Documentation/power/runtime_pm.rst

14 Support for runtime power management (runtime PM) of I/O devices is provided
15 at the power management core (PM core) level by means of:
18   put their PM-related work items.  It is strongly recommended that pm_wq be
19   used for queuing all work items related to runtime PM, because this allows
24 * A number of runtime PM fields in the 'power' member of 'struct device' (which
25   is of the type 'struct dev_pm_info', defined in include/linux/pm.h) that can
26   be used for synchronizing runtime PM operations with one another.
28 * Three device runtime PM callbacks in 'struct dev_pm_ops' (defined in
29   include/linux/pm.h).
32   used for carrying out runtime PM operations in such a way that the
33   synchronization between them is taken care of by the PM core.  Bus types and
36 The runtime PM callbacks present in 'struct dev_pm_ops', the device runtime PM
38 runtime PM are described below.
40 2. Device Runtime PM Callbacks
43 There are three device runtime PM callbacks defined in 'struct dev_pm_ops'::
54 are executed by the PM core for the device's subsystem that may be either of
57   1. PM domain of the device, if the device's PM domain object, dev->pm_domain,
60   2. Device type of the device, if both dev->type and dev->type->pm are present.
62   3. Device class of the device, if both dev->class and dev->class->pm are
65   4. Bus type of the device, if both dev->bus and dev->bus->pm are present.
68 callback, the PM core will invoke the corresponding driver callback stored in
69 dev->driver->pm directly (if present).
71 The PM core always checks which callback to use in the order given above, so the
72 priority order of callbacks from high to low is: PM domain, device type, class
74 a low-priority one.  The PM domain, bus type, device type and class callbacks
79 the PM core that it is safe to run the ->runtime_suspend(), ->runtime_resume()
89 PM core's point of view it is not necessary to implement a ->runtime_suspend()
94     if invoked directly) has completed successfully for the given device, the PM
99     PM status of a device after successful execution of the suspend callback is
102   * If the suspend callback returns -EBUSY or -EAGAIN, the device's runtime PM
107     -EAGAIN, the PM core regards this as a fatal error and will refuse to run
109     is directly set to  either 'active', or 'suspended' (the PM core provides
124 PM core's point of view it is not necessary to implement a ->runtime_resume()
129     invoked directly) has completed successfully, the PM core regards the device
131     I/O operations as needed.  The runtime PM status of the device is then
134   * If the resume callback returns an error code, the PM core regards this as a
137     'suspended' (by means of special helper functions provided by the PM core
141 executed by the PM core whenever the device appears to be idle, which is
142 indicated to the PM core by two counters, the device's usage counter and the
146     the PM core and it turns out to be equal to zero, the other counter is
147     checked.  If that counter also is equal to zero, the PM core executes the
155 0, then the PM core will attempt to carry out a runtime suspend of the device,
161 error return codes are ignored by the PM core.
163 The helper functions provided by the PM core, described in Section 4, guarantee
164 that the following constraints are met with respect to runtime PM callbacks for
