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Rafael J. Wysocki62052ab2011-07-06 10:52:13 +02001Runtime Power Management Framework for I/O Devices
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +02002
Rafael J. Wysocki9659cc02011-02-18 23:20:21 +01003(C) 2009-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
Alan Stern7490e442010-09-25 23:35:15 +02004(C) 2010 Alan Stern <stern@rowland.harvard.edu>
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +02005
61. Introduction
7
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +02008Support for runtime power management (runtime PM) of I/O devices is provided
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +02009at the power management core (PM core) level by means of:
10
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
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +020013 used for queuing all work items related to runtime PM, because this allows
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +020014 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.
17
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +020018* A number of runtime PM fields in the 'power' member of 'struct device' (which
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +020019 is of the type 'struct dev_pm_info', defined in include/linux/pm.h) that can
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +020020 be used for synchronizing runtime PM operations with one another.
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +020021
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +020022* Three device runtime PM callbacks in 'struct dev_pm_ops' (defined in
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +020023 include/linux/pm.h).
24
25* A set of helper functions defined in drivers/base/power/runtime.c that can be
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +020026 used for carrying out runtime PM operations in such a way that the
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +020027 synchronization between them is taken care of by the PM core. Bus types and
28 device drivers are encouraged to use these functions.
29
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +020030The runtime PM callbacks present in 'struct dev_pm_ops', the device runtime PM
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +020031fields of 'struct dev_pm_info' and the core helper functions provided for
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +020032runtime PM are described below.
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +020033
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200342. Device Runtime PM Callbacks
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +020035
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +020036There are three device runtime PM callbacks defined in 'struct dev_pm_ops':
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +020037
38struct dev_pm_ops {
39 ...
40 int (*runtime_suspend)(struct device *dev);
41 int (*runtime_resume)(struct device *dev);
Rafael J. Wysockie1b19032009-12-03 21:04:08 +010042 int (*runtime_idle)(struct device *dev);
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +020043 ...
44};
45
Ming Lei2fb242a2011-10-09 11:40:25 +080046The ->runtime_suspend(), ->runtime_resume() and ->runtime_idle() callbacks
Rafael J. Wysocki5841eb62011-11-23 21:18:39 +010047are executed by the PM core for the device's subsystem that may be either of
48the following:
49
50 1. PM domain of the device, if the device's PM domain object, dev->pm_domain,
51 is present.
52
53 2. Device type of the device, if both dev->type and dev->type->pm are present.
54
55 3. Device class of the device, if both dev->class and dev->class->pm are
56 present.
57
58 4. Bus type of the device, if both dev->bus and dev->bus->pm are present.
59
Rafael J. Wysocki35cd1332011-12-18 00:34:13 +010060If the subsystem chosen by applying the above rules doesn't provide the relevant
61callback, the PM core will invoke the corresponding driver callback stored in
62dev->driver->pm directly (if present).
63
Rafael J. Wysocki5841eb62011-11-23 21:18:39 +010064The PM core always checks which callback to use in the order given above, so the
65priority order of callbacks from high to low is: PM domain, device type, class
66and bus type. Moreover, the high-priority one will always take precedence over
67a low-priority one. The PM domain, bus type, device type and class callbacks
68are referred to as subsystem-level callbacks in what follows.
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +020069
Alan Sternc7b61de2010-12-01 00:14:42 +010070By default, the callbacks are always invoked in process context with interrupts
Rafael J. Wysocki35cd1332011-12-18 00:34:13 +010071enabled. However, the pm_runtime_irq_safe() helper function can be used to tell
72the PM core that it is safe to run the ->runtime_suspend(), ->runtime_resume()
73and ->runtime_idle() callbacks for the given device in atomic context with
74interrupts disabled. This implies that the callback routines in question must
75not block or sleep, but it also means that the synchronous helper functions
76listed at the end of Section 4 may be used for that device within an interrupt
77handler or generally in an atomic context.
Alan Sternc7b61de2010-12-01 00:14:42 +010078
Rafael J. Wysocki35cd1332011-12-18 00:34:13 +010079The subsystem-level suspend callback, if present, is _entirely_ _responsible_
80for handling the suspend of the device as appropriate, which may, but need not
81include executing the device driver's own ->runtime_suspend() callback (from the
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +010082PM core's point of view it is not necessary to implement a ->runtime_suspend()
83callback in a device driver as long as the subsystem-level suspend callback
84knows what to do to handle the device).
