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

	Run-time Power Management Framework for I/O Devices
	
	(C) 2009 Rafael J. Wysocki <rjw[AT]sisk.pl>, Novell Inc[DOT]
	
	1. Introduction
	
	Support for run-time power management (run-time PM) of I/O devices is provided
	at the power management core (PM core) level by means of:
	
	* The power management workqueue pm_wq in which bus types and device drivers can
	  put their PM-related work items.  It is strongly recommended that pm_wq be
	  used for queuing all work items related to run-time PM, because this allows
	  them to be synchronized with system-wide power transitions (suspend to RAM,
	  hibernation and resume from system sleep states).  pm_wq is declared in
	  include/linux/pm_runtime.h and defined in kernel/power/main.c.
	
	* A number of run-time PM fields in the 'power' member of 'struct device' (which
	  is of the type 'struct dev_pm_info', defined in include/linux/pm.h) that can
	  be used for synchronizing run-time PM operations with one another.
	
	* Three device run-time PM callbacks in 'struct dev_pm_ops' (defined in
	  include/linux/pm.h).
	
	* A set of helper functions defined in drivers/base/power/runtime.c that can be
	  used for carrying out run-time PM operations in such a way that the
	  synchronization between them is taken care of by the PM core.  Bus types and
	  device drivers are encouraged to use these functions.
	
	The run-time PM callbacks present in 'struct dev_pm_ops', the device run-time PM
	fields of 'struct dev_pm_info' and the core helper functions provided for
	run-time PM are described below.
	
	2. Device Run-time PM Callbacks
	
	There are three device run-time PM callbacks defined in 'struct dev_pm_ops':
	
	struct dev_pm_ops {
		...
		int (*runtime_suspend)(struct device *dev);
		int (*runtime_resume)(struct device *dev);
		int (*runtime_idle)(struct device *dev);
		...
	};
	
	The ->runtime_suspend(), ->runtime_resume() and ->runtime_idle() callbacks are
	executed by the PM core for either the bus type, or device type (if the bus
	type's callback is not defined), or device class (if the bus type's and device
	type's callbacks are not defined) of given device.  The bus type, device type
	and device class callbacks are referred to as subsystem-level callbacks in what
	follows.
	
	The subsystem-level suspend callback is _entirely_ _responsible_ for handling
	the suspend of the device as appropriate, which may, but need not include
	executing the device driver's own ->runtime_suspend() callback (from the
	PM core's point of view it is not necessary to implement a ->runtime_suspend()
	callback in a device driver as long as the subsystem-level suspend callback
	knows what to do to handle the device).
	
	  * Once the subsystem-level suspend callback has completed successfully
	    for given device, the PM core regards the device as suspended, which need
	    not mean that the device has been put into a low power state.  It is
	    supposed to mean, however, that the device will not process data and will
	    not communicate with the CPU(s) and RAM until the subsystem-level resume
	    callback is executed for it.  The run-time PM status of a device after
	    successful execution of the subsystem-level suspend callback is 'suspended'.
	
	  * If the subsystem-level suspend callback returns -EBUSY or -EAGAIN,
	    the device's run-time PM status is 'active', which means that the device
	    _must_ be fully operational afterwards.
	
	  * If the subsystem-level suspend callback returns an error code different
	    from -EBUSY or -EAGAIN, the PM core regards this as a fatal error and will
	    refuse to run the helper functions described in Section 4 for the device,
	    until the status of it is directly set either to 'active', or to 'suspended'
	    (the PM core provides special helper functions for this purpose).
	
	In particular, if the driver requires remote wake-up capability (i.e. hardware
	mechanism allowing the device to request a change of its power state, such as
	PCI PME) for proper functioning and device_run_wake() returns 'false' for the
	device, then ->runtime_suspend() should return -EBUSY.  On the other hand, if
	device_run_wake() returns 'true' for the device and the device is put into a low
	power state during the execution of the subsystem-level suspend callback, it is
	expected that remote wake-up will be enabled for the device.  Generally, remote
	wake-up should be enabled for all input devices put into a low power state at
	run time.
	
