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 the device's subsystem that may be either of
  the following:
  
    1. PM domain of the device, if the device's PM domain object, dev->pm_domain,
       is present.
  
    2. Device type of the device, if both dev->type and dev->type->pm are present.
  
    3. Device class of the device, if both dev->class and dev->class->pm are
       present.
  
    4. Bus type of the device, if both dev->bus and dev->bus->pm are present.
  
  If the subsystem chosen by applying the above rules doesn't provide the relevant
  callback, the PM core will invoke the corresponding driver callback stored in
  dev->driver->pm directly (if present).
  
  The PM core always checks which callback to use in the order given above, so the
  priority order of callbacks from high to low is: PM domain, device type, class
  and bus type.  Moreover, the high-priority one will always take precedence over
  a low-priority one.  The PM domain, bus type, device type and class callbacks
  are referred to as subsystem-level callbacks in what follows.
  
  By default, the callbacks are always invoked in process context with interrupts
  enabled.  However, the pm_runtime_irq_safe() helper function can be used to tell
  the PM core that it is safe to run the ->runtime_suspend(), ->runtime_resume()
  and ->runtime_idle() callbacks for the given device in atomic context with
  interrupts disabled.  This implies that the callback routines in question must
  not block or sleep, but it also means that the synchronous helper functions
  listed at the end of Section 4 may be used for that device within an interrupt
  handler or generally in an atomic context.
  
  The subsystem-level suspend callback, if present, 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 (or the driver suspend callback,
      if invoked directly) has completed successfully for the given device, the PM
      core regards the device as suspended, which need not mean that it 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 appropriate resume callback is executed for it.  The runtime
      PM status of a device after successful execution of the suspend callback is
      'suspended'.
  
    * If the suspend callback returns -EBUSY or -EAGAIN, the device's runtime PM
      status remains 'active', which means that the device _must_ be fully
      operational afterwards.
  
    * If the suspend callback returns an error code different from -EBUSY and
      -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 its status
      is directly set to  either'active', or 'suspended' (the PM core provides
      special helper functions for this purpose).
  
  In particular, if the driver requires remote wakeup 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 suspend callback, it is expected
  that remote wakeup will be enabled for the device.  Generally, remote wakeup
  should be enabled for all input devices put into low-power states at run time.
  
  The subsystem-level resume callback, if present, 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 (or the driver resume callback, if
      invoked directly) 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 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 to either 'active', or
      'suspended' (by means of special helper functions provided by the PM core
      for this purpose).
  
  The idle callback (a subsystem-level one, if present, or the driver one) 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
      idle callback with the device as its argument.
  
  The action performed by the idle callback is totally dependent on the subsystem
  (or driver) 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.  If there is no idle callback, or if the callback returns
  0, then the PM core will attempt to carry out a runtime suspend of the device,
  also respecting devices configured for autosuspend.  In essence this means a
  call to pm_runtime_autosuspend() (do note that drivers needs to update the
  device last busy mark, pm_runtime_mark_last_busy(), to control the delay under
  this circumstance).  To prevent this (for example, if the callback routine has
  started a delayed suspend), the routine must return a non-zero value.  Negative
  error return codes are 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 runtime PM callbacks for
  one device:
  
  (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 an
        error code on failure, where -EINPROGRESS means that ->runtime_idle() is
        already being executed; if there is no callback or the callback returns 0
        then run pm_runtime_autosuspend(dev) and return its result
  
    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_active(struct device *dev);
      - return true if the device's runtime PM status is 'active' or its
        'power.disable_depth' field is not equal to zero, or false otherwise
  
    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().
  
  It may be desirable to suspend the device once ->probe() has finished.
  Therefore the driver core uses the asyncronous pm_request_idle() to submit a
  request to execute the subsystem-level idle callback for the device at that
  time.  A driver that makes use of the runtime autosuspend feature, may want to
  update the last busy mark before returning from ->probe().
  
  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() with 'false' as the second argument for every device
      right before executing the subsystem-level .suspend_late() callback for it.
  
    * During system resume it calls pm_runtime_enable() and pm_runtime_put()
      for every device right after executing the subsystem-level .resume_early()
      callback 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_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.
  
  Under some circumstances a driver or subsystem may want to prevent a device
  from autosuspending immediately, even though the usage counter is zero and the
  autosuspend delay time has expired.  If the ->runtime_suspend() callback
  returns -EAGAIN or -EBUSY, and if the next autosuspend delay expiration time is
  in the future (as it normally would be if the callback invoked
  pm_runtime_mark_last_busy()), the PM core will automatically reschedule the
  autosuspend.  The ->runtime_suspend() callback can't do this rescheduling
  itself because no suspend requests of any kind are accepted while the device is
  suspending (i.e., while the callback is running).
  
  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.