USB: documentation for USB power management

			Power Management for USB

		 Alan Stern <stern@rowland.harvard.edu>

			    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:

	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.

	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 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

	(although the interfaces will be in the same altsettings as

	before the suspend).

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;

2.6.23 doesn't do this.

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

suspending the other interfaces.  The USB core doesn't allow this; all

interfaces are suspended when the device itself is suspended and all

interfaces are resumed when the device is resumed.  It isn't possible

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

	---------------------------------------------------

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

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 three functions:

	int  usb_autopm_get_interface(struct usb_interface *intf);

	void usb_autopm_put_interface(struct usb_interface *intf);

	int  usb_autopm_set_interface(struct usb_interface *intf);

The functions work by maintaining a counter in the usb_interface

structure.  When intf->pm_usage_count is > 0 then the interface is

deemed to be busy, and the kernel will not autosuspend the interface's

device.  When intf->pm_usage_count is <= 0 then the interface is

considered to be idle, and the kernel may autosuspend the device.

(There is a similar pm_usage_count field in struct usb_device,

associated with the device itself rather than any of its interfaces.

This field is used only by the USB core.)

The driver owns intf->pm_usage_count; it can modify the value however

and whenever it likes.  A nice aspect of the usb_autopm_* routines is

that the changes they make are protected by the usb_device structure's

PM mutex (udev->pm_mutex); however drivers may change pm_usage_count

without holding the mutex.

	usb_autopm_get_interface() increments pm_usage_count and

	attempts an autoresume if the new value is > 0 and the

	device is suspended.

	usb_autopm_put_interface() decrements pm_usage_count and

	attempts an autosuspend if the new value is <= 0 and the

	device isn't suspended.

	usb_autopm_set_interface() leaves pm_usage_count alone.

	It attempts an autoresume if the value is > 0 and the device

	is suspended, and it attempts an autosuspend if the value is

	<= 0 and the device isn't suspended.

There also are a couple of utility routines drivers can use:

	usb_autopm_enable() sets pm_usage_cnt to 1 and then calls

	usb_autopm_set_interface(), which will attempt an autoresume.

	usb_autopm_disable() sets pm_usage_cnt to 0 and then calls

	usb_autopm_set_interface(), which will attempt an autosuspend.

The conventional 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 patterns are possible.

The autosuspend attempts mentioned above will often fail for one

reason or another.  For example, the power/level 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 delayed workqueue

routine is automatically set up to carry out the operation when the

autosuspend idle-delay has expired.

Autoresume attempts also can fail.  This will happen if power/level is

set to "suspend" or if the device doesn't manage to resume properly.

Unlike autosuspend, there's no delay for an autoresume.

	Other parts of the driver interface

	-----------------------------------

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

device if remote wakeup isn't available or has been disabled through

the power/wakeup attribute.  (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 method, at which

time the device is guaranteed not to be autosuspended.)

The usb_autopm_* routines have to run in a sleepable process context;

they must not be called from an interrupt handler or while holding a

spinlock.  In fact, the entire autosuspend mechanism is not well geared

toward interrupt-driven operation.  However there is one thing a

driver can do in an interrupt handler:

	usb_mark_last_busy(struct usb_device *udev);

This sets udev->last_busy to the current time.  udev->last_busy is the

field used for idle-delay calculations; updating it will cause any

pending autosuspend to be moved back.  The usb_autopm_* routines will

also set the last_busy field to the current time.

Calling urb_mark_last_busy() from within an URB completion handler is

subject to races: The kernel may have just finished deciding the

device has been idle for long enough but not yet gotten around to

calling the driver's suspend method.  The driver would have to be

responsible for synchronizing its suspend method with its URB

completion handler and causing the autosuspend to fail with -EBUSY if

an URB had completed too recently.

External suspend calls should never be allowed to fail in this way,

only autosuspend calls.  The driver can tell them apart by checking

udev->auto_pm; this flag will be set to 1 for internal PM events

(autosuspend or autoresume) and 0 for external PM events.

