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2 Runtime Power Management Framework for I/O Devices
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5 (C) 2009-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
7 (C) 2010 Alan Stern <stern@rowland.harvard.edu>
9 (C) 2014 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
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:
17 * The power management workqueue pm_wq in which bus types and device drivers can
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
20 them to be synchronized with system-wide power transitions (suspend to RAM,
21 hibernation and resume from system sleep states). pm_wq is declared in
22 include/linux/pm_runtime.h and defined in kernel/power/main.c.
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
31 * A set of helper functions defined in drivers/base/power/runtime.c that can be
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
34 device drivers are encouraged to use these functions.
36 The runtime PM callbacks present in 'struct dev_pm_ops', the device runtime PM
37 fields of 'struct dev_pm_info' and the core helper functions provided for
38 runtime PM are described below.
40 2. Device Runtime PM Callbacks
41 ==============================
43 There are three device runtime PM callbacks defined in 'struct dev_pm_ops'::
47 int (*runtime_suspend)(struct device *dev);
48 int (*runtime_resume)(struct device *dev);
49 int (*runtime_idle)(struct device *dev);
53 The ->runtime_suspend(), ->runtime_resume() and ->runtime_idle() callbacks
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.
67 If the subsystem chosen by applying the above rules doesn't provide the relevant
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
73 and bus type. Moreover, the high-priority one will always take precedence over
74 a low-priority one. The PM domain, bus type, device type and class callbacks
75 are referred to as subsystem-level callbacks in what follows.
77 By default, the callbacks are always invoked in process context with interrupts
78 enabled. However, the pm_runtime_irq_safe() helper function can be used to tell
79 the PM core that it is safe to run the ->runtime_suspend(), ->runtime_resume()
80 and ->runtime_idle() callbacks for the given device in atomic context with
81 interrupts disabled. This implies that the callback routines in question must
82 not block or sleep, but it also means that the synchronous helper functions
83 listed at the end of Section 4 may be used for that device within an interrupt
84 handler or generally in an atomic context.
86 The subsystem-level suspend callback, if present, is _entirely_ _responsible_
87 for handling the suspend of the device as appropriate, which may, but need not
88 include executing the device driver's own ->runtime_suspend() callback (from the
89 PM core's point of view it is not necessary to implement a ->runtime_suspend()
90 callback in a device driver as long as the subsystem-level suspend callback
91 knows what to do to handle the device).
93 * Once the subsystem-level suspend callback (or the driver suspend callback,
94 if invoked directly) has completed successfully for the given device, the PM
95 core regards the device as suspended, which need not mean that it has been
96 put into a low power state. It is supposed to mean, however, that the
97 device will not process data and will not communicate with the CPU(s) and
98 RAM until the appropriate resume callback is executed for it. The runtime
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
103 status remains 'active', which means that the device _must_ be fully
104 operational afterwards.
106 * If the suspend callback returns an error code different from -EBUSY and
107 -EAGAIN, the PM core regards this as a fatal error and will refuse to run
108 the helper functions described in Section 4 for the device until its status
109 is directly set to either 'active', or 'suspended' (the PM core provides
110 special helper functions for this purpose).
112 In particular, if the driver requires remote wakeup capability (i.e. hardware
113 mechanism allowing the device to request a change of its power state, such as
114 PCI PME) for proper functioning and device_can_wakeup() returns 'false' for the
115 device, then ->runtime_suspend() should return -EBUSY. On the other hand, if
116 device_can_wakeup() returns 'true' for the device and the device is put into a
117 low-power state during the execution of the suspend callback, it is expected
118 that remote wakeup will be enabled for the device. Generally, remote wakeup
119 should be enabled for all input devices put into low-power states at run time.
121 The subsystem-level resume callback, if present, is **entirely responsible** for
122 handling the resume of the device as appropriate, which may, but need not
123 include executing the device driver's own ->runtime_resume() callback (from the
124 PM core's point of view it is not necessary to implement a ->runtime_resume()
125 callback in a device driver as long as the subsystem-level resume callback knows
126 what to do to handle the device).
128 * Once the subsystem-level resume callback (or the driver resume callback, if
129 invoked directly) has completed successfully, the PM core regards the device
130 as fully operational, which means that the device _must_ be able to complete
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
135 fatal error and will refuse to run the helper functions described in Section
136 4 for the device, until its status is directly set to either 'active', or
137 'suspended' (by means of special helper functions provided by the PM core
140 The idle callback (a subsystem-level one, if present, or the driver one) is
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
143 counter of 'active' children of the device.
145 * If any of these counters is decreased using a helper function provided by
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
148 idle callback with the device as its argument.
