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Rafael J. Wysocki5e928f72009-08-18 23:38:32 +02001Run-time Power Management Framework for I/O Devices
2
3(C) 2009 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
4
51. Introduction
6
7Support for run-time power management (run-time PM) of I/O devices is provided
8at the power management core (PM core) level by means of:
9
10* The power management workqueue pm_wq in which bus types and device drivers can
11 put their PM-related work items. It is strongly recommended that pm_wq be
12 used for queuing all work items related to run-time PM, because this allows
13 them to be synchronized with system-wide power transitions (suspend to RAM,
14 hibernation and resume from system sleep states). pm_wq is declared in
15 include/linux/pm_runtime.h and defined in kernel/power/main.c.
16
17* A number of run-time PM fields in the 'power' member of 'struct device' (which
18 is of the type 'struct dev_pm_info', defined in include/linux/pm.h) that can
19 be used for synchronizing run-time PM operations with one another.
20
21* Three device run-time PM callbacks in 'struct dev_pm_ops' (defined in
22 include/linux/pm.h).
23
24* A set of helper functions defined in drivers/base/power/runtime.c that can be
25 used for carrying out run-time PM operations in such a way that the
26 synchronization between them is taken care of by the PM core. Bus types and
27 device drivers are encouraged to use these functions.
28
29The run-time PM callbacks present in 'struct dev_pm_ops', the device run-time PM
30fields of 'struct dev_pm_info' and the core helper functions provided for
31run-time PM are described below.
32
332. Device Run-time PM Callbacks
34
35There are three device run-time PM callbacks defined in 'struct dev_pm_ops':
36
37struct dev_pm_ops {
38 ...
39 int (*runtime_suspend)(struct device *dev);
40 int (*runtime_resume)(struct device *dev);
Rafael J. Wysockie1b19032009-12-03 21:04:08 +010041 int (*runtime_idle)(struct device *dev);
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +020042 ...
43};
44
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +010045The ->runtime_suspend(), ->runtime_resume() and ->runtime_idle() callbacks are
46executed by the PM core for either the bus type, or device type (if the bus
47type's callback is not defined), or device class (if the bus type's and device
48type's callbacks are not defined) of given device. The bus type, device type
49and device class callbacks are referred to as subsystem-level callbacks in what
50follows.
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +020051
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +010052The subsystem-level suspend callback is _entirely_ _responsible_ for handling
53the suspend of the device as appropriate, which may, but need not include
54executing the device driver's own ->runtime_suspend() callback (from the
55PM core's point of view it is not necessary to implement a ->runtime_suspend()
56callback in a device driver as long as the subsystem-level suspend callback
57knows what to do to handle the device).
58
59 * Once the subsystem-level suspend callback has completed successfully
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +020060 for given device, the PM core regards the device as suspended, which need
61 not mean that the device has been put into a low power state. It is
62 supposed to mean, however, that the device will not process data and will
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +010063 not communicate with the CPU(s) and RAM until the subsystem-level resume
64 callback is executed for it. The run-time PM status of a device after
65 successful execution of the subsystem-level suspend callback is 'suspended'.
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +020066
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +010067 * If the subsystem-level suspend callback returns -EBUSY or -EAGAIN,
68 the device's run-time PM status is 'active', which means that the device
69 _must_ be fully operational afterwards.
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +020070
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +010071 * If the subsystem-level suspend callback returns an error code different
72 from -EBUSY or -EAGAIN, the PM core regards this as a fatal error and will
73 refuse to run the helper functions described in Section 4 for the device,
74 until the status of it is directly set either to 'active', or to 'suspended'
75 (the PM core provides special helper functions for this purpose).
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +020076
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +010077In particular, if the driver requires remote wake-up capability (i.e. hardware
78mechanism allowing the device to request a change of its power state, such as
79PCI PME) for proper functioning and device_run_wake() returns 'false' for the
80device, then ->runtime_suspend() should return -EBUSY. On the other hand, if
81device_run_wake() returns 'true' for the device and the device is put into a low
82power state during the execution of the subsystem-level suspend callback, it is
83expected that remote wake-up will be enabled for the device. Generally, remote
84wake-up should be enabled for all input devices put into a low power state at
85run time.