Many of the ingredients in the autosuspend framework are oriented

towards interfaces: The usb_interface structure contains the

pm_usage_cnt field, and the usb_autopm_* routines take an interface

pointer as their argument.  But somewhat confusingly, a few of the

pieces (usb_mark_last_busy() and udev->auto_pm) use the usb_device

structure instead.  Drivers need to keep this straight; they can call

interface_to_usbdev() to find the device structure for a given

interface.

	Locking requirements

	--------------------

All three suspend/resume methods are always called while holding the

usb_device's PM mutex.  For external events -- but not necessarily for

autosuspend or autoresume -- the device semaphore (udev->dev.sem) will

also be held.  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 internal

suspend/resume events as well.

If a driver wants to block all suspend/resume calls during some

critical section, it can simply acquire udev->pm_mutex.

Alternatively, if the critical section might call some of the

usb_autopm_* routines, the driver can avoid deadlock by doing:

	down(&udev->dev.sem);

	rc = usb_autopm_get_interface(intf);

and at the end of the critical section:

	if (!rc)

		usb_autopm_put_interface(intf);

	up(&udev->dev.sem);

Holding the device semaphore will block all external PM calls, and the

usb_autopm_get_interface() will prevent any internal PM calls, even if

it fails.  (Exercise: Why?)

The rules for locking order are:

	Never acquire any device semaphore while holding any PM mutex.

	Never acquire udev->pm_mutex while holding the PM mutex for

	a device that isn't a descendant of udev.

In other words, PM mutexes should only be acquired going up the device

tree, and they should be acquired only after locking all the device

semaphores you need to hold.  These rules don't matter to drivers very

much; they usually affect just the USB core.

Still, drivers do need to be careful.  For example, many drivers use a

private mutex to synchronize their normal I/O activities with their

disconnect method.  Now if the driver supports autosuspend then it

must call usb_autopm_put_interface() from somewhere -- maybe from its

close method.  It should make the call while holding the private mutex,

since a driver shouldn't call any of the usb_autopm_* functions for an

interface from which it has been unbound.

But the usb_autpm_* routines always acquire the device's PM mutex, and

consequently the locking order has to be: private mutex first, PM

mutex second.  Since the suspend method is always called with the PM

mutex held, it mustn't try to acquire the private mutex.  It has to

synchronize with the driver's I/O activities in some other way.

	Interaction between dynamic PM and system PM

	--------------------------------------------

Dynamic power management and system power management can interact in

a couple of ways.

Firstly, a device may already be manually suspended or autosuspended

when a system suspend occurs.  Since system suspends are supposed to

be as transparent as possible, the device should remain suspended

following the system resume.  The 2.6.23 kernel obeys this principle

for manually suspended devices but not for autosuspended devices; they

do get resumed when the system wakes up.  (Presumably they will be

autosuspended again after their idle-delay time expires.)  In later

kernels this behavior will be fixed.

(There is an exception.  If a device would undergo a reset-resume

instead of a normal resume, and the device is enabled for remote

wakeup, then the reset-resume takes place even if the device was

already suspended when the system suspend began.  The justification is

that a reset-resume is a kind of remote-wakeup event.  Or to put it

another way, a device which needs a reset won't be able to generate

normal remote-wakeup signals, so it ought to be resumed immediately.)

Secondly, a dynamic power-management event may occur as a system

suspend is underway.  The window for this is short, since system

suspends don't take long (a few seconds usually), but it can happen.

For example, a suspended device may send a remote-wakeup signal while

the system is suspending.  The remote wakeup may succeed, which would

cause the system suspend to abort.  If the remote wakeup doesn't

succeed, it may still remain active and thus cause the system to

resume as soon as the system suspend is complete.  Or the remote

wakeup may fail and get lost.  Which outcome occurs depends on timing

and on the hardware and firmware design.

More interestingly, a device might undergo a manual resume or

autoresume during system suspend.  With current kernels this shouldn't

happen, because manual resumes must be initiated by userspace and

autoresumes happen in response to I/O requests, but all user processes

and I/O should be quiescent during a system suspend -- thanks to the

freezer.  However there are plans to do away with the freezer, which

would mean these things would become possible.  If and when this comes

about, the USB core will carefully arrange matters so that either type

of resume will block until the entire system has resumed.