Runtime Power Management Framework for I/O Devices

(C) 2009-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
(C) 2010 Alan Stern <stern@rowland.harvard.edu>

1. Introduction

Support for runtime power management (runtime 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 runtime 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 runtime 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 runtime PM operations with one another.

* Three device runtime 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 runtime 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 runtime PM callbacks present in 'struct dev_pm_ops', the device runtime PM
fields of 'struct dev_pm_info' and the core helper functions provided for
runtime PM are described below.

2. Device Runtime PM Callbacks

There are three device runtime 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 power domain, or the device type
(if the device power domain's struct dev_pm_ops does not exist), or the class
(if the device power domain's and type's struct dev_pm_ops object does not
exist), or the bus type (if the device power domain's, type's and class'
struct dev_pm_ops objects do not exist) of the given device, so the priority
order of callbacks from high to low is that power domain callbacks, device
type callbacks, class callbacks and bus type callbacks, and the high priority
one will take precedence over low priority one. The bus type, device type and
class callbacks are referred to as subsystem-level callbacks in what follows,
and generally speaking, the power domain callbacks are used for representing
power domains within a SoC.

By default, the callbacks are always invoked in process context with interrupts
enabled.  However, subsystems can use the pm_runtime_irq_safe() helper function
to tell the PM core that a device's ->runtime_suspend() and ->runtime_resume()
callbacks should be invoked in atomic context with interrupts disabled.
This implies that these callback routines must not block or sleep, but it also
means that the synchronous helper functions listed at the end of Section 4 can
be used within an interrupt handler or in an atomic context.

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 runtime 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 runtime 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 runtime 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 runtime
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 runtime 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 runtime
    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,
    except for scheduled autosuspends.

3. Runtime PM Device Fields

The following device runtime 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 and autosuspend requests

  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. runtime 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 runtime wake-up events

  enum rpm_status runtime_status;
    - the runtime 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

  unsigned int no_callbacks;
    - indicates that the device does not use the runtime PM callbacks (see
      Section 8); it may be modified only by the pm_runtime_no_callbacks()
      helper function

  unsigned int irq_safe;
    - indicates that the ->runtime_suspend() and ->runtime_resume() callbacks
      will be invoked with the spinlock held and interrupts disabled

  unsigned int use_autosuspend;
    - indicates that the device's driver supports delayed autosuspend (see
      Section 9); it may be modified only by the
      pm_runtime{_dont}_use_autosuspend() helper functions

  unsigned int timer_autosuspends;
    - indicates that the PM core should attempt to carry out an autosuspend
      when the timer expires rather than a normal suspend

  int autosuspend_delay;
    - the delay time (in milliseconds) to be used for autosuspend

  unsigned long last_busy;
    - the time (in jiffies) when the pm_runtime_mark_last_busy() helper
      function was last called for this device; used in calculating inactivity
      periods for autosuspend

All of the above fields are members of the 'power' member of 'struct device'.

4. Runtime PM Device Helper Functions

The following runtime 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 runtime PM fields in 'struct dev_pm_info'

  void pm_runtime_remove(struct device *dev);
    - make sure that the runtime 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 runtime 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 and -EACCES means that
      'power.disable_depth' is different from 0

  int pm_runtime_autosuspend(struct device *dev);
    - same as pm_runtime_suspend() except that the autosuspend delay is taken
      into account; if pm_runtime_autosuspend_expiration() says the delay has
      not yet expired then an autosuspend is scheduled for the appropriate time
      and 0 is returned

  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 runtime 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, and -EACCES means that 'power.disable_depth' is
      different from 0

  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_request_autosuspend(struct device *dev);
    - schedule the execution of the subsystem-level suspend callback for the
      device when the autosuspend delay has expired; if the delay has already
      expired then the work item is queued up immediately

  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
      runtime 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 runtime 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; if the result is 0 then run
      pm_request_idle(dev) and return its result

  int pm_runtime_put_autosuspend(struct device *dev);
    - decrement the device's usage counter; if the result is 0 then run
      pm_request_autosuspend(dev) and return its result

  int pm_runtime_put_sync(struct device *dev);
    - decrement the device's usage counter; if the result is 0 then run
      pm_runtime_idle(dev) and return its result

  int pm_runtime_put_sync_suspend(struct device *dev);
    - decrement the device's usage counter; if the result is 0 then run
      pm_runtime_suspend(dev) and return its result

  int pm_runtime_put_sync_autosuspend(struct device *dev);
    - decrement the device's usage counter; if the result is 0 then run
      pm_runtime_autosuspend(dev) and return its result

