1 Run-time Power Management Framework for I/O Devices
3 (C) 2009-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
4 (C) 2010 Alan Stern <stern@rowland.harvard.edu>
8 Support for run-time power management (run-time PM) of I/O devices is provided
9 at the power management core (PM core) level by means of:
11 * The power management workqueue pm_wq in which bus types and device drivers can
12 put their PM-related work items. It is strongly recommended that pm_wq be
13 used for queuing all work items related to run-time PM, because this allows
14 them to be synchronized with system-wide power transitions (suspend to RAM,
15 hibernation and resume from system sleep states). pm_wq is declared in
16 include/linux/pm_runtime.h and defined in kernel/power/main.c.
18 * A number of run-time PM fields in the 'power' member of 'struct device' (which
19 is of the type 'struct dev_pm_info', defined in include/linux/pm.h) that can
20 be used for synchronizing run-time PM operations with one another.
22 * Three device run-time PM callbacks in 'struct dev_pm_ops' (defined in
25 * A set of helper functions defined in drivers/base/power/runtime.c that can be
26 used for carrying out run-time PM operations in such a way that the
27 synchronization between them is taken care of by the PM core. Bus types and
28 device drivers are encouraged to use these functions.
30 The run-time PM callbacks present in 'struct dev_pm_ops', the device run-time PM
31 fields of 'struct dev_pm_info' and the core helper functions provided for
32 run-time PM are described below.
34 2. Device Run-time PM Callbacks
36 There are three device run-time PM callbacks defined in 'struct dev_pm_ops':
40 int (*runtime_suspend)(struct device *dev);
41 int (*runtime_resume)(struct device *dev);
42 int (*runtime_idle)(struct device *dev);
46 The ->runtime_suspend(), ->runtime_resume() and ->runtime_idle() callbacks are
47 executed by the PM core for either the device type, or the class (if the device
48 type's struct dev_pm_ops object does not exist), or the bus type (if the
49 device type's and class' struct dev_pm_ops objects do not exist) of the given
50 device (this allows device types to override callbacks provided by bus types or
51 classes if necessary). The bus type, device type and class callbacks are
52 referred to as subsystem-level callbacks in what follows.
54 By default, the callbacks are always invoked in process context with interrupts
55 enabled. However, subsystems can use the pm_runtime_irq_safe() helper function
56 to tell the PM core that a device's ->runtime_suspend() and ->runtime_resume()
57 callbacks should be invoked in atomic context with interrupts disabled
58 (->runtime_idle() is still invoked the default way). This implies that these
59 callback routines must not block or sleep, but it also means that the
60 synchronous helper functions listed at the end of Section 4 can be used within
61 an interrupt handler or in an atomic context.
63 The subsystem-level suspend callback is _entirely_ _responsible_ for handling
64 the suspend of the device as appropriate, which may, but need not include
65 executing the device driver's own ->runtime_suspend() callback (from the
66 PM core's point of view it is not necessary to implement a ->runtime_suspend()
67 callback in a device driver as long as the subsystem-level suspend callback
68 knows what to do to handle the device).
70 * Once the subsystem-level suspend callback has completed successfully
71 for given device, the PM core regards the device as suspended, which need
72 not mean that the device has been put into a low power state. It is
73 supposed to mean, however, that the device will not process data and will
74 not communicate with the CPU(s) and RAM until the subsystem-level resume
75 callback is executed for it. The run-time PM status of a device after
76 successful execution of the subsystem-level suspend callback is 'suspended'.
78 * If the subsystem-level suspend callback returns -EBUSY or -EAGAIN,
79 the device's run-time PM status is 'active', which means that the device
80 _must_ be fully operational afterwards.
82 * If the subsystem-level suspend callback returns an error code different
83 from -EBUSY or -EAGAIN, the PM core regards this as a fatal error and will
84 refuse to run the helper functions described in Section 4 for the device,
85 until the status of it is directly set either to 'active', or to 'suspended'
86 (the PM core provides special helper functions for this purpose).
88 In particular, if the driver requires remote wake-up capability (i.e. hardware
89 mechanism allowing the device to request a change of its power state, such as
90 PCI PME) for proper functioning and device_run_wake() returns 'false' for the
91 device, then ->runtime_suspend() should return -EBUSY. On the other hand, if
92 device_run_wake() returns 'true' for the device and the device is put into a low
93 power state during the execution of the subsystem-level suspend callback, it is
94 expected that remote wake-up will be enabled for the device. Generally, remote
95 wake-up should be enabled for all input devices put into a low power state at
98 The subsystem-level resume callback is _entirely_ _responsible_ for handling the
99 resume of the device as appropriate, which may, but need not include executing
100 the device driver's own ->runtime_resume() callback (from the PM core's point of
101 view it is not necessary to implement a ->runtime_resume() callback in a device
102 driver as long as the subsystem-level resume callback knows what to do to handle
105 * Once the subsystem-level resume callback has completed successfully, the PM
106 core regards the device as fully operational, which means that the device
107 _must_ be able to complete I/O operations as needed. The run-time PM status
108 of the device is then 'active'.
