1 Run-time Power Management Framework for I/O Devices
3 (C) 2009 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 bus type, or device type (if the bus
48 type's callback is not defined), or device class (if the bus type's and device
49 type's callbacks are not defined) of given device. The bus type, device type
50 and device class callbacks are referred to as subsystem-level callbacks in what
53 The subsystem-level suspend callback is _entirely_ _responsible_ for handling
54 the suspend of the device as appropriate, which may, but need not include
55 executing the device driver's own ->runtime_suspend() callback (from the
56 PM core's point of view it is not necessary to implement a ->runtime_suspend()
57 callback in a device driver as long as the subsystem-level suspend callback
58 knows what to do to handle the device).
60 * Once the subsystem-level suspend callback has completed successfully
61 for given device, the PM core regards the device as suspended, which need
62 not mean that the device has been put into a low power state. It is
63 supposed to mean, however, that the device will not process data and will
64 not communicate with the CPU(s) and RAM until the subsystem-level resume
65 callback is executed for it. The run-time PM status of a device after
66 successful execution of the subsystem-level suspend callback is 'suspended'.
68 * If the subsystem-level suspend callback returns -EBUSY or -EAGAIN,
69 the device's run-time PM status is 'active', which means that the device
70 _must_ be fully operational afterwards.
72 * If the subsystem-level suspend callback returns an error code different
73 from -EBUSY or -EAGAIN, the PM core regards this as a fatal error and will
74 refuse to run the helper functions described in Section 4 for the device,
75 until the status of it is directly set either to 'active', or to 'suspended'
76 (the PM core provides special helper functions for this purpose).
78 In particular, if the driver requires remote wake-up capability (i.e. hardware
79 mechanism allowing the device to request a change of its power state, such as
80 PCI PME) for proper functioning and device_run_wake() returns 'false' for the
81 device, then ->runtime_suspend() should return -EBUSY. On the other hand, if
82 device_run_wake() returns 'true' for the device and the device is put into a low
83 power state during the execution of the subsystem-level suspend callback, it is
84 expected that remote wake-up will be enabled for the device. Generally, remote
85 wake-up should be enabled for all input devices put into a low power state at
88 The subsystem-level resume callback is _entirely_ _responsible_ for handling the
89 resume of the device as appropriate, which may, but need not include executing
90 the device driver's own ->runtime_resume() callback (from the PM core's point of
91 view it is not necessary to implement a ->runtime_resume() callback in a device
92 driver as long as the subsystem-level resume callback knows what to do to handle
95 * Once the subsystem-level resume callback has completed successfully, the PM
96 core regards the device as fully operational, which means that the device
97 _must_ be able to complete I/O operations as needed. The run-time PM status
98 of the device is then 'active'.
100 * If the subsystem-level resume callback returns an error code, the PM core
101 regards this as a fatal error and will refuse to run the helper functions
102 described in Section 4 for the device, until its status is directly set
103 either to 'active' or to 'suspended' (the PM core provides special helper
104 functions for this purpose).
106 The subsystem-level idle callback is executed by the PM core whenever the device
107 appears to be idle, which is indicated to the PM core by two counters, the
108 device's usage counter and the counter of 'active' children of the device.
110 * If any of these counters is decreased using a helper function provided by
111 the PM core and it turns out to be equal to zero, the other counter is
112 checked. If that counter also is equal to zero, the PM core executes the
113 subsystem-level idle callback with the device as an argument.
115 The action performed by a subsystem-level idle callback is totally dependent on
116 the subsystem in question, but the expected and recommended action is to check
117 if the device can be suspended (i.e. if all of the conditions necessary for
118 suspending the device are satisfied) and to queue up a suspend request for the
119 device in that case. The value returned by this callback is ignored by the PM
122 The helper functions provided by the PM core, described in Section 4, guarantee
123 that the following constraints are met with respect to the bus type's run-time
126 (1) The callbacks are mutually exclusive (e.g. it is forbidden to execute
127 ->runtime_suspend() in parallel with ->runtime_resume() or with another
128 instance of ->runtime_suspend() for the same device) with the exception that
129 ->runtime_suspend() or ->runtime_resume() can be executed in parallel with
130 ->runtime_idle() (although ->runtime_idle() will not be started while any
131 of the other callbacks is being executed for the same device).
133 (2) ->runtime_idle() and ->runtime_suspend() can only be executed for 'active'
134 devices (i.e. the PM core will only execute ->runtime_idle() or
135 ->runtime_suspend() for the devices the run-time PM status of which is
138 (3) ->runtime_idle() and ->runtime_suspend() can only be executed for a device
139 the usage counter of which is equal to zero _and_ either the counter of
140 'active' children of which is equal to zero, or the 'power.ignore_children'
141 flag of which is set.