  void pm_runtime_enable(struct device *dev);
    - decrement the device's 'power.disable_depth' field; if that field is equal
      to zero, the runtime PM helper functions can execute subsystem-level
      callbacks described in Section 2 for the device

  int pm_runtime_disable(struct device *dev);
    - increment the device's 'power.disable_depth' field (if the value of that
      field was previously zero, this prevents subsystem-level runtime PM
      callbacks from being run for the device), make sure that all of the pending
      runtime 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

  int pm_runtime_barrier(struct device *dev);
    - check if there's a resume request pending for the device and resume it
      (synchronously) in that case, cancel any other pending runtime PM requests
      regarding it and wait for all runtime PM operations on it in progress to
      complete; 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 runtime
      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 runtime
      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' and its
      'power.disable_depth' field is equal to zero, or false otherwise

  bool pm_runtime_status_suspended(struct device *dev);
    - return true if the device's runtime PM status is 'suspended'

  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)

  void pm_runtime_no_callbacks(struct device *dev);
    - set the power.no_callbacks flag for the device and remove the runtime
      PM attributes from /sys/devices/.../power (or prevent them from being
      added when the device is registered)

  void pm_runtime_irq_safe(struct device *dev);
    - set the power.irq_safe flag for the device, causing the runtime-PM
      callbacks to be invoked with interrupts off

  void pm_runtime_mark_last_busy(struct device *dev);
    - set the power.last_busy field to the current time

  void pm_runtime_use_autosuspend(struct device *dev);
    - set the power.use_autosuspend flag, enabling autosuspend delays

  void pm_runtime_dont_use_autosuspend(struct device *dev);
    - clear the power.use_autosuspend flag, disabling autosuspend delays

  void pm_runtime_set_autosuspend_delay(struct device *dev, int delay);
    - set the power.autosuspend_delay value to 'delay' (expressed in
      milliseconds); if 'delay' is negative then runtime suspends are
      prevented

  unsigned long pm_runtime_autosuspend_expiration(struct device *dev);
    - calculate the time when the current autosuspend delay period will expire,
      based on power.last_busy and power.autosuspend_delay; if the delay time
      is 1000 ms or larger then the expiration time is rounded up to the
      nearest second; returns 0 if the delay period has already expired or
      power.use_autosuspend isn't set, otherwise returns the expiration time
      in jiffies

It is safe to execute the following helper functions from interrupt context:

pm_request_idle()
pm_request_autosuspend()
pm_schedule_suspend()
pm_request_resume()
pm_runtime_get_noresume()
pm_runtime_get()
pm_runtime_put_noidle()
pm_runtime_put()
pm_runtime_put_autosuspend()
pm_runtime_enable()
pm_suspend_ignore_children()
pm_runtime_set_active()
pm_runtime_set_suspended()
pm_runtime_suspended()
pm_runtime_mark_last_busy()
pm_runtime_autosuspend_expiration()

If pm_runtime_irq_safe() has been called for a device then the following helper
functions may also be used in interrupt context:

pm_runtime_idle()
pm_runtime_suspend()
pm_runtime_autosuspend()
pm_runtime_resume()
pm_runtime_get_sync()
pm_runtime_put_sync()
pm_runtime_put_sync_suspend()
pm_runtime_put_sync_autosuspend()

5. Runtime PM Initialization, Device Probing and Removal

Initially, the runtime PM is disabled for all devices, which means that the
majority of the runtime 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 runtime 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
runtime 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 runtime 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
runtime 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 runtime PM
status should be changed back to 'suspended' with the help of
pm_runtime_set_suspended().

If the default initial runtime 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 runtime PM has to be
enabled earlier by calling pm_runtime_enable().

If the device bus type's or driver's ->probe() 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 driver core before executing ->probe().  Still, it may be
desirable to suspend the device as soon as ->probe() has finished, so the driver
core uses pm_runtime_put_sync() to invoke the subsystem-level idle callback for
the device at that time.

Moreover, the driver core prevents runtime PM callbacks from racing with the bus
notifier callback in __device_release_driver(), which is necessary, because the
notifier is used by some subsystems to carry out operations affecting the
runtime PM functionality.  It does so by calling pm_runtime_get_sync() before
driver_sysfs_remove() and the BUS_NOTIFY_UNBIND_DRIVER notifications.  This
resumes the device if it's in the suspended state and prevents it from
being suspended again while those routines are being executed.

To allow bus types and drivers to put devices into the suspended state by
calling pm_runtime_suspend() from their ->remove() routines, the driver core
executes pm_runtime_put_sync() after running the BUS_NOTIFY_UNBIND_DRIVER
notifications in __device_release_driver().  This requires bus types and
drivers to make their ->remove() callbacks avoid races with runtime PM directly,
but also it allows of more flexibility in the handling of devices during the
removal of their drivers.