110 * If the subsystem-level resume callback returns an error code, the PM core
111 regards this as a fatal error and will refuse to run the helper functions
112 described in Section 4 for the device, until its status is directly set
113 either to 'active' or to 'suspended' (the PM core provides special helper
114 functions for this purpose).
116 The subsystem-level idle callback is executed by the PM core whenever the device
117 appears to be idle, which is indicated to the PM core by two counters, the
118 device's usage counter and the counter of 'active' children of the device.
120 * If any of these counters is decreased using a helper function provided by
121 the PM core and it turns out to be equal to zero, the other counter is
122 checked. If that counter also is equal to zero, the PM core executes the
123 subsystem-level idle callback with the device as an argument.
125 The action performed by a subsystem-level idle callback is totally dependent on
126 the subsystem in question, but the expected and recommended action is to check
127 if the device can be suspended (i.e. if all of the conditions necessary for
128 suspending the device are satisfied) and to queue up a suspend request for the
129 device in that case. The value returned by this callback is ignored by the PM
132 The helper functions provided by the PM core, described in Section 4, guarantee
133 that the following constraints are met with respect to the bus type's run-time
136 (1) The callbacks are mutually exclusive (e.g. it is forbidden to execute
137 ->runtime_suspend() in parallel with ->runtime_resume() or with another
138 instance of ->runtime_suspend() for the same device) with the exception that
139 ->runtime_suspend() or ->runtime_resume() can be executed in parallel with
140 ->runtime_idle() (although ->runtime_idle() will not be started while any
141 of the other callbacks is being executed for the same device).
143 (2) ->runtime_idle() and ->runtime_suspend() can only be executed for 'active'
144 devices (i.e. the PM core will only execute ->runtime_idle() or
145 ->runtime_suspend() for the devices the run-time PM status of which is
148 (3) ->runtime_idle() and ->runtime_suspend() can only be executed for a device
149 the usage counter of which is equal to zero _and_ either the counter of
150 'active' children of which is equal to zero, or the 'power.ignore_children'
151 flag of which is set.
153 (4) ->runtime_resume() can only be executed for 'suspended' devices (i.e. the
154 PM core will only execute ->runtime_resume() for the devices the run-time
155 PM status of which is 'suspended').
157 Additionally, the helper functions provided by the PM core obey the following
160 * If ->runtime_suspend() is about to be executed or there's a pending request
161 to execute it, ->runtime_idle() will not be executed for the same device.
163 * A request to execute or to schedule the execution of ->runtime_suspend()
164 will cancel any pending requests to execute ->runtime_idle() for the same
167 * If ->runtime_resume() is about to be executed or there's a pending request
168 to execute it, the other callbacks will not be executed for the same device.
170 * A request to execute ->runtime_resume() will cancel any pending or
171 scheduled requests to execute the other callbacks for the same device,
172 except for scheduled autosuspends.
174 3. Run-time PM Device Fields
176 The following device run-time PM fields are present in 'struct dev_pm_info', as
177 defined in include/linux/pm.h:
179 struct timer_list suspend_timer;
180 - timer used for scheduling (delayed) suspend and autosuspend requests
182 unsigned long timer_expires;
183 - timer expiration time, in jiffies (if this is different from zero, the
184 timer is running and will expire at that time, otherwise the timer is not
187 struct work_struct work;
188 - work structure used for queuing up requests (i.e. work items in pm_wq)
190 wait_queue_head_t wait_queue;
191 - wait queue used if any of the helper functions needs to wait for another
195 - lock used for synchronisation
197 atomic_t usage_count;
198 - the usage counter of the device
200 atomic_t child_count;
201 - the count of 'active' children of the device
203 unsigned int ignore_children;
204 - if set, the value of child_count is ignored (but still updated)
206 unsigned int disable_depth;
207 - used for disabling the helper funcions (they work normally if this is
208 equal to zero); the initial value of it is 1 (i.e. run-time PM is
209 initially disabled for all devices)
211 unsigned int runtime_error;
212 - if set, there was a fatal error (one of the callbacks returned error code
213 as described in Section 2), so the helper funtions will not work until
214 this flag is cleared; this is the error code returned by the failing
217 unsigned int idle_notification;
218 - if set, ->runtime_idle() is being executed
220 unsigned int request_pending;
221 - if set, there's a pending request (i.e. a work item queued up into pm_wq)
