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Based on kernel version 2.6.33. Page generated on 2010-02-24 15:37 EST.

				Power Management for USB
	
			 Alan Stern <stern[AT]rowland.harvard[DOT]edu>
	
				    November 10, 2009
	
	
	
		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 occur when the kernel decides to suspend an idle
	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.
	
	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 resume (also triggered by userspace), and
	remote wakeup (triggered by the device).  Internal events are those
	triggered within the USB stack: autosuspend and autoresume.  Note that
	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 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 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.)
	
			"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
			suspended and autoresume was not allowed.  This
			setting is no longer supported.)
	
		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 negative values mean
			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).
	
	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
	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
	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 six functions:
	
		int  usb_autopm_get_interface(struct usb_interface *intf);
		void usb_autopm_put_interface(struct usb_interface *intf);
		int  usb_autopm_get_interface_async(struct usb_interface *intf);
		void usb_autopm_put_interface_async(struct usb_interface *intf);
		void usb_autopm_get_interface_no_resume(struct usb_interface *intf);
		void usb_autopm_put_interface_no_suspend(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.)
	
	Drivers must not modify intf->pm_usage_count directly; its value
	should be changed only be using the functions listed above.  Drivers
	are responsible for insuring that the overall change to pm_usage_count
	during their lifetime balances out to 0 (it may be necessary for the
	disconnect method to call usb_autopm_put_interface() one or more times
	to fulfill this requirement).  The first two routines use the PM mutex
	in struct usb_device for mutual exclusion; drivers using the async
	routines are responsible for their own synchronization and mutual
	exclusion.
	
		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_get_interface_async() and
		usb_autopm_put_interface_async() do almost the same things as
		their non-async counterparts.  The differences are: they do
		not acquire the PM mutex, and they use a workqueue to do 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 not generally not yet be in the desired state.
	
		usb_autopm_get_interface_no_resume() and
		usb_autopm_put_interface_no_suspend() merely increment or
		decrement the pm_usage_count value; they do not attempt to
		carry out an autoresume or an autosuspend.  Hence they can be
		called in an atomic context.
	
	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, although failure would mean that
	the device is no longer present or operating 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 synchronous 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
	the PM_EVENT_AUTO bit in the message.event argument to the suspend
	method; this bit will be set for internal PM events (autosuspend) and
	clear 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 (i.e., usb_mark_last_busy()) 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. Note that
	calls to resume may be triggered indirectly. Block IO due to memory
	allocations can make the vm subsystem resume a device. Thus while
	holding this lock you must not allocate memory with GFP_KERNEL or
	GFP_NOFS.
	
	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.

• [ usb ]
• acm.txt
• anchors.txt
• authorization.txt
• callbacks.txt
• CREDITS
• dma.txt
• ehci.txt
• error-codes.txt
• gadget_printer.txt
• gadget_serial.txt
• hiddev.txt
• hotplug.txt
• iuu_phoenix.txt
• linux.inf
• misc_usbsevseg.txt
• mtouchusb.txt
• ohci.txt
• persist.txt
• power-management.txt
• proc_usb_info.txt
• rio.txt
• URB.txt
• usb-help.txt
• usb-serial.txt
• usbmon.txt
• wusb-cbaf
• WUSB-Design-overview.txt
•