150 The action performed by the idle callback is totally dependent on the subsystem
151 (or driver) in question, but the expected and recommended action is to check
152 if the device can be suspended (i.e. if all of the conditions necessary for
153 suspending the device are satisfied) and to queue up a suspend request for the
154 device in that case. If there is no idle callback, or if the callback returns
155 0, then the PM core will attempt to carry out a runtime suspend of the device,
156 also respecting devices configured for autosuspend. In essence this means a
157 call to pm_runtime_autosuspend() (do note that drivers needs to update the
158 device last busy mark, pm_runtime_mark_last_busy(), to control the delay under
159 this circumstance). To prevent this (for example, if the callback routine has
160 started a delayed suspend), the routine must return a non-zero value. Negative
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
167 (1) The callbacks are mutually exclusive (e.g. it is forbidden to execute
168 ->runtime_suspend() in parallel with ->runtime_resume() or with another
169 instance of ->runtime_suspend() for the same device) with the exception that
170 ->runtime_suspend() or ->runtime_resume() can be executed in parallel with
171 ->runtime_idle() (although ->runtime_idle() will not be started while any
172 of the other callbacks is being executed for the same device).
174 (2) ->runtime_idle() and ->runtime_suspend() can only be executed for 'active'
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
179 (3) ->runtime_idle() and ->runtime_suspend() can only be executed for a device
180 the usage counter of which is equal to zero _and_ either the counter of
181 'active' children of which is equal to zero, or the 'power.ignore_children'
182 flag of which is set.
184 (4) ->runtime_resume() can only be executed for 'suspended' devices (i.e. the
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
191 * If ->runtime_suspend() is about to be executed or there's a pending request
192 to execute it, ->runtime_idle() will not be executed for the same device.
194 * A request to execute or to schedule the execution of ->runtime_suspend()
195 will cancel any pending requests to execute ->runtime_idle() for the same
198 * If ->runtime_resume() is about to be executed or there's a pending request
199 to execute it, the other callbacks will not be executed for the same device.
201 * A request to execute ->runtime_resume() will cancel any pending or
202 scheduled requests to execute the other callbacks for the same device,
203 except for scheduled autosuspends.
205 3. Runtime PM Device Fields
206 ===========================
208 The following device runtime PM fields are present in 'struct dev_pm_info', as
209 defined in include/linux/pm.h:
211 `struct timer_list suspend_timer;`
212 - timer used for scheduling (delayed) suspend and autosuspend requests
214 `unsigned long timer_expires;`
215 - timer expiration time, in jiffies (if this is different from zero, the
216 timer is running and will expire at that time, otherwise the timer is not
219 `struct work_struct work;`
220 - work structure used for queuing up requests (i.e. work items in pm_wq)
222 `wait_queue_head_t wait_queue;`
223 - wait queue used if any of the helper functions needs to wait for another
227 - lock used for synchronization
229 `atomic_t usage_count;`
230 - the usage counter of the device
232 `atomic_t child_count;`
233 - the count of 'active' children of the device
235 `unsigned int ignore_children;`
236 - if set, the value of child_count is ignored (but still updated)
238 `unsigned int disable_depth;`
239 - used for disabling the helper functions (they work normally if this is
240 equal to zero); the initial value of it is 1 (i.e. runtime PM is
241 initially disabled for all devices)
244 - if set, there was a fatal error (one of the callbacks returned error code
245 as described in Section 2), so the helper functions will not work until
246 this flag is cleared; this is the error code returned by the failing
249 `unsigned int idle_notification;`
250 - if set, ->runtime_idle() is being executed
252 `unsigned int request_pending;`
253 - if set, there's a pending request (i.e. a work item queued up into pm_wq)
255 `enum rpm_request request;`
256 - type of request that's pending (valid if request_pending is set)
258 `unsigned int deferred_resume;`
259 - set if ->runtime_resume() is about to be run while ->runtime_suspend() is
260 being executed for that device and it is not practical to wait for the
261 suspend to complete; means "start a resume as soon as you've suspended"
