.  When the counter is > 0
-then the interface is deemed to be busy, and the kernel will not
-autosuspend the interface's device.  When the usage counter is = 0
-then the interface is considered to be idle, and the kernel may
-autosuspend the device.
-
-Drivers need not be concerned about balancing changes to the usage
-counter; the USB core will undo any remaining "get"s when a driver
-is unbound from its interface.  As a corollary, drivers must not call
-any of the usb_autopm_* functions after their disconnect() routine has
-returned.
-
-Drivers using the async routines are responsible for their own
-synchronization and mutual exclusion.
-
-	usb_autopm_get_interface() increments the usage counter and
-	does an autoresume if the device is suspended.  If the
-	autoresume fails, the counter is decremented back.
-
-	usb_autopm_put_interface() decrements the usage counter and
-	attempts an autosuspend if the new value is = 0.
-
-	usb_autopm_get_interface_async() and
-	usb_autopm_put_interface_async() do almost the same things as
-	their non-async counterparts.  The big difference is that they
-	use a workqueue to do the resume or suspend part of their
-	jobs.  As a result they can be called in an atomic context,
-	such as an URB's completion handler, but when they return the
-	device will generally not yet be in the desired state.
-
-	usb_autopm_get_interface_no_resume() and
-	usb_autopm_put_interface_no_suspend() merely increment or
-	decrement the usage counter; they do not attempt to carry out
-	an autoresume or an autosuspend.  Hence they can be called in
-	an atomic context.
-
-The simplest usage pattern is that a driver calls
-usb_autopm_get_interface() in its open routine and
-usb_autopm_put_interface() in its close or release routine.  But other
-patterns are possible.
-
-The autosuspend attempts mentioned above will often fail for one
-reason or another.  For example, the power/control attribute might be
-set to "on", or another interface in the same device might not be
-idle.  This is perfectly normal.  If the reason for failure was that
-the device hasn't been idle for long enough, a timer is scheduled to
-carry out the operation automatically when the autosuspend idle-delay
-has expired.
-
-Autoresume attempts also can fail, although failure would mean that
-the device is no longer present or operating properly.  Unlike
-autosuspend, there's no idle-delay for an autoresume.
-
-
-	Other parts of the driver interface
-	-----------------------------------
-
-Drivers can enable autosuspend for their devices by calling
-
-	usb_enable_autosuspend(struct usb_device *udev);
-
-in their probe() routine, if they know that the device is capable of
-suspending and resuming correctly.  This is exactly equivalent to
-writing "auto" to the device's power/control attribute.  Likewise,
-drivers can disable autosuspend by calling
-
-	usb_disable_autosuspend(struct usb_device *udev);
-
-This is exactly the same as writing "on" to the power/control attribute.
-
-Sometimes a driver needs to make sure that remote wakeup is enabled
-during autosuspend.  For example, there's not much point
-autosuspending a keyboard if the user can't cause the keyboard to do a
-remote wakeup by typing on it.  If the driver sets
-intf->needs_remote_wakeup to 1, the kernel won't autosuspend the
-device if remote wakeup isn't available.  (If the device is already
-autosuspended, though, setting this flag won't cause the kernel to
-autoresume it.  Normally a driver would set this flag in its probe
-method, at which time the device is guaranteed not to be
-autosuspended.)
-
-If a driver does its I/O asynchronously in interrupt context, it
-should call usb_autopm_get_interface_async() before starting output and
-usb_autopm_put_interface_async() when the output queue drains.  When
-it receives an input event, it should call
-
-	usb_mark_last_busy(struct usb_device *udev);
-
-in the event handler.  This tells the PM core that the device was just
-busy and therefore the next autosuspend idle-delay expiration should
-be pushed back.  Many of the usb_autopm_* routines also make this call,
-so drivers need to worry only when interrupt-driven input arrives.
-
-Asynchronous operation is always subject to races.  For example, a
-driver may call the usb_autopm_get_interface_async() routine at a time
-when the core has just finished deciding the device has been idle for
-long enough but not yet gotten around to calling the driver's suspend
-method.  The suspend method must be responsible for synchronizing with
-the I/O request routine and the URB completion handler; it should
-cause autosuspends to fail with -EBUSY if the driver needs to use the
-device.
-
-External suspend calls should never be allowed to fail in this way,
-only autosuspend calls.  The driver can tell them apart by applying
-the PMSG_IS_AUTO() macro to the message argument to the suspend
-method; it will return True for internal PM events (autosuspend) and
-False for external PM events.