175     devices (i.e. the PM core will only execute ->runtime_idle() or
176     ->runtime_suspend() for the devices the runtime PM status of which is
185     PM core will only execute ->runtime_resume() for the devices the runtime
186     PM status of which is 'suspended').
188 Additionally, the helper functions provided by the PM core obey the following
205 3. Runtime PM Device Fields
208 The following device runtime PM fields are present in 'struct dev_pm_info', as
209 defined in include/linux/pm.h:
240       equal to zero); the initial value of it is 1 (i.e. runtime PM is
264     - the runtime PM status of the device; this field's initial value is
266       PM core as 'suspended', regardless of its real hardware status
269     - the last runtime PM status of the device captured before disabling runtime
270       PM for it (invalid initially and when disable_depth is 0)
279     - indicates that the device does not use the runtime PM callbacks (see
293     - indicates that the PM core should attempt to carry out an autosuspend
306 4. Runtime PM Device Helper Functions
309 The following runtime PM helper functions are defined in
313     - initialize the device runtime PM fields in 'struct dev_pm_info'
316     - make sure that the runtime PM of the device will be disabled after
327       success, 1 if the device's runtime PM status was already 'suspended', or
340       success, 1 if the device's runtime PM status is already 'active' (also if
351       runtime PM status was already 'active') or the error code from
368       item is queued up immediately); returns 0 on success, 1 if the device's PM
377       success, 1 if the device's runtime PM status was already 'active', or
397       runtime PM status is RPM_ACTIVE and the runtime PM usage counter is
403       runtime PM status is RPM_ACTIVE, and either ign_usage_count is true
432       to zero, the runtime PM helper functions can execute subsystem-level
437       field was previously zero, this prevents subsystem-level runtime PM
439       pending runtime PM operations on the device are either completed or
446       (synchronously) in that case, cancel any other pending runtime PM requests
447       regarding it and wait for all runtime PM operations on it in progress to
457       PM status to 'active' and update its parent's counter of 'active'
465       PM status to 'suspended' and update its parent's counter of 'active'
471     - return true if the device's runtime PM status is 'active' or its
475     - return true if the device's runtime PM status is 'suspended' and its
479     - return true if the device's runtime PM status is 'suspended'
493       PM attributes from /sys/devices/.../power (or prevent them from being
497     - set the power.irq_safe flag for the device, causing the runtime-PM
502       the runtime-PM callbacks to be invoked with interrupts off
565 5. Runtime PM Initialization, Device Probing and Removal
568 Initially, the runtime PM is disabled for all devices, which means that the
569 majority of the runtime PM helper functions described in Section 4 will return
572 In addition to that, the initial runtime PM status of all devices is
575 runtime PM status must be changed to 'active', with the help of
578 However, if the device has a parent and the parent's runtime PM is enabled,
581 parent won't be able to suspend at run time, using the PM core's helper
583 runtime PM is still disabled (i.e. pm_runtime_enable() hasn't been called for
586 should be called for it too as soon as reasonably possible or its runtime PM
590 If the default initial runtime PM status of the device (i.e. 'suspended')
592 ->probe() callback will likely need to wake it up using one of the PM core's
594 should be used.  Of course, for this purpose the device's runtime PM has to be
609 Moreover, the driver core prevents runtime PM callbacks from racing with the bus
612 runtime PM functionality.  It does so by calling pm_runtime_get_sync() before
621 drivers to make their ->remove() callbacks avoid races with runtime PM directly,
625 Drivers in ->remove() callback should undo the runtime PM changes done
634 Namely, during the initialization the driver can make sure that the runtime PM
641 6. Runtime PM and System Sleep
644 Runtime PM and system sleep (i.e., system suspend and hibernation, also known
649 The device may have different wake-up settings for runtime PM and system sleep.
678 brought back to full power during resume, then its runtime PM status will have
686 The PM core always increments the runtime usage counter before calling the
688 Hence disabling runtime PM temporarily like this will not cause any runtime
705 To this end, the PM core provides a mechanism allowing some coordination between
707 callback returns a positive number for a device, that indicates to the PM core
710 left in runtime suspend.  If that happens, the PM core will not execute any
714 related to hibernation (see Documentation/driver-api/pm/devices.rst for more
717 The PM core does its best to reduce the probability of race conditions between
718 the runtime PM and system suspend/resume (and hibernation) callbacks by carrying
724     subsystem-level .suspend() callback for it.  In addition to that the PM core
737 management callbacks provided by the PM core, defined in
760       if successful, change the device's runtime PM status to 'active'
797       if successful, change the device's runtime PM status to 'active'
802 These functions are the defaults used by the PM core if a subsystem doesn't
812 UNIVERSAL_DEV_PM_OPS macro defined in include/linux/pm.h (possibly setting its
822 need of runtime PM callbacks; if the callbacks did exist, ->runtime_suspend()
826 Subsystems can tell the PM core about these devices by calling
830 prevent the non-debugging runtime PM sysfs attributes from being created.
832 When power.no_callbacks is set, the PM core will not invoke the
837 As a consequence, the PM core will never directly inform the device's subsystem
844 the runtime PM callbacks needs to be implemented, a platform dependent PM
847 in subsystems/drivers, the PM core allows runtime PM callbacks to be
848 unassigned. More precisely, if a callback pointer is NULL, the PM core will act
865 the appropriate PM routines); rather it means that runtime suspends will
895 pm_runtime_mark_last_busy()), the PM core will automatically reschedule the
901 However such use inevitably involves races, because the PM core can't