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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 2017-04-05 10:23:05 -03:00 committed by Jonathan Corbet
parent 3b38e4f218
commit 3db5f406e4
2 changed files with 214 additions and 191 deletions
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1
Documentation/driver-api/usb/index.rst
View File

@ -9,6 +9,7 @@ Linux USB API
anchors anchors
bulk-streams bulk-streams
callbacks callbacks
power-management
writing_usb_driver writing_usb_driver
writing_musb_glue_layer writing_musb_glue_layer

404
Documentation/usb/power-management.txt → Documentation/driver-api/usb/power-management.rst
View File

@ -1,10 +1,12 @@
Power Management for USB .. _usb-power-management:
Alan Stern <stern@rowland.harvard.edu> Power Management for USB
~~~~~~~~~~~~~~~~~~~~~~~~
Last-updated: February 2014
:Author: Alan Stern <stern@rowland.harvard.edu>
:Date: Last-updated: February 2014
..
Contents: Contents:
--------- ---------
* What is Power Management? * What is Power Management?
@ -25,14 +27,14 @@
* Suggested Userspace Port Power Policy * Suggested Userspace Port Power Policy
What is Power Management? What is Power Management?
------------------------- -------------------------
Power Management (PM) is the practice of saving energy by suspending Power Management (PM) is the practice of saving energy by suspending
parts of a computer system when they aren't being used. While a parts of a computer system when they aren't being used. While a
component is "suspended" it is in a nonfunctional low-power state; it component is ``suspended`` it is in a nonfunctional low-power state; it
might even be turned off completely. A suspended component can be might even be turned off completely. A suspended component can be
"resumed" (returned to a functional full-power state) when the kernel ``resumed`` (returned to a functional full-power state) when the kernel
needs to use it. (There also are forms of PM in which components are needs to use it. (There also are forms of PM in which components are
placed in a less functional but still usable state instead of being placed in a less functional but still usable state instead of being
suspended; an example would be reducing the CPU's clock rate. This suspended; an example would be reducing the CPU's clock rate. This
@ -44,22 +46,25 @@ device is turned off while the system as a whole remains running, we
call it a "dynamic suspend" (also known as a "runtime suspend" or call it a "dynamic suspend" (also known as a "runtime suspend" or
"selective suspend"). This document concentrates mostly on how "selective suspend"). This document concentrates mostly on how
dynamic PM is implemented in the USB subsystem, although system PM is dynamic PM is implemented in the USB subsystem, although system PM is
covered to some extent (see Documentation/power/*.txt for more covered to some extent (see ``Documentation/power/*.txt`` for more
information about system PM). information about system PM).
System PM support is present only if the kernel was built with CONFIG_SUSPEND System PM support is present only if the kernel was built with
or CONFIG_HIBERNATION enabled. Dynamic PM support for USB is present whenever ``CONFIG_SUSPEND`` or ``CONFIG_HIBERNATION`` enabled. Dynamic PM support
the kernel was built with CONFIG_PM enabled.
for USB is present whenever
the kernel was built with ``CONFIG_PM`` enabled.
[Historically, dynamic PM support for USB was present only if the [Historically, dynamic PM support for USB was present only if the
kernel had been built with CONFIG_USB_SUSPEND enabled (which depended on kernel had been built with ``CONFIG_USB_SUSPEND`` enabled (which depended on
CONFIG_PM_RUNTIME). Starting with the 3.10 kernel release, dynamic PM support ``CONFIG_PM_RUNTIME``). Starting with the 3.10 kernel release, dynamic PM
for USB was present whenever the kernel was built with CONFIG_PM_RUNTIME support for USB was present whenever the kernel was built with
enabled. The CONFIG_USB_SUSPEND option had been eliminated.] ``CONFIG_PM_RUNTIME`` enabled. The ``CONFIG_USB_SUSPEND`` option had been
eliminated.]
What is Remote Wakeup? What is Remote Wakeup?
---------------------- ----------------------
When a device has been suspended, it generally doesn't resume until When a device has been suspended, it generally doesn't resume until
the computer tells it to. Likewise, if the entire computer has been the computer tells it to. Likewise, if the entire computer has been
@ -76,8 +81,8 @@ event. Examples include a suspended keyboard resuming when a key is
pressed, or a suspended USB hub resuming when a device is plugged in. pressed, or a suspended USB hub resuming when a device is plugged in.
