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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
45The ->runtime_suspend() callback is executed by the PM core for the bus type of
46the device being suspended. The bus type's callback is then _entirely_
47_responsible_ for handling the device as appropriate, which may, but need not
48include executing the device driver's own ->runtime_suspend() callback (from the
49PM core's point of view it is not necessary to implement a ->runtime_suspend()
50callback in a device driver as long as the bus type's ->runtime_suspend() knows
51what to do to handle the device).
52
53 * Once the bus type's ->runtime_suspend() callback has completed successfully
54 for given device, the PM core regards the device as suspended, which need
55 not mean that the device has been put into a low power state. It is
56 supposed to mean, however, that the device will not process data and will
57 not communicate with the CPU(s) and RAM until its bus type's
58 ->runtime_resume() callback is executed for it. The run-time PM status of
59 a device after successful execution of its bus type's ->runtime_suspend()
60 callback is 'suspended'.
61
62 * If the bus type's ->runtime_suspend() callback returns -EBUSY or -EAGAIN,
63 the device's run-time PM status is supposed to be 'active', which means that
64 the device _must_ be fully operational afterwards.
65
66 * If the bus type's ->runtime_suspend() callback returns an error code
67 different from -EBUSY or -EAGAIN, the PM core regards this as a fatal
68 error and will refuse to run the helper functions described in Section 4
69 for the device, until the status of it is directly set either to 'active'
70 or to 'suspended' (the PM core provides special helper functions for this
71 purpose).
72
73In particular, if the driver requires remote wakeup capability for proper
Rafael J. Wysocki7a1a8eb2009-12-03 21:19:18 +010074functioning and device_run_wake() returns 'false' for the device, then
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +020075->runtime_suspend() should return -EBUSY. On the other hand, if
Rafael J. Wysocki7a1a8eb2009-12-03 21:19:18 +010076device_run_wake() returns 'true' for the device and the device is put
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +020077into a low power state during the execution of its bus type's
78->runtime_suspend(), it is expected that remote wake-up (i.e. hardware mechanism
79allowing the device to request a change of its power state, such as PCI PME)
80will be enabled for the device. Generally, remote wake-up should be enabled
81for all input devices put into a low power state at run time.
82
83The ->runtime_resume() callback is executed by the PM core for the bus type of
84the device being woken up. The bus type's callback is then _entirely_
85_responsible_ for handling the device as appropriate, which may, but need not
86include executing the device driver's own ->runtime_resume() callback (from the
87PM core's point of view it is not necessary to implement a ->runtime_resume()
88callback in a device driver as long as the bus type's ->runtime_resume() knows
89what to do to handle the device).
90
91 * Once the bus type's ->runtime_resume() callback has completed successfully,
92 the PM core regards the device as fully operational, which means that the
93 device _must_ be able to complete I/O operations as needed. The run-time
94 PM status of the device is then 'active'.
95
96 * If the bus type's ->runtime_resume() callback returns an error code, the PM
97 core regards this as a fatal error and will refuse to run the helper
98 functions described in Section 4 for the device, until its status is
99 directly set either to 'active' or to 'suspended' (the PM core provides
100 special helper functions for this purpose).
101
102The ->runtime_idle() callback is executed by the PM core for the bus type of
103given device whenever the device appears to be idle, which is indicated to the
104PM core by two counters, the device's usage counter and the counter of 'active'
105children of the device.
106
107 * If any of these counters is decreased using a helper function provided by
108 the PM core and it turns out to be equal to zero, the other counter is
109 checked. If that counter also is equal to zero, the PM core executes the
110 device bus type's ->runtime_idle() callback (with the device as an
111 argument).
112
113The action performed by a bus type's ->runtime_idle() callback is totally
114dependent on the bus type in question, but the expected and recommended action
115is to check if the device can be suspended (i.e. if all of the conditions
116necessary for suspending the device are satisfied) and to queue up a suspend
Rafael J. Wysockie1b19032009-12-03 21:04:08 +0100117request for the device in that case. The value returned by this callback is
118ignored by the PM core.