223 enum rpm_request request;
224 - type of request that's pending (valid if request_pending is set)
226 unsigned int deferred_resume;
227 - set if ->runtime_resume() is about to be run while ->runtime_suspend() is
228 being executed for that device and it is not practical to wait for the
229 suspend to complete; means "start a resume as soon as you've suspended"
231 unsigned int run_wake;
232 - set if the device is capable of generating run-time wake-up events
234 enum rpm_status runtime_status;
235 - the run-time PM status of the device; this field's initial value is
236 RPM_SUSPENDED, which means that each device is initially regarded by the
237 PM core as 'suspended', regardless of its real hardware status
239 unsigned int runtime_auto;
240 - if set, indicates that the user space has allowed the device driver to
241 power manage the device at run time via the /sys/devices/.../power/control
242 interface; it may only be modified with the help of the pm_runtime_allow()
243 and pm_runtime_forbid() helper functions
245 unsigned int no_callbacks;
246 - indicates that the device does not use the run-time PM callbacks (see
247 Section 8); it may be modified only by the pm_runtime_no_callbacks()
250 unsigned int irq_safe;
251 - indicates that the ->runtime_suspend() and ->runtime_resume() callbacks
252 will be invoked with the spinlock held and interrupts disabled
254 unsigned int use_autosuspend;
255 - indicates that the device's driver supports delayed autosuspend (see
256 Section 9); it may be modified only by the
257 pm_runtime{_dont}_use_autosuspend() helper functions
259 unsigned int timer_autosuspends;
260 - indicates that the PM core should attempt to carry out an autosuspend
261 when the timer expires rather than a normal suspend
263 int autosuspend_delay;
264 - the delay time (in milliseconds) to be used for autosuspend
266 unsigned long last_busy;
267 - the time (in jiffies) when the pm_runtime_mark_last_busy() helper
268 function was last called for this device; used in calculating inactivity
269 periods for autosuspend
271 All of the above fields are members of the 'power' member of 'struct device'.
273 4. Run-time PM Device Helper Functions
275 The following run-time PM helper functions are defined in
276 drivers/base/power/runtime.c and include/linux/pm_runtime.h:
278 void pm_runtime_init(struct device *dev);
279 - initialize the device run-time PM fields in 'struct dev_pm_info'
281 void pm_runtime_remove(struct device *dev);
282 - make sure that the run-time PM of the device will be disabled after
283 removing the device from device hierarchy
285 int pm_runtime_idle(struct device *dev);
286 - execute the subsystem-level idle callback for the device; returns 0 on
287 success or error code on failure, where -EINPROGRESS means that
288 ->runtime_idle() is already being executed
290 int pm_runtime_suspend(struct device *dev);
291 - execute the subsystem-level suspend callback for the device; returns 0 on
292 success, 1 if the device's run-time PM status was already 'suspended', or
293 error code on failure, where -EAGAIN or -EBUSY means it is safe to attempt
294 to suspend the device again in future and -EACCES means that
295 'power.disable_depth' is different from 0
297 int pm_runtime_autosuspend(struct device *dev);
298 - same as pm_runtime_suspend() except that the autosuspend delay is taken
299 into account; if pm_runtime_autosuspend_expiration() says the delay has
300 not yet expired then an autosuspend is scheduled for the appropriate time
303 int pm_runtime_resume(struct device *dev);
304 - execute the subsystem-level resume callback for the device; returns 0 on
305 success, 1 if the device's run-time PM status was already 'active' or
306 error code on failure, where -EAGAIN means it may be safe to attempt to
307 resume the device again in future, but 'power.runtime_error' should be
308 checked additionally, and -EACCES means that 'power.disable_depth' is
311 int pm_request_idle(struct device *dev);
312 - submit a request to execute the subsystem-level idle callback for the
313 device (the request is represented by a work item in pm_wq); returns 0 on
314 success or error code if the request has not been queued up
316 int pm_request_autosuspend(struct device *dev);
317 - schedule the execution of the subsystem-level suspend callback for the
318 device when the autosuspend delay has expired; if the delay has already
319 expired then the work item is queued up immediately
321 int pm_schedule_suspend(struct device *dev, unsigned int delay);
322 - schedule the execution of the subsystem-level suspend callback for the
323 device in future, where 'delay' is the time to wait before queuing up a
324 suspend work item in pm_wq, in milliseconds (if 'delay' is zero, the work
325 item is queued up immediately); returns 0 on success, 1 if the device's PM
326 run-time status was already 'suspended', or error code if the request
327 hasn't been scheduled (or queued up if 'delay' is 0); if the execution of