When is a USB device idle? When is a USB device idle?
-------------------------- --------------------------
A device is idle whenever the kernel thinks it's not busy doing A device is idle whenever the kernel thinks it's not busy doing
anything important and thus is a candidate for being suspended. The anything important and thus is a candidate for being suspended. The
@ -92,11 +97,11 @@ If a USB device has no driver, its usbfs file isn't open, and it isn't
being accessed through sysfs, then it definitely is idle. being accessed through sysfs, then it definitely is idle.
Forms of dynamic PM Forms of dynamic PM
------------------- -------------------
Dynamic suspends occur when the kernel decides to suspend an idle Dynamic suspends occur when the kernel decides to suspend an idle
device. This is called "autosuspend" for short. In general, a device device. This is called ``autosuspend`` for short. In general, a device
won't be autosuspended unless it has been idle for some minimum period won't be autosuspended unless it has been idle for some minimum period
of time, the so-called idle-delay time. of time, the so-called idle-delay time.
@ -125,51 +130,51 @@ all dynamic suspend events are internal; external agents are not
allowed to issue dynamic suspends. allowed to issue dynamic suspends.
The user interface for dynamic PM The user interface for dynamic PM
--------------------------------- ---------------------------------
The user interface for controlling dynamic PM is located in the power/ The user interface for controlling dynamic PM is located in the ``power/``
subdirectory of each USB device's sysfs directory, that is, in subdirectory of each USB device's sysfs directory, that is, in
/sys/bus/usb/devices/.../power/ where "..." is the device's ID. The ``/sys/bus/usb/devices/.../power/`` where "..." is the device's ID. The
relevant attribute files are: wakeup, control, and relevant attribute files are: wakeup, control, and
autosuspend_delay_ms. (There may also be a file named "level"; this ``autosuspend_delay_ms``. (There may also be a file named ``level``; this
file was deprecated as of the 2.6.35 kernel and replaced by the file was deprecated as of the 2.6.35 kernel and replaced by the
"control" file. In 2.6.38 the "autosuspend" file will be deprecated ``control`` file. In 2.6.38 the ``autosuspend`` file will be deprecated
and replaced by the "autosuspend_delay_ms" file. The only difference and replaced by the ``autosuspend_delay_ms`` file. The only difference
is that the newer file expresses the delay in milliseconds whereas the is that the newer file expresses the delay in milliseconds whereas the
older file uses seconds. Confusingly, both files are present in 2.6.37 older file uses seconds. Confusingly, both files are present in 2.6.37
but only "autosuspend" works.) but only ``autosuspend`` works.)
power/wakeup ``power/wakeup``
This file is empty if the device does not support This file is empty if the device does not support
remote wakeup. Otherwise the file contains either the remote wakeup. Otherwise the file contains either the
word "enabled" or the word "disabled", and you can word ``enabled`` or the word ``disabled``, and you can
write those words to the file. The setting determines write those words to the file. The setting determines
whether or not remote wakeup will be enabled when the whether or not remote wakeup will be enabled when the
device is next suspended. (If the setting is changed device is next suspended. (If the setting is changed
while the device is suspended, the change won't take while the device is suspended, the change won't take
effect until the following suspend.) effect until the following suspend.)
power/control ``power/control``
This file contains one of two words: "on" or "auto". This file contains one of two words: ``on`` or ``auto``.
You can write those words to the file to change the You can write those words to the file to change the
device's setting. device's setting.
"on" means that the device should be resumed and - ``on`` means that the device should be resumed and
autosuspend is not allowed. (Of course, system autosuspend is not allowed. (Of course, system
suspends are still allowed.) suspends are still allowed.)
"auto" is the normal state in which the kernel is - ``auto`` is the normal state in which the kernel is
allowed to autosuspend and autoresume the device. allowed to autosuspend and autoresume the device.
(In kernels up to 2.6.32, you could also specify (In kernels up to 2.6.32, you could also specify
"suspend", meaning that the device should remain ``suspend``, meaning that the device should remain
suspended and autoresume was not allowed. This suspended and autoresume was not allowed. This
setting is no longer supported.) setting is no longer supported.)
power/autosuspend_delay_ms ``power/autosuspend_delay_ms``
This file contains an integer value, which is the This file contains an integer value, which is the
number of milliseconds the device should remain idle number of milliseconds the device should remain idle
@ -180,31 +185,31 @@ but only "autosuspend" works.)
number to the file to change the autosuspend number to the file to change the autosuspend
idle-delay time. idle-delay time.