	The subsystem-level resume callback is _entirely_ _responsible_ for handling the
	resume of the device as appropriate, which may, but need not include executing
	the device driver's own ->runtime_resume() callback (from the PM core's point of
	view it is not necessary to implement a ->runtime_resume() callback in a device
	driver as long as the subsystem-level resume callback knows what to do to handle
	the device).
	
	  * Once the subsystem-level resume callback has completed successfully, the PM
	    core regards the device as fully operational, which means that the device
	    _must_ be able to complete I/O operations as needed.  The run-time PM status
	    of the device is then 'active'.
	
	  * If the subsystem-level resume callback returns an error code, the PM core
	    regards this as a fatal error and will refuse to run the helper functions
	    described in Section 4 for the device, until its status is directly set
	    either to 'active' or to 'suspended' (the PM core provides special helper
	    functions for this purpose).
	
	The subsystem-level idle callback is executed by the PM core whenever the device
	appears to be idle, which is indicated to the PM core by two counters, the
	device's usage counter and the counter of 'active' children of the device.
	
	  * If any of these counters is decreased using a helper function provided by
	    the PM core and it turns out to be equal to zero, the other counter is
	    checked.  If that counter also is equal to zero, the PM core executes the
	    subsystem-level idle callback with the device as an argument.
	
	The action performed by a subsystem-level idle callback is totally dependent on
	the subsystem in question, but the expected and recommended action is to check
	if the device can be suspended (i.e. if all of the conditions necessary for
	suspending the device are satisfied) and to queue up a suspend request for the
	device in that case.  The value returned by this callback is ignored by the PM
	core.
	
	The helper functions provided by the PM core, described in Section 4, guarantee
	that the following constraints are met with respect to the bus type's run-time
	PM callbacks:
	
	(1) The callbacks are mutually exclusive (e.g. it is forbidden to execute
	    ->runtime_suspend() in parallel with ->runtime_resume() or with another
	    instance of ->runtime_suspend() for the same device) with the exception that
	    ->runtime_suspend() or ->runtime_resume() can be executed in parallel with
	    ->runtime_idle() (although ->runtime_idle() will not be started while any
	    of the other callbacks is being executed for the same device).
	
	(2) ->runtime_idle() and ->runtime_suspend() can only be executed for 'active'
	    devices (i.e. the PM core will only execute ->runtime_idle() or
	    ->runtime_suspend() for the devices the run-time PM status of which is
	    'active').
	
	(3) ->runtime_idle() and ->runtime_suspend() can only be executed for a device
	    the usage counter of which is equal to zero _and_ either the counter of
	    'active' children of which is equal to zero, or the 'power.ignore_children'
	    flag of which is set.
	
	(4) ->runtime_resume() can only be executed for 'suspended' devices  (i.e. the
	    PM core will only execute ->runtime_resume() for the devices the run-time
	    PM status of which is 'suspended').
	
	Additionally, the helper functions provided by the PM core obey the following
	rules:
	
	  * If ->runtime_suspend() is about to be executed or there's a pending request
	    to execute it, ->runtime_idle() will not be executed for the same device.
	
	  * A request to execute or to schedule the execution of ->runtime_suspend()
	    will cancel any pending requests to execute ->runtime_idle() for the same
	    device.
	
	  * If ->runtime_resume() is about to be executed or there's a pending request
	    to execute it, the other callbacks will not be executed for the same device.
	
	  * A request to execute ->runtime_resume() will cancel any pending or
	    scheduled requests to execute the other callbacks for the same device.
	
