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ReStructuredText
795 lines
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ReStructuredText
.. _usb-power-management:
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Power Management for USB
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~~~~~~~~~~~~~~~~~~~~~~~~
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:Author: Alan Stern <stern@rowland.harvard.edu>
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:Date: Last-updated: February 2014
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..
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Contents:
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---------
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* What is Power Management?
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* What is Remote Wakeup?
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* When is a USB device idle?
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* Forms of dynamic PM
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* The user interface for dynamic PM
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* Changing the default idle-delay time
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* Warnings
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* The driver interface for Power Management
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* The driver interface for autosuspend and autoresume
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* Other parts of the driver interface
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* Mutual exclusion
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* Interaction between dynamic PM and system PM
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* xHCI hardware link PM
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* USB Port Power Control
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* User Interface for Port Power Control
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* Suggested Userspace Port Power Policy
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What is Power Management?
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-------------------------
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Power Management (PM) is the practice of saving energy by suspending
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parts of a computer system when they aren't being used. While a
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component is ``suspended`` it is in a nonfunctional low-power state; it
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might even be turned off completely. A suspended component can be
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``resumed`` (returned to a functional full-power state) when the kernel
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needs to use it. (There also are forms of PM in which components are
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placed in a less functional but still usable state instead of being
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suspended; an example would be reducing the CPU's clock rate. This
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document will not discuss those other forms.)
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When the parts being suspended include the CPU and most of the rest of
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the system, we speak of it as a "system suspend". When a particular
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device is turned off while the system as a whole remains running, we
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call it a "dynamic suspend" (also known as a "runtime suspend" or
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"selective suspend"). This document concentrates mostly on how
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dynamic PM is implemented in the USB subsystem, although system PM is
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covered to some extent (see ``Documentation/power/*.txt`` for more
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information about system PM).
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System PM support is present only if the kernel was built with
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``CONFIG_SUSPEND`` or ``CONFIG_HIBERNATION`` enabled. Dynamic PM support
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for USB is present whenever
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the kernel was built with ``CONFIG_PM`` enabled.
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[Historically, dynamic PM support for USB was present only if the
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kernel had been built with ``CONFIG_USB_SUSPEND`` enabled (which depended on
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``CONFIG_PM_RUNTIME``). Starting with the 3.10 kernel release, dynamic PM
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support for USB was present whenever the kernel was built with
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``CONFIG_PM_RUNTIME`` enabled. The ``CONFIG_USB_SUSPEND`` option had been
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eliminated.]
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What is Remote Wakeup?
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----------------------
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When a device has been suspended, it generally doesn't resume until
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the computer tells it to. Likewise, if the entire computer has been
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suspended, it generally doesn't resume until the user tells it to, say
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by pressing a power button or opening the cover.
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However some devices have the capability of resuming by themselves, or
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asking the kernel to resume them, or even telling the entire computer
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to resume. This capability goes by several names such as "Wake On
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LAN"; we will refer to it generically as "remote wakeup". When a
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device is enabled for remote wakeup and it is suspended, it may resume
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itself (or send a request to be resumed) in response to some external
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event. Examples include a suspended keyboard resuming when a key is
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pressed, or a suspended USB hub resuming when a device is plugged in.
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When is a USB device idle?
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--------------------------
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A device is idle whenever the kernel thinks it's not busy doing
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anything important and thus is a candidate for being suspended. The
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exact definition depends on the device's driver; drivers are allowed
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to declare that a device isn't idle even when there's no actual
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communication taking place. (For example, a hub isn't considered idle
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unless all the devices plugged into that hub are already suspended.)
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In addition, a device isn't considered idle so long as a program keeps
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its usbfs file open, whether or not any I/O is going on.
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If a USB device has no driver, its usbfs file isn't open, and it isn't
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being accessed through sysfs, then it definitely is idle.
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Forms of dynamic PM
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-------------------
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Dynamic suspends occur when the kernel decides to suspend an idle
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device. This is called ``autosuspend`` for short. In general, a device
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won't be autosuspended unless it has been idle for some minimum period
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of time, the so-called idle-delay time.
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Of course, nothing the kernel does on its own initiative should
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prevent the computer or its devices from working properly. If a
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device has been autosuspended and a program tries to use it, the
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kernel will automatically resume the device (autoresume). For the
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same reason, an autosuspended device will usually have remote wakeup
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enabled, if the device supports remote wakeup.
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It is worth mentioning that many USB drivers don't support
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autosuspend. In fact, at the time of this writing (Linux 2.6.23) the
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only drivers which do support it are the hub driver, kaweth, asix,
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usblp, usblcd, and usb-skeleton (which doesn't count). If a
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non-supporting driver is bound to a device, the device won't be
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autosuspended. In effect, the kernel pretends the device is never
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idle.