Writing "-1" to power/autosuspend_delay_ms and writing "on" to Writing ``-1`` to ``power/autosuspend_delay_ms`` and writing ``on`` to
power/control do essentially the same thing -- they both prevent the ``power/control`` do essentially the same thing -- they both prevent the
device from being autosuspended. Yes, this is a redundancy in the device from being autosuspended. Yes, this is a redundancy in the
API. API.
(In 2.6.21 writing "0" to power/autosuspend would prevent the device (In 2.6.21 writing ``0`` to ``power/autosuspend`` would prevent the device
from being autosuspended; the behavior was changed in 2.6.22. The from being autosuspended; the behavior was changed in 2.6.22. The
power/autosuspend attribute did not exist prior to 2.6.21, and the ``power/autosuspend`` attribute did not exist prior to 2.6.21, and the
power/level attribute did not exist prior to 2.6.22. power/control ``power/level`` attribute did not exist prior to 2.6.22. ``power/control``
was added in 2.6.34, and power/autosuspend_delay_ms was added in was added in 2.6.34, and ``power/autosuspend_delay_ms`` was added in
2.6.37 but did not become functional until 2.6.38.) 2.6.37 but did not become functional until 2.6.38.)
Changing the default idle-delay time Changing the default idle-delay time
------------------------------------ ------------------------------------
The default autosuspend idle-delay time (in seconds) is controlled by The default autosuspend idle-delay time (in seconds) is controlled by
a module parameter in usbcore. You can specify the value when usbcore a module parameter in usbcore. You can specify the value when usbcore
is loaded. For example, to set it to 5 seconds instead of 2 you would is loaded. For example, to set it to 5 seconds instead of 2 you would
do: do::
modprobe usbcore autosuspend=5 modprobe usbcore autosuspend=5
Equivalently, you could add to a configuration file in /etc/modprobe.d Equivalently, you could add to a configuration file in /etc/modprobe.d
a line saying: a line saying::
options usbcore autosuspend=5 options usbcore autosuspend=5
@ -214,14 +219,14 @@ image. To alter the parameter value you would have to rebuild that
image. image.
If usbcore is compiled into the kernel rather than built as a loadable If usbcore is compiled into the kernel rather than built as a loadable
module, you can add module, you can add::
usbcore.autosuspend=5 usbcore.autosuspend=5
to the kernel's boot command line. to the kernel's boot command line.
Finally, the parameter value can be changed while the system is Finally, the parameter value can be changed while the system is
running. If you do: running. If you do::
echo 5 >/sys/module/usbcore/parameters/autosuspend echo 5 >/sys/module/usbcore/parameters/autosuspend
@ -234,8 +239,8 @@ autosuspend of any USB device. This has the benefit of allowing you
then to enable autosuspend for selected devices. then to enable autosuspend for selected devices.
Warnings Warnings
-------- --------
The USB specification states that all USB devices must support power The USB specification states that all USB devices must support power
management. Nevertheless, the sad fact is that many devices do not management. Nevertheless, the sad fact is that many devices do not
@ -246,7 +251,7 @@ among printers and scanners, but plenty of other types of device have
the same deficiency. the same deficiency.
For this reason, by default the kernel disables autosuspend (the For this reason, by default the kernel disables autosuspend (the
power/control attribute is initialized to "on") for all devices other ``power/control`` attribute is initialized to ``on``) for all devices other
than hubs. Hubs, at least, appear to be reasonably well-behaved in than hubs. Hubs, at least, appear to be reasonably well-behaved in
this regard. this regard.
@ -284,30 +289,30 @@ device by suspending it at the wrong time. (Highly unlikely, but
possible.) Take care. possible.) Take care.