143 (4) ->runtime_resume() can only be executed for 'suspended' devices (i.e. the
144 PM core will only execute ->runtime_resume() for the devices the run-time
145 PM status of which is 'suspended').
147 Additionally, the helper functions provided by the PM core obey the following
150 * If ->runtime_suspend() is about to be executed or there's a pending request
151 to execute it, ->runtime_idle() will not be executed for the same device.
153 * A request to execute or to schedule the execution of ->runtime_suspend()
154 will cancel any pending requests to execute ->runtime_idle() for the same
157 * If ->runtime_resume() is about to be executed or there's a pending request
158 to execute it, the other callbacks will not be executed for the same device.
160 * A request to execute ->runtime_resume() will cancel any pending or
161 scheduled requests to execute the other callbacks for the same device.
163 3. Run-time PM Device Fields
165 The following device run-time PM fields are present in 'struct dev_pm_info', as
166 defined in include/linux/pm.h:
168 struct timer_list suspend_timer;
169 - timer used for scheduling (delayed) suspend request
171 unsigned long timer_expires;
172 - timer expiration time, in jiffies (if this is different from zero, the
173 timer is running and will expire at that time, otherwise the timer is not
176 struct work_struct work;
177 - work structure used for queuing up requests (i.e. work items in pm_wq)
179 wait_queue_head_t wait_queue;
180 - wait queue used if any of the helper functions needs to wait for another
184 - lock used for synchronisation
186 atomic_t usage_count;
187 - the usage counter of the device
189 atomic_t child_count;
190 - the count of 'active' children of the device
192 unsigned int ignore_children;
193 - if set, the value of child_count is ignored (but still updated)
195 unsigned int disable_depth;
196 - used for disabling the helper funcions (they work normally if this is
197 equal to zero); the initial value of it is 1 (i.e. run-time PM is
198 initially disabled for all devices)
200 unsigned int runtime_error;
201 - if set, there was a fatal error (one of the callbacks returned error code
202 as described in Section 2), so the helper funtions will not work until
203 this flag is cleared; this is the error code returned by the failing
206 unsigned int idle_notification;
207 - if set, ->runtime_idle() is being executed
209 unsigned int request_pending;
210 - if set, there's a pending request (i.e. a work item queued up into pm_wq)
212 enum rpm_request request;
213 - type of request that's pending (valid if request_pending is set)
215 unsigned int deferred_resume;
216 - set if ->runtime_resume() is about to be run while ->runtime_suspend() is
217 being executed for that device and it is not practical to wait for the
218 suspend to complete; means "start a resume as soon as you've suspended"
220 unsigned int run_wake;
221 - set if the device is capable of generating run-time wake-up events
223 enum rpm_status runtime_status;
224 - the run-time PM status of the device; this field's initial value is
225 RPM_SUSPENDED, which means that each device is initially regarded by the
226 PM core as 'suspended', regardless of its real hardware status
228 unsigned int runtime_auto;
229 - if set, indicates that the user space has allowed the device driver to
230 power manage the device at run time via the /sys/devices/.../power/control
231 interface; it may only be modified with the help of the pm_runtime_allow()
232 and pm_runtime_forbid() helper functions
234 unsigned int no_callbacks;