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
runtime power management of the device until the user space turns it on.
Namely, during the initialization the driver can make sure that the runtime 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. Runtime PM and System Sleep

Runtime 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 runtime PM and system sleep.
For example, remote wake-up may be enabled for runtime 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 runtime suspend and system sleep.

During system resume, the simplest approach is to bring all devices back to full
power, even if they had been suspended before the system suspend 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 runtime resume in the near future anyway.

If the device had been suspended before the system suspend began and it's
brought back to full power during resume, then its runtime 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 runtime usage counter before calling the
->suspend() callback and decrements it after calling the ->resume() callback.
Hence disabling runtime PM temporarily like this will not cause any runtime
suspend attempts to be permanently lost.  If the usage count goes to zero
following the return of the ->resume() callback, the ->runtime_idle() callback
will be invoked as usual.

On some systems, however, system sleep is not entered through a global firmware
or hardware operation.  Instead, all hardware components are put into low-power
states directly by the kernel in a coordinated way.  Then, the system sleep
state effectively follows from the states the hardware components end up in
and the system is woken up from that state by a hardware interrupt or a similar
mechanism entirely under the kernel's control.  As a result, the kernel never
gives control away and the states of all devices during resume are precisely
known to it.  If that is the case and none of the situations listed above takes
place (in particular, if the system is not waking up from hibernation), it may
be more efficient to leave the devices that had been suspended before the system
suspend began in the suspended state.

The PM core does its best to reduce the probability of race conditions between
the runtime PM and system suspend/resume (and hibernation) callbacks by carrying
out the following operations:

  * During system suspend it calls pm_runtime_get_noresume() and
    pm_runtime_barrier() for every device right before executing the
    subsystem-level .suspend() callback for it.  In addition to that it calls
    pm_runtime_disable() for every device right after executing the
    subsystem-level .suspend() callback for it.

  * During system resume it calls pm_runtime_enable() and pm_runtime_put_sync()
    for every device right before and right after executing the subsystem-level
    .resume() callback for it, respectively.

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_suspend_noirq(struct device *dev);
    - if pm_runtime_suspended(dev) returns "false", invoke the ->suspend_noirq()
      callback provided by the device's 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_resume_noirq(struct device *dev);
    - invoke the ->resume_noirq() callback provided by the driver of this device

  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_freeze_noirq(struct device *dev);
    - if pm_runtime_suspended(dev) returns "false", invoke the ->freeze_noirq()
      callback provided by the device's 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_thaw_noirq(struct device *dev);
    - if pm_runtime_suspended(dev) returns "false", invoke the ->thaw_noirq()
      callback provided by the device's 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_poweroff_noirq(struct device *dev);
    - if pm_runtime_suspended(dev) returns "false", run the ->poweroff_noirq()
      callback provided by the device's 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'

  int pm_generic_restore_noirq(struct device *dev);
    - invoke the ->restore_noirq() callback provided by the device's driver

These functions can be assigned to the ->runtime_idle(), ->runtime_suspend(),
->runtime_resume(), ->suspend(), ->suspend_noirq(), ->resume(),
->resume_noirq(), ->freeze(), ->freeze_noirq(), ->thaw(), ->thaw_noirq(),
->poweroff(), ->poweroff_noirq(), ->restore(), ->restore_noirq() 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 runtime suspend callback, and similarly for system resume, thaw,
restore, and runtime 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).

8. "No-Callback" Devices

Some "devices" are only logical sub-devices of their parent and cannot be
power-managed on their own.  (The prototype example is a USB interface.  Entire
USB devices can go into low-power mode or send wake-up requests, but neither is
possible for individual interfaces.)  The drivers for these devices have no
need of runtime PM callbacks; if the callbacks did exist, ->runtime_suspend()
and ->runtime_resume() would always return 0 without doing anything else and
->runtime_idle() would always call pm_runtime_suspend().

Subsystems can tell the PM core about these devices by calling
pm_runtime_no_callbacks().  This should be done after the device structure is
initialized and before it is registered (although after device registration is
also okay).  The routine will set the device's power.no_callbacks flag and
prevent the non-debugging runtime PM sysfs attributes from being created.

When power.no_callbacks is set, the PM core will not invoke the
->runtime_idle(), ->runtime_suspend(), or ->runtime_resume() callbacks.
Instead it will assume that suspends and resumes always succeed and that idle
devices should be suspended.