328 ->runtime_suspend() is already scheduled and not yet expired, the new
329 value of 'delay' will be used as the time to wait
331 int pm_request_resume(struct device *dev);
332 - submit a request to execute the subsystem-level resume callback for the
333 device (the request is represented by a work item in pm_wq); returns 0 on
334 success, 1 if the device's run-time PM status was already 'active', or
335 error code if the request hasn't been queued up
337 void pm_runtime_get_noresume(struct device *dev);
338 - increment the device's usage counter
340 int pm_runtime_get(struct device *dev);
341 - increment the device's usage counter, run pm_request_resume(dev) and
344 int pm_runtime_get_sync(struct device *dev);
345 - increment the device's usage counter, run pm_runtime_resume(dev) and
348 void pm_runtime_put_noidle(struct device *dev);
349 - decrement the device's usage counter
351 int pm_runtime_put(struct device *dev);
352 - decrement the device's usage counter; if the result is 0 then run
353 pm_request_idle(dev) and return its result
355 int pm_runtime_put_autosuspend(struct device *dev);
356 - decrement the device's usage counter; if the result is 0 then run
357 pm_request_autosuspend(dev) and return its result
359 int pm_runtime_put_sync(struct device *dev);
360 - decrement the device's usage counter; if the result is 0 then run
361 pm_runtime_idle(dev) and return its result
363 int pm_runtime_put_sync_suspend(struct device *dev);
364 - decrement the device's usage counter; if the result is 0 then run
365 pm_runtime_suspend(dev) and return its result
367 int pm_runtime_put_sync_autosuspend(struct device *dev);
368 - decrement the device's usage counter; if the result is 0 then run
369 pm_runtime_autosuspend(dev) and return its result
371 void pm_runtime_enable(struct device *dev);
372 - decrement the device's 'power.disable_depth' field; if that field is equal
373 to zero, the run-time PM helper functions can execute subsystem-level
374 callbacks described in Section 2 for the device
376 int pm_runtime_disable(struct device *dev);
377 - increment the device's 'power.disable_depth' field (if the value of that
378 field was previously zero, this prevents subsystem-level runtime PM
379 callbacks from being run for the device), make sure that all of the pending
380 run-time PM operations on the device are either completed or canceled;
381 returns 1 if there was a resume request pending and it was necessary to
382 execute the subsystem-level resume callback for the device to satisfy that
383 request, otherwise 0 is returned
385 int pm_runtime_barrier(struct device *dev);
386 - check if there's a resume request pending for the device and resume it
387 (synchronously) in that case, cancel any other pending runtime PM requests
388 regarding it and wait for all runtime PM operations on it in progress to
389 complete; returns 1 if there was a resume request pending and it was
390 necessary to execute the subsystem-level resume callback for the device to
391 satisfy that request, otherwise 0 is returned
393 void pm_suspend_ignore_children(struct device *dev, bool enable);
394 - set/unset the power.ignore_children flag of the device
396 int pm_runtime_set_active(struct device *dev);
397 - clear the device's 'power.runtime_error' flag, set the device's run-time
398 PM status to 'active' and update its parent's counter of 'active'
399 children as appropriate (it is only valid to use this function if
400 'power.runtime_error' is set or 'power.disable_depth' is greater than
401 zero); it will fail and return error code if the device has a parent
402 which is not active and the 'power.ignore_children' flag of which is unset
404 void pm_runtime_set_suspended(struct device *dev);
405 - clear the device's 'power.runtime_error' flag, set the device's run-time
406 PM status to 'suspended' and update its parent's counter of 'active'
407 children as appropriate (it is only valid to use this function if
408 'power.runtime_error' is set or 'power.disable_depth' is greater than
411 bool pm_runtime_suspended(struct device *dev);
412 - return true if the device's runtime PM status is 'suspended' and its
413 'power.disable_depth' field is equal to zero, or false otherwise
415 void pm_runtime_allow(struct device *dev);
416 - set the power.runtime_auto flag for the device and decrease its usage
417 counter (used by the /sys/devices/.../power/control interface to
418 effectively allow the device to be power managed at run time)
420 void pm_runtime_forbid(struct device *dev);
421 - unset the power.runtime_auto flag for the device and increase its usage
422 counter (used by the /sys/devices/.../power/control interface to
423 effectively prevent the device from being power managed at run time)
425 void pm_runtime_no_callbacks(struct device *dev);
426 - set the power.no_callbacks flag for the device and remove the run-time
427 PM attributes from /sys/devices/.../power (or prevent them from being