85
Rafael J. Wysocki35cd1332011-12-18 00:34:13 +010086 * Once the subsystem-level suspend callback (or the driver suspend callback,
87 if invoked directly) has completed successfully for the given device, the PM
88 core regards the device as suspended, which need not mean that it has been
89 put into a low power state. It is supposed to mean, however, that the
90 device will not process data and will not communicate with the CPU(s) and
91 RAM until the appropriate resume callback is executed for it. The runtime
92 PM status of a device after successful execution of the suspend callback is
93 'suspended'.
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +020094
Rafael J. Wysocki35cd1332011-12-18 00:34:13 +010095 * If the suspend callback returns -EBUSY or -EAGAIN, the device's runtime PM
96 status remains 'active', which means that the device _must_ be fully
97 operational afterwards.
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +020098
Rafael J. Wysocki35cd1332011-12-18 00:34:13 +010099 * If the suspend callback returns an error code different from -EBUSY and
100 -EAGAIN, the PM core regards this as a fatal error and will refuse to run
101 the helper functions described in Section 4 for the device until its status
102 is directly set to either'active', or 'suspended' (the PM core provides
103 special helper functions for this purpose).
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200104
Rafael J. Wysocki35cd1332011-12-18 00:34:13 +0100105In particular, if the driver requires remote wakeup capability (i.e. hardware
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +0100106mechanism allowing the device to request a change of its power state, such as
107PCI PME) for proper functioning and device_run_wake() returns 'false' for the
108device, then ->runtime_suspend() should return -EBUSY. On the other hand, if
Rafael J. Wysocki35cd1332011-12-18 00:34:13 +0100109device_run_wake() returns 'true' for the device and the device is put into a
110low-power state during the execution of the suspend callback, it is expected
111that remote wakeup will be enabled for the device. Generally, remote wakeup
112should be enabled for all input devices put into low-power states at run time.
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200113
Rafael J. Wysocki35cd1332011-12-18 00:34:13 +0100114The subsystem-level resume callback, if present, is _entirely_ _responsible_ for
115handling the resume of the device as appropriate, which may, but need not
116include executing the device driver's own ->runtime_resume() callback (from the
117PM core's point of view it is not necessary to implement a ->runtime_resume()
118callback in a device driver as long as the subsystem-level resume callback knows
119what to do to handle the device).
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200120
Rafael J. Wysocki35cd1332011-12-18 00:34:13 +0100121 * Once the subsystem-level resume callback (or the driver resume callback, if
122 invoked directly) has completed successfully, the PM core regards the device
123 as fully operational, which means that the device _must_ be able to complete
124 I/O operations as needed. The runtime PM status of the device is then
125 'active'.
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200126
Rafael J. Wysocki35cd1332011-12-18 00:34:13 +0100127 * If the resume callback returns an error code, the PM core regards this as a
128 fatal error and will refuse to run the helper functions described in Section
129 4 for the device, until its status is directly set to either 'active', or
130 'suspended' (by means of special helper functions provided by the PM core
131 for this purpose).
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200132
Rafael J. Wysocki35cd1332011-12-18 00:34:13 +0100133The idle callback (a subsystem-level one, if present, or the driver one) is
134executed by the PM core whenever the device appears to be idle, which is
135indicated to the PM core by two counters, the device's usage counter and the
136counter of 'active' children of the device.
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200137
138 * If any of these counters is decreased using a helper function provided by
139 the PM core and it turns out to be equal to zero, the other counter is
140 checked. If that counter also is equal to zero, the PM core executes the
Rafael J. Wysocki35cd1332011-12-18 00:34:13 +0100141 idle callback with the device as its argument.
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200142
Rafael J. Wysocki35cd1332011-12-18 00:34:13 +0100143The action performed by the idle callback is totally dependent on the subsystem
144(or driver) in question, but the expected and recommended action is to check
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +0100145if the device can be suspended (i.e. if all of the conditions necessary for
146suspending the device are satisfied) and to queue up a suspend request for the
147device in that case. The value returned by this callback is ignored by the PM
148core.
