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>
This commit is contained in:
Mauro Carvalho Chehab
Jonathan Corbet
Родитель
3b38e4f218
Коммит
3db5f406e4
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@ -9,6 +9,7 @@ Linux USB API
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anchors
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bulk-streams
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callbacks
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power-management
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writing_usb_driver
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writing_musb_glue_layer
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@ -1,10 +1,12 @@
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Power Management for USB
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.. _usb-power-management:
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Alan Stern <stern@rowland.harvard.edu>
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Last-updated: February 2014
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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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@ -25,14 +27,14 @@
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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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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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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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``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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@ -44,22 +46,25 @@ 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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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 CONFIG_SUSPEND
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or CONFIG_HIBERNATION enabled. Dynamic PM support for USB is present whenever
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the kernel was built with CONFIG_PM enabled.
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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 support
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for USB was present whenever the kernel was built with CONFIG_PM_RUNTIME
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enabled. The CONFIG_USB_SUSPEND option had been eliminated.]
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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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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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@ -76,8 +81,8 @@ 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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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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@ -92,11 +97,11 @@ 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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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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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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@ -125,51 +130,51 @@ 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 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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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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``/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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``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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``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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but only ``autosuspend`` works.)
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power/wakeup
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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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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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``power/control``
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This file contains one of two words: "on" or "auto".
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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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- ``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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- ``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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``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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``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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@ -180,31 +185,31 @@ but only "autosuspend" works.)
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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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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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(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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``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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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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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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a line saying::
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options usbcore autosuspend=5
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@ -214,14 +219,14 @@ 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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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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running. If you do::
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echo 5 >/sys/module/usbcore/parameters/autosuspend
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@ -234,8 +239,8 @@ 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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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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@ -246,7 +251,7 @@ 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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``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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@ -284,30 +289,30 @@ 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 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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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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|
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methods in its usb_driver structure, and the reset_resume method is
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optional. The methods' jobs are quite simple:
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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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|
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The suspend method is called to warn the driver that the
|
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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 (usb_kill_urb()) and not submit any more.
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outstanding URBs (:c:func:`usb_kill_urb`) and not submit any more.
|
||||
|
||||
The resume method is called to tell the driver that the
|
||||
- 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
|
||||
operation. URBs may once more be submitted.
|
||||
|
||||
The reset_resume method is called to tell the driver that
|
||||
- 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
|
||||
|
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@ -315,22 +320,22 @@ optional. The methods' jobs are quite simple:
|
|||
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
|
||||
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
|
||||
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;
|
||||
``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
|
||||
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
|
||||
|
|
@ -341,15 +346,15 @@ 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
|
||||
---------------------------------------------------
|
||||
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
|
||||
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:
|
||||
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);
|
||||
|
|
@ -368,41 +373,41 @@ 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.
|
||||
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
|
||||
: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.
|
||||
|
||||
usb_autopm_put_interface() decrements the usage counter and
|
||||
:c:func:`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
|
||||
: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.
|
||||
|
||||
usb_autopm_get_interface_no_resume() and
|
||||
usb_autopm_put_interface_no_suspend() merely increment or
|
||||
: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
|
||||
usb_autopm_get_interface() in its open routine and
|
||||
usb_autopm_put_interface() in its close or release routine. But other
|
||||
: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
|
||||
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
|
||||
|
|
@ -413,37 +418,37 @@ 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
|
||||
-----------------------------------
|
||||
Other parts of the driver interface
|
||||
-----------------------------------
|
||||
|
||||
Drivers can enable autosuspend for their devices by calling
|
||||
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
|
||||
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
|
||||
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.
|
||||
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
|
||||
``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
|
||||
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
|
||||
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);
|
||||
|
||||
|
|
@ -453,41 +458,41 @@ 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
|
||||
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
|
||||
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
|
||||
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
|
||||
----------------
|
||||
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
|
||||
``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
|
||||
: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 usb_autopm_get_interface() will prevent any
|
||||
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
|
||||
--------------------------------------------
|
||||
Interaction between dynamic PM and system PM
|
||||
--------------------------------------------
|
||||
|
||||
Dynamic power management and system power management can interact in
|
||||
a couple of ways.
|
||||
|
|
@ -512,8 +517,8 @@ wakeup may fail and get lost. Which outcome occurs depends on timing
|
|||
and on the hardware and firmware design.
|
||||
|
||||
|
||||
xHCI hardware link PM
|
||||
---------------------
|
||||
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
|
||||
|
|
@ -522,11 +527,11 @@ 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/`` 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
|
||||
``power/usb2_hardware_lpm``
|
||||
|
||||
When a USB2 device which support LPM is plugged to a
|
||||
xHCI host root hub which support software LPM, the
|
||||
|
|
@ -537,8 +542,8 @@ relevant attribute files are usb2_hardware_lpm and usb3_hardware_lpm.
|
|||
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
|
||||
``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
|
||||
|
|
@ -550,29 +555,31 @@ relevant attribute files are usb2_hardware_lpm and usb3_hardware_lpm.
|
|||
indicating whether or not USB3 hardware LPM U1 or U2
|
||||
is enabled for the device.
|
||||
|
||||
USB Port Power Control
|
||||
----------------------
|
||||
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.
|
||||
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)
|
||||
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]:
|
||||
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 [3].
|
||||
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*.
|
||||
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.
|
||||
.. 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
|
||||
|
|
@ -581,38 +588,49 @@ 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
|
||||
.. [#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
|
||||
-------------------------------------
|
||||
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
|
||||
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).
|
||||
``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.
|
||||
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).
|
||||
these files are relative to a usb hub device (prefix)::
|
||||
|
||||
prefix=/sys/devices/pci0000:00/0000:00:14.0/usb3/3-1
|
||||
|
||||
|
|
@ -631,10 +649,10 @@ these files are relative to a usb hub device (prefix).
|
|||
|
||||
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.
|
||||
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
|
||||
$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
|
||||
|
|
@ -645,24 +663,26 @@ 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
|
||||
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.
|
||||
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
|
||||
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
|
||||
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).
|
||||
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:
|
||||
|
||||
``<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
|
||||
|
|
@ -670,24 +690,24 @@ Sysfs files relevant for port power control:
|
|||
'1' the port will remain active/powered regardless of
|
||||
the stats of descendants. Defaults to 1.
|
||||
|
||||
<hubdev-portX>/power/runtime_status:
|
||||
``<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:
|
||||
``<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
|
||||
``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
|
||||
``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
|
||||
|
|
@ -698,48 +718,50 @@ Sysfs files relevant for port power control:
|
|||
powering off, or to activate the port prior to enabling
|
||||
connection via a switch.
|
||||
|
||||
"not used" refers to an internal port that is expected
|
||||
``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
|
||||
``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.
|
||||
.. note::
|
||||
|
||||
NOTE2: Take care in clearing pm_qos_no_power_off. Once
|
||||
power is off this port will
|
||||
not respond to new connect events.
|
||||
- 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
|
||||
``<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
|
||||
``<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
|
||||
this value is ``0`` (quirky devices), port poweroff is
|
||||
disabled.
|
||||
|
||||
<child>/driver/unbind:
|
||||
``<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:
|
||||
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
|
||||
|
|
@ -747,14 +769,14 @@ Summary of poweroff pre-requisite settings relative to a port device:
|
|||
echo auto > <child>/power/control
|
||||
echo 1 > <child>/power/persist # this is the default value
|
||||
|
||||
Suggested Userspace Port Power Policy
|
||||
-------------------------------------
|
||||
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
|
||||
``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
|
||||
|
|
@ -764,7 +786,7 @@ 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
|
||||
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
|
||||
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