428 added when the device is registered)
430 void pm_runtime_irq_safe(struct device *dev);
431 - set the power.irq_safe flag for the device, causing the runtime-PM
432 suspend and resume callbacks (but not the idle callback) to be invoked
433 with interrupts disabled
435 void pm_runtime_mark_last_busy(struct device *dev);
436 - set the power.last_busy field to the current time
438 void pm_runtime_use_autosuspend(struct device *dev);
439 - set the power.use_autosuspend flag, enabling autosuspend delays
441 void pm_runtime_dont_use_autosuspend(struct device *dev);
442 - clear the power.use_autosuspend flag, disabling autosuspend delays
444 void pm_runtime_set_autosuspend_delay(struct device *dev, int delay);
445 - set the power.autosuspend_delay value to 'delay' (expressed in
446 milliseconds); if 'delay' is negative then run-time suspends are
449 unsigned long pm_runtime_autosuspend_expiration(struct device *dev);
450 - calculate the time when the current autosuspend delay period will expire,
451 based on power.last_busy and power.autosuspend_delay; if the delay time
452 is 1000 ms or larger then the expiration time is rounded up to the
453 nearest second; returns 0 if the delay period has already expired or
454 power.use_autosuspend isn't set, otherwise returns the expiration time
457 It is safe to execute the following helper functions from interrupt context:
460 pm_request_autosuspend()
461 pm_schedule_suspend()
463 pm_runtime_get_noresume()
465 pm_runtime_put_noidle()
467 pm_runtime_put_autosuspend()
469 pm_suspend_ignore_children()
470 pm_runtime_set_active()
471 pm_runtime_set_suspended()
472 pm_runtime_suspended()
473 pm_runtime_mark_last_busy()
474 pm_runtime_autosuspend_expiration()
476 If pm_runtime_irq_safe() has been called for a device then the following helper
477 functions may also be used in interrupt context:
480 pm_runtime_autosuspend()
482 pm_runtime_get_sync()
483 pm_runtime_put_sync_suspend()
485 5. Run-time PM Initialization, Device Probing and Removal
487 Initially, the run-time PM is disabled for all devices, which means that the
488 majority of the run-time PM helper funtions described in Section 4 will return
489 -EAGAIN until pm_runtime_enable() is called for the device.
491 In addition to that, the initial run-time PM status of all devices is
492 'suspended', but it need not reflect the actual physical state of the device.
493 Thus, if the device is initially active (i.e. it is able to process I/O), its
494 run-time PM status must be changed to 'active', with the help of
495 pm_runtime_set_active(), before pm_runtime_enable() is called for the device.
497 However, if the device has a parent and the parent's run-time PM is enabled,
498 calling pm_runtime_set_active() for the device will affect the parent, unless
499 the parent's 'power.ignore_children' flag is set. Namely, in that case the
500 parent won't be able to suspend at run time, using the PM core's helper
501 functions, as long as the child's status is 'active', even if the child's
502 run-time PM is still disabled (i.e. pm_runtime_enable() hasn't been called for
503 the child yet or pm_runtime_disable() has been called for it). For this reason,
504 once pm_runtime_set_active() has been called for the device, pm_runtime_enable()
505 should be called for it too as soon as reasonably possible or its run-time PM
506 status should be changed back to 'suspended' with the help of
507 pm_runtime_set_suspended().
509 If the default initial run-time PM status of the device (i.e. 'suspended')
510 reflects the actual state of the device, its bus type's or its driver's
511 ->probe() callback will likely need to wake it up using one of the PM core's
512 helper functions described in Section 4. In that case, pm_runtime_resume()
513 should be used. Of course, for this purpose the device's run-time PM has to be
514 enabled earlier by calling pm_runtime_enable().
516 If the device bus type's or driver's ->probe() callback runs
517 pm_runtime_suspend() or pm_runtime_idle() or their asynchronous counterparts,
518 they will fail returning -EAGAIN, because the device's usage counter is
519 incremented by the driver core before executing ->probe(). Still, it may be
520 desirable to suspend the device as soon as ->probe() has finished, so the driver
521 core uses pm_runtime_put_sync() to invoke the subsystem-level idle callback for
522 the device at that time.
524 Moreover, the driver core prevents runtime PM callbacks from racing with the bus
525 notifier callback in __device_release_driver(), which is necessary, because the
526 notifier is used by some subsystems to carry out operations affecting the
527 runtime PM functionality. It does so by calling pm_runtime_get_sync() before
528 driver_sysfs_remove() and the BUS_NOTIFY_UNBIND_DRIVER notifications. This
529 resumes the device if it's in the suspended state and prevents it from
530 being suspended again while those routines are being executed.
