Documentation / usb / power-management.txt

Custom Search

Based on kernel version 2.6.34. Page generated on 2010-05-31 16:03 EST.

				Power Management for USB
	
			 Alan Stern <stern[AT]rowland.harvard[DOT]edu>
	
				    December 11, 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 (which depends on
	CONFIG_PM_RUNTIME).  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.
	
	If a driver knows that its device has proper suspend/resume support,
	it can enable autosuspend all by itself.  For example, the video
	driver for a laptop's webcam might do this, since these devices are
	rarely used and so should normally be autosuspended.
	
	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 usage counter in the
	usb_interface's embedded device structure.  When the counter is > 0
	then the interface is deemed to be busy, and the kernel will not
	autosuspend the interface's device.  When the usage counter is = 0
	then the interface is considered to be idle, and the kernel may
	autosuspend the device.
	
	(There is a similar usage counter field in struct usb_device,
	associated with the device itself rather than any of its interfaces.
	This counter is used only by the USB core.)
	
	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 diconnect() 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
		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
		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
		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
		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
	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 timer is scheduled to
	carry out the operation automatically 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 idle-delay for an autoresume.
	
	
		Other parts of the driver interface
		-----------------------------------
	
	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
	suspending and resuming correctly.  This is exactly equivalent to
	writing "auto" to the device's power/level 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/level 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
	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.)
	
	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
	
		usb_mark_last_busy(struct usb_device *udev);
	
	in the event handler.  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.  Most
	of the usb_autopm_* routines will also set the last_busy field to the
	current time.
	
	Asynchronous operation is always subject to races.  For example, a
	driver may call one of the usb_autopm_*_interface_async() routines 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 the output 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 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.
	
	
		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
	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
	critical section).  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?)
	
	
		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 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.  But this theory may not work out well in practice; over time
	the kernel's behavior in this regard has changed.
	
	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
•