	3. Run-time PM Device Fields
	
	The following device run-time PM fields are present in 'struct dev_pm_info', as
	defined in include/linux/pm.h:
	
	  struct timer_list suspend_timer;
	    - timer used for scheduling (delayed) suspend request
	
	  unsigned long timer_expires;
	    - timer expiration time, in jiffies (if this is different from zero, the
	      timer is running and will expire at that time, otherwise the timer is not
	      running)
	
	  struct work_struct work;
	    - work structure used for queuing up requests (i.e. work items in pm_wq)
	
	  wait_queue_head_t wait_queue;
	    - wait queue used if any of the helper functions needs to wait for another
	      one to complete
	
	  spinlock_t lock;
	    - lock used for synchronisation
	
	  atomic_t usage_count;
	    - the usage counter of the device
	
	  atomic_t child_count;
	    - the count of 'active' children of the device
	
	  unsigned int ignore_children;
	    - if set, the value of child_count is ignored (but still updated)
	
	  unsigned int disable_depth;
	    - used for disabling the helper funcions (they work normally if this is
	      equal to zero); the initial value of it is 1 (i.e. run-time PM is
	      initially disabled for all devices)
	
	  unsigned int runtime_error;
	    - if set, there was a fatal error (one of the callbacks returned error code
	      as described in Section 2), so the helper funtions will not work until
	      this flag is cleared; this is the error code returned by the failing
	      callback
	
	  unsigned int idle_notification;
	    - if set, ->runtime_idle() is being executed
	
	  unsigned int request_pending;
	    - if set, there's a pending request (i.e. a work item queued up into pm_wq)
	
	  enum rpm_request request;
	    - type of request that's pending (valid if request_pending is set)
	
	  unsigned int deferred_resume;
	    - set if ->runtime_resume() is about to be run while ->runtime_suspend() is
	      being executed for that device and it is not practical to wait for the
	      suspend to complete; means "start a resume as soon as you've suspended"
	
	  unsigned int run_wake;
	    - set if the device is capable of generating run-time wake-up events
	
	  enum rpm_status runtime_status;
	    - the run-time PM status of the device; this field's initial value is
	      RPM_SUSPENDED, which means that each device is initially regarded by the
	      PM core as 'suspended', regardless of its real hardware status
	
	  unsigned int runtime_auto;
	    - if set, indicates that the user space has allowed the device driver to
	      power manage the device at run time via the /sys/devices/.../power/control
	      interface; it may only be modified with the help of the pm_runtime_allow()
	      and pm_runtime_forbid() helper functions
	
	All of the above fields are members of the 'power' member of 'struct device'.
	
	4. Run-time PM Device Helper Functions
	
	The following run-time PM helper functions are defined in
	drivers/base/power/runtime.c and include/linux/pm_runtime.h:
	
	  void pm_runtime_init(struct device *dev);
	    - initialize the device run-time PM fields in 'struct dev_pm_info'
	
	  void pm_runtime_remove(struct device *dev);
	    - make sure that the run-time PM of the device will be disabled after
	      removing the device from device hierarchy
	
	  int pm_runtime_idle(struct device *dev);
	    - execute the subsystem-level idle callback for the device; returns 0 on
	      success or error code on failure, where -EINPROGRESS means that
	      ->runtime_idle() is already being executed
	
	  int pm_runtime_suspend(struct device *dev);
	    - execute the subsystem-level suspend callback for the device; returns 0 on
	      success, 1 if the device's run-time PM status was already 'suspended', or
	      error code on failure, where -EAGAIN or -EBUSY means it is safe to attempt
	      to suspend the device again in future
	
	  int pm_runtime_resume(struct device *dev);
	    - execute the subsystem-level resume callback for the device; returns 0 on
	      success, 1 if the device's run-time PM status was already 'active' or
	      error code on failure, where -EAGAIN means it may be safe to attempt to
	      resume the device again in future, but 'power.runtime_error' should be
	      checked additionally
	
	  int pm_request_idle(struct device *dev);
	    - submit a request to execute the subsystem-level idle callback for the
	      device (the request is represented by a work item in pm_wq); returns 0 on
	      success or error code if the request has not been queued up
	
	  int pm_schedule_suspend(struct device *dev, unsigned int delay);
	    - schedule the execution of the subsystem-level suspend callback for the
	      device in future, where 'delay' is the time to wait before queuing up a
	      suspend work item in pm_wq, in milliseconds (if 'delay' is zero, the work
	      item is queued up immediately); returns 0 on success, 1 if the device's PM
	      run-time status was already 'suspended', or error code if the request
	      hasn't been scheduled (or queued up if 'delay' is 0); if the execution of
	      ->runtime_suspend() is already scheduled and not yet expired, the new
	      value of 'delay' will be used as the time to wait
	