263 `enum rpm_status runtime_status;`
264 - the runtime PM status of the device; this field's initial value is
265 RPM_SUSPENDED, which means that each device is initially regarded by the
266 PM core as 'suspended', regardless of its real hardware status
268 `unsigned int runtime_auto;`
269 - if set, indicates that the user space has allowed the device driver to
270 power manage the device at run time via the /sys/devices/.../power/control
271 `interface;` it may only be modified with the help of the
272 pm_runtime_allow() and pm_runtime_forbid() helper functions
274 `unsigned int no_callbacks;`
275 - indicates that the device does not use the runtime PM callbacks (see
276 Section 8); it may be modified only by the pm_runtime_no_callbacks()
279 `unsigned int irq_safe;`
280 - indicates that the ->runtime_suspend() and ->runtime_resume() callbacks
281 will be invoked with the spinlock held and interrupts disabled
283 `unsigned int use_autosuspend;`
284 - indicates that the device's driver supports delayed autosuspend (see
285 Section 9); it may be modified only by the
286 pm_runtime{_dont}_use_autosuspend() helper functions
288 `unsigned int timer_autosuspends;`
289 - indicates that the PM core should attempt to carry out an autosuspend
290 when the timer expires rather than a normal suspend
292 `int autosuspend_delay;`
293 - the delay time (in milliseconds) to be used for autosuspend
295 `unsigned long last_busy;`
296 - the time (in jiffies) when the pm_runtime_mark_last_busy() helper
297 function was last called for this device; used in calculating inactivity
298 periods for autosuspend
300 All of the above fields are members of the 'power' member of 'struct device'.
302 4. Runtime PM Device Helper Functions
303 =====================================
305 The following runtime PM helper functions are defined in
306 drivers/base/power/runtime.c and include/linux/pm_runtime.h:
308 `void pm_runtime_init(struct device *dev);`
309 - initialize the device runtime PM fields in 'struct dev_pm_info'
311 `void pm_runtime_remove(struct device *dev);`
312 - make sure that the runtime PM of the device will be disabled after
313 removing the device from device hierarchy
315 `int pm_runtime_idle(struct device *dev);`
316 - execute the subsystem-level idle callback for the device; returns an
317 error code on failure, where -EINPROGRESS means that ->runtime_idle() is
318 already being executed; if there is no callback or the callback returns 0
319 then run pm_runtime_autosuspend(dev) and return its result
321 `int pm_runtime_suspend(struct device *dev);`
322 - execute the subsystem-level suspend callback for the device; returns 0 on
323 success, 1 if the device's runtime PM status was already 'suspended', or
324 error code on failure, where -EAGAIN or -EBUSY means it is safe to attempt
325 to suspend the device again in future and -EACCES means that
326 'power.disable_depth' is different from 0
328 `int pm_runtime_autosuspend(struct device *dev);`
329 - same as pm_runtime_suspend() except that the autosuspend delay is taken
330 `into account;` if pm_runtime_autosuspend_expiration() says the delay has
331 not yet expired then an autosuspend is scheduled for the appropriate time
334 `int pm_runtime_resume(struct device *dev);`
335 - execute the subsystem-level resume callback for the device; returns 0 on
336 success, 1 if the device's runtime PM status was already 'active' or
337 error code on failure, where -EAGAIN means it may be safe to attempt to
338 resume the device again in future, but 'power.runtime_error' should be
339 checked additionally, and -EACCES means that 'power.disable_depth' is
342 `int pm_request_idle(struct device *dev);`
343 - submit a request to execute the subsystem-level idle callback for the
344 device (the request is represented by a work item in pm_wq); returns 0 on
345 success or error code if the request has not been queued up
347 `int pm_request_autosuspend(struct device *dev);`
348 - schedule the execution of the subsystem-level suspend callback for the
349 device when the autosuspend delay has expired; if the delay has already
350 expired then the work item is queued up immediately
352 `int pm_schedule_suspend(struct device *dev, unsigned int delay);`
353 - schedule the execution of the subsystem-level suspend callback for the
354 device in future, where 'delay' is the time to wait before queuing up a
355 suspend work item in pm_wq, in milliseconds (if 'delay' is zero, the work
356 item is queued up immediately); returns 0 on success, 1 if the device's PM
357 runtime status was already 'suspended', or error code if the request
358 hasn't been scheduled (or queued up if 'delay' is 0); if the execution of
359 ->runtime_suspend() is already scheduled and not yet expired, the new
360 value of 'delay' will be used as the time to wait
362 `int pm_request_resume(struct device *dev);`
363 - submit a request to execute the subsystem-level resume callback for the
364 device (the request is represented by a work item in pm_wq); returns 0 on
365 success, 1 if the device's runtime PM status was already 'active', or
366 error code if the request hasn't been queued up
368 `void pm_runtime_get_noresume(struct device *dev);`
369 - increment the device's usage counter
371 `int pm_runtime_get(struct device *dev);`
372 - increment the device's usage counter, run pm_request_resume(dev) and
375 `int pm_runtime_get_sync(struct device *dev);`
376 - increment the device's usage counter, run pm_runtime_resume(dev) and
379 `int pm_runtime_get_if_in_use(struct device *dev);`
380 - return -EINVAL if 'power.disable_depth' is nonzero; otherwise, if the
381 runtime PM status is RPM_ACTIVE and the runtime PM usage counter is
382 nonzero, increment the counter and return 1; otherwise return 0 without