The driver interface for Power Management The driver interface for Power Management
----------------------------------------- -----------------------------------------
The requirements for a USB driver to support external power management The requirements for a USB driver to support external power management
are pretty modest; the driver need only define are pretty modest; the driver need only define::
.suspend .suspend
.resume .resume
.reset_resume .reset_resume
methods in its usb_driver structure, and the reset_resume method is methods in its :c:type:`usb_driver` structure, and the ``reset_resume`` method
optional. The methods' jobs are quite simple: is optional. The methods' jobs are quite simple:
The suspend method is called to warn the driver that the - The ``suspend`` method is called to warn the driver that the
device is going to be suspended. If the driver returns a device is going to be suspended. If the driver returns a
negative error code, the suspend will be aborted. Normally negative error code, the suspend will be aborted. Normally
the driver will return 0, in which case it must cancel all the driver will return 0, in which case it must cancel all
outstanding URBs (usb_kill_urb()) and not submit any more. 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
device has been resumed and the driver can return to normal device has been resumed and the driver can return to normal
operation. URBs may once more be submitted. 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 device has been resumed and it also has been reset.
The driver should redo any necessary device initialization, The driver should redo any necessary device initialization,
since the device has probably lost most or all of its state since the device has probably lost most or all of its state
@ -315,22 +320,22 @@ optional. The methods' jobs are quite simple:
before the suspend). before the suspend).
If the device is disconnected or powered down while it is suspended, If the device is disconnected or powered down while it is suspended,
the disconnect method will be called instead of the resume or the ``disconnect`` method will be called instead of the ``resume`` or
reset_resume method. This is also quite likely to happen when ``reset_resume`` method. This is also quite likely to happen when
waking up from hibernation, as many systems do not maintain suspend waking up from hibernation, as many systems do not maintain suspend
current to the USB host controllers during hibernation. (It's current to the USB host controllers during hibernation. (It's
possible to work around the hibernation-forces-disconnect problem by possible to work around the hibernation-forces-disconnect problem by
using the USB Persist facility.) using the USB Persist facility.)
The reset_resume method is used by the USB Persist facility (see The ``reset_resume`` method is used by the USB Persist facility (see
Documentation/usb/persist.txt) and it can also be used under certain ``Documentation/usb/persist.txt``) and it can also be used under certain
circumstances when CONFIG_USB_PERSIST is not enabled. Currently, if a circumstances when ``CONFIG_USB_PERSIST`` is not enabled. Currently, if a
device is reset during a resume and the driver does not have 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 ``reset_resume`` method, the driver won't receive any notification about
the resume. Later kernels will call the driver's disconnect method; the resume. Later kernels will call the driver's ``disconnect`` method;
2.6.23 doesn't do this. 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 methods get called when the interfaces are suspended or resumed. In
principle one might want to suspend some interfaces on a device (i.e., principle one might want to suspend some interfaces on a device (i.e.,
force the drivers for those interface to stop all activity) without 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. 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 To support autosuspend and autoresume, a driver should implement all
three of the methods listed above. In addition, a driver indicates 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 in its usb_driver structure. It is then responsible for informing the
USB core whenever one of its interfaces becomes busy or idle. 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); int usb_autopm_get_interface(struct usb_interface *intf);
void usb_autopm_put_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 Drivers need not be concerned about balancing changes to the usage
counter; the USB core will undo any remaining "get"s when a driver 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 is unbound from its interface. As a corollary, drivers must not call
any of the usb_autopm_* functions after their disconnect() routine has any of the ``usb_autopm_*`` functions after their ``disconnect``
returned. routine has returned.
Drivers using the async routines are responsible for their own Drivers using the async routines are responsible for their own
synchronization and mutual exclusion. 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 does an autoresume if the device is suspended. If the
autoresume fails, the counter is decremented back. 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. attempts an autosuspend if the new value is = 0.
usb_autopm_get_interface_async() and :c:func:`usb_autopm_get_interface_async` and
usb_autopm_put_interface_async() do almost the same things as :c:func:`usb_autopm_put_interface_async` do almost the same things as
their non-async counterparts. The big difference is that they their non-async counterparts. The big difference is that they
use a workqueue to do the resume or suspend part of their use a workqueue to do the resume or suspend part of their
jobs. As a result they can be called in an atomic context, jobs. As a result they can be called in an atomic context,
such as an URB's completion handler, but when they return the such as an URB's completion handler, but when they return the
device will generally not yet be in the desired state. device will generally not yet be in the desired state.
usb_autopm_get_interface_no_resume() and :c:func:`usb_autopm_get_interface_no_resume` and
usb_autopm_put_interface_no_suspend() merely increment or :c:func:`usb_autopm_put_interface_no_suspend` merely increment or
decrement the usage counter; they do not attempt to carry out decrement the usage counter; they do not attempt to carry out
an autoresume or an autosuspend. Hence they can be called in an autoresume or an autosuspend. Hence they can be called in
an atomic context. an atomic context.