235 - indicates that the device does not use the run-time PM callbacks (see
236 Section 8); it may be modified only by the pm_runtime_no_callbacks()
239 All of the above fields are members of the 'power' member of 'struct device'.
241 4. Run-time PM Device Helper Functions
243 The following run-time PM helper functions are defined in
244 drivers/base/power/runtime.c and include/linux/pm_runtime.h:
246 void pm_runtime_init(struct device *dev);
247 - initialize the device run-time PM fields in 'struct dev_pm_info'
249 void pm_runtime_remove(struct device *dev);
250 - make sure that the run-time PM of the device will be disabled after
251 removing the device from device hierarchy
253 int pm_runtime_idle(struct device *dev);
254 - execute the subsystem-level idle callback for the device; returns 0 on
255 success or error code on failure, where -EINPROGRESS means that
256 ->runtime_idle() is already being executed
258 int pm_runtime_suspend(struct device *dev);
259 - execute the subsystem-level suspend callback for the device; returns 0 on
260 success, 1 if the device's run-time PM status was already 'suspended', or
261 error code on failure, where -EAGAIN or -EBUSY means it is safe to attempt
262 to suspend the device again in future
264 int pm_runtime_resume(struct device *dev);
265 - execute the subsystem-level resume callback for the device; returns 0 on
266 success, 1 if the device's run-time PM status was already 'active' or
267 error code on failure, where -EAGAIN means it may be safe to attempt to
268 resume the device again in future, but 'power.runtime_error' should be
271 int pm_request_idle(struct device *dev);
272 - submit a request to execute the subsystem-level idle callback for the
273 device (the request is represented by a work item in pm_wq); returns 0 on
274 success or error code if the request has not been queued up
276 int pm_schedule_suspend(struct device *dev, unsigned int delay);
277 - schedule the execution of the subsystem-level suspend callback for the
278 device in future, where 'delay' is the time to wait before queuing up a
279 suspend work item in pm_wq, in milliseconds (if 'delay' is zero, the work
280 item is queued up immediately); returns 0 on success, 1 if the device's PM
281 run-time status was already 'suspended', or error code if the request
282 hasn't been scheduled (or queued up if 'delay' is 0); if the execution of
283 ->runtime_suspend() is already scheduled and not yet expired, the new
284 value of 'delay' will be used as the time to wait
286 int pm_request_resume(struct device *dev);
287 - submit a request to execute the subsystem-level resume callback for the
288 device (the request is represented by a work item in pm_wq); returns 0 on
289 success, 1 if the device's run-time PM status was already 'active', or
290 error code if the request hasn't been queued up
292 void pm_runtime_get_noresume(struct device *dev);
293 - increment the device's usage counter
295 int pm_runtime_get(struct device *dev);
296 - increment the device's usage counter, run pm_request_resume(dev) and
299 int pm_runtime_get_sync(struct device *dev);
300 - increment the device's usage counter, run pm_runtime_resume(dev) and
303 void pm_runtime_put_noidle(struct device *dev);
304 - decrement the device's usage counter
306 int pm_runtime_put(struct device *dev);
307 - decrement the device's usage counter, run pm_request_idle(dev) and return
310 int pm_runtime_put_sync(struct device *dev);
311 - decrement the device's usage counter, run pm_runtime_idle(dev) and return
314 void pm_runtime_enable(struct device *dev);
315 - enable the run-time PM helper functions to run the device bus type's
316 run-time PM callbacks described in Section 2
318 int pm_runtime_disable(struct device *dev);
319 - prevent the run-time PM helper functions from running subsystem-level
320 run-time PM callbacks for the device, make sure that all of the pending
321 run-time PM operations on the device are either completed or canceled;
322 returns 1 if there was a resume request pending and it was necessary to
323 execute the subsystem-level resume callback for the device to satisfy that
324 request, otherwise 0 is returned
326 void pm_suspend_ignore_children(struct device *dev, bool enable);
327 - set/unset the power.ignore_children flag of the device
329 int pm_runtime_set_active(struct device *dev);
330 - clear the device's 'power.runtime_error' flag, set the device's run-time
331 PM status to 'active' and update its parent's counter of 'active'
332 children as appropriate (it is only valid to use this function if
333 'power.runtime_error' is set or 'power.disable_depth' is greater than
334 zero); it will fail and return error code if the device has a parent
335 which is not active and the 'power.ignore_children' flag of which is unset
337 void pm_runtime_set_suspended(struct device *dev);
338 - clear the device's 'power.runtime_error' flag, set the device's run-time
339 PM status to 'suspended' and update its parent's counter of 'active'
340 children as appropriate (it is only valid to use this function if
341 'power.runtime_error' is set or 'power.disable_depth' is greater than
344 bool pm_runtime_suspended(struct device *dev);
345 - return true if the device's runtime PM status is 'suspended', or false
348 void pm_runtime_allow(struct device *dev);
349 - set the power.runtime_auto flag for the device and decrease its usage
350 counter (used by the /sys/devices/.../power/control interface to
351 effectively allow the device to be power managed at run time)
353 void pm_runtime_forbid(struct device *dev);
354 - unset the power.runtime_auto flag for the device and increase its usage
355 counter (used by the /sys/devices/.../power/control interface to
356 effectively prevent the device from being power managed at run time)
358 void pm_runtime_no_callbacks(struct device *dev);
359 - set the power.no_callbacks flag for the device and remove the run-time
360 PM attributes from /sys/devices/.../power (or prevent them from being
361 added when the device is registered)
363 It is safe to execute the following helper functions from interrupt context:
366 pm_schedule_suspend()
368 pm_runtime_get_noresume()
370 pm_runtime_put_noidle()
372 pm_suspend_ignore_children()
373 pm_runtime_set_active()
374 pm_runtime_set_suspended()
377 5. Run-time PM Initialization, Device Probing and Removal
379 Initially, the run-time PM is disabled for all devices, which means that the
380 majority of the run-time PM helper funtions described in Section 4 will return
381 -EAGAIN until pm_runtime_enable() is called for the device.