385 `int pm_runtime_get_if_active(struct device *dev, bool ign_usage_count);`
386 - return -EINVAL if 'power.disable_depth' is nonzero; otherwise, if the
387 runtime PM status is RPM_ACTIVE, and either ign_usage_count is true
388 or the device's usage_count is non-zero, increment the counter and
389 return 1; otherwise return 0 without changing the counter
391 `void pm_runtime_put_noidle(struct device *dev);`
392 - decrement the device's usage counter
394 `int pm_runtime_put(struct device *dev);`
395 - decrement the device's usage counter; if the result is 0 then run
396 pm_request_idle(dev) and return its result
398 `int pm_runtime_put_autosuspend(struct device *dev);`
399 - decrement the device's usage counter; if the result is 0 then run
400 pm_request_autosuspend(dev) and return its result
402 `int pm_runtime_put_sync(struct device *dev);`
403 - decrement the device's usage counter; if the result is 0 then run
404 pm_runtime_idle(dev) and return its result
406 `int pm_runtime_put_sync_suspend(struct device *dev);`
407 - decrement the device's usage counter; if the result is 0 then run
408 pm_runtime_suspend(dev) and return its result
410 `int pm_runtime_put_sync_autosuspend(struct device *dev);`
411 - decrement the device's usage counter; if the result is 0 then run
412 pm_runtime_autosuspend(dev) and return its result
414 `void pm_runtime_enable(struct device *dev);`
415 - decrement the device's 'power.disable_depth' field; if that field is equal
416 to zero, the runtime PM helper functions can execute subsystem-level
417 callbacks described in Section 2 for the device
419 `int pm_runtime_disable(struct device *dev);`
420 - increment the device's 'power.disable_depth' field (if the value of that
421 field was previously zero, this prevents subsystem-level runtime PM
422 callbacks from being run for the device), make sure that all of the
423 pending runtime PM operations on the device are either completed or
424 canceled; returns 1 if there was a resume request pending and it was
425 necessary to execute the subsystem-level resume callback for the device
426 to satisfy that request, otherwise 0 is returned
428 `int pm_runtime_barrier(struct device *dev);`
429 - check if there's a resume request pending for the device and resume it
430 (synchronously) in that case, cancel any other pending runtime PM requests
431 regarding it and wait for all runtime PM operations on it in progress to
432 complete; returns 1 if there was a resume request pending and it was
433 necessary to execute the subsystem-level resume callback for the device to
434 satisfy that request, otherwise 0 is returned
436 `void pm_suspend_ignore_children(struct device *dev, bool enable);`
437 - set/unset the power.ignore_children flag of the device
439 `int pm_runtime_set_active(struct device *dev);`
440 - clear the device's 'power.runtime_error' flag, set the device's runtime
441 PM status to 'active' and update its parent's counter of 'active'
442 children as appropriate (it is only valid to use this function if
443 'power.runtime_error' is set or 'power.disable_depth' is greater than
444 zero); it will fail and return error code if the device has a parent
445 which is not active and the 'power.ignore_children' flag of which is unset
447 `void pm_runtime_set_suspended(struct device *dev);`
448 - clear the device's 'power.runtime_error' flag, set the device's runtime
449 PM status to 'suspended' and update its parent's counter of 'active'
450 children as appropriate (it is only valid to use this function if
451 'power.runtime_error' is set or 'power.disable_depth' is greater than
454 `bool pm_runtime_active(struct device *dev);`
455 - return true if the device's runtime PM status is 'active' or its
456 'power.disable_depth' field is not equal to zero, or false otherwise
458 `bool pm_runtime_suspended(struct device *dev);`
459 - return true if the device's runtime PM status is 'suspended' and its
460 'power.disable_depth' field is equal to zero, or false otherwise
462 `bool pm_runtime_status_suspended(struct device *dev);`
463 - return true if the device's runtime PM status is 'suspended'
465 `void pm_runtime_allow(struct device *dev);`
466 - set the power.runtime_auto flag for the device and decrease its usage
467 counter (used by the /sys/devices/.../power/control interface to
468 effectively allow the device to be power managed at run time)
470 `void pm_runtime_forbid(struct device *dev);`
471 - unset the power.runtime_auto flag for the device and increase its usage
472 counter (used by the /sys/devices/.../power/control interface to
473 effectively prevent the device from being power managed at run time)
475 `void pm_runtime_no_callbacks(struct device *dev);`
476 - set the power.no_callbacks flag for the device and remove the runtime
477 PM attributes from /sys/devices/.../power (or prevent them from being
478 added when the device is registered)
480 `void pm_runtime_irq_safe(struct device *dev);`
481 - set the power.irq_safe flag for the device, causing the runtime-PM
482 callbacks to be invoked with interrupts off
484 `bool pm_runtime_is_irq_safe(struct device *dev);`
485 - return true if power.irq_safe flag was set for the device, causing
486 the runtime-PM callbacks to be invoked with interrupts off
488 `void pm_runtime_mark_last_busy(struct device *dev);`
489 - set the power.last_busy field to the current time
491 `void pm_runtime_use_autosuspend(struct device *dev);`
492 - set the power.use_autosuspend flag, enabling autosuspend delays; call
493 pm_runtime_get_sync if the flag was previously cleared and
494 power.autosuspend_delay is negative
496 `void pm_runtime_dont_use_autosuspend(struct device *dev);`
497 - clear the power.use_autosuspend flag, disabling autosuspend delays;