As a consequence, the PM core will never directly inform the device's subsystem
or driver about runtime power changes.  Instead, the driver for the device's
parent must take responsibility for telling the device's driver when the
parent's power state changes.

9. Autosuspend, or automatically-delayed suspends

Changing a device's power state isn't free; it requires both time and energy.
A device should be put in a low-power state only when there's some reason to
think it will remain in that state for a substantial time.  A common heuristic
says that a device which hasn't been used for a while is liable to remain
unused; following this advice, drivers should not allow devices to be suspended
at runtime until they have been inactive for some minimum period.  Even when
the heuristic ends up being non-optimal, it will still prevent devices from
"bouncing" too rapidly between low-power and full-power states.

The term "autosuspend" is an historical remnant.  It doesn't mean that the
device is automatically suspended (the subsystem or driver still has to call
the appropriate PM routines); rather it means that runtime suspends will
automatically be delayed until the desired period of inactivity has elapsed.

Inactivity is determined based on the power.last_busy field.  Drivers should
call pm_runtime_mark_last_busy() to update this field after carrying out I/O,
typically just before calling pm_runtime_put_autosuspend().  The desired length
of the inactivity period is a matter of policy.  Subsystems can set this length
initially by calling pm_runtime_set_autosuspend_delay(), but after device
registration the length should be controlled by user space, using the
/sys/devices/.../power/autosuspend_delay_ms attribute.

In order to use autosuspend, subsystems or drivers must call
pm_runtime_use_autosuspend() (preferably before registering the device), and
thereafter they should use the various *_autosuspend() helper functions instead
of the non-autosuspend counterparts:

	Instead of: pm_runtime_suspend    use: pm_runtime_autosuspend;
	Instead of: pm_schedule_suspend   use: pm_request_autosuspend;
	Instead of: pm_runtime_put        use: pm_runtime_put_autosuspend;
	Instead of: pm_runtime_put_sync   use: pm_runtime_put_sync_autosuspend.

Drivers may also continue to use the non-autosuspend helper functions; they
will behave normally, not taking the autosuspend delay into account.
Similarly, if the power.use_autosuspend field isn't set then the autosuspend
helper functions will behave just like the non-autosuspend counterparts.

The implementation is well suited for asynchronous use in interrupt contexts.
However such use inevitably involves races, because the PM core can't
synchronize ->runtime_suspend() callbacks with the arrival of I/O requests.
This synchronization must be handled by the driver, using its private lock.
Here is a schematic pseudo-code example:

	foo_read_or_write(struct foo_priv *foo, void *data)
	{
		lock(&foo->private_lock);
		add_request_to_io_queue(foo, data);
		if (foo->num_pending_requests++ == 0)
			pm_runtime_get(&foo->dev);
		if (!foo->is_suspended)
			foo_process_next_request(foo);
		unlock(&foo->private_lock);
	}

	foo_io_completion(struct foo_priv *foo, void *req)
	{
		lock(&foo->private_lock);
		if (--foo->num_pending_requests == 0) {
			pm_runtime_mark_last_busy(&foo->dev);
			pm_runtime_put_autosuspend(&foo->dev);
		} else {
			foo_process_next_request(foo);
		}
		unlock(&foo->private_lock);
		/* Send req result back to the user ... */
	}

	int foo_runtime_suspend(struct device *dev)
	{
		struct foo_priv foo = container_of(dev, ...);
		int ret = 0;

		lock(&foo->private_lock);
		if (foo->num_pending_requests > 0) {
			ret = -EBUSY;
		} else {
			/* ... suspend the device ... */
			foo->is_suspended = 1;
		}
		unlock(&foo->private_lock);
		return ret;
	}

	int foo_runtime_resume(struct device *dev)
	{
		struct foo_priv foo = container_of(dev, ...);

		lock(&foo->private_lock);
		/* ... resume the device ... */
		foo->is_suspended = 0;
		pm_runtime_mark_last_busy(&foo->dev);
		if (foo->num_pending_requests > 0)
			foo_process_requests(foo);
		unlock(&foo->private_lock);
		return 0;
	}

The important point is that after foo_io_completion() asks for an autosuspend,
the foo_runtime_suspend() callback may race with foo_read_or_write().
Therefore foo_runtime_suspend() has to check whether there are any pending I/O
requests (while holding the private lock) before allowing the suspend to
proceed.

In addition, the power.autosuspend_delay field can be changed by user space at
any time.  If a driver cares about this, it can call
pm_runtime_autosuspend_expiration() from within the ->runtime_suspend()
callback while holding its private lock.  If the function returns a nonzero
value then the delay has not yet expired and the callback should return
-EAGAIN.