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +020086
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +010087The subsystem-level resume callback is _entirely_ _responsible_ for handling the
88resume of the device as appropriate, which may, but need not include executing
89the device driver's own ->runtime_resume() callback (from the PM core's point of
90view it is not necessary to implement a ->runtime_resume() callback in a device
91driver as long as the subsystem-level resume callback knows what to do to handle
92the device).
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +020093
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +010094 * Once the subsystem-level resume callback has completed successfully, the PM
95 core regards the device as fully operational, which means that the device
96 _must_ be able to complete I/O operations as needed. The run-time PM status
97 of the device is then 'active'.
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +020098
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +010099 * If the subsystem-level resume callback returns an error code, the PM core
100 regards this as a fatal error and will refuse to run the helper functions
101 described in Section 4 for the device, until its status is directly set
102 either to 'active' or to 'suspended' (the PM core provides special helper
103 functions for this purpose).
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200104
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +0100105The subsystem-level idle callback is executed by the PM core whenever the device
106appears to be idle, which is indicated to the PM core by two counters, the
107device's usage counter and the counter of 'active' children of the device.
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200108
109 * If any of these counters is decreased using a helper function provided by
110 the PM core and it turns out to be equal to zero, the other counter is
111 checked. If that counter also is equal to zero, the PM core executes the
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +0100112 subsystem-level idle callback with the device as an argument.
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200113
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +0100114The action performed by a subsystem-level idle callback is totally dependent on
115the subsystem in question, but the expected and recommended action is to check
116if the device can be suspended (i.e. if all of the conditions necessary for
117suspending the device are satisfied) and to queue up a suspend request for the
118device in that case. The value returned by this callback is ignored by the PM
119core.
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200120
121The helper functions provided by the PM core, described in Section 4, guarantee
122that the following constraints are met with respect to the bus type's run-time
123PM callbacks:
124
125(1) The callbacks are mutually exclusive (e.g. it is forbidden to execute
126 ->runtime_suspend() in parallel with ->runtime_resume() or with another
127 instance of ->runtime_suspend() for the same device) with the exception that
128 ->runtime_suspend() or ->runtime_resume() can be executed in parallel with
129 ->runtime_idle() (although ->runtime_idle() will not be started while any
130 of the other callbacks is being executed for the same device).
131
132(2) ->runtime_idle() and ->runtime_suspend() can only be executed for 'active'
133 devices (i.e. the PM core will only execute ->runtime_idle() or
134 ->runtime_suspend() for the devices the run-time PM status of which is
135 'active').
136
137(3) ->runtime_idle() and ->runtime_suspend() can only be executed for a device
138 the usage counter of which is equal to zero _and_ either the counter of
139 'active' children of which is equal to zero, or the 'power.ignore_children'
140 flag of which is set.
141
142(4) ->runtime_resume() can only be executed for 'suspended' devices (i.e. the
143 PM core will only execute ->runtime_resume() for the devices the run-time
144 PM status of which is 'suspended').
145
146Additionally, the helper functions provided by the PM core obey the following
147rules:
148
149 * If ->runtime_suspend() is about to be executed or there's a pending request
150 to execute it, ->runtime_idle() will not be executed for the same device.
151
152 * A request to execute or to schedule the execution of ->runtime_suspend()
153 will cancel any pending requests to execute ->runtime_idle() for the same
154 device.
155
156 * If ->runtime_resume() is about to be executed or there's a pending request
157 to execute it, the other callbacks will not be executed for the same device.
158
159 * A request to execute ->runtime_resume() will cancel any pending or
160 scheduled requests to execute the other callbacks for the same device.