498 decrement the device's usage counter if the flag was previously set and
499 power.autosuspend_delay is negative; call pm_runtime_idle
501 `void pm_runtime_set_autosuspend_delay(struct device *dev, int delay);`
502 - set the power.autosuspend_delay value to 'delay' (expressed in
503 milliseconds); if 'delay' is negative then runtime suspends are
504 prevented; if power.use_autosuspend is set, pm_runtime_get_sync may be
505 called or the device's usage counter may be decremented and
506 pm_runtime_idle called depending on if power.autosuspend_delay is
507 changed to or from a negative value; if power.use_autosuspend is clear,
508 pm_runtime_idle is called
510 `unsigned long pm_runtime_autosuspend_expiration(struct device *dev);`
511 - calculate the time when the current autosuspend delay period will expire,
512 based on power.last_busy and power.autosuspend_delay; if the delay time
513 is 1000 ms or larger then the expiration time is rounded up to the
514 nearest second; returns 0 if the delay period has already expired or
515 power.use_autosuspend isn't set, otherwise returns the expiration time
518 It is safe to execute the following helper functions from interrupt context:
521 - pm_request_autosuspend()
522 - pm_schedule_suspend()
523 - pm_request_resume()
524 - pm_runtime_get_noresume()
526 - pm_runtime_put_noidle()
528 - pm_runtime_put_autosuspend()
529 - pm_runtime_enable()
530 - pm_suspend_ignore_children()
531 - pm_runtime_set_active()
532 - pm_runtime_set_suspended()
533 - pm_runtime_suspended()
534 - pm_runtime_mark_last_busy()
535 - pm_runtime_autosuspend_expiration()
537 If pm_runtime_irq_safe() has been called for a device then the following helper
538 functions may also be used in interrupt context:
541 - pm_runtime_suspend()
542 - pm_runtime_autosuspend()
543 - pm_runtime_resume()
544 - pm_runtime_get_sync()
545 - pm_runtime_put_sync()
546 - pm_runtime_put_sync_suspend()
547 - pm_runtime_put_sync_autosuspend()
549 5. Runtime PM Initialization, Device Probing and Removal
550 ========================================================
552 Initially, the runtime PM is disabled for all devices, which means that the
553 majority of the runtime PM helper functions described in Section 4 will return
554 -EAGAIN until pm_runtime_enable() is called for the device.
556 In addition to that, the initial runtime PM status of all devices is
557 'suspended', but it need not reflect the actual physical state of the device.
558 Thus, if the device is initially active (i.e. it is able to process I/O), its
559 runtime PM status must be changed to 'active', with the help of
560 pm_runtime_set_active(), before pm_runtime_enable() is called for the device.
562 However, if the device has a parent and the parent's runtime PM is enabled,
563 calling pm_runtime_set_active() for the device will affect the parent, unless
564 the parent's 'power.ignore_children' flag is set. Namely, in that case the
565 parent won't be able to suspend at run time, using the PM core's helper
566 functions, as long as the child's status is 'active', even if the child's
567 runtime PM is still disabled (i.e. pm_runtime_enable() hasn't been called for
568 the child yet or pm_runtime_disable() has been called for it). For this reason,
569 once pm_runtime_set_active() has been called for the device, pm_runtime_enable()
570 should be called for it too as soon as reasonably possible or its runtime PM
571 status should be changed back to 'suspended' with the help of
572 pm_runtime_set_suspended().
574 If the default initial runtime PM status of the device (i.e. 'suspended')
575 reflects the actual state of the device, its bus type's or its driver's
576 ->probe() callback will likely need to wake it up using one of the PM core's
577 helper functions described in Section 4. In that case, pm_runtime_resume()
578 should be used. Of course, for this purpose the device's runtime PM has to be
579 enabled earlier by calling pm_runtime_enable().
581 Note, if the device may execute pm_runtime calls during the probe (such as
582 if it is registers with a subsystem that may call back in) then the
583 pm_runtime_get_sync() call paired with a pm_runtime_put() call will be
584 appropriate to ensure that the device is not put back to sleep during the
585 probe. This can happen with systems such as the network device layer.
587 It may be desirable to suspend the device once ->probe() has finished.
588 Therefore the driver core uses the asynchronous pm_request_idle() to submit a
589 request to execute the subsystem-level idle callback for the device at that
590 time. A driver that makes use of the runtime autosuspend feature, may want to
591 update the last busy mark before returning from ->probe().
593 Moreover, the driver core prevents runtime PM callbacks from racing with the bus
594 notifier callback in __device_release_driver(), which is necessary, because the
595 notifier is used by some subsystems to carry out operations affecting the
596 runtime PM functionality. It does so by calling pm_runtime_get_sync() before
597 driver_sysfs_remove() and the BUS_NOTIFY_UNBIND_DRIVER notifications. This
598 resumes the device if it's in the suspended state and prevents it from
599 being suspended again while those routines are being executed.
601 To allow bus types and drivers to put devices into the suspended state by
602 calling pm_runtime_suspend() from their ->remove() routines, the driver core
603 executes pm_runtime_put_sync() after running the BUS_NOTIFY_UNBIND_DRIVER
604 notifications in __device_release_driver(). This requires bus types and
605 drivers to make their ->remove() callbacks avoid races with runtime PM directly,
606 but also it allows of more flexibility in the handling of devices during the
607 removal of their drivers.
