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authorRafael J. Wysocki <rjw@sisk.pl>2009-08-18 17:38:32 -0400
committerRafael J. Wysocki <rjw@sisk.pl>2009-08-22 18:04:44 -0400
commit5e928f77a09a07f9dd595bb8a489965d69a83458 (patch)
treeef53ec90fa3214fd22e36b07c11c06b09e373d8d /Documentation/power/runtime_pm.txt
parent8400146d0dc03590bba051399e4bb7e1cbf1c010 (diff)
PM: Introduce core framework for run-time PM of I/O devices (rev. 17)
Introduce a core framework for run-time power management of I/O devices. Add device run-time PM fields to 'struct dev_pm_info' and device run-time PM callbacks to 'struct dev_pm_ops'. Introduce a run-time PM workqueue and define some device run-time PM helper functions at the core level. Document all these things. Special thanks to Alan Stern for his help with the design and multiple detailed reviews of the pereceding versions of this patch and to Magnus Damm for testing feedback. Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl> Acked-by: Magnus Damm <damm@igel.co.jp>
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1Run-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);
41 void (*runtime_idle)(struct device *dev);
42 ...
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
74functioning and device_may_wakeup() returns 'false' for the device, then
75->runtime_suspend() should return -EBUSY. On the other hand, if
76device_may_wakeup() returns 'true' for the device and the device is put
77into 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
117request for the device in that case.
118
119The helper functions provided by the PM core, described in Section 4, guarantee
120that the following constraints are met with respect to the bus type's run-time
121PM callbacks:
122
123(1) The callbacks are mutually exclusive (e.g. it is forbidden to execute
124 ->runtime_suspend() in parallel with ->runtime_resume() or with another
125 instance of ->runtime_suspend() for the same device) with the exception that
126 ->runtime_suspend() or ->runtime_resume() can be executed in parallel with
127 ->runtime_idle() (although ->runtime_idle() will not be started while any
128 of the other callbacks is being executed for the same device).
129
130(2) ->runtime_idle() and ->runtime_suspend() can only be executed for 'active'
131 devices (i.e. the PM core will only execute ->runtime_idle() or
132 ->runtime_suspend() for the devices the run-time PM status of which is
133 'active').
134
135(3) ->runtime_idle() and ->runtime_suspend() can only be executed for a device
136 the usage counter of which is equal to zero _and_ either the counter of
137 'active' children of which is equal to zero, or the 'power.ignore_children'
138 flag of which is set.
139
140(4) ->runtime_resume() can only be executed for 'suspended' devices (i.e. the
141 PM core will only execute ->runtime_resume() for the devices the run-time
142 PM status of which is 'suspended').
143
144Additionally, the helper functions provided by the PM core obey the following
145rules:
146
147 * If ->runtime_suspend() is about to be executed or there's a pending request
148 to execute it, ->runtime_idle() will not be executed for the same device.
149
150 * A request to execute or to schedule the execution of ->runtime_suspend()
151 will cancel any pending requests to execute ->runtime_idle() for the same
152 device.
153
154 * If ->runtime_resume() is about to be executed or there's a pending request
155 to execute it, the other callbacks will not be executed for the same device.
156
157 * A request to execute ->runtime_resume() will cancel any pending or
158 scheduled requests to execute the other callbacks for the same device.