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200119
120The helper functions provided by the PM core, described in Section 4, guarantee
121that the following constraints are met with respect to the bus type's run-time
122PM callbacks:
123
124(1) The callbacks are mutually exclusive (e.g. it is forbidden to execute
125 ->runtime_suspend() in parallel with ->runtime_resume() or with another
126 instance of ->runtime_suspend() for the same device) with the exception that
127 ->runtime_suspend() or ->runtime_resume() can be executed in parallel with
128 ->runtime_idle() (although ->runtime_idle() will not be started while any
129 of the other callbacks is being executed for the same device).
130
131(2) ->runtime_idle() and ->runtime_suspend() can only be executed for 'active'
132 devices (i.e. the PM core will only execute ->runtime_idle() or
133 ->runtime_suspend() for the devices the run-time PM status of which is
134 'active').
135
136(3) ->runtime_idle() and ->runtime_suspend() can only be executed for a device
137 the usage counter of which is equal to zero _and_ either the counter of
138 'active' children of which is equal to zero, or the 'power.ignore_children'
139 flag of which is set.
140
141(4) ->runtime_resume() can only be executed for 'suspended' devices (i.e. the
142 PM core will only execute ->runtime_resume() for the devices the run-time
143 PM status of which is 'suspended').
144
145Additionally, the helper functions provided by the PM core obey the following
146rules:
147
148 * If ->runtime_suspend() is about to be executed or there's a pending request
149 to execute it, ->runtime_idle() will not be executed for the same device.
150
151 * A request to execute or to schedule the execution of ->runtime_suspend()
152 will cancel any pending requests to execute ->runtime_idle() for the same
153 device.
154
155 * If ->runtime_resume() is about to be executed or there's a pending request
156 to execute it, the other callbacks will not be executed for the same device.
157
158 * A request to execute ->runtime_resume() will cancel any pending or
159 scheduled requests to execute the other callbacks for the same device.
160
1613. Run-time PM Device Fields
162
163The following device run-time PM fields are present in 'struct dev_pm_info', as
164defined in include/linux/pm.h:
165
166 struct timer_list suspend_timer;
167 - timer used for scheduling (delayed) suspend request
168
169 unsigned long timer_expires;
170 - timer expiration time, in jiffies (if this is different from zero, the
171 timer is running and will expire at that time, otherwise the timer is not
172 running)
173
174 struct work_struct work;
175 - work structure used for queuing up requests (i.e. work items in pm_wq)
176
177 wait_queue_head_t wait_queue;
178 - wait queue used if any of the helper functions needs to wait for another
179 one to complete
180
181 spinlock_t lock;
182 - lock used for synchronisation
183
184 atomic_t usage_count;
185 - the usage counter of the device
186
187 atomic_t child_count;
188 - the count of 'active' children of the device
189
190 unsigned int ignore_children;
191 - if set, the value of child_count is ignored (but still updated)
192
193 unsigned int disable_depth;
194 - used for disabling the helper funcions (they work normally if this is
195 equal to zero); the initial value of it is 1 (i.e. run-time PM is
196 initially disabled for all devices)
197
198 unsigned int runtime_error;
199 - if set, there was a fatal error (one of the callbacks returned error code
200 as described in Section 2), so the helper funtions will not work until
201 this flag is cleared; this is the error code returned by the failing
202 callback
203
204 unsigned int idle_notification;
205 - if set, ->runtime_idle() is being executed
206
207 unsigned int request_pending;
208 - if set, there's a pending request (i.e. a work item queued up into pm_wq)
209
210 enum rpm_request request;
211 - type of request that's pending (valid if request_pending is set)
212
213 unsigned int deferred_resume;
214 - set if ->runtime_resume() is about to be run while ->runtime_suspend() is
215 being executed for that device and it is not practical to wait for the
216 suspend to complete; means "start a resume as soon as you've suspended"
217
Rafael J. Wysocki7a1a8eb2009-12-03 21:19:18 +0100218 unsigned int run_wake;
219 - set if the device is capable of generating run-time wake-up events
220
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200221 enum rpm_status runtime_status;
222 - the run-time PM status of the device; this field's initial value is
223 RPM_SUSPENDED, which means that each device is initially regarded by the
224 PM core as 'suspended', regardless of its real hardware status
225
226All of the above fields are members of the 'power' member of 'struct device'.