-
-
-	Mutual exclusion
-	----------------
-
-For external events -- but not necessarily for autosuspend or
-autoresume -- the device semaphore (udev->dev.sem) will be held when a
-suspend or resume method is called.  This implies that external
-suspend/resume events are mutually exclusive with calls to probe,
-disconnect, pre_reset, and post_reset; the USB core guarantees that
-this is true of autosuspend/autoresume events as well.
-
-If a driver wants to block all suspend/resume calls during some
-critical section, the best way is to lock the device and call
-usb_autopm_get_interface() (and do the reverse at the end of the
-critical section).  Holding the device semaphore will block all
-external PM calls, and the usb_autopm_get_interface() will prevent any
-internal PM calls, even if it fails.  (Exercise: Why?)
-
-
-	Interaction between dynamic PM and system PM
-	--------------------------------------------
-
-Dynamic power management and system power management can interact in
-a couple of ways.
-
-Firstly, a device may already be autosuspended when a system suspend
-occurs.  Since system suspends are supposed to be as transparent as
-possible, the device should remain suspended following the system
-resume.  But this theory may not work out well in practice; over time
-the kernel's behavior in this regard has changed.  As of 2.6.37 the
-policy is to resume all devices during a system resume and let them
-handle their own runtime suspends afterward.
-
-Secondly, a dynamic power-management event may occur as a system
-suspend is underway.  The window for this is short, since system
-suspends don't take long (a few seconds usually), but it can happen.
-For example, a suspended device may send a remote-wakeup signal while
-the system is suspending.  The remote wakeup may succeed, which would
-cause the system suspend to abort.  If the remote wakeup doesn't
-succeed, it may still remain active and thus cause the system to
-resume as soon as the system suspend is complete.  Or the remote
-wakeup may fail and get lost.  Which outcome occurs depends on timing
-and on the hardware and firmware design.
-
-
-	xHCI hardware link PM
-	---------------------
-
-xHCI host controller provides hardware link power management to usb2.0
-(xHCI 1.0 feature) and usb3.0 devices which support link PM. By
-enabling hardware LPM, the host can automatically put the device into
-lower power state(L1 for usb2.0 devices, or U1/U2 for usb3.0 devices),
-which state device can enter and resume very quickly.
-
-The user interface for controlling hardware LPM is located in the
-power/ subdirectory of each USB device's sysfs directory, that is, in
-/sys/bus/usb/devices/.../power/ where "..." is the device's ID. The
-relevant attribute files are usb2_hardware_lpm and usb3_hardware_lpm.
-
-	power/usb2_hardware_lpm
-
-		When a USB2 device which support LPM is plugged to a
-		xHCI host root hub which support software LPM, the
-		host will run a software LPM test for it; if the device
-		enters L1 state and resume successfully and the host
-		supports USB2 hardware LPM, this file will show up and
-		driver will enable hardware LPM	for the device. You
-		can write y/Y/1 or n/N/0 to the file to	enable/disable
-		USB2 hardware LPM manually. This is for	test purpose mainly.
-
-	power/usb3_hardware_lpm_u1
-	power/usb3_hardware_lpm_u2
-
-		When a USB 3.0 lpm-capable device is plugged in to a
-		xHCI host which supports link PM, it will check if U1
-		and U2 exit latencies have been set in the BOS
-		descriptor; if the check is passed and the host
-		supports USB3 hardware LPM, USB3 hardware LPM will be
-		enabled for the device and these files will be created.
-		The files hold a string value (enable or disable)
-		indicating whether or not USB3 hardware LPM U1 or U2
-		is enabled for the device.
-
-	USB Port Power Control
-	----------------------
-
-In addition to suspending endpoint devices and enabling hardware
-controlled link power management, the USB subsystem also has the
-capability to disable power to ports under some conditions.  Power is
-controlled through Set/ClearPortFeature(PORT_POWER) requests to a hub.
-In the case of a root or platform-internal hub the host controller
-driver translates PORT_POWER requests into platform firmware (ACPI)
-method calls to set the port power state. For more background see the
-Linux Plumbers Conference 2012 slides [1] and video [2]:
-
-Upon receiving a ClearPortFeature(PORT_POWER) request a USB port is
-logically off, and may trigger the actual loss of VBUS to the port [3].
-VBUS may be maintained in the case where a hub gangs multiple ports into
-a shared power well causing power to remain until all ports in the gang
-are turned off.  VBUS may also be maintained by hub ports configured for
-a charging application.  In any event a logically off port will lose
-connection with its device, not respond to hotplug events, and not
-respond to remote wakeup events*.
