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提交 3db5f406 编写于 作者: M Mauro Carvalho Chehab 提交者： Jonathan Corbet
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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: NMauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>
上级 3b38e4f2
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Documentation/driver-api/usb/index.rst
浏览文件 @ 3db5f406
......@@ -9,6 +9,7 @@ Linux USB API
anchors
bulk-streams
callbacks
power-management
writing_usb_driver
writing_musb_glue_layer
......
Documentation/usb/power-management.txt → Documentation/driver-api/usb/power-management.rst
浏览文件 @ 3db5f406
Power Management for USB
.. _usb-power-management:
Alan Stern <stern@rowland.harvard.edu>
Last-updated: February 2014
Power Management for USB
~~~~~~~~~~~~~~~~~~~~~~~~
:Author: Alan Stern <stern@rowland.harvard.edu>
:Date: Last-updated: February 2014
..
Contents:
---------
* What is Power Management?
......@@ -25,14 +27,14 @@
* Suggested Userspace Port Power Policy
What is Power Management?
-------------------------
What is Power Management?
-------------------------
Power Management (PM) is the practice of saving energy by suspending
parts of a computer system when they aren't being used. While a
component is "suspended" it is in a nonfunctional low-power state; it
component is ``suspended`` it is in a nonfunctional low-power state; it
might even be turned off completely. A suspended component can be
"resumed" (returned to a functional full-power state) when the kernel
``resumed`` (returned to a functional full-power state) when the kernel
needs to use it. (There also are forms of PM in which components are
placed in a less functional but still usable state instead of being
suspended; an example would be reducing the CPU's clock rate. This
......@@ -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
"selective suspend"). This document concentrates mostly on how
dynamic PM is implemented in the USB subsystem, although system PM is
covered to some extent (see Documentation/power/*.txt for more
covered to some extent (see ``Documentation/power/*.txt`` for more
information about system PM).
System PM support is present only if the kernel was built with CONFIG_SUSPEND
or CONFIG_HIBERNATION enabled. Dynamic PM support for USB is present whenever
the kernel was built with CONFIG_PM enabled.
System PM support is present only if the kernel was built with
``CONFIG_SUSPEND`` or ``CONFIG_HIBERNATION`` enabled. Dynamic PM support
for USB is present whenever
the kernel was built with ``CONFIG_PM`` enabled.
[Historically, dynamic PM support for USB was present only if the
kernel had been built with CONFIG_USB_SUSPEND enabled (which depended on
CONFIG_PM_RUNTIME). Starting with the 3.10 kernel release, dynamic PM support
for USB was present whenever the kernel was built with CONFIG_PM_RUNTIME
enabled. The CONFIG_USB_SUSPEND option had been eliminated.]
kernel had been built with ``CONFIG_USB_SUSPEND`` enabled (which depended on
``CONFIG_PM_RUNTIME``). Starting with the 3.10 kernel release, dynamic PM
support for USB was present whenever the kernel was built with
``CONFIG_PM_RUNTIME`` enabled. The ``CONFIG_USB_SUSPEND`` option had been
eliminated.]
What is Remote Wakeup?
----------------------
What is Remote Wakeup?
----------------------
When a device has been suspended, it generally doesn't resume until
the computer tells it to. Likewise, if the entire computer has been
......@@ -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.
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
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.
Forms of dynamic PM
-------------------
Forms of dynamic PM
-------------------
Dynamic suspends occur when the kernel decides to suspend an idle
device. This is called "autosuspend" for short. In general, a device
device. This is called ``autosuspend`` for short. In general, a device
won't be autosuspended unless it has been idle for some minimum period
of time, the so-called idle-delay time.
......@@ -125,51 +130,51 @@ all dynamic suspend events are internal; external agents are not
allowed to issue dynamic suspends.
The user interface for dynamic PM
---------------------------------
The user interface for 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
/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
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
"control" file. In 2.6.38 the "autosuspend" file will be deprecated
and replaced by the "autosuspend_delay_ms" file. The only difference
``control`` file. In 2.6.38 the ``autosuspend`` file will be deprecated
and replaced by the ``autosuspend_delay_ms`` file. The only difference
is that the newer file expresses the delay in milliseconds whereas the
older file uses seconds. Confusingly, both files are present in 2.6.37
but only "autosuspend" works.)
but only ``autosuspend`` works.)
power/wakeup
``power/wakeup``
This file is empty if the device does not support
remote wakeup. Otherwise the file contains either the
word "enabled" or the word "disabled", and you can
word ``enabled`` or the word ``disabled``, and you can
write those words to the file. The setting determines
whether or not remote wakeup will be enabled when the
device is next suspended. (If the setting is changed
while the device is suspended, the change won't take
effect until the following suspend.)
power/control
``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
device's setting.
