Documentation/driver-api/usb/power-management.rst

.. _usb-power-management:

Power Management for USB
~~~~~~~~~~~~~~~~~~~~~~~~

:Author: Alan Stern <stern@rowland.harvard.edu>
:Date: 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 :c:type:`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 (:c:func:`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
:ref:`usb-persist`) 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.

	:c:func:`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.

	:c:func:`usb_autopm_put_interface` decrements the usage counter and
	attempts an autosuspend if the new value is = 0.

	:c:func:`usb_autopm_get_interface_async` and
	:c:func:`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.

	:c:func:`usb_autopm_get_interface_no_resume` and
	:c:func:`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
:c:func:`usb_autopm_get_interface` in its open routine and
:c:func:`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 :c:func:`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 :c:func:`usb_autopm_get_interface_async` before starting output and
:c:func:`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 :c:func:`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 :c:func:`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
:c:func:`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 :c:func:`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 [#f1]_ and video [#f2]_:

Upon receiving a ``ClearPortFeature(PORT_POWER)`` request a USB port is
logically off, and may trigger the actual loss of VBUS to the port [#f3]_.
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.

.. [#f1]

  http://dl.dropbox.com/u/96820575/sarah-sharp-lpt-port-power-off2-mini.pdf

.. [#f2]

  http://linuxplumbers.ubicast.tv/videos/usb-port-power-off-kerneluserspace-api/

.. [#f3]

  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 :c:type:`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.

		.. note::

			- since we are relying on the BIOS to get this ACPI
			  information correct, the USB port descriptions may
			  be missing or wrong.

			- 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.