-
-WARNING: turning off a port may result in the inability to hot add a device.
-Please see "User Interface for Port Power Control" for details.
-
-As far as the effect on the device itself it is similar to what a device
-goes through during system suspend, i.e. the power session is lost.  Any
-USB device or driver that misbehaves with system suspend will be
-similarly affected by a port power cycle event.  For this reason the
-implementation shares the same device recovery path (and honors the same
-quirks) as the system resume path for the hub.
-
-[1]: http://dl.dropbox.com/u/96820575/sarah-sharp-lpt-port-power-off2-mini.pdf
-[2]: http://linuxplumbers.ubicast.tv/videos/usb-port-power-off-kerneluserspace-api/
-[3]: USB 3.1 Section 10.12
-* wakeup note: if a device is configured to send wakeup events the port
-  power control implementation will block poweroff attempts on that
-  port.
-
-
-	User Interface for Port Power Control
-	-------------------------------------
-
-The port power control mechanism uses the PM runtime system.  Poweroff is
-requested by clearing the power/pm_qos_no_power_off flag of the port device
-(defaults to 1).  If the port is disconnected it will immediately receive a
-ClearPortFeature(PORT_POWER) request.  Otherwise, it will honor the pm runtime
-rules and require the attached child device and all descendants to be suspended.
-This mechanism is dependent on the hub advertising port power switching in its
-hub descriptor (wHubCharacteristics logical power switching mode field).
-
-Note, some interface devices/drivers do not support autosuspend.  Userspace may
-need to unbind the interface drivers before the usb_device will suspend.  An
-unbound interface device is suspended by default.  When unbinding, be careful
-to unbind interface drivers, not the driver of the parent usb device.  Also,
-leave hub interface drivers bound.  If the driver for the usb device (not
-interface) is unbound the kernel is no longer able to resume the device.  If a
-hub interface driver is unbound, control of its child ports is lost and all
-attached child-devices will disconnect.  A good rule of thumb is that if the
-'driver/module' link for a device points to /sys/module/usbcore then unbinding
-it will interfere with port power control.
-
-Example of the relevant files for port power control.  Note, in this example
-these files are relative to a usb hub device (prefix).
-
-     prefix=/sys/devices/pci0000:00/0000:00:14.0/usb3/3-1
-
-                      attached child device +
-                  hub port device +         |
-     hub interface device +       |         |
-                          v       v         v
-                  $prefix/3-1:1.0/3-1-port1/device
-
-     $prefix/3-1:1.0/3-1-port1/power/pm_qos_no_power_off
-     $prefix/3-1:1.0/3-1-port1/device/power/control
-     $prefix/3-1:1.0/3-1-port1/device/3-1.1:<intf0>/driver/unbind
-     $prefix/3-1:1.0/3-1-port1/device/3-1.1:<intf1>/driver/unbind
-     ...
-     $prefix/3-1:1.0/3-1-port1/device/3-1.1:<intfN>/driver/unbind
-
-In addition to these files some ports may have a 'peer' link to a port on
-another hub.  The expectation is that all superspeed ports have a
-hi-speed peer.
-
-$prefix/3-1:1.0/3-1-port1/peer -> ../../../../usb2/2-1/2-1:1.0/2-1-port1
-../../../../usb2/2-1/2-1:1.0/2-1-port1/peer -> ../../../../usb3/3-1/3-1:1.0/3-1-port1
-
-Distinct from 'companion ports', or 'ehci/xhci shared switchover ports'
-peer ports are simply the hi-speed and superspeed interface pins that
-are combined into a single usb3 connector.  Peer ports share the same
-ancestor XHCI device.
-
-While a superspeed port is powered off a device may downgrade its
-connection and attempt to connect to the hi-speed pins.  The
-implementation takes steps to prevent this:
-
-1/ Port suspend is sequenced to guarantee that hi-speed ports are powered-off
-   before their superspeed peer is permitted to power-off.  The implication is
-   that the setting pm_qos_no_power_off to zero on a superspeed port may not cause
-   the port to power-off until its highspeed peer has gone to its runtime suspend
-   state.  Userspace must take care to order the suspensions if it wants to
-   guarantee that a superspeed port will power-off.
-
-2/ Port resume is sequenced to force a superspeed port to power-on prior to its
-   highspeed peer.
-
-3/ Port resume always triggers an attached child device to resume.  After a
-   power session is lost the device may have been removed, or need reset.