227
2284. Run-time PM Device Helper Functions
229
230The following run-time PM helper functions are defined in
231drivers/base/power/runtime.c and include/linux/pm_runtime.h:
232
233 void pm_runtime_init(struct device *dev);
234 - initialize the device run-time PM fields in 'struct dev_pm_info'
235
236 void pm_runtime_remove(struct device *dev);
237 - make sure that the run-time PM of the device will be disabled after
238 removing the device from device hierarchy
239
240 int pm_runtime_idle(struct device *dev);
241 - execute ->runtime_idle() for the device's bus type; returns 0 on success
242 or error code on failure, where -EINPROGRESS means that ->runtime_idle()
243 is already being executed
244
245 int pm_runtime_suspend(struct device *dev);
246 - execute ->runtime_suspend() for the device's bus type; returns 0 on
247 success, 1 if the device's run-time PM status was already 'suspended', or
248 error code on failure, where -EAGAIN or -EBUSY means it is safe to attempt
249 to suspend the device again in future
250
251 int pm_runtime_resume(struct device *dev);
252 - execute ->runtime_resume() for the device's bus type; returns 0 on
253 success, 1 if the device's run-time PM status was already 'active' or
254 error code on failure, where -EAGAIN means it may be safe to attempt to
255 resume the device again in future, but 'power.runtime_error' should be
256 checked additionally
257
258 int pm_request_idle(struct device *dev);
259 - submit a request to execute ->runtime_idle() for the device's bus type
260 (the request is represented by a work item in pm_wq); returns 0 on success
261 or error code if the request has not been queued up
262
263 int pm_schedule_suspend(struct device *dev, unsigned int delay);
264 - schedule the execution of ->runtime_suspend() for the device's bus type
265 in future, where 'delay' is the time to wait before queuing up a suspend
266 work item in pm_wq, in milliseconds (if 'delay' is zero, the work item is
267 queued up immediately); returns 0 on success, 1 if the device's PM
268 run-time status was already 'suspended', or error code if the request
269 hasn't been scheduled (or queued up if 'delay' is 0); if the execution of
270 ->runtime_suspend() is already scheduled and not yet expired, the new
271 value of 'delay' will be used as the time to wait
272
273 int pm_request_resume(struct device *dev);
274 - submit a request to execute ->runtime_resume() for the device's bus type
275 (the request is represented by a work item in pm_wq); returns 0 on
276 success, 1 if the device's run-time PM status was already 'active', or
277 error code if the request hasn't been queued up
278
279 void pm_runtime_get_noresume(struct device *dev);
280 - increment the device's usage counter
281
282 int pm_runtime_get(struct device *dev);
283 - increment the device's usage counter, run pm_request_resume(dev) and
284 return its result
285
286 int pm_runtime_get_sync(struct device *dev);
287 - increment the device's usage counter, run pm_runtime_resume(dev) and
288 return its result
289
290 void pm_runtime_put_noidle(struct device *dev);
291 - decrement the device's usage counter
292
293 int pm_runtime_put(struct device *dev);
294 - decrement the device's usage counter, run pm_request_idle(dev) and return
295 its result
296
297 int pm_runtime_put_sync(struct device *dev);
298 - decrement the device's usage counter, run pm_runtime_idle(dev) and return
299 its result
300
301 void pm_runtime_enable(struct device *dev);
302 - enable the run-time PM helper functions to run the device bus type's
303 run-time PM callbacks described in Section 2
304
305 int pm_runtime_disable(struct device *dev);
306 - prevent the run-time PM helper functions from running the device bus
307 type's run-time PM callbacks, make sure that all of the pending run-time
308 PM operations on the device are either completed or canceled; returns
309 1 if there was a resume request pending and it was necessary to execute