161
1623. Run-time PM Device Fields
163
164The following device run-time PM fields are present in 'struct dev_pm_info', as
165defined in include/linux/pm.h:
166
167 struct timer_list suspend_timer;
168 - timer used for scheduling (delayed) suspend request
169
170 unsigned long timer_expires;
171 - timer expiration time, in jiffies (if this is different from zero, the
172 timer is running and will expire at that time, otherwise the timer is not
173 running)
174
175 struct work_struct work;
176 - work structure used for queuing up requests (i.e. work items in pm_wq)
177
178 wait_queue_head_t wait_queue;
179 - wait queue used if any of the helper functions needs to wait for another
180 one to complete
181
182 spinlock_t lock;
183 - lock used for synchronisation
184
185 atomic_t usage_count;
186 - the usage counter of the device
187
188 atomic_t child_count;
189 - the count of 'active' children of the device
190
191 unsigned int ignore_children;
192 - if set, the value of child_count is ignored (but still updated)
193
194 unsigned int disable_depth;
195 - used for disabling the helper funcions (they work normally if this is
196 equal to zero); the initial value of it is 1 (i.e. run-time PM is
197 initially disabled for all devices)
198
199 unsigned int runtime_error;
200 - if set, there was a fatal error (one of the callbacks returned error code
201 as described in Section 2), so the helper funtions will not work until
202 this flag is cleared; this is the error code returned by the failing
203 callback
204
205 unsigned int idle_notification;
206 - if set, ->runtime_idle() is being executed
207
208 unsigned int request_pending;
209 - if set, there's a pending request (i.e. a work item queued up into pm_wq)
210
211 enum rpm_request request;
212 - type of request that's pending (valid if request_pending is set)
213
214 unsigned int deferred_resume;
215 - set if ->runtime_resume() is about to be run while ->runtime_suspend() is
216 being executed for that device and it is not practical to wait for the
217 suspend to complete; means "start a resume as soon as you've suspended"
218
Rafael J. Wysocki7a1a8eb2009-12-03 21:19:18 +0100219 unsigned int run_wake;
220 - set if the device is capable of generating run-time wake-up events
221
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200222 enum rpm_status runtime_status;
223 - the run-time PM status of the device; this field's initial value is
224 RPM_SUSPENDED, which means that each device is initially regarded by the
225 PM core as 'suspended', regardless of its real hardware status
226
227All of the above fields are members of the 'power' member of 'struct device'.
228
2294. Run-time PM Device Helper Functions
230
231The following run-time PM helper functions are defined in
232drivers/base/power/runtime.c and include/linux/pm_runtime.h:
233
234 void pm_runtime_init(struct device *dev);
235 - initialize the device run-time PM fields in 'struct dev_pm_info'
236
237 void pm_runtime_remove(struct device *dev);
238 - make sure that the run-time PM of the device will be disabled after
239 removing the device from device hierarchy
240
241 int pm_runtime_idle(struct device *dev);
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +0100242 - execute the subsystem-level idle callback for the device; returns 0 on
243 success or error code on failure, where -EINPROGRESS means that
244 ->runtime_idle() is already being executed
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200245
246 int pm_runtime_suspend(struct device *dev);
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +0100247 - execute the subsystem-level suspend callback for the device; returns 0 on
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200248 success, 1 if the device's run-time PM status was already 'suspended', or
249 error code on failure, where -EAGAIN or -EBUSY means it is safe to attempt
250 to suspend the device again in future
251
252 int pm_runtime_resume(struct device *dev);
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +0100253 - execute the subsystem-leve resume callback for the device; returns 0 on
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200254 success, 1 if the device's run-time PM status was already 'active' or
255 error code on failure, where -EAGAIN means it may be safe to attempt to
256 resume the device again in future, but 'power.runtime_error' should be
257 checked additionally
258
259 int pm_request_idle(struct device *dev);
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +0100260 - submit a request to execute the subsystem-level idle callback for the
