path: root/Documentation/power/runtime_pm.rst

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Runtime Power Management Framework for I/O Devices
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(C) 2009-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.

(C) 2010 Alan Stern <stern@rowland.harvard.edu>

(C) 2014 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>

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_can_wakeup() returns 'false' for the
device, then ->runtime_suspend() should return -EBUSY.  On the other hand, if
device_can_wakeup() 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 synchronization

  `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 functions (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)

  `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 functions 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 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

  `int pm_runtime_get_if_in_use(struct device *dev);`
    - return -EINVAL if 'power.disable_depth' is nonzero; otherwise, if the
      runtime PM status is RPM_ACTIVE and the runtime PM usage counter is
      nonzero, increment the counter and return 1; otherwise return 0 without
      changing the counter

  `int pm_runtime_get_if_active(struct device *dev, bool ign_usage_count);`
    - return -EINVAL if 'power.disable_depth' is nonzero; otherwise, if the
      runtime PM status is RPM_ACTIVE, and either ign_usage_count is true
      or the device's usage_count is non-zero, increment the counter and
      return 1; otherwise return 0 without changing the counter

  `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