310 ->runtime_resume() for the device's bus type to satisfy that request,
311 otherwise 0 is returned
312
313 void pm_suspend_ignore_children(struct device *dev, bool enable);
314 - set/unset the power.ignore_children flag of the device
315
316 int pm_runtime_set_active(struct device *dev);
317 - clear the device's 'power.runtime_error' flag, set the device's run-time
318 PM status to 'active' and update its parent's counter of 'active'
319 children as appropriate (it is only valid to use this function if
320 'power.runtime_error' is set or 'power.disable_depth' is greater than
321 zero); it will fail and return error code if the device has a parent
322 which is not active and the 'power.ignore_children' flag of which is unset
323
324 void pm_runtime_set_suspended(struct device *dev);
325 - clear the device's 'power.runtime_error' flag, set the device's run-time
326 PM status to 'suspended' and update its parent's counter of 'active'
327 children as appropriate (it is only valid to use this function if
328 'power.runtime_error' is set or 'power.disable_depth' is greater than
329 zero)
330
331It is safe to execute the following helper functions from interrupt context:
332
333pm_request_idle()
334pm_schedule_suspend()
335pm_request_resume()
336pm_runtime_get_noresume()
337pm_runtime_get()
338pm_runtime_put_noidle()
339pm_runtime_put()
340pm_suspend_ignore_children()
341pm_runtime_set_active()
342pm_runtime_set_suspended()
343pm_runtime_enable()
344
3455. Run-time PM Initialization, Device Probing and Removal
346
347Initially, the run-time PM is disabled for all devices, which means that the
348majority of the run-time PM helper funtions described in Section 4 will return
349-EAGAIN until pm_runtime_enable() is called for the device.
350
351In addition to that, the initial run-time PM status of all devices is
352'suspended', but it need not reflect the actual physical state of the device.
353Thus, if the device is initially active (i.e. it is able to process I/O), its
354run-time PM status must be changed to 'active', with the help of
355pm_runtime_set_active(), before pm_runtime_enable() is called for the device.
356
357However, if the device has a parent and the parent's run-time PM is enabled,
358calling pm_runtime_set_active() for the device will affect the parent, unless
359the parent's 'power.ignore_children' flag is set. Namely, in that case the
360parent won't be able to suspend at run time, using the PM core's helper
361functions, as long as the child's status is 'active', even if the child's
362run-time PM is still disabled (i.e. pm_runtime_enable() hasn't been called for
363the child yet or pm_runtime_disable() has been called for it). For this reason,
364once pm_runtime_set_active() has been called for the device, pm_runtime_enable()
365should be called for it too as soon as reasonably possible or its run-time PM
366status should be changed back to 'suspended' with the help of
367pm_runtime_set_suspended().
368
369If the default initial run-time PM status of the device (i.e. 'suspended')
370reflects the actual state of the device, its bus type's or its driver's
371->probe() callback will likely need to wake it up using one of the PM core's
372helper functions described in Section 4. In that case, pm_runtime_resume()
373should be used. Of course, for this purpose the device's run-time PM has to be
374enabled earlier by calling pm_runtime_enable().
375
376If the device bus type's or driver's ->probe() or ->remove() callback runs
377pm_runtime_suspend() or pm_runtime_idle() or their asynchronous counterparts,
378they will fail returning -EAGAIN, because the device's usage counter is
379incremented by the core before executing ->probe() and ->remove(). Still, it
380may be desirable to suspend the device as soon as ->probe() or ->remove() has
381finished, so the PM core uses pm_runtime_idle_sync() to invoke the device bus
382type's ->runtime_idle() callback at that time.