261 device (the request is represented by a work item in pm_wq); returns 0 on
262 success or error code if the request has not been queued up
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200263
264 int pm_schedule_suspend(struct device *dev, unsigned int delay);
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +0100265 - schedule the execution of the subsystem-level suspend callback for the
266 device in future, where 'delay' is the time to wait before queuing up a
267 suspend work item in pm_wq, in milliseconds (if 'delay' is zero, the work
268 item is queued up immediately); returns 0 on success, 1 if the device's PM
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200269 run-time status was already 'suspended', or error code if the request
270 hasn't been scheduled (or queued up if 'delay' is 0); if the execution of
271 ->runtime_suspend() is already scheduled and not yet expired, the new
272 value of 'delay' will be used as the time to wait
273
274 int pm_request_resume(struct device *dev);
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +0100275 - submit a request to execute the subsystem-level resume callback for the
276 device (the request is represented by a work item in pm_wq); returns 0 on
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200277 success, 1 if the device's run-time PM status was already 'active', or
278 error code if the request hasn't been queued up
279
280 void pm_runtime_get_noresume(struct device *dev);
281 - increment the device's usage counter
282
283 int pm_runtime_get(struct device *dev);
284 - increment the device's usage counter, run pm_request_resume(dev) and
285 return its result
286
287 int pm_runtime_get_sync(struct device *dev);
288 - increment the device's usage counter, run pm_runtime_resume(dev) and
289 return its result
290
291 void pm_runtime_put_noidle(struct device *dev);
292 - decrement the device's usage counter
293
294 int pm_runtime_put(struct device *dev);
295 - decrement the device's usage counter, run pm_request_idle(dev) and return
296 its result
297
298 int pm_runtime_put_sync(struct device *dev);
299 - decrement the device's usage counter, run pm_runtime_idle(dev) and return
300 its result
301
302 void pm_runtime_enable(struct device *dev);
303 - enable the run-time PM helper functions to run the device bus type's
304 run-time PM callbacks described in Section 2
305
306 int pm_runtime_disable(struct device *dev);
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +0100307 - prevent the run-time PM helper functions from running subsystem-level
308 run-time PM callbacks for the device, make sure that all of the pending
309 run-time PM operations on the device are either completed or canceled;
310 returns 1 if there was a resume request pending and it was necessary to
311 execute the subsystem-level resume callback for the device to satisfy that
312 request, otherwise 0 is returned
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200313
314 void pm_suspend_ignore_children(struct device *dev, bool enable);
315 - set/unset the power.ignore_children flag of the device
316
317 int pm_runtime_set_active(struct device *dev);
318 - clear the device's 'power.runtime_error' flag, set the device's run-time
319 PM status to 'active' and update its parent's counter of 'active'
320 children as appropriate (it is only valid to use this function if
321 'power.runtime_error' is set or 'power.disable_depth' is greater than
322 zero); it will fail and return error code if the device has a parent
323 which is not active and the 'power.ignore_children' flag of which is unset
324
325 void pm_runtime_set_suspended(struct device *dev);
326 - clear the device's 'power.runtime_error' flag, set the device's run-time
327 PM status to 'suspended' and update its parent's counter of 'active'
328 children as appropriate (it is only valid to use this function if
329 'power.runtime_error' is set or 'power.disable_depth' is greater than
330 zero)
331
332It is safe to execute the following helper functions from interrupt context:
333
334pm_request_idle()
335pm_schedule_suspend()
336pm_request_resume()
337pm_runtime_get_noresume()
338pm_runtime_get()
339pm_runtime_put_noidle()
340pm_runtime_put()
341pm_suspend_ignore_children()
342pm_runtime_set_active()
343pm_runtime_set_suspended()
344pm_runtime_enable()
345
3465. Run-time PM Initialization, Device Probing and Removal
347
348Initially, the run-time PM is disabled for all devices, which means that the
349majority of the run-time PM helper funtions described in Section 4 will return
350-EAGAIN until pm_runtime_enable() is called for the device.
