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+Run-time Power Management Framework for I/O Devices
+
+(C) 2009 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
+
+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);
+	void (*runtime_idle)(struct device *dev);
+	...
+};
+
+The ->runtime_suspend() callback is executed by the PM core for the bus type of
+the device being suspended.  The bus type's callback is then _entirely_
+_responsible_ for handling 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 bus type's ->runtime_suspend() knows
+what to do to handle the device).
+
+  * Once the bus type's ->runtime_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 its bus type's
+    ->runtime_resume() callback is executed for it.  The run-time PM status of
+    a device after successful execution of its bus type's ->runtime_suspend()
+    callback is 'suspended'.
+
+  * If the bus type's ->runtime_suspend() callback returns -EBUSY or -EAGAIN,
+    the device's run-time PM status is supposed to be 'active', which means that
+    the device _must_ be fully operational afterwards.
+
+  * If the bus type's ->runtime_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 wakeup capability for proper
+functioning and device_may_wakeup() returns 'false' for the device, then
+->runtime_suspend() should return -EBUSY.  On the other hand, if
+device_may_wakeup() returns 'true' for the device and the device is put
+into a low power state during the execution of its bus type's
+->runtime_suspend(), it is expected that remote wake-up (i.e. hardware mechanism
+allowing the device to request a change of its power state, such as PCI PME)
+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 ->runtime_resume() callback is executed by the PM core for the bus type of
+the device being woken up.  The bus type's callback is then _entirely_
+_responsible_ for handling 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 bus type's ->runtime_resume() knows
+what to do to handle the device).
+
+  * Once the bus type's ->runtime_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 bus type's ->runtime_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 ->runtime_idle() callback is executed by the PM core for the bus type of
+given device 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
+    device bus type's ->runtime_idle() callback (with the device as an
+    argument).
+
+The action performed by a bus type's ->runtime_idle() callback is totally
+dependent on the bus type 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 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"
+
+  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
+
+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 ->runtime_idle() for the device's bus type; 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 ->runtime_suspend() for the device's bus type; 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 ->runtime_resume() for the device's bus type; 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 ->runtime_idle() for the device's bus type
+      (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 ->runtime_suspend() for the device's bus type
+      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 ->runtime_resume() for the device's bus type
+      (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 the device bus
+      type's run-time PM callbacks, 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
+      ->runtime_resume() for the device's bus type 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)
+
+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 device bus
+type's ->runtime_idle() callback at that time.