532 To allow bus types and drivers to put devices into the suspended state by
533 calling pm_runtime_suspend() from their ->remove() routines, the driver core
534 executes pm_runtime_put_sync() after running the BUS_NOTIFY_UNBIND_DRIVER
535 notifications in __device_release_driver(). This requires bus types and
536 drivers to make their ->remove() callbacks avoid races with runtime PM directly,
537 but also it allows of more flexibility in the handling of devices during the
538 removal of their drivers.
540 The user space can effectively disallow the driver of the device to power manage
541 it at run time by changing the value of its /sys/devices/.../power/control
542 attribute to "on", which causes pm_runtime_forbid() to be called. In principle,
543 this mechanism may also be used by the driver to effectively turn off the
544 run-time power management of the device until the user space turns it on.
545 Namely, during the initialization the driver can make sure that the run-time PM
546 status of the device is 'active' and call pm_runtime_forbid(). It should be
547 noted, however, that if the user space has already intentionally changed the
548 value of /sys/devices/.../power/control to "auto" to allow the driver to power
549 manage the device at run time, the driver may confuse it by using
550 pm_runtime_forbid() this way.
552 6. Run-time PM and System Sleep
554 Run-time PM and system sleep (i.e., system suspend and hibernation, also known
555 as suspend-to-RAM and suspend-to-disk) interact with each other in a couple of
556 ways. If a device is active when a system sleep starts, everything is
557 straightforward. But what should happen if the device is already suspended?
559 The device may have different wake-up settings for run-time PM and system sleep.
560 For example, remote wake-up may be enabled for run-time suspend but disallowed
561 for system sleep (device_may_wakeup(dev) returns 'false'). When this happens,
562 the subsystem-level system suspend callback is responsible for changing the
563 device's wake-up setting (it may leave that to the device driver's system
564 suspend routine). It may be necessary to resume the device and suspend it again
565 in order to do so. The same is true if the driver uses different power levels
566 or other settings for run-time suspend and system sleep.
568 During system resume, the simplest approach is to bring all devices back to full
569 power, even if they had been suspended before the system suspend began. There
570 are several reasons for this, including:
572 * The device might need to switch power levels, wake-up settings, etc.
574 * Remote wake-up events might have been lost by the firmware.
576 * The device's children may need the device to be at full power in order
577 to resume themselves.
579 * The driver's idea of the device state may not agree with the device's
580 physical state. This can happen during resume from hibernation.
582 * The device might need to be reset.
584 * Even though the device was suspended, if its usage counter was > 0 then most
585 likely it would need a run-time resume in the near future anyway.
587 If the device had been suspended before the system suspend began and it's
588 brought back to full power during resume, then its run-time PM status will have
589 to be updated to reflect the actual post-system sleep status. The way to do
592 pm_runtime_disable(dev);
593 pm_runtime_set_active(dev);
594 pm_runtime_enable(dev);
596 The PM core always increments the run-time usage counter before calling the
597 ->suspend() callback and decrements it after calling the ->resume() callback.
598 Hence disabling run-time PM temporarily like this will not cause any runtime
599 suspend attempts to be permanently lost. If the usage count goes to zero
600 following the return of the ->resume() callback, the ->runtime_idle() callback
601 will be invoked as usual.
603 On some systems, however, system sleep is not entered through a global firmware
604 or hardware operation. Instead, all hardware components are put into low-power
605 states directly by the kernel in a coordinated way. Then, the system sleep
606 state effectively follows from the states the hardware components end up in
607 and the system is woken up from that state by a hardware interrupt or a similar
608 mechanism entirely under the kernel's control. As a result, the kernel never
609 gives control away and the states of all devices during resume are precisely
610 known to it. If that is the case and none of the situations listed above takes
611 place (in particular, if the system is not waking up from hibernation), it may
612 be more efficient to leave the devices that had been suspended before the system
613 suspend began in the suspended state.
615 The PM core does its best to reduce the probability of race conditions between
616 the runtime PM and system suspend/resume (and hibernation) callbacks by carrying
617 out the following operations:
619 * During system suspend it calls pm_runtime_get_noresume() and
620 pm_runtime_barrier() for every device right before executing the
621 subsystem-level .suspend() callback for it. In addition to that it calls
622 pm_runtime_disable() for every device right after executing the
623 subsystem-level .suspend() callback for it.
625 * During system resume it calls pm_runtime_enable() and pm_runtime_put_sync()
626 for every device right before and right after executing the subsystem-level
627 .resume() callback for it, respectively.
629 7. Generic subsystem callbacks
631 Subsystems may wish to conserve code space by using the set of generic power
632 management callbacks provided by the PM core, defined in
633 driver/base/power/generic_ops.c:
635 int pm_generic_runtime_idle(struct device *dev);
636 - invoke the ->runtime_idle() callback provided by the driver of this
637 device, if defined, and call pm_runtime_suspend() for this device if the
638 return value is 0 or the callback is not defined
640 int pm_generic_runtime_suspend(struct device *dev);
641 - invoke the ->runtime_suspend() callback provided by the driver of this
642 device and return its result, or return -EINVAL if not defined
644 int pm_generic_runtime_resume(struct device *dev);
645 - invoke the ->runtime_resume() callback provided by the driver of this
646 device and return its result, or return -EINVAL if not defined
648 int pm_generic_suspend(struct device *dev);
649 - if the device has not been suspended at run time, invoke the ->suspend()
650 callback provided by its driver and return its result, or return 0 if not
653 int pm_generic_suspend_noirq(struct device *dev);
654 - if pm_runtime_suspended(dev) returns "false", invoke the ->suspend_noirq()
655 callback provided by the device's driver and return its result, or return
658 int pm_generic_resume(struct device *dev);
659 - invoke the ->resume() callback provided by the driver of this device and,
660 if successful, change the device's runtime PM status to 'active'
662 int pm_generic_resume_noirq(struct device *dev);
663 - invoke the ->resume_noirq() callback provided by the driver of this device
665 int pm_generic_freeze(struct device *dev);
666 - if the device has not been suspended at run time, invoke the ->freeze()
667 callback provided by its driver and return its result, or return 0 if not
670 int pm_generic_freeze_noirq(struct device *dev);
671 - if pm_runtime_suspended(dev) returns "false", invoke the ->freeze_noirq()
672 callback provided by the device's driver and return its result, or return
675 int pm_generic_thaw(struct device *dev);
676 - if the device has not been suspended at run time, invoke the ->thaw()
677 callback provided by its driver and return its result, or return 0 if not
680 int pm_generic_thaw_noirq(struct device *dev);
681 - if pm_runtime_suspended(dev) returns "false", invoke the ->thaw_noirq()
682 callback provided by the device's driver and return its result, or return
685 int pm_generic_poweroff(struct device *dev);
686 - if the device has not been suspended at run time, invoke the ->poweroff()
687 callback provided by its driver and return its result, or return 0 if not
690 int pm_generic_poweroff_noirq(struct device *dev);
691 - if pm_runtime_suspended(dev) returns "false", run the ->poweroff_noirq()
692 callback provided by the device's driver and return its result, or return
695 int pm_generic_restore(struct device *dev);
696 - invoke the ->restore() callback provided by the driver of this device and,
697 if successful, change the device's runtime PM status to 'active'
699 int pm_generic_restore_noirq(struct device *dev);
700 - invoke the ->restore_noirq() callback provided by the device's driver
702 These functions can be assigned to the ->runtime_idle(), ->runtime_suspend(),
703 ->runtime_resume(), ->suspend(), ->suspend_noirq(), ->resume(),
704 ->resume_noirq(), ->freeze(), ->freeze_noirq(), ->thaw(), ->thaw_noirq(),
705 ->poweroff(), ->poweroff_noirq(), ->restore(), ->restore_noirq() callback
706 pointers in the subsystem-level dev_pm_ops structures.
708 If a subsystem wishes to use all of them at the same time, it can simply assign
709 the GENERIC_SUBSYS_PM_OPS macro, defined in include/linux/pm.h, to its
710 dev_pm_ops structure pointer.
712 Device drivers that wish to use the same function as a system suspend, freeze,
713 poweroff and run-time suspend callback, and similarly for system resume, thaw,
714 restore, and run-time resume, can achieve this with the help of the
715 UNIVERSAL_DEV_PM_OPS macro defined in include/linux/pm.h (possibly setting its
716 last argument to NULL).
718 8. "No-Callback" Devices
720 Some "devices" are only logical sub-devices of their parent and cannot be
721 power-managed on their own. (The prototype example is a USB interface. Entire
722 USB devices can go into low-power mode or send wake-up requests, but neither is
723 possible for individual interfaces.) The drivers for these devices have no
724 need of run-time PM callbacks; if the callbacks did exist, ->runtime_suspend()
725 and ->runtime_resume() would always return 0 without doing anything else and
726 ->runtime_idle() would always call pm_runtime_suspend().
728 Subsystems can tell the PM core about these devices by calling
729 pm_runtime_no_callbacks(). This should be done after the device structure is
730 initialized and before it is registered (although after device registration is
731 also okay). The routine will set the device's power.no_callbacks flag and
732 prevent the non-debugging run-time PM sysfs attributes from being created.
734 When power.no_callbacks is set, the PM core will not invoke the
735 ->runtime_idle(), ->runtime_suspend(), or ->runtime_resume() callbacks.
736 Instead it will assume that suspends and resumes always succeed and that idle
737 devices should be suspended.