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We can categorize power management events in two broad classes:
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external and internal. External events are those triggered by some
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agent outside the USB stack: system suspend/resume (triggered by
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userspace), manual dynamic resume (also triggered by userspace), and
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remote wakeup (triggered by the device). Internal events are those
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triggered within the USB stack: autosuspend and autoresume. Note that
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all dynamic suspend events are internal; external agents are not
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allowed to issue dynamic suspends.
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The user interface for dynamic PM
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---------------------------------
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The user interface for controlling dynamic PM is located in the ``power/``
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subdirectory of each USB device's sysfs directory, that is, in
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``/sys/bus/usb/devices/.../power/`` where "..." is the device's ID. The
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relevant attribute files are: wakeup, control, and
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``autosuspend_delay_ms``. (There may also be a file named ``level``; this
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file was deprecated as of the 2.6.35 kernel and replaced by the
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``control`` file. In 2.6.38 the ``autosuspend`` file will be deprecated
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and replaced by the ``autosuspend_delay_ms`` file. The only difference
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is that the newer file expresses the delay in milliseconds whereas the
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older file uses seconds. Confusingly, both files are present in 2.6.37
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but only ``autosuspend`` works.)
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``power/wakeup``
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This file is empty if the device does not support
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remote wakeup. Otherwise the file contains either the
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word ``enabled`` or the word ``disabled``, and you can
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write those words to the file. The setting determines
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whether or not remote wakeup will be enabled when the
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device is next suspended. (If the setting is changed
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while the device is suspended, the change won't take
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effect until the following suspend.)
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``power/control``
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This file contains one of two words: ``on`` or ``auto``.
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You can write those words to the file to change the
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device's setting.
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- ``on`` means that the device should be resumed and
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autosuspend is not allowed. (Of course, system
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suspends are still allowed.)
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- ``auto`` is the normal state in which the kernel is
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allowed to autosuspend and autoresume the device.
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(In kernels up to 2.6.32, you could also specify
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``suspend``, meaning that the device should remain
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suspended and autoresume was not allowed. This
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setting is no longer supported.)
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``power/autosuspend_delay_ms``
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This file contains an integer value, which is the
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number of milliseconds the device should remain idle
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before the kernel will autosuspend it (the idle-delay
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time). The default is 2000. 0 means to autosuspend
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as soon as the device becomes idle, and negative
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values mean never to autosuspend. You can write a
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number to the file to change the autosuspend
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idle-delay time.
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Writing ``-1`` to ``power/autosuspend_delay_ms`` and writing ``on`` to
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``power/control`` do essentially the same thing -- they both prevent the
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device from being autosuspended. Yes, this is a redundancy in the
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API.
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(In 2.6.21 writing ``0`` to ``power/autosuspend`` would prevent the device
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from being autosuspended; the behavior was changed in 2.6.22. The
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``power/autosuspend`` attribute did not exist prior to 2.6.21, and the
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``power/level`` attribute did not exist prior to 2.6.22. ``power/control``
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was added in 2.6.34, and ``power/autosuspend_delay_ms`` was added in
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2.6.37 but did not become functional until 2.6.38.)
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Changing the default idle-delay time
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------------------------------------
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The default autosuspend idle-delay time (in seconds) is controlled by
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a module parameter in usbcore. You can specify the value when usbcore
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is loaded. For example, to set it to 5 seconds instead of 2 you would
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do::
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modprobe usbcore autosuspend=5
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Equivalently, you could add to a configuration file in /etc/modprobe.d
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a line saying::
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options usbcore autosuspend=5
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Some distributions load the usbcore module very early during the boot
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process, by means of a program or script running from an initramfs
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image. To alter the parameter value you would have to rebuild that
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image.
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If usbcore is compiled into the kernel rather than built as a loadable
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module, you can add::
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usbcore.autosuspend=5
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to the kernel's boot command line.
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Finally, the parameter value can be changed while the system is
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running. If you do::
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echo 5 >/sys/module/usbcore/parameters/autosuspend
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then each new USB device will have its autosuspend idle-delay
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initialized to 5. (The idle-delay values for already existing devices
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will not be affected.)
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Setting the initial default idle-delay to -1 will prevent any
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autosuspend of any USB device. This has the benefit of allowing you
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then to enable autosuspend for selected devices.
