power-management.txt

linux 内核源代码

			Power Management for USB

		 Alan Stern <[e-mail]>

			    October 5, 2007



	What is Power Management?
	-------------------------

Power Management (PM) is the practice of saving energy by suspending
parts of a computer system when they aren't being used.  While a
component is "suspended" it is in a nonfunctional low-power state; it
might even be turned off completely.  A suspended component can be
"resumed" (returned to a functional full-power state) when the kernel
needs to use it.  (There also are forms of PM in which components are
placed in a less functional but still usable state instead of being
suspended; an example would be reducing the CPU's clock rate.  This
document will not discuss those other forms.)

When the parts being suspended include the CPU and most of the rest of
the system, we speak of it as a "system suspend".  When a particular
device is turned off while the system as a whole remains running, we
call it a "dynamic suspend" (also known as a "runtime suspend" or
"selective suspend").  This document concentrates mostly on how
dynamic PM is implemented in the USB subsystem, although system PM is
covered to some extent (see Documentation/power/*.txt for more
information about system PM).

Note: Dynamic PM support for USB is present only if the kernel was
built with CONFIG_USB_SUSPEND enabled.  System PM support is present
only if the kernel was built with CONFIG_SUSPEND or CONFIG_HIBERNATION
enabled.


	What is Remote Wakeup?
	----------------------

When a device has been suspended, it generally doesn't resume until
the computer tells it to.  Likewise, if the entire computer has been
suspended, it generally doesn't resume until the user tells it to, say
by pressing a power button or opening the cover.

However some devices have the capability of resuming by themselves, or
asking the kernel to resume them, or even telling the entire computer
to resume.  This capability goes by several names such as "Wake On
LAN"; we will refer to it generically as "remote wakeup".  When a
device is enabled for remote wakeup and it is suspended, it may resume
itself (or send a request to be resumed) in response to some external
event.  Examples include a suspended keyboard resuming when a key is
pressed, or a suspended USB hub resuming when a device is plugged in.


	When is a USB device idle?
	--------------------------

A device is idle whenever the kernel thinks it's not busy doing
anything important and thus is a candidate for being suspended.  The
exact definition depends on the device's driver; drivers are allowed
to declare that a device isn't idle even when there's no actual
communication taking place.  (For example, a hub isn't considered idle
unless all the devices plugged into that hub are already suspended.)
In addition, a device isn't considered idle so long as a program keeps
its usbfs file open, whether or not any I/O is going on.

If a USB device has no driver, its usbfs file isn't open, and it isn't
being accessed through sysfs, then it definitely is idle.


	Forms of dynamic PM
	-------------------

Dynamic suspends can occur in two ways: manual and automatic.
"Manual" means that the user has told the kernel to suspend a device,
whereas "automatic" means that the kernel has decided all by itself to
suspend a device.  Automatic suspend is called "autosuspend" for
short.  In general, a device won't be autosuspended unless it has been
idle for some minimum period of time, the so-called idle-delay time.

Of course, nothing the kernel does on its own initiative should
prevent the computer or its devices from working properly.  If a
device has been autosuspended and a program tries to use it, the
kernel will automatically resume the device (autoresume).  For the
same reason, an autosuspended device will usually have remote wakeup
enabled, if the device supports remote wakeup.

It is worth mentioning that many USB drivers don't support
autosuspend.  In fact, at the time of this writing (Linux 2.6.23) the
only drivers which do support it are the hub driver, kaweth, asix,
usblp, usblcd, and usb-skeleton (which doesn't count).  If a
non-supporting driver is bound to a device, the device won't be
autosuspended.  In effect, the kernel pretends the device is never
idle.

We can categorize power management events in two broad classes:
external and internal.  External events are those triggered by some
agent outside the USB stack: system suspend/resume (triggered by
userspace), manual dynamic suspend/resume (also triggered by
userspace), and remote wakeup (triggered by the device).  Internal
events are those triggered within the USB stack: autosuspend and
autoresume.


	The user interface for dynamic PM
	---------------------------------

The user interface for controlling dynamic PM is located in the power/
subdirectory of each USB device's sysfs directory, that is, in
/sys/bus/usb/devices/.../power/ where "..." is the device's ID.  The
relevant attribute files are: wakeup, level, and autosuspend.

	power/wakeup

		This file is empty if the device does not support
		remote wakeup.  Otherwise the file contains either the
		word "enabled" or the word "disabled", and you can
		write those words to the file.  The setting determines
		whether or not remote wakeup will be enabled when the
		device is next suspended.  (If the setting is changed
		while the device is suspended, the change won't take
		effect until the following suspend.)

	power/level

		This file contains one of three words: "on", "auto",
		or "suspend".  You can write those words to the file
		to change the device's setting.