"on" means that the device should be resumed and
autosuspend is not allowed. (Of course, system
suspends are still allowed.)
- ``on`` means that the device should be resumed and
autosuspend is not allowed. (Of course, system
suspends are still allowed.)
"auto" is the normal state in which the kernel is
allowed to autosuspend and autoresume the device.
- ``auto`` is the normal state in which the kernel is
allowed to autosuspend and autoresume the device.
(In kernels up to 2.6.32, you could also specify
"suspend", meaning that the device should remain
``suspend``, meaning that the device should remain
suspended and autoresume was not allowed. This
setting is no longer supported.)
power/autosuspend_delay_ms
``power/autosuspend_delay_ms``
This file contains an integer value, which is the
number of milliseconds the device should remain idle
......@@ -180,31 +185,31 @@ but only "autosuspend" works.)
number to the file to change the autosuspend
idle-delay time.
Writing "-1" to power/autosuspend_delay_ms and writing "on" to
power/control do essentially the same thing -- they both prevent the
Writing ``-1`` to ``power/autosuspend_delay_ms`` and writing ``on`` to
``power/control`` do essentially the same thing -- they both prevent the
device from being autosuspended. Yes, this is a redundancy in the
API.
(In 2.6.21 writing "0" to power/autosuspend would prevent the device
(In 2.6.21 writing ``0`` to ``power/autosuspend`` would prevent the device
from being autosuspended; the behavior was changed in 2.6.22. The
power/autosuspend attribute did not exist prior to 2.6.21, and the
power/level attribute did not exist prior to 2.6.22. power/control
was added in 2.6.34, and power/autosuspend_delay_ms was added in
``power/autosuspend`` attribute did not exist prior to 2.6.21, and the
``power/level`` attribute did not exist prior to 2.6.22. ``power/control``
was added in 2.6.34, and ``power/autosuspend_delay_ms`` was added in
2.6.37 but did not become functional until 2.6.38.)
Changing the default idle-delay time
------------------------------------
Changing the default idle-delay time
------------------------------------
The default autosuspend idle-delay time (in seconds) is controlled by
a module parameter in usbcore. You can specify the value when usbcore
is loaded. For example, to set it to 5 seconds instead of 2 you would
do:
do::
modprobe usbcore autosuspend=5
Equivalently, you could add to a configuration file in /etc/modprobe.d
a line saying:
a line saying::
options usbcore autosuspend=5
......@@ -214,14 +219,14 @@ image. To alter the parameter value you would have to rebuild that
image.
If usbcore is compiled into the kernel rather than built as a loadable
module, you can add
module, you can add::
usbcore.autosuspend=5
to the kernel's boot command line.
Finally, the parameter value can be changed while the system is
running. If you do:
running. If you do::
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.
Warnings
--------
Warnings
--------
The USB specification states that all USB devices must support power
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.
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
this regard.
......@@ -284,30 +289,30 @@ device by suspending it at the wrong time. (Highly unlikely, but
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
are pretty modest; the driver need only define
are pretty modest; the driver need only define::
.suspend
.resume
.reset_resume
methods in its usb_driver structure, and the reset_resume method is
optional. The methods' jobs are quite simple:
methods in its :c:type:`usb_driver` structure, and the ``reset_resume`` method
is optional. The methods' jobs are quite simple:
The suspend method is called to warn the driver that the
- The ``suspend`` method is called to warn the driver that the
device is going to be suspended. If the driver returns a
negative error code, the suspend will be aborted. Normally
the driver will return 0, in which case it must cancel all
outstanding URBs (usb_kill_urb()) and not submit any more.
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
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
......@@ -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::
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.
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::
- since we are relying on the BIOS to get this ACPI
information correct, the USB port descriptions may
be missing or wrong.
NOTE2: Take care in clearing pm_qos_no_power_off. Once
power is off this port will
not respond to new connect events.
- 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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