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Warnings
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--------
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The USB specification states that all USB devices must support power
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management. Nevertheless, the sad fact is that many devices do not
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support it very well. You can suspend them all right, but when you
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try to resume them they disconnect themselves from the USB bus or
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they stop working entirely. This seems to be especially prevalent
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among printers and scanners, but plenty of other types of device have
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the same deficiency.
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For this reason, by default the kernel disables autosuspend (the
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``power/control`` attribute is initialized to ``on``) for all devices other
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than hubs. Hubs, at least, appear to be reasonably well-behaved in
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this regard.
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(In 2.6.21 and 2.6.22 this wasn't the case. Autosuspend was enabled
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by default for almost all USB devices. A number of people experienced
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problems as a result.)
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This means that non-hub devices won't be autosuspended unless the user
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or a program explicitly enables it. As of this writing there aren't
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any widespread programs which will do this; we hope that in the near
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future device managers such as HAL will take on this added
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responsibility. In the meantime you can always carry out the
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necessary operations by hand or add them to a udev script. You can
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also change the idle-delay time; 2 seconds is not the best choice for
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every device.
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If a driver knows that its device has proper suspend/resume support,
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it can enable autosuspend all by itself. For example, the video
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driver for a laptop's webcam might do this (in recent kernels they
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do), since these devices are rarely used and so should normally be
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autosuspended.
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Sometimes it turns out that even when a device does work okay with
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autosuspend there are still problems. For example, the usbhid driver,
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which manages keyboards and mice, has autosuspend support. Tests with
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a number of keyboards show that typing on a suspended keyboard, while
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causing the keyboard to do a remote wakeup all right, will nonetheless
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frequently result in lost keystrokes. Tests with mice show that some
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of them will issue a remote-wakeup request in response to button
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presses but not to motion, and some in response to neither.
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The kernel will not prevent you from enabling autosuspend on devices
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that can't handle it. It is even possible in theory to damage a
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device by suspending it at the wrong time. (Highly unlikely, but
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possible.) Take care.
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The driver interface for Power Management
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-----------------------------------------
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The requirements for a USB driver to support external power management
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are pretty modest; the driver need only define::
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.suspend
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.resume
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.reset_resume
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methods in its :c:type:`usb_driver` structure, and the ``reset_resume`` method
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is optional. The methods' jobs are quite simple:
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- The ``suspend`` method is called to warn the driver that the
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device is going to be suspended. If the driver returns a
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negative error code, the suspend will be aborted. Normally
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the driver will return 0, in which case it must cancel all
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outstanding URBs (:c:func:`usb_kill_urb`) and not submit any more.
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- The ``resume`` method is called to tell the driver that the
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device has been resumed and the driver can return to normal
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operation. URBs may once more be submitted.
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- The ``reset_resume`` method is called to tell the driver that
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the device has been resumed and it also has been reset.
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The driver should redo any necessary device initialization,
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since the device has probably lost most or all of its state
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(although the interfaces will be in the same altsettings as
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before the suspend).
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If the device is disconnected or powered down while it is suspended,
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the ``disconnect`` method will be called instead of the ``resume`` or
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``reset_resume`` method. This is also quite likely to happen when
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waking up from hibernation, as many systems do not maintain suspend
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current to the USB host controllers during hibernation. (It's
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possible to work around the hibernation-forces-disconnect problem by
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using the USB Persist facility.)
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The ``reset_resume`` method is used by the USB Persist facility (see
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:ref:`usb-persist`) and it can also be used under certain
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circumstances when ``CONFIG_USB_PERSIST`` is not enabled. Currently, if a
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device is reset during a resume and the driver does not have a
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``reset_resume`` method, the driver won't receive any notification about
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the resume. Later kernels will call the driver's ``disconnect`` method;
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2.6.23 doesn't do this.
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USB drivers are bound to interfaces, so their ``suspend`` and ``resume``
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methods get called when the interfaces are suspended or resumed. In
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principle one might want to suspend some interfaces on a device (i.e.,
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force the drivers for those interface to stop all activity) without
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suspending the other interfaces. The USB core doesn't allow this; all
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interfaces are suspended when the device itself is suspended and all
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interfaces are resumed when the device is resumed. It isn't possible
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to suspend or resume some but not all of a device's interfaces. The
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closest you can come is to unbind the interfaces' drivers.