383 In addition to that, the initial run-time PM status of all devices is
384 'suspended', but it need not reflect the actual physical state of the device.
385 Thus, if the device is initially active (i.e. it is able to process I/O), its
386 run-time PM status must be changed to 'active', with the help of
387 pm_runtime_set_active(), before pm_runtime_enable() is called for the device.
389 However, if the device has a parent and the parent's run-time PM is enabled,
390 calling pm_runtime_set_active() for the device will affect the parent, unless
391 the parent's 'power.ignore_children' flag is set. Namely, in that case the
392 parent won't be able to suspend at run time, using the PM core's helper
393 functions, as long as the child's status is 'active', even if the child's
394 run-time PM is still disabled (i.e. pm_runtime_enable() hasn't been called for
395 the child yet or pm_runtime_disable() has been called for it). For this reason,
396 once pm_runtime_set_active() has been called for the device, pm_runtime_enable()
397 should be called for it too as soon as reasonably possible or its run-time PM
398 status should be changed back to 'suspended' with the help of
399 pm_runtime_set_suspended().
401 If the default initial run-time PM status of the device (i.e. 'suspended')
402 reflects the actual state of the device, its bus type's or its driver's
403 ->probe() callback will likely need to wake it up using one of the PM core's
404 helper functions described in Section 4. In that case, pm_runtime_resume()
405 should be used. Of course, for this purpose the device's run-time PM has to be
406 enabled earlier by calling pm_runtime_enable().
408 If the device bus type's or driver's ->probe() or ->remove() callback runs
409 pm_runtime_suspend() or pm_runtime_idle() or their asynchronous counterparts,
410 they will fail returning -EAGAIN, because the device's usage counter is
411 incremented by the core before executing ->probe() and ->remove(). Still, it
412 may be desirable to suspend the device as soon as ->probe() or ->remove() has
413 finished, so the PM core uses pm_runtime_idle_sync() to invoke the
414 subsystem-level idle callback for the device at that time.
416 The user space can effectively disallow the driver of the device to power manage
417 it at run time by changing the value of its /sys/devices/.../power/control
418 attribute to "on", which causes pm_runtime_forbid() to be called. In principle,
419 this mechanism may also be used by the driver to effectively turn off the
420 run-time power management of the device until the user space turns it on.
421 Namely, during the initialization the driver can make sure that the run-time PM
422 status of the device is 'active' and call pm_runtime_forbid(). It should be
423 noted, however, that if the user space has already intentionally changed the
424 value of /sys/devices/.../power/control to "auto" to allow the driver to power
425 manage the device at run time, the driver may confuse it by using
426 pm_runtime_forbid() this way.
428 6. Run-time PM and System Sleep
430 Run-time PM and system sleep (i.e., system suspend and hibernation, also known
431 as suspend-to-RAM and suspend-to-disk) interact with each other in a couple of
432 ways. If a device is active when a system sleep starts, everything is
433 straightforward. But what should happen if the device is already suspended?
435 The device may have different wake-up settings for run-time PM and system sleep.
436 For example, remote wake-up may be enabled for run-time suspend but disallowed
437 for system sleep (device_may_wakeup(dev) returns 'false'). When this happens,
438 the subsystem-level system suspend callback is responsible for changing the
439 device's wake-up setting (it may leave that to the device driver's system
440 suspend routine). It may be necessary to resume the device and suspend it again
441 in order to do so. The same is true if the driver uses different power levels
442 or other settings for run-time suspend and system sleep.
444 During system resume, devices generally should be brought back to full power,
445 even if they were suspended before the system sleep began. There are several
446 reasons for this, including:
448 * The device might need to switch power levels, wake-up settings, etc.
450 * Remote wake-up events might have been lost by the firmware.
452 * The device's children may need the device to be at full power in order
453 to resume themselves.
455 * The driver's idea of the device state may not agree with the device's
456 physical state. This can happen during resume from hibernation.
458 * The device might need to be reset.
460 * Even though the device was suspended, if its usage counter was > 0 then most
461 likely it would need a run-time resume in the near future anyway.