	  int pm_request_resume(struct device *dev);
	    - submit a request to execute the subsystem-level resume callback for the
	      device (the request is represented by a work item in pm_wq); returns 0 on
	      success, 1 if the device's run-time PM status was already 'active', or
	      error code if the request hasn't been queued up
	
	  void pm_runtime_get_noresume(struct device *dev);
	    - increment the device's usage counter
	
	  int pm_runtime_get(struct device *dev);
	    - increment the device's usage counter, run pm_request_resume(dev) and
	      return its result
	
	  int pm_runtime_get_sync(struct device *dev);
	    - increment the device's usage counter, run pm_runtime_resume(dev) and
	      return its result
	
	  void pm_runtime_put_noidle(struct device *dev);
	    - decrement the device's usage counter
	
	  int pm_runtime_put(struct device *dev);
	    - decrement the device's usage counter, run pm_request_idle(dev) and return
	      its result
	
	  int pm_runtime_put_sync(struct device *dev);
	    - decrement the device's usage counter, run pm_runtime_idle(dev) and return
	      its result
	
	  void pm_runtime_enable(struct device *dev);
	    - enable the run-time PM helper functions to run the device bus type's
	      run-time PM callbacks described in Section 2
	
	  int pm_runtime_disable(struct device *dev);
	    - prevent the run-time PM helper functions from running subsystem-level
	      run-time PM callbacks for the device, make sure that all of the pending
	      run-time PM operations on the device are either completed or canceled;
	      returns 1 if there was a resume request pending and it was necessary to
	      execute the subsystem-level resume callback for the device to satisfy that
	      request, otherwise 0 is returned
	
	  void pm_suspend_ignore_children(struct device *dev, bool enable);
	    - set/unset the power.ignore_children flag of the device
	
	  int pm_runtime_set_active(struct device *dev);
	    - clear the device's 'power.runtime_error' flag, set the device's run-time
	      PM status to 'active' and update its parent's counter of 'active'
	      children as appropriate (it is only valid to use this function if
	      'power.runtime_error' is set or 'power.disable_depth' is greater than
	      zero); it will fail and return error code if the device has a parent
	      which is not active and the 'power.ignore_children' flag of which is unset
	
	  void pm_runtime_set_suspended(struct device *dev);
	    - clear the device's 'power.runtime_error' flag, set the device's run-time
	      PM status to 'suspended' and update its parent's counter of 'active'
	      children as appropriate (it is only valid to use this function if
	      'power.runtime_error' is set or 'power.disable_depth' is greater than
	      zero)
	
	  bool pm_runtime_suspended(struct device *dev);
	    - return true if the device's runtime PM status is 'suspended', or false
	      otherwise
	
	  void pm_runtime_allow(struct device *dev);
	    - set the power.runtime_auto flag for the device and decrease its usage
	      counter (used by the /sys/devices/.../power/control interface to
	      effectively allow the device to be power managed at run time)
	
	  void pm_runtime_forbid(struct device *dev);
	    - unset the power.runtime_auto flag for the device and increase its usage
	      counter (used by the /sys/devices/.../power/control interface to
	      effectively prevent the device from being power managed at run time)
	
	It is safe to execute the following helper functions from interrupt context:
	
	pm_request_idle()
	pm_schedule_suspend()
	pm_request_resume()
	pm_runtime_get_noresume()
	pm_runtime_get()
	pm_runtime_put_noidle()
	pm_runtime_put()
	pm_suspend_ignore_children()
	pm_runtime_set_active()
	pm_runtime_set_suspended()
	pm_runtime_enable()
	
	5. Run-time PM Initialization, Device Probing and Removal
	
	Initially, the run-time PM is disabled for all devices, which means that the
	majority of the run-time PM helper funtions described in Section 4 will return
	-EAGAIN until pm_runtime_enable() is called for the device.
	