351
352In addition to that, the initial run-time PM status of all devices is
353'suspended', but it need not reflect the actual physical state of the device.
354Thus, if the device is initially active (i.e. it is able to process I/O), its
355run-time PM status must be changed to 'active', with the help of
356pm_runtime_set_active(), before pm_runtime_enable() is called for the device.
357
358However, if the device has a parent and the parent's run-time PM is enabled,
359calling pm_runtime_set_active() for the device will affect the parent, unless
360the parent's 'power.ignore_children' flag is set. Namely, in that case the
361parent won't be able to suspend at run time, using the PM core's helper
362functions, as long as the child's status is 'active', even if the child's
363run-time PM is still disabled (i.e. pm_runtime_enable() hasn't been called for
364the child yet or pm_runtime_disable() has been called for it). For this reason,
365once pm_runtime_set_active() has been called for the device, pm_runtime_enable()
366should be called for it too as soon as reasonably possible or its run-time PM
367status should be changed back to 'suspended' with the help of
368pm_runtime_set_suspended().
369
370If the default initial run-time PM status of the device (i.e. 'suspended')
371reflects the actual state of the device, its bus type's or its driver's
372->probe() callback will likely need to wake it up using one of the PM core's
373helper functions described in Section 4. In that case, pm_runtime_resume()
374should be used. Of course, for this purpose the device's run-time PM has to be
375enabled earlier by calling pm_runtime_enable().
376
377If the device bus type's or driver's ->probe() or ->remove() callback runs
378pm_runtime_suspend() or pm_runtime_idle() or their asynchronous counterparts,
379they will fail returning -EAGAIN, because the device's usage counter is
380incremented by the core before executing ->probe() and ->remove(). Still, it
381may be desirable to suspend the device as soon as ->probe() or ->remove() has
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +0100382finished, so the PM core uses pm_runtime_idle_sync() to invoke the
383subsystem-level idle callback for the device at that time.
Alan Sternf1212ae2009-12-22 20:43:40 +0100384
3856. Run-time PM and System Sleep
386
387Run-time PM and system sleep (i.e., system suspend and hibernation, also known
388as suspend-to-RAM and suspend-to-disk) interact with each other in a couple of
389ways. If a device is active when a system sleep starts, everything is
390straightforward. But what should happen if the device is already suspended?
391
392The device may have different wake-up settings for run-time PM and system sleep.
393For example, remote wake-up may be enabled for run-time suspend but disallowed
394for system sleep (device_may_wakeup(dev) returns 'false'). When this happens,
395the subsystem-level system suspend callback is responsible for changing the
396device's wake-up setting (it may leave that to the device driver's system
397suspend routine). It may be necessary to resume the device and suspend it again
398in order to do so. The same is true if the driver uses different power levels
399or other settings for run-time suspend and system sleep.
400
401During system resume, devices generally should be brought back to full power,
402even if they were suspended before the system sleep began. There are several
403reasons for this, including:
404
405 * The device might need to switch power levels, wake-up settings, etc.
406
407 * Remote wake-up events might have been lost by the firmware.
408
409 * The device's children may need the device to be at full power in order
410 to resume themselves.
411
412 * The driver's idea of the device state may not agree with the device's
413 physical state. This can happen during resume from hibernation.
414
415 * The device might need to be reset.
416
417 * Even though the device was suspended, if its usage counter was > 0 then most
418 likely it would need a run-time resume in the near future anyway.
419
420 * Always going back to full power is simplest.
421
422If the device was suspended before the sleep began, then its run-time PM status
423will have to be updated to reflect the actual post-system sleep status. The way
424to do this is:
425
426 pm_runtime_disable(dev);
427 pm_runtime_set_active(dev);
428 pm_runtime_enable(dev);
429
430The PM core always increments the run-time usage counter before calling the
431->prepare() callback and decrements it after calling the ->complete() callback.
432Hence disabling run-time PM temporarily like this will not cause any run-time
433suspend callbacks to be lost.