159
1603. Run-time PM Device Fields
161
162The following device run-time PM fields are present in 'struct dev_pm_info', as
163defined in include/linux/pm.h:
164
165 struct timer_list suspend_timer;
166 - timer used for scheduling (delayed) suspend request
167
168 unsigned long timer_expires;
169 - timer expiration time, in jiffies (if this is different from zero, the
170 timer is running and will expire at that time, otherwise the timer is not
171 running)
172
173 struct work_struct work;
174 - work structure used for queuing up requests (i.e. work items in pm_wq)
175
176 wait_queue_head_t wait_queue;
177 - wait queue used if any of the helper functions needs to wait for another
178 one to complete
179
180 spinlock_t lock;
181 - lock used for synchronisation
182
183 atomic_t usage_count;
184 - the usage counter of the device
185
186 atomic_t child_count;
187 - the count of 'active' children of the device
188
189 unsigned int ignore_children;
190 - if set, the value of child_count is ignored (but still updated)
191
192 unsigned int disable_depth;
193 - used for disabling the helper funcions (they work normally if this is
194 equal to zero); the initial value of it is 1 (i.e. run-time PM is
195 initially disabled for all devices)
196
197 unsigned int runtime_error;
198 - if set, there was a fatal error (one of the callbacks returned error code
199 as described in Section 2), so the helper funtions will not work until
200 this flag is cleared; this is the error code returned by the failing
201 callback
202
203 unsigned int idle_notification;
204 - if set, ->runtime_idle() is being executed
205
206 unsigned int request_pending;
207 - if set, there's a pending request (i.e. a work item queued up into pm_wq)
208
209 enum rpm_request request;
210 - type of request that's pending (valid if request_pending is set)
211
212 unsigned int deferred_resume;
213 - set if ->runtime_resume() is about to be run while ->runtime_suspend() is
214 being executed for that device and it is not practical to wait for the
215 suspend to complete; means "start a resume as soon as you've suspended"
216
217 enum rpm_status runtime_status;
218 - the run-time PM status of the device; this field's initial value is
219 RPM_SUSPENDED, which means that each device is initially regarded by the
220 PM core as 'suspended', regardless of its real hardware status
221
222All of the above fields are members of the 'power' member of 'struct device'.
223
2244. Run-time PM Device Helper Functions
225
226The following run-time PM helper functions are defined in
227drivers/base/power/runtime.c and include/linux/pm_runtime.h:
228
229 void pm_runtime_init(struct device *dev);
230 - initialize the device run-time PM fields in 'struct dev_pm_info'
231
232 void pm_runtime_remove(struct device *dev);
233 - make sure that the run-time PM of the device will be disabled after
234 removing the device from device hierarchy
235
236 int pm_runtime_idle(struct device *dev);
237 - execute ->runtime_idle() for the device's bus type; returns 0 on success
238 or error code on failure, where -EINPROGRESS means that ->runtime_idle()
239 is already being executed
240
241 int pm_runtime_suspend(struct device *dev);
242 - execute ->runtime_suspend() for the device's bus type; returns 0 on
243 success, 1 if the device's run-time PM status was already 'suspended', or
244 error code on failure, where -EAGAIN or -EBUSY means it is safe to attempt
245 to suspend the device again in future
246
247 int pm_runtime_resume(struct device *dev);
248 - execute ->runtime_resume() for the device's bus type; returns 0 on
249 success, 1 if the device's run-time PM status was already 'active' or
250 error code on failure, where -EAGAIN means it may be safe to attempt to
251 resume the device again in future, but 'power.runtime_error' should be
252 checked additionally
253
254 int pm_request_idle(struct device *dev);
255 - submit a request to execute ->runtime_idle() for the device's bus type
256 (the request is represented by a work item in pm_wq); returns 0 on success
257 or error code if the request has not been queued up
258
259 int pm_schedule_suspend(struct device *dev, unsigned int delay);
260 - schedule the execution of ->runtime_suspend() for the device's bus type
261 in future, where 'delay' is the time to wait before queuing up a suspend
262 work item in pm_wq, in milliseconds (if 'delay' is zero, the work item is
263 queued up immediately); returns 0 on success, 1 if the device's PM
264 run-time status was already 'suspended', or error code if the request
265 hasn't been scheduled (or queued up if 'delay' is 0); if the execution of
266 ->runtime_suspend() is already scheduled and not yet expired, the new
267 value of 'delay' will be used as the time to wait
268
269 int pm_request_resume(struct device *dev);
270 - submit a request to execute ->runtime_resume() for the device's bus type
271 (the request is represented by a work item in pm_wq); returns 0 on
272 success, 1 if the device's run-time PM status was already 'active', or
273 error code if the request hasn't been queued up
274
275 void pm_runtime_get_noresume(struct device *dev);
276 - increment the device's usage counter