		"on" means that the device should be resumed and
		autosuspend is not allowed.  (Of course, system
		suspends are still allowed.)

		"auto" is the normal state in which the kernel is
		allowed to autosuspend and autoresume the device.

		"suspend" means that the device should remain
		suspended, and autoresume is not allowed.  (But remote
		wakeup may still be allowed, since it is controlled
		separately by the power/wakeup attribute.)

	power/autosuspend

		This file contains an integer value, which is the
		number of seconds the device should remain idle before
		the kernel will autosuspend it (the idle-delay time).
		The default is 2.  0 means to autosuspend as soon as
		the device becomes idle, and -1 means never to
		autosuspend.  You can write a number to the file to
		change the autosuspend idle-delay time.

Writing "-1" to power/autosuspend and writing "on" to power/level do
essentially the same thing -- they both prevent the device from being
autosuspended.  Yes, this is a redundancy in the API.

(In 2.6.21 writing "0" to power/autosuspend would prevent the device
from being autosuspended; the behavior was changed in 2.6.22.  The
power/autosuspend attribute did not exist prior to 2.6.21, and the
power/level attribute did not exist prior to 2.6.22.)


	Changing the default idle-delay time
	------------------------------------

The default autosuspend idle-delay time is controlled by a module
parameter in usbcore.  You can specify the value when usbcore is
loaded.  For example, to set it to 5 seconds instead of 2 you would
do:

	modprobe usbcore autosuspend=5

Equivalently, you could add to /etc/modprobe.conf a line saying:

	options usbcore autosuspend=5

Some distributions load the usbcore module very early during the boot
process, by means of a program or script running from an initramfs
image.  To alter the parameter value you would have to rebuild that
image.

If usbcore is compiled into the kernel rather than built as a loadable
module, you can add

	usbcore.autosuspend=5

to the kernel's boot command line.

Finally, the parameter value can be changed while the system is
running.  If you do:

	echo 5 >/sys/module/usbcore/parameters/autosuspend

then each new USB device will have its autosuspend idle-delay
initialized to 5.  (The idle-delay values for already existing devices
will not be affected.)

Setting the initial default idle-delay to -1 will prevent any
autosuspend of any USB device.  This is a simple alternative to
disabling CONFIG_USB_SUSPEND and rebuilding the kernel, and it has the
added benefit of allowing you to enable autosuspend for selected
devices.


	Warnings
	--------

The USB specification states that all USB devices must support power
management.  Nevertheless, the sad fact is that many devices do not
support it very well.  You can suspend them all right, but when you
try to resume them they disconnect themselves from the USB bus or
they stop working entirely.  This seems to be especially prevalent
among printers and scanners, but plenty of other types of device have
the same deficiency.

For this reason, by default the kernel disables autosuspend (the
power/level attribute is initialized to "on") for all devices other
than hubs.  Hubs, at least, appear to be reasonably well-behaved in
this regard.

(In 2.6.21 and 2.6.22 this wasn't the case.  Autosuspend was enabled
by default for almost all USB devices.  A number of people experienced
problems as a result.)

This means that non-hub devices won't be autosuspended unless the user
or a program explicitly enables it.  As of this writing there aren't
any widespread programs which will do this; we hope that in the near
future device managers such as HAL will take on this added
responsibility.  In the meantime you can always carry out the
necessary operations by hand or add them to a udev script.  You can
also change the idle-delay time; 2 seconds is not the best choice for
every device.

Sometimes it turns out that even when a device does work okay with
autosuspend there are still problems.  For example, there are
experimental patches adding autosuspend support to the usbhid driver,
which manages keyboards and mice, among other things.  Tests with a
number of keyboards showed that typing on a suspended keyboard, while
causing the keyboard to do a remote wakeup all right, would
nonetheless frequently result in lost keystrokes.  Tests with mice
showed that some of them would issue a remote-wakeup request in
response to button presses but not to motion, and some in response to
neither.

The kernel will not prevent you from enabling autosuspend on devices
that can't handle it.  It is even possible in theory to damage a
device by suspending it at the wrong time -- for example, suspending a
USB hard disk might cause it to spin down without parking the heads.
(Highly unlikely, but possible.)  Take care.


	The driver interface for Power Management
	-----------------------------------------

The requirements for a USB driver to support external power management
are pretty modest; the driver need only define

	.suspend
	.resume
	.reset_resume

methods in its usb_driver structure, and the reset_resume method is
optional.  The methods' jobs are quite simple: