Power Management for USB
  
  		 Alan Stern <stern@rowland.harvard.edu>
  
  			    October 28, 2010
  
  
  
  	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.
  
  (Starting with the 3.10 kernel release, dynamic PM support for USB is
  present whenever the kernel was built with CONFIG_PM_RUNTIME enabled.
  The CONFIG_USB_SUSPEND option has been eliminated.)
  
  
  	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, control, and
  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
  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.)
  
  	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/control
  
  		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_delay_ms
  
  		This file contains an integer value, which is the
  		number of milliseconds the device should remain idle
  		before the kernel will autosuspend it (the idle-delay
  		time).  The default is 2000.  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_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
  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
  2.6.37 but did not become functional until 2.6.38.)
  
  
  	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:
  
  	modprobe usbcore autosuspend=5
  
  Equivalently, you could add to a configuration file in /etc/modprobe.d
  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 has the benefit of allowing you
  then 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/control 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 (in recent kernels they
  do), 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, the usbhid driver,
  which manages keyboards and mice, has autosuspend support.  Tests with
  a number of keyboards show that typing on a suspended keyboard, while
  causing the keyboard to do a remote wakeup all right, will nonetheless
  frequently result in lost keystrokes.  Tests with mice show that some
  of them will 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.  (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.
  
  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.
  
  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/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
  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/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.
  
  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.  (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 tells the PM core that the device was just
  busy and therefore the next autosuspend idle-delay expiration should
  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
  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 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
  method; it will return True for internal PM events (autosuspend) and
  False 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.  As of 2.6.37 the
  policy is to resume all devices during a system resume and let them
  handle their own runtime suspends afterward.
  
  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.
  
  
  	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
  enabling hardware LPM, the host can automatically put the device into
  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 USB2 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 is 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
  		host will run a software LPM test for it; if the device
  		enters L1 state and resume successfully and the host
  		supports USB2 hardware LPM, this file will show up and
  		driver will enable hardware LPM	for the device. You
  		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.