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The driver interface for autosuspend and autoresume
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---------------------------------------------------
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To support autosuspend and autoresume, a driver should implement all
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three of the methods listed above. In addition, a driver indicates
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that it supports autosuspend by setting the ``.supports_autosuspend`` flag
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in its usb_driver structure. It is then responsible for informing the
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USB core whenever one of its interfaces becomes busy or idle. The
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driver does so by calling these six functions::
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int usb_autopm_get_interface(struct usb_interface *intf);
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void usb_autopm_put_interface(struct usb_interface *intf);
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int usb_autopm_get_interface_async(struct usb_interface *intf);
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void usb_autopm_put_interface_async(struct usb_interface *intf);
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void usb_autopm_get_interface_no_resume(struct usb_interface *intf);
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void usb_autopm_put_interface_no_suspend(struct usb_interface *intf);
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The functions work by maintaining a usage counter in the
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usb_interface's embedded device structure. When the counter is > 0
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then the interface is deemed to be busy, and the kernel will not
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autosuspend the interface's device. When the usage counter is = 0
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then the interface is considered to be idle, and the kernel may
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autosuspend the device.
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Drivers need not be concerned about balancing changes to the usage
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counter; the USB core will undo any remaining "get"s when a driver
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is unbound from its interface. As a corollary, drivers must not call
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any of the ``usb_autopm_*`` functions after their ``disconnect``
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routine has returned.
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Drivers using the async routines are responsible for their own
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synchronization and mutual exclusion.
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:c:func:`usb_autopm_get_interface` increments the usage counter and
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does an autoresume if the device is suspended. If the
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autoresume fails, the counter is decremented back.
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:c:func:`usb_autopm_put_interface` decrements the usage counter and
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attempts an autosuspend if the new value is = 0.
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:c:func:`usb_autopm_get_interface_async` and
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:c:func:`usb_autopm_put_interface_async` do almost the same things as
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their non-async counterparts. The big difference is that they
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use a workqueue to do the resume or suspend part of their
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jobs. As a result they can be called in an atomic context,
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such as an URB's completion handler, but when they return the
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device will generally not yet be in the desired state.
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:c:func:`usb_autopm_get_interface_no_resume` and
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:c:func:`usb_autopm_put_interface_no_suspend` merely increment or
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decrement the usage counter; they do not attempt to carry out
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an autoresume or an autosuspend. Hence they can be called in
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an atomic context.
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The simplest usage pattern is that a driver calls
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:c:func:`usb_autopm_get_interface` in its open routine and
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:c:func:`usb_autopm_put_interface` in its close or release routine. But other
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patterns are possible.
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The autosuspend attempts mentioned above will often fail for one
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reason or another. For example, the ``power/control`` attribute might be
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set to ``on``, or another interface in the same device might not be
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idle. This is perfectly normal. If the reason for failure was that
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the device hasn't been idle for long enough, a timer is scheduled to
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carry out the operation automatically when the autosuspend idle-delay
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has expired.
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Autoresume attempts also can fail, although failure would mean that
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the device is no longer present or operating properly. Unlike
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autosuspend, there's no idle-delay for an autoresume.
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Other parts of the driver interface
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-----------------------------------
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Drivers can enable autosuspend for their devices by calling::
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usb_enable_autosuspend(struct usb_device *udev);
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in their :c:func:`probe` routine, if they know that the device is capable of
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suspending and resuming correctly. This is exactly equivalent to
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writing ``auto`` to the device's ``power/control`` attribute. Likewise,
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drivers can disable autosuspend by calling::
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usb_disable_autosuspend(struct usb_device *udev);
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This is exactly the same as writing ``on`` to the ``power/control`` attribute.
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Sometimes a driver needs to make sure that remote wakeup is enabled
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during autosuspend. For example, there's not much point
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autosuspending a keyboard if the user can't cause the keyboard to do a
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remote wakeup by typing on it. If the driver sets
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``intf->needs_remote_wakeup`` to 1, the kernel won't autosuspend the
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device if remote wakeup isn't available. (If the device is already
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autosuspended, though, setting this flag won't cause the kernel to
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autoresume it. Normally a driver would set this flag in its ``probe``
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method, at which time the device is guaranteed not to be
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autosuspended.)
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If a driver does its I/O asynchronously in interrupt context, it
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should call :c:func:`usb_autopm_get_interface_async` before starting output and
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:c:func:`usb_autopm_put_interface_async` when the output queue drains. When
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it receives an input event, it should call::
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usb_mark_last_busy(struct usb_device *udev);
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in the event handler. This tells the PM core that the device was just
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busy and therefore the next autosuspend idle-delay expiration should
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be pushed back. Many of the usb_autopm_* routines also make this call,
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so drivers need to worry only when interrupt-driven input arrives.
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Asynchronous operation is always subject to races. For example, a
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driver may call the :c:func:`usb_autopm_get_interface_async` routine at a time
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when the core has just finished deciding the device has been idle for
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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.
|