	In addition to that, the initial run-time PM status of all devices is
	'suspended', but it need not reflect the actual physical state of the device.
	Thus, if the device is initially active (i.e. it is able to process I/O), its
	run-time PM status must be changed to 'active', with the help of
	pm_runtime_set_active(), before pm_runtime_enable() is called for the device.
	
	However, if the device has a parent and the parent's run-time PM is enabled,
	calling pm_runtime_set_active() for the device will affect the parent, unless
	the parent's 'power.ignore_children' flag is set.  Namely, in that case the
	parent won't be able to suspend at run time, using the PM core's helper
	functions, as long as the child's status is 'active', even if the child's
	run-time PM is still disabled (i.e. pm_runtime_enable() hasn't been called for
	the child yet or pm_runtime_disable() has been called for it).  For this reason,
	once pm_runtime_set_active() has been called for the device, pm_runtime_enable()
	should be called for it too as soon as reasonably possible or its run-time PM
	status should be changed back to 'suspended' with the help of
	pm_runtime_set_suspended().
	
	If the default initial run-time PM status of the device (i.e. 'suspended')
	reflects the actual state of the device, its bus type's or its driver's
	->probe() callback will likely need to wake it up using one of the PM core's
	helper functions described in Section 4.  In that case, pm_runtime_resume()
	should be used.  Of course, for this purpose the device's run-time PM has to be
	enabled earlier by calling pm_runtime_enable().
	
	If the device bus type's or driver's ->probe() or ->remove() callback runs
	pm_runtime_suspend() or pm_runtime_idle() or their asynchronous counterparts,
	they will fail returning -EAGAIN, because the device's usage counter is
	incremented by the core before executing ->probe() and ->remove().  Still, it
	may be desirable to suspend the device as soon as ->probe() or ->remove() has
	finished, so the PM core uses pm_runtime_idle_sync() to invoke the
	subsystem-level idle callback for the device at that time.
	
	The user space can effectively disallow the driver of the device to power manage
	it at run time by changing the value of its /sys/devices/.../power/control
	attribute to "on", which causes pm_runtime_forbid() to be called.  In principle,
	this mechanism may also be used by the driver to effectively turn off the
	run-time power management of the device until the user space turns it on.
	Namely, during the initialization the driver can make sure that the run-time PM
	status of the device is 'active' and call pm_runtime_forbid().  It should be
	noted, however, that if the user space has already intentionally changed the
	value of /sys/devices/.../power/control to "auto" to allow the driver to power
	manage the device at run time, the driver may confuse it by using
	pm_runtime_forbid() this way.
	
	6. Run-time PM and System Sleep
	
	Run-time PM and system sleep (i.e., system suspend and hibernation, also known
	as suspend-to-RAM and suspend-to-disk) interact with each other in a couple of
	ways.  If a device is active when a system sleep starts, everything is
	straightforward.  But what should happen if the device is already suspended?
	
	The device may have different wake-up settings for run-time PM and system sleep.
	For example, remote wake-up may be enabled for run-time suspend but disallowed
	for system sleep (device_may_wakeup(dev) returns 'false').  When this happens,
	the subsystem-level system suspend callback is responsible for changing the
	device's wake-up setting (it may leave that to the device driver's system
	suspend routine).  It may be necessary to resume the device and suspend it again
	in order to do so.  The same is true if the driver uses different power levels
	or other settings for run-time suspend and system sleep.
	
	During system resume, devices generally should be brought back to full power,
	even if they were suspended before the system sleep began.  There are several
	reasons for this, including:
	
	  * The device might need to switch power levels, wake-up settings, etc.
	
	  * Remote wake-up events might have been lost by the firmware.
	
	  * The device's children may need the device to be at full power in order
	    to resume themselves.
	
	  * The driver's idea of the device state may not agree with the device's
	    physical state.  This can happen during resume from hibernation.
	
	  * The device might need to be reset.
	