609 Drivers in ->remove() callback should undo the runtime PM changes done
610 in ->probe(). Usually this means calling pm_runtime_disable(),
611 pm_runtime_dont_use_autosuspend() etc.
613 The user space can effectively disallow the driver of the device to power manage
614 it at run time by changing the value of its /sys/devices/.../power/control
615 attribute to "on", which causes pm_runtime_forbid() to be called. In principle,
616 this mechanism may also be used by the driver to effectively turn off the
617 runtime power management of the device until the user space turns it on.
618 Namely, during the initialization the driver can make sure that the runtime PM
619 status of the device is 'active' and call pm_runtime_forbid(). It should be
620 noted, however, that if the user space has already intentionally changed the
621 value of /sys/devices/.../power/control to "auto" to allow the driver to power
622 manage the device at run time, the driver may confuse it by using
623 pm_runtime_forbid() this way.
625 6. Runtime PM and System Sleep
626 ==============================
628 Runtime PM and system sleep (i.e., system suspend and hibernation, also known
629 as suspend-to-RAM and suspend-to-disk) interact with each other in a couple of
630 ways. If a device is active when a system sleep starts, everything is
631 straightforward. But what should happen if the device is already suspended?
633 The device may have different wake-up settings for runtime PM and system sleep.
634 For example, remote wake-up may be enabled for runtime suspend but disallowed
635 for system sleep (device_may_wakeup(dev) returns 'false'). When this happens,
636 the subsystem-level system suspend callback is responsible for changing the
637 device's wake-up setting (it may leave that to the device driver's system
638 suspend routine). It may be necessary to resume the device and suspend it again
639 in order to do so. The same is true if the driver uses different power levels
640 or other settings for runtime suspend and system sleep.
642 During system resume, the simplest approach is to bring all devices back to full
643 power, even if they had been suspended before the system suspend began. There
644 are several reasons for this, including:
646 * The device might need to switch power levels, wake-up settings, etc.
648 * Remote wake-up events might have been lost by the firmware.
650 * The device's children may need the device to be at full power in order
651 to resume themselves.
653 * The driver's idea of the device state may not agree with the device's
654 physical state. This can happen during resume from hibernation.
656 * The device might need to be reset.
658 * Even though the device was suspended, if its usage counter was > 0 then most
659 likely it would need a runtime resume in the near future anyway.
661 If the device had been suspended before the system suspend began and it's
662 brought back to full power during resume, then its runtime PM status will have
663 to be updated to reflect the actual post-system sleep status. The way to do
666 - pm_runtime_disable(dev);
667 - pm_runtime_set_active(dev);
668 - pm_runtime_enable(dev);
670 The PM core always increments the runtime usage counter before calling the
671 ->suspend() callback and decrements it after calling the ->resume() callback.
672 Hence disabling runtime PM temporarily like this will not cause any runtime
673 suspend attempts to be permanently lost. If the usage count goes to zero
674 following the return of the ->resume() callback, the ->runtime_idle() callback
675 will be invoked as usual.
677 On some systems, however, system sleep is not entered through a global firmware
678 or hardware operation. Instead, all hardware components are put into low-power
679 states directly by the kernel in a coordinated way. Then, the system sleep
680 state effectively follows from the states the hardware components end up in
681 and the system is woken up from that state by a hardware interrupt or a similar
682 mechanism entirely under the kernel's control. As a result, the kernel never
683 gives control away and the states of all devices during resume are precisely
684 known to it. If that is the case and none of the situations listed above takes
685 place (in particular, if the system is not waking up from hibernation), it may
686 be more efficient to leave the devices that had been suspended before the system
687 suspend began in the suspended state.
689 To this end, the PM core provides a mechanism allowing some coordination between
690 different levels of device hierarchy. Namely, if a system suspend .prepare()
691 callback returns a positive number for a device, that indicates to the PM core
692 that the device appears to be runtime-suspended and its state is fine, so it
693 may be left in runtime suspend provided that all of its descendants are also
694 left in runtime suspend. If that happens, the PM core will not execute any
695 system suspend and resume callbacks for all of those devices, except for the
696 complete callback, which is then entirely responsible for handling the device
697 as appropriate. This only applies to system suspend transitions that are not
698 related to hibernation (see Documentation/driver-api/pm/devices.rst for more
701 The PM core does its best to reduce the probability of race conditions between
702 the runtime PM and system suspend/resume (and hibernation) callbacks by carrying
703 out the following operations:
705 * During system suspend pm_runtime_get_noresume() is called for every device
706 right before executing the subsystem-level .prepare() callback for it and
707 pm_runtime_barrier() is called for every device right before executing the
708 subsystem-level .suspend() callback for it. In addition to that the PM core
709 calls __pm_runtime_disable() with 'false' as the second argument for every
710 device right before executing the subsystem-level .suspend_late() callback
713 * During system resume pm_runtime_enable() and pm_runtime_put() are called for
714 every device right after executing the subsystem-level .resume_early()
715 callback and right after executing the subsystem-level .complete() callback
716 for it, respectively.