739 As a consequence, the PM core will never directly inform the device's subsystem
740 or driver about run-time power changes. Instead, the driver for the device's
741 parent must take responsibility for telling the device's driver when the
742 parent's power state changes.
744 9. Autosuspend, or automatically-delayed suspends
746 Changing a device's power state isn't free; it requires both time and energy.
747 A device should be put in a low-power state only when there's some reason to
748 think it will remain in that state for a substantial time. A common heuristic
749 says that a device which hasn't been used for a while is liable to remain
750 unused; following this advice, drivers should not allow devices to be suspended
751 at run-time until they have been inactive for some minimum period. Even when
752 the heuristic ends up being non-optimal, it will still prevent devices from
753 "bouncing" too rapidly between low-power and full-power states.
755 The term "autosuspend" is an historical remnant. It doesn't mean that the
756 device is automatically suspended (the subsystem or driver still has to call
757 the appropriate PM routines); rather it means that run-time suspends will
758 automatically be delayed until the desired period of inactivity has elapsed.
760 Inactivity is determined based on the power.last_busy field. Drivers should
761 call pm_runtime_mark_last_busy() to update this field after carrying out I/O,
762 typically just before calling pm_runtime_put_autosuspend(). The desired length
763 of the inactivity period is a matter of policy. Subsystems can set this length
764 initially by calling pm_runtime_set_autosuspend_delay(), but after device
765 registration the length should be controlled by user space, using the
766 /sys/devices/.../power/autosuspend_delay_ms attribute.
768 In order to use autosuspend, subsystems or drivers must call
769 pm_runtime_use_autosuspend() (preferably before registering the device), and
770 thereafter they should use the various *_autosuspend() helper functions instead
771 of the non-autosuspend counterparts:
773 Instead of: pm_runtime_suspend use: pm_runtime_autosuspend;
774 Instead of: pm_schedule_suspend use: pm_request_autosuspend;
775 Instead of: pm_runtime_put use: pm_runtime_put_autosuspend;
776 Instead of: pm_runtime_put_sync use: pm_runtime_put_sync_autosuspend.
778 Drivers may also continue to use the non-autosuspend helper functions; they
779 will behave normally, not taking the autosuspend delay into account.
780 Similarly, if the power.use_autosuspend field isn't set then the autosuspend
781 helper functions will behave just like the non-autosuspend counterparts.
783 The implementation is well suited for asynchronous use in interrupt contexts.
784 However such use inevitably involves races, because the PM core can't
785 synchronize ->runtime_suspend() callbacks with the arrival of I/O requests.
786 This synchronization must be handled by the driver, using its private lock.
787 Here is a schematic pseudo-code example:
789 foo_read_or_write(struct foo_priv *foo, void *data)
791 lock(&foo->private_lock);
792 add_request_to_io_queue(foo, data);
793 if (foo->num_pending_requests++ == 0)
794 pm_runtime_get(&foo->dev);
795 if (!foo->is_suspended)
796 foo_process_next_request(foo);
797 unlock(&foo->private_lock);
800 foo_io_completion(struct foo_priv *foo, void *req)
802 lock(&foo->private_lock);
803 if (--foo->num_pending_requests == 0) {
804 pm_runtime_mark_last_busy(&foo->dev);
805 pm_runtime_put_autosuspend(&foo->dev);
807 foo_process_next_request(foo);
809 unlock(&foo->private_lock);
810 /* Send req result back to the user ... */
813 int foo_runtime_suspend(struct device *dev)
815 struct foo_priv foo = container_of(dev, ...);
818 lock(&foo->private_lock);
819 if (foo->num_pending_requests > 0) {
822 /* ... suspend the device ... */
823 foo->is_suspended = 1;
825 unlock(&foo->private_lock);
829 int foo_runtime_resume(struct device *dev)
831 struct foo_priv foo = container_of(dev, ...);
833 lock(&foo->private_lock);
834 /* ... resume the device ... */
835 foo->is_suspended = 0;
836 pm_runtime_mark_last_busy(&foo->dev);
837 if (foo->num_pending_requests > 0)
838 foo_process_requests(foo);
839 unlock(&foo->private_lock);
843 The important point is that after foo_io_completion() asks for an autosuspend,
844 the foo_runtime_suspend() callback may race with foo_read_or_write().
845 Therefore foo_runtime_suspend() has to check whether there are any pending I/O
846 requests (while holding the private lock) before allowing the suspend to
849 In addition, the power.autosuspend_delay field can be changed by user space at
850 any time. If a driver cares about this, it can call
851 pm_runtime_autosuspend_expiration() from within the ->runtime_suspend()
852 callback while holding its private lock. If the function returns a nonzero
853 value then the delay has not yet expired and the callback should return