-   Resuming the child device when the parent port regains power resolves those
-   states and clamps the maximum port power cycle frequency at the rate the child
-   device can suspend (autosuspend-delay) and resume (reset-resume latency).
-
-Sysfs files relevant for port power control:
-	<hubdev-portX>/power/pm_qos_no_power_off:
-		This writable flag controls the state of an idle port.
-		Once all children and descendants have suspended the
-		port may suspend/poweroff provided that
-		pm_qos_no_power_off is '0'.  If pm_qos_no_power_off is
-		'1' the port will remain active/powered regardless of
-		the stats of descendants.  Defaults to 1.
-
-	<hubdev-portX>/power/runtime_status:
-		This file reflects whether the port is 'active' (power is on)
-		or 'suspended' (logically off).  There is no indication to
-		userspace whether VBUS is still supplied.
-
-	<hubdev-portX>/connect_type:
-		An advisory read-only flag to userspace indicating the
-		location and connection type of the port.  It returns
-		one of four values 'hotplug', 'hardwired', 'not used',
-		and 'unknown'.  All values, besides unknown, are set by
-		platform firmware.
-
-		"hotplug" indicates an externally connectable/visible
-		port on the platform.  Typically userspace would choose
-		to keep such a port powered to handle new device
-		connection events.
-
-		"hardwired" refers to a port that is not visible but
-		connectable. Examples are internal ports for USB
-		bluetooth that can be disconnected via an external
-		switch or a port with a hardwired USB camera.  It is
-		expected to be safe to allow these ports to suspend
-		provided pm_qos_no_power_off is coordinated with any
-		switch that gates connections.  Userspace must arrange
-		for the device to be connected prior to the port
-		powering off, or to activate the port prior to enabling
-		connection via a switch.
-
-		"not used" refers to an internal port that is expected
-		to never have a device connected to it.  These may be
-		empty internal ports, or ports that are not physically
-		exposed on a platform.  Considered safe to be
-		powered-off at all times.
-
-		"unknown" means platform firmware does not provide
-		information for this port.  Most commonly refers to
-		external hub ports which should be considered 'hotplug'
-		for policy decisions.
-
-		NOTE1: since we are relying on the BIOS to get this ACPI
-		information correct, the USB port descriptions may be
-		missing or wrong.
-
-		NOTE2: Take care in clearing pm_qos_no_power_off.  Once
-		power is off this port will
-		not respond to new connect events.
-
-	Once a child device is attached additional constraints are
-	applied before the port is allowed to poweroff.
-
-	<child>/power/control:
-		Must be 'auto', and the port will not
-		power down until <child>/power/runtime_status
-		reflects the 'suspended' state.  Default
-		value is controlled by child device driver.
-
-	<child>/power/persist:
-		This defaults to '1' for most devices and indicates if
-		kernel can persist the device's configuration across a
-		power session loss (suspend / port-power event).  When
-		this value is '0' (quirky devices), port poweroff is
-		disabled.
-
-	<child>/driver/unbind:
-		Wakeup capable devices will block port poweroff.  At
-		this time the only mechanism to clear the usb-internal
-		wakeup-capability for an interface device is to unbind
-		its driver.
-
-Summary of poweroff pre-requisite settings relative to a port device:
-
-	echo 0 > power/pm_qos_no_power_off
-	echo 0 > peer/power/pm_qos_no_power_off # if it exists
-	echo auto > power/control # this is the default value
-	echo auto > <child>/power/control
-	echo 1 > <child>/power/persist # this is the default value
-
-	Suggested Userspace Port Power Policy
-	-------------------------------------
-
-As noted above userspace needs to be careful and deliberate about what
-ports are enabled for poweroff.
-
-The default configuration is that all ports start with
-power/pm_qos_no_power_off set to '1' causing ports to always remain
-active.
-
-Given confidence in the platform firmware's description of the ports
-(ACPI _PLD record for a port populates 'connect_type') userspace can
-clear pm_qos_no_power_off for all 'not used' ports.  The same can be
-done for 'hardwired' ports provided poweroff is coordinated with any
-connection switch for the port.
-
-A more aggressive userspace policy is to enable USB port power off for
-all ports (set <hubdev-portX>/power/pm_qos_no_power_off to '0') when
-some external factor indicates the user has stopped interacting with the
-system.  For example, a distro may want to enable power off all USB
-ports when the screen blanks, and re-power them when the screen becomes
-active.  Smart phones and tablets may want to power off USB ports when
-the user pushes the power button.