463 * Always going back to full power is simplest.
465 If the device was suspended before the sleep began, then its run-time PM status
466 will have to be updated to reflect the actual post-system sleep status. The way
469 pm_runtime_disable(dev);
470 pm_runtime_set_active(dev);
471 pm_runtime_enable(dev);
473 The PM core always increments the run-time usage counter before calling the
474 ->prepare() callback and decrements it after calling the ->complete() callback.
475 Hence disabling run-time PM temporarily like this will not cause any run-time
476 suspend callbacks to be lost.
478 7. Generic subsystem callbacks
480 Subsystems may wish to conserve code space by using the set of generic power
481 management callbacks provided by the PM core, defined in
482 driver/base/power/generic_ops.c:
484 int pm_generic_runtime_idle(struct device *dev);
485 - invoke the ->runtime_idle() callback provided by the driver of this
486 device, if defined, and call pm_runtime_suspend() for this device if the
487 return value is 0 or the callback is not defined
489 int pm_generic_runtime_suspend(struct device *dev);
490 - invoke the ->runtime_suspend() callback provided by the driver of this
491 device and return its result, or return -EINVAL if not defined
493 int pm_generic_runtime_resume(struct device *dev);
494 - invoke the ->runtime_resume() callback provided by the driver of this
495 device and return its result, or return -EINVAL if not defined
497 int pm_generic_suspend(struct device *dev);
498 - if the device has not been suspended at run time, invoke the ->suspend()
499 callback provided by its driver and return its result, or return 0 if not
502 int pm_generic_resume(struct device *dev);
503 - invoke the ->resume() callback provided by the driver of this device and,
504 if successful, change the device's runtime PM status to 'active'
506 int pm_generic_freeze(struct device *dev);
507 - if the device has not been suspended at run time, invoke the ->freeze()
508 callback provided by its driver and return its result, or return 0 if not
511 int pm_generic_thaw(struct device *dev);
512 - if the device has not been suspended at run time, invoke the ->thaw()
513 callback provided by its driver and return its result, or return 0 if not
516 int pm_generic_poweroff(struct device *dev);
517 - if the device has not been suspended at run time, invoke the ->poweroff()
518 callback provided by its driver and return its result, or return 0 if not
521 int pm_generic_restore(struct device *dev);
522 - invoke the ->restore() callback provided by the driver of this device and,
523 if successful, change the device's runtime PM status to 'active'
525 These functions can be assigned to the ->runtime_idle(), ->runtime_suspend(),
526 ->runtime_resume(), ->suspend(), ->resume(), ->freeze(), ->thaw(), ->poweroff(),
527 or ->restore() callback pointers in the subsystem-level dev_pm_ops structures.
529 If a subsystem wishes to use all of them at the same time, it can simply assign
530 the GENERIC_SUBSYS_PM_OPS macro, defined in include/linux/pm.h, to its
531 dev_pm_ops structure pointer.
533 Device drivers that wish to use the same function as a system suspend, freeze,
534 poweroff and run-time suspend callback, and similarly for system resume, thaw,
535 restore, and run-time resume, can achieve this with the help of the
536 UNIVERSAL_DEV_PM_OPS macro defined in include/linux/pm.h (possibly setting its
537 last argument to NULL).
539 8. "No-Callback" Devices
541 Some "devices" are only logical sub-devices of their parent and cannot be
542 power-managed on their own. (The prototype example is a USB interface. Entire
543 USB devices can go into low-power mode or send wake-up requests, but neither is
544 possible for individual interfaces.) The drivers for these devices have no
545 need of run-time PM callbacks; if the callbacks did exist, ->runtime_suspend()
546 and ->runtime_resume() would always return 0 without doing anything else and
547 ->runtime_idle() would always call pm_runtime_suspend().
549 Subsystems can tell the PM core about these devices by calling
550 pm_runtime_no_callbacks(). This should be done after the device structure is
551 initialized and before it is registered (although after device registration is
552 also okay). The routine will set the device's power.no_callbacks flag and
553 prevent the non-debugging run-time PM sysfs attributes from being created.
555 When power.no_callbacks is set, the PM core will not invoke the
556 ->runtime_idle(), ->runtime_suspend(), or ->runtime_resume() callbacks.
557 Instead it will assume that suspends and resumes always succeed and that idle
558 devices should be suspended.
560 As a consequence, the PM core will never directly inform the device's subsystem
561 or driver about run-time power changes. Instead, the driver for the device's
562 parent must take responsibility for telling the device's driver when the
563 parent's power state changes.