	  * Even though the device was suspended, if its usage counter was > 0 then most
	    likely it would need a run-time resume in the near future anyway.
	
	  * Always going back to full power is simplest.
	
	If the device was suspended before the sleep began, then its run-time PM status
	will have to be updated to reflect the actual post-system sleep status.  The way
	to do this is:
	
		pm_runtime_disable(dev);
		pm_runtime_set_active(dev);
		pm_runtime_enable(dev);
	
	The PM core always increments the run-time usage counter before calling the
	->prepare() callback and decrements it after calling the ->complete() callback.
	Hence disabling run-time PM temporarily like this will not cause any run-time
	suspend callbacks to be lost.
	
	7. Generic subsystem callbacks
	
	Subsystems may wish to conserve code space by using the set of generic power
	management callbacks provided by the PM core, defined in
	driver/base/power/generic_ops.c:
	
	  int pm_generic_runtime_idle(struct device *dev);
	    - invoke the ->runtime_idle() callback provided by the driver of this
	      device, if defined, and call pm_runtime_suspend() for this device if the
	      return value is 0 or the callback is not defined
	
	  int pm_generic_runtime_suspend(struct device *dev);
	    - invoke the ->runtime_suspend() callback provided by the driver of this
	      device and return its result, or return -EINVAL if not defined
	
	  int pm_generic_runtime_resume(struct device *dev);
	    - invoke the ->runtime_resume() callback provided by the driver of this
	      device and return its result, or return -EINVAL if not defined
	
	  int pm_generic_suspend(struct device *dev);
	    - if the device has not been suspended at run time, invoke the ->suspend()
	      callback provided by its driver and return its result, or return 0 if not
	      defined
	
	  int pm_generic_resume(struct device *dev);
	    - invoke the ->resume() callback provided by the driver of this device and,
	      if successful, change the device's runtime PM status to 'active'
	
	  int pm_generic_freeze(struct device *dev);
	    - if the device has not been suspended at run time, invoke the ->freeze()
	      callback provided by its driver and return its result, or return 0 if not
	      defined
	
	  int pm_generic_thaw(struct device *dev);
	    - if the device has not been suspended at run time, invoke the ->thaw()
	      callback provided by its driver and return its result, or return 0 if not
	      defined
	
	  int pm_generic_poweroff(struct device *dev);
	    - if the device has not been suspended at run time, invoke the ->poweroff()
	      callback provided by its driver and return its result, or return 0 if not
	      defined
	
	  int pm_generic_restore(struct device *dev);
	    - invoke the ->restore() callback provided by the driver of this device and,
	      if successful, change the device's runtime PM status to 'active'
	
	These functions can be assigned to the ->runtime_idle(), ->runtime_suspend(),
	->runtime_resume(), ->suspend(), ->resume(), ->freeze(), ->thaw(), ->poweroff(),
	or ->restore() callback pointers in the subsystem-level dev_pm_ops structures.
	
	If a subsystem wishes to use all of them at the same time, it can simply assign
	the GENERIC_SUBSYS_PM_OPS macro, defined in include/linux/pm.h, to its
	dev_pm_ops structure pointer.
	
	Device drivers that wish to use the same function as a system suspend, freeze,
	poweroff and run-time suspend callback, and similarly for system resume, thaw,
	restore, and run-time resume, can achieve this with the help of the
	UNIVERSAL_DEV_PM_OPS macro defined in include/linux/pm.h (possibly setting its
	last argument to NULL).

• [ power ]
• 00-INDEX
• apm-acpi.txt
• basic-pm-debugging.txt
• devices.txt
• drivers-testing.txt
• freezing-of-tasks.txt
• interface.txt
• notifiers.txt
• pci.txt
• pm_qos_interface.txt
• power_supply_class.txt
• [ regulator ]
• runtime_pm.txt
• s2ram.txt
• states.txt
• swsusp-and-swap-files.txt
• swsusp-dmcrypt.txt
• swsusp.txt
• tricks.txt
• userland-swsusp.txt
• video.txt
• video_extension.txt
•