718 7. Generic subsystem callbacks
720 Subsystems may wish to conserve code space by using the set of generic power
721 management callbacks provided by the PM core, defined in
722 driver/base/power/generic_ops.c:
724 `int pm_generic_runtime_suspend(struct device *dev);`
725 - invoke the ->runtime_suspend() callback provided by the driver of this
726 device and return its result, or return 0 if not defined
728 `int pm_generic_runtime_resume(struct device *dev);`
729 - invoke the ->runtime_resume() callback provided by the driver of this
730 device and return its result, or return 0 if not defined
732 `int pm_generic_suspend(struct device *dev);`
733 - if the device has not been suspended at run time, invoke the ->suspend()
734 callback provided by its driver and return its result, or return 0 if not
737 `int pm_generic_suspend_noirq(struct device *dev);`
738 - if pm_runtime_suspended(dev) returns "false", invoke the ->suspend_noirq()
739 callback provided by the device's driver and return its result, or return
742 `int pm_generic_resume(struct device *dev);`
743 - invoke the ->resume() callback provided by the driver of this device and,
744 if successful, change the device's runtime PM status to 'active'
746 `int pm_generic_resume_noirq(struct device *dev);`
747 - invoke the ->resume_noirq() callback provided by the driver of this device
749 `int pm_generic_freeze(struct device *dev);`
750 - if the device has not been suspended at run time, invoke the ->freeze()
751 callback provided by its driver and return its result, or return 0 if not
754 `int pm_generic_freeze_noirq(struct device *dev);`
755 - if pm_runtime_suspended(dev) returns "false", invoke the ->freeze_noirq()
756 callback provided by the device's driver and return its result, or return
759 `int pm_generic_thaw(struct device *dev);`
760 - if the device has not been suspended at run time, invoke the ->thaw()
761 callback provided by its driver and return its result, or return 0 if not
764 `int pm_generic_thaw_noirq(struct device *dev);`
765 - if pm_runtime_suspended(dev) returns "false", invoke the ->thaw_noirq()
766 callback provided by the device's driver and return its result, or return
769 `int pm_generic_poweroff(struct device *dev);`
770 - if the device has not been suspended at run time, invoke the ->poweroff()
771 callback provided by its driver and return its result, or return 0 if not
774 `int pm_generic_poweroff_noirq(struct device *dev);`
775 - if pm_runtime_suspended(dev) returns "false", run the ->poweroff_noirq()
776 callback provided by the device's driver and return its result, or return
779 `int pm_generic_restore(struct device *dev);`
780 - invoke the ->restore() callback provided by the driver of this device and,
781 if successful, change the device's runtime PM status to 'active'
783 `int pm_generic_restore_noirq(struct device *dev);`
784 - invoke the ->restore_noirq() callback provided by the device's driver
786 These functions are the defaults used by the PM core, if a subsystem doesn't
787 provide its own callbacks for ->runtime_idle(), ->runtime_suspend(),
788 ->runtime_resume(), ->suspend(), ->suspend_noirq(), ->resume(),
789 ->resume_noirq(), ->freeze(), ->freeze_noirq(), ->thaw(), ->thaw_noirq(),
790 ->poweroff(), ->poweroff_noirq(), ->restore(), ->restore_noirq() in the
791 subsystem-level dev_pm_ops structure.
793 Device drivers that wish to use the same function as a system suspend, freeze,
794 poweroff and runtime suspend callback, and similarly for system resume, thaw,
795 restore, and runtime resume, can achieve this with the help of the
796 UNIVERSAL_DEV_PM_OPS macro defined in include/linux/pm.h (possibly setting its
797 last argument to NULL).
799 8. "No-Callback" Devices
800 ========================
802 Some "devices" are only logical sub-devices of their parent and cannot be
803 power-managed on their own. (The prototype example is a USB interface. Entire
804 USB devices can go into low-power mode or send wake-up requests, but neither is
805 possible for individual interfaces.) The drivers for these devices have no
806 need of runtime PM callbacks; if the callbacks did exist, ->runtime_suspend()
807 and ->runtime_resume() would always return 0 without doing anything else and
808 ->runtime_idle() would always call pm_runtime_suspend().
810 Subsystems can tell the PM core about these devices by calling
811 pm_runtime_no_callbacks(). This should be done after the device structure is
812 initialized and before it is registered (although after device registration is
813 also okay). The routine will set the device's power.no_callbacks flag and
814 prevent the non-debugging runtime PM sysfs attributes from being created.
816 When power.no_callbacks is set, the PM core will not invoke the
817 ->runtime_idle(), ->runtime_suspend(), or ->runtime_resume() callbacks.
818 Instead it will assume that suspends and resumes always succeed and that idle
819 devices should be suspended.
821 As a consequence, the PM core will never directly inform the device's subsystem
822 or driver about runtime power changes. Instead, the driver for the device's
823 parent must take responsibility for telling the device's driver when the
824 parent's power state changes.