277
278 int pm_runtime_get(struct device *dev);
279 - increment the device's usage counter, run pm_request_resume(dev) and
280 return its result
281
282 int pm_runtime_get_sync(struct device *dev);
283 - increment the device's usage counter, run pm_runtime_resume(dev) and
284 return its result
285
286 void pm_runtime_put_noidle(struct device *dev);
287 - decrement the device's usage counter
288
289 int pm_runtime_put(struct device *dev);
290 - decrement the device's usage counter, run pm_request_idle(dev) and return
291 its result
292
293 int pm_runtime_put_sync(struct device *dev);
294 - decrement the device's usage counter, run pm_runtime_idle(dev) and return
295 its result
296
297 void pm_runtime_enable(struct device *dev);
298 - enable the run-time PM helper functions to run the device bus type's
299 run-time PM callbacks described in Section 2
300
301 int pm_runtime_disable(struct device *dev);
302 - prevent the run-time PM helper functions from running the device bus
303 type's run-time PM callbacks, make sure that all of the pending run-time
304 PM operations on the device are either completed or canceled; returns
305 1 if there was a resume request pending and it was necessary to execute
306 ->runtime_resume() for the device's bus type to satisfy that request,
307 otherwise 0 is returned
308
309 void pm_suspend_ignore_children(struct device *dev, bool enable);
310 - set/unset the power.ignore_children flag of the device
311
312 int pm_runtime_set_active(struct device *dev);
313 - clear the device's 'power.runtime_error' flag, set the device's run-time
314 PM status to 'active' and update its parent's counter of 'active'
315 children as appropriate (it is only valid to use this function if
316 'power.runtime_error' is set or 'power.disable_depth' is greater than
317 zero); it will fail and return error code if the device has a parent
318 which is not active and the 'power.ignore_children' flag of which is unset
319
320 void pm_runtime_set_suspended(struct device *dev);
321 - clear the device's 'power.runtime_error' flag, set the device's run-time
322 PM status to 'suspended' and update its parent's counter of 'active'
323 children as appropriate (it is only valid to use this function if
324 'power.runtime_error' is set or 'power.disable_depth' is greater than
325 zero)
326
327It is safe to execute the following helper functions from interrupt context:
328
329pm_request_idle()
330pm_schedule_suspend()
331pm_request_resume()
332pm_runtime_get_noresume()
333pm_runtime_get()
334pm_runtime_put_noidle()
335pm_runtime_put()
336pm_suspend_ignore_children()
337pm_runtime_set_active()
338pm_runtime_set_suspended()
339pm_runtime_enable()
340
3415. Run-time PM Initialization, Device Probing and Removal
342
343Initially, the run-time PM is disabled for all devices, which means that the
344majority of the run-time PM helper funtions described in Section 4 will return
345-EAGAIN until pm_runtime_enable() is called for the device.
346
347In addition to that, the initial run-time PM status of all devices is
348'suspended', but it need not reflect the actual physical state of the device.
349Thus, if the device is initially active (i.e. it is able to process I/O), its
350run-time PM status must be changed to 'active', with the help of
351pm_runtime_set_active(), before pm_runtime_enable() is called for the device.
352
353However, if the device has a parent and the parent's run-time PM is enabled,
354calling pm_runtime_set_active() for the device will affect the parent, unless
355the parent's 'power.ignore_children' flag is set. Namely, in that case the
356parent won't be able to suspend at run time, using the PM core's helper
357functions, as long as the child's status is 'active', even if the child's
358run-time PM is still disabled (i.e. pm_runtime_enable() hasn't been called for
359the child yet or pm_runtime_disable() has been called for it). For this reason,
360once pm_runtime_set_active() has been called for the device, pm_runtime_enable()
361should be called for it too as soon as reasonably possible or its run-time PM
362status should be changed back to 'suspended' with the help of
363pm_runtime_set_suspended().
364
365If the default initial run-time PM status of the device (i.e. 'suspended')
366reflects the actual state of the device, its bus type's or its driver's
367->probe() callback will likely need to wake it up using one of the PM core's
368helper functions described in Section 4. In that case, pm_runtime_resume()
369should be used. Of course, for this purpose the device's run-time PM has to be
370enabled earlier by calling pm_runtime_enable().
371
372If the device bus type's or driver's ->probe() or ->remove() callback runs
373pm_runtime_suspend() or pm_runtime_idle() or their asynchronous counterparts,
374they will fail returning -EAGAIN, because the device's usage counter is
375incremented by the core before executing ->probe() and ->remove(). Still, it
376may be desirable to suspend the device as soon as ->probe() or ->remove() has
377finished, so the PM core uses pm_runtime_idle_sync() to invoke the device bus
378type's ->runtime_idle() callback at that time.