The simplest usage pattern is that a driver calls The simplest usage pattern is that a driver calls
usb_autopm_get_interface() in its open routine and :c:func:`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_put_interface` in its close or release routine. But other
patterns are possible. patterns are possible.
The autosuspend attempts mentioned above will often fail for one The autosuspend attempts mentioned above will often fail for one
reason or another. For example, the power/control attribute might 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 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 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 the device hasn't been idle for long enough, a timer is scheduled to
carry out the operation automatically when the autosuspend idle-delay 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. 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); 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 suspending and resuming correctly. This is exactly equivalent to
writing "auto" to the device's power/control attribute. Likewise, writing ``auto`` to the device's ``power/control`` attribute. Likewise,
drivers can disable autosuspend by calling drivers can disable autosuspend by calling::
usb_disable_autosuspend(struct usb_device *udev); 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 Sometimes a driver needs to make sure that remote wakeup is enabled
during autosuspend. For example, there's not much point during autosuspend. For example, there's not much point
autosuspending a keyboard if the user can't cause the keyboard to do a autosuspending a keyboard if the user can't cause the keyboard to do a
remote wakeup by typing on it. If the driver sets 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 device if remote wakeup isn't available. (If the device is already
autosuspended, though, setting this flag won't cause the kernel to 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 method, at which time the device is guaranteed not to be
autosuspended.) autosuspended.)
If a driver does its I/O asynchronously in interrupt context, it If a driver does its I/O asynchronously in interrupt context, it
should call usb_autopm_get_interface_async() before starting output and should call :c:func:`usb_autopm_get_interface_async` before starting output and
usb_autopm_put_interface_async() when the output queue drains. When :c:func:`usb_autopm_put_interface_async` when the output queue drains. When
it receives an input event, it should call it receives an input event, it should call::
usb_mark_last_busy(struct usb_device *udev); 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. so drivers need to worry only when interrupt-driven input arrives.
Asynchronous operation is always subject to races. For example, a 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 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 long enough but not yet gotten around to calling the driver's ``suspend``
method. The suspend method must be responsible for synchronizing with method. The ``suspend`` method must be responsible for synchronizing with
the I/O request routine and the URB completion handler; it should 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 cause autosuspends to fail with -EBUSY if the driver needs to use the
device. device.
External suspend calls should never be allowed to fail in this way, External suspend calls should never be allowed to fail in this way,
only autosuspend calls. The driver can tell them apart by applying 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 method; it will return True for internal PM events (autosuspend) and
False for external PM events. False for external PM events.
Mutual exclusion Mutual exclusion
---------------- ----------------
For external events -- but not necessarily for autosuspend or For external events -- but not necessarily for autosuspend or
autoresume -- the device semaphore (udev->dev.sem) will be held when a autoresume -- the device semaphore (udev->dev.sem) will be held when a
suspend or resume method is called. This implies that external ``suspend`` or ``resume`` method is called. This implies that external
suspend/resume events are mutually exclusive with calls to probe, suspend/resume events are mutually exclusive with calls to ``probe``,
disconnect, pre_reset, and post_reset; the USB core guarantees that ``disconnect``, ``pre_reset``, and ``post_reset``; the USB core guarantees that
this is true of autosuspend/autoresume events as well. this is true of autosuspend/autoresume events as well.
If a driver wants to block all suspend/resume calls during some If a driver wants to block all suspend/resume calls during some
critical section, the best way is to lock the device and call 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 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?) 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 Dynamic power management and system power management can interact in
a couple of ways. 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. 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 host controller provides hardware link power management to usb2.0
(xHCI 1.0 feature) and usb3.0 devices which support link PM. By (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. which state device can enter and resume very quickly.