826 9. Autosuspend, or automatically-delayed suspends
827 =================================================
829 Changing a device's power state isn't free; it requires both time and energy.
830 A device should be put in a low-power state only when there's some reason to
831 think it will remain in that state for a substantial time. A common heuristic
832 says that a device which hasn't been used for a while is liable to remain
833 unused; following this advice, drivers should not allow devices to be suspended
834 at runtime until they have been inactive for some minimum period. Even when
835 the heuristic ends up being non-optimal, it will still prevent devices from
836 "bouncing" too rapidly between low-power and full-power states.
838 The term "autosuspend" is an historical remnant. It doesn't mean that the
839 device is automatically suspended (the subsystem or driver still has to call
840 the appropriate PM routines); rather it means that runtime suspends will
841 automatically be delayed until the desired period of inactivity has elapsed.
843 Inactivity is determined based on the power.last_busy field. Drivers should
844 call pm_runtime_mark_last_busy() to update this field after carrying out I/O,
845 typically just before calling pm_runtime_put_autosuspend(). The desired length
846 of the inactivity period is a matter of policy. Subsystems can set this length
847 initially by calling pm_runtime_set_autosuspend_delay(), but after device
848 registration the length should be controlled by user space, using the
849 /sys/devices/.../power/autosuspend_delay_ms attribute.
851 In order to use autosuspend, subsystems or drivers must call
852 pm_runtime_use_autosuspend() (preferably before registering the device), and
853 thereafter they should use the various `*_autosuspend()` helper functions
854 instead of the non-autosuspend counterparts::
856 Instead of: pm_runtime_suspend use: pm_runtime_autosuspend;
857 Instead of: pm_schedule_suspend use: pm_request_autosuspend;
858 Instead of: pm_runtime_put use: pm_runtime_put_autosuspend;
859 Instead of: pm_runtime_put_sync use: pm_runtime_put_sync_autosuspend.
861 Drivers may also continue to use the non-autosuspend helper functions; they
862 will behave normally, which means sometimes taking the autosuspend delay into
863 account (see pm_runtime_idle).
865 Under some circumstances a driver or subsystem may want to prevent a device
866 from autosuspending immediately, even though the usage counter is zero and the
867 autosuspend delay time has expired. If the ->runtime_suspend() callback
868 returns -EAGAIN or -EBUSY, and if the next autosuspend delay expiration time is
869 in the future (as it normally would be if the callback invoked
870 pm_runtime_mark_last_busy()), the PM core will automatically reschedule the
871 autosuspend. The ->runtime_suspend() callback can't do this rescheduling
872 itself because no suspend requests of any kind are accepted while the device is
873 suspending (i.e., while the callback is running).
875 The implementation is well suited for asynchronous use in interrupt contexts.
876 However such use inevitably involves races, because the PM core can't
877 synchronize ->runtime_suspend() callbacks with the arrival of I/O requests.
878 This synchronization must be handled by the driver, using its private lock.
879 Here is a schematic pseudo-code example::
881 foo_read_or_write(struct foo_priv *foo, void *data)
883 lock(&foo->private_lock);
884 add_request_to_io_queue(foo, data);
885 if (foo->num_pending_requests++ == 0)
886 pm_runtime_get(&foo->dev);
887 if (!foo->is_suspended)
888 foo_process_next_request(foo);
889 unlock(&foo->private_lock);
892 foo_io_completion(struct foo_priv *foo, void *req)
894 lock(&foo->private_lock);
895 if (--foo->num_pending_requests == 0) {
896 pm_runtime_mark_last_busy(&foo->dev);
897 pm_runtime_put_autosuspend(&foo->dev);
899 foo_process_next_request(foo);
901 unlock(&foo->private_lock);
902 /* Send req result back to the user ... */
905 int foo_runtime_suspend(struct device *dev)
907 struct foo_priv foo = container_of(dev, ...);
910 lock(&foo->private_lock);
911 if (foo->num_pending_requests > 0) {
914 /* ... suspend the device ... */
915 foo->is_suspended = 1;
917 unlock(&foo->private_lock);
921 int foo_runtime_resume(struct device *dev)
923 struct foo_priv foo = container_of(dev, ...);
925 lock(&foo->private_lock);
926 /* ... resume the device ... */
927 foo->is_suspended = 0;
928 pm_runtime_mark_last_busy(&foo->dev);
929 if (foo->num_pending_requests > 0)
930 foo_process_next_request(foo);
931 unlock(&foo->private_lock);
935 The important point is that after foo_io_completion() asks for an autosuspend,
936 the foo_runtime_suspend() callback may race with foo_read_or_write().
937 Therefore foo_runtime_suspend() has to check whether there are any pending I/O
938 requests (while holding the private lock) before allowing the suspend to
941 In addition, the power.autosuspend_delay field can be changed by user space at
942 any time. If a driver cares about this, it can call
943 pm_runtime_autosuspend_expiration() from within the ->runtime_suspend()
944 callback while holding its private lock. If the function returns a nonzero
945 value then the delay has not yet expired and the callback should return