The user interface for controlling hardware LPM is located in the The user interface for controlling hardware LPM is located in the
power/ subdirectory of each USB device's sysfs directory, that is, in ``power/`` subdirectory of each USB device's sysfs directory, that is, in
/sys/bus/usb/devices/.../power/ where "..." is the device's ID. The ``/sys/bus/usb/devices/.../power/`` where "..." is the device's ID. The
relevant attribute files are usb2_hardware_lpm and usb3_hardware_lpm. 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 When a USB2 device which support LPM is plugged to a
xHCI host root hub which support software LPM, the 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 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. USB2 hardware LPM manually. This is for test purpose mainly.
power/usb3_hardware_lpm_u1 ``power/usb3_hardware_lpm_u1``
power/usb3_hardware_lpm_u2 ``power/usb3_hardware_lpm_u2``
When a USB 3.0 lpm-capable device is plugged in to a When a USB 3.0 lpm-capable device is plugged in to a
xHCI host which supports link PM, it will check if U1 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 indicating whether or not USB3 hardware LPM U1 or U2
is enabled for the device. is enabled for the device.
USB Port Power Control USB Port Power Control
---------------------- ----------------------
In addition to suspending endpoint devices and enabling hardware In addition to suspending endpoint devices and enabling hardware
controlled link power management, the USB subsystem also has the controlled link power management, the USB subsystem also has the
capability to disable power to ports under some conditions. Power is 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 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 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 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]. 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 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 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 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 a charging application. In any event a logically off port will lose
connection with its device, not respond to hotplug events, and not 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. .. warning::
Please see "User Interface for Port Power Control" for details.
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 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 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 implementation shares the same device recovery path (and honors the same
quirks) as the system resume path for the hub. quirks) as the system resume path for the hub.
[1]: http://dl.dropbox.com/u/96820575/sarah-sharp-lpt-port-power-off2-mini.pdf .. [#f1]
[2]: http://linuxplumbers.ubicast.tv/videos/usb-port-power-off-kerneluserspace-api/
[3]: USB 3.1 Section 10.12 http://dl.dropbox.com/u/96820575/sarah-sharp-lpt-port-power-off2-mini.pdf
* wakeup note: if a device is configured to send wakeup events the port
.. [#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 power control implementation will block poweroff attempts on that
port. 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 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 (defaults to 1). If the port is disconnected it will immediately receive a
ClearPortFeature(PORT_POWER) request. Otherwise, it will honor the pm runtime ``ClearPortFeature(PORT_POWER)`` request. Otherwise, it will honor the pm
rules and require the attached child device and all descendants to be suspended. runtime rules and require the attached child device and all descendants to be
This mechanism is dependent on the hub advertising port power switching in its suspended. This mechanism is dependent on the hub advertising port power
hub descriptor (wHubCharacteristics logical power switching mode field). switching in its hub descriptor (wHubCharacteristics logical power switching
mode field).
Note, some interface devices/drivers do not support autosuspend. Userspace may Note, some interface devices/drivers do not support autosuspend. Userspace may
need to unbind the interface drivers before the usb_device will suspend. An need to unbind the interface drivers before the :c:type:`usb_device` will
unbound interface device is suspended by default. When unbinding, be careful suspend. An unbound interface device is suspended by default. When unbinding,
to unbind interface drivers, not the driver of the parent usb device. Also, be careful to unbind interface drivers, not the driver of the parent usb
leave hub interface drivers bound. If the driver for the usb device (not device. Also, leave hub interface drivers bound. If the driver for the usb
interface) is unbound the kernel is no longer able to resume the device. If a device (not interface) is unbound the kernel is no longer able to resume the
hub interface driver is unbound, control of its child ports is lost and all device. If a hub interface driver is unbound, control of its child ports is
attached child-devices will disconnect. A good rule of thumb is that if the lost and all attached child-devices will disconnect. A good rule of thumb is
'driver/module' link for a device points to /sys/module/usbcore then unbinding that if the 'driver/module' link for a device points to
it will interfere with port power control. ``/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 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 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 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 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 $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 ../../../../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' Distinct from 'companion ports', or 'ehci/xhci shared switchover ports'
peer ports are simply the hi-speed and superspeed interface pins that 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 connection and attempt to connect to the hi-speed pins. The
implementation takes steps to prevent this: 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 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 that the setting ``pm_qos_no_power_off`` to zero on a superspeed port may
the port to power-off until its highspeed peer has gone to its runtime suspend not cause the port to power-off until its highspeed peer has gone to its
state. Userspace must take care to order the suspensions if it wants to runtime suspend state. Userspace must take care to order the suspensions
guarantee that a superspeed port will power-off. 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. 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. 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 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 states and clamps the maximum port power cycle frequency at the rate the
device can suspend (autosuspend-delay) and resume (reset-resume latency). child device can suspend (autosuspend-delay) and resume (reset-resume
latency).
Sysfs files relevant for port power control: 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. This writable flag controls the state of an idle port.
Once all children and descendants have suspended the Once all children and descendants have suspended the
port may suspend/poweroff provided that 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 '1' the port will remain active/powered regardless of
the stats of descendants. Defaults to 1. 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) This file reflects whether the port is 'active' (power is on)
or 'suspended' (logically off). There is no indication to or 'suspended' (logically off). There is no indication to
userspace whether VBUS is still supplied. userspace whether VBUS is still supplied.
<hubdev-portX>/connect_type: ``<hubdev-portX>/connect_type``:
An advisory read-only flag to userspace indicating the An advisory read-only flag to userspace indicating the
location and connection type of the port. It returns location and connection type of the port. It returns
one of four values 'hotplug', 'hardwired', 'not used', one of four values 'hotplug', 'hardwired', 'not used',
and 'unknown'. All values, besides unknown, are set by and 'unknown'. All values, besides unknown, are set by
platform firmware. platform firmware.
"hotplug" indicates an externally connectable/visible ``hotplug`` indicates an externally connectable/visible
port on the platform. Typically userspace would choose port on the platform. Typically userspace would choose
to keep such a port powered to handle new device to keep such a port powered to handle new device
connection events. 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 connectable. Examples are internal ports for USB
bluetooth that can be disconnected via an external bluetooth that can be disconnected via an external
switch or a port with a hardwired USB camera. It is 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 powering off, or to activate the port prior to enabling
connection via a switch. 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 to never have a device connected to it. These may be
empty internal ports, or ports that are not physically empty internal ports, or ports that are not physically
exposed on a platform. Considered safe to be exposed on a platform. Considered safe to be
powered-off at all times. 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 information for this port. Most commonly refers to
external hub ports which should be considered 'hotplug' external hub ports which should be considered 'hotplug'
for policy decisions. for policy decisions.
NOTE1: since we are relying on the BIOS to get this ACPI .. note::
information correct, the USB port descriptions may be
missing or wrong.
NOTE2: Take care in clearing pm_qos_no_power_off. Once - since we are relying on the BIOS to get this ACPI
power is off this port will information correct, the USB port descriptions may
not respond to new connect events. 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 Once a child device is attached additional constraints are
applied before the port is allowed to poweroff. applied before the port is allowed to poweroff.
<child>/power/control: ``<child>/power/control``:
Must be 'auto', and the port will not Must be ``auto``, and the port will not
power down until <child>/power/runtime_status power down until ``<child>/power/runtime_status``
reflects the 'suspended' state. Default reflects the 'suspended' state. Default
value is controlled by child device driver. value is controlled by child device driver.
<child>/power/persist: ``<child>/power/persist``:
This defaults to '1' for most devices and indicates if This defaults to ``1`` for most devices and indicates if
kernel can persist the device's configuration across a kernel can persist the device's configuration across a
power session loss (suspend / port-power event). When 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. disabled.
<child>/driver/unbind: ``<child>/driver/unbind``:
Wakeup capable devices will block port poweroff. At Wakeup capable devices will block port poweroff. At
this time the only mechanism to clear the usb-internal this time the only mechanism to clear the usb-internal
wakeup-capability for an interface device is to unbind wakeup-capability for an interface device is to unbind
its driver. 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 > power/pm_qos_no_power_off
echo 0 > peer/power/pm_qos_no_power_off # if it exists 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 auto > <child>/power/control
echo 1 > <child>/power/persist # this is the default value 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 As noted above userspace needs to be careful and deliberate about what
ports are enabled for poweroff. ports are enabled for poweroff.
The default configuration is that all ports start with 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. active.
Given confidence in the platform firmware's description of the ports 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. connection switch for the port.
A more aggressive userspace policy is to enable USB port power off for 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 some external factor indicates the user has stopped interacting with the
system. For example, a distro may want to enable power off all USB 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 ports when the screen blanks, and re-power them when the screen becomes