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200149
150The helper functions provided by the PM core, described in Section 4, guarantee
Rafael J. Wysocki35cd1332011-12-18 00:34:13 +0100151that the following constraints are met with respect to runtime PM callbacks for
152one device:
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200153
154(1) The callbacks are mutually exclusive (e.g. it is forbidden to execute
155 ->runtime_suspend() in parallel with ->runtime_resume() or with another
156 instance of ->runtime_suspend() for the same device) with the exception that
157 ->runtime_suspend() or ->runtime_resume() can be executed in parallel with
158 ->runtime_idle() (although ->runtime_idle() will not be started while any
159 of the other callbacks is being executed for the same device).
160
161(2) ->runtime_idle() and ->runtime_suspend() can only be executed for 'active'
162 devices (i.e. the PM core will only execute ->runtime_idle() or
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200163 ->runtime_suspend() for the devices the runtime PM status of which is
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200164 'active').
165
166(3) ->runtime_idle() and ->runtime_suspend() can only be executed for a device
167 the usage counter of which is equal to zero _and_ either the counter of
168 'active' children of which is equal to zero, or the 'power.ignore_children'
169 flag of which is set.
170
171(4) ->runtime_resume() can only be executed for 'suspended' devices (i.e. the
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200172 PM core will only execute ->runtime_resume() for the devices the runtime
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200173 PM status of which is 'suspended').
174
175Additionally, the helper functions provided by the PM core obey the following
176rules:
177
178 * If ->runtime_suspend() is about to be executed or there's a pending request
179 to execute it, ->runtime_idle() will not be executed for the same device.
180
181 * A request to execute or to schedule the execution of ->runtime_suspend()
182 will cancel any pending requests to execute ->runtime_idle() for the same
183 device.
184
185 * If ->runtime_resume() is about to be executed or there's a pending request
186 to execute it, the other callbacks will not be executed for the same device.
187
188 * A request to execute ->runtime_resume() will cancel any pending or
Alan Stern15bcb91d2010-09-25 23:35:21 +0200189 scheduled requests to execute the other callbacks for the same device,
190 except for scheduled autosuspends.
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200191
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +02001923. Runtime PM Device Fields
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200193
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200194The following device runtime PM fields are present in 'struct dev_pm_info', as
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200195defined in include/linux/pm.h:
196
197 struct timer_list suspend_timer;
Alan Stern15bcb91d2010-09-25 23:35:21 +0200198 - timer used for scheduling (delayed) suspend and autosuspend requests
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200199
200 unsigned long timer_expires;
201 - timer expiration time, in jiffies (if this is different from zero, the
202 timer is running and will expire at that time, otherwise the timer is not
203 running)
204
205 struct work_struct work;
206 - work structure used for queuing up requests (i.e. work items in pm_wq)
207
208 wait_queue_head_t wait_queue;
209 - wait queue used if any of the helper functions needs to wait for another
210 one to complete
211
212 spinlock_t lock;
213 - lock used for synchronisation
214
215 atomic_t usage_count;
216 - the usage counter of the device
217
218 atomic_t child_count;
219 - the count of 'active' children of the device
220
221 unsigned int ignore_children;
222 - if set, the value of child_count is ignored (but still updated)
223
224 unsigned int disable_depth;
225 - used for disabling the helper funcions (they work normally if this is
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200226 equal to zero); the initial value of it is 1 (i.e. runtime PM is
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200227 initially disabled for all devices)
228
229 unsigned int runtime_error;
230 - if set, there was a fatal error (one of the callbacks returned error code
231 as described in Section 2), so the helper funtions will not work until
232 this flag is cleared; this is the error code returned by the failing
233 callback
234
235 unsigned int idle_notification;
236 - if set, ->runtime_idle() is being executed
237
238 unsigned int request_pending;
239 - if set, there's a pending request (i.e. a work item queued up into pm_wq)
240
241 enum rpm_request request;
242 - type of request that's pending (valid if request_pending is set)
243
244 unsigned int deferred_resume;
245 - set if ->runtime_resume() is about to be run while ->runtime_suspend() is
246 being executed for that device and it is not practical to wait for the
247 suspend to complete; means "start a resume as soon as you've suspended"
248
Rafael J. Wysocki7a1a8eb2009-12-03 21:19:18 +0100249 unsigned int run_wake;
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200250 - set if the device is capable of generating runtime wake-up events
Rafael J. Wysocki7a1a8eb2009-12-03 21:19:18 +0100251
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200252 enum rpm_status runtime_status;
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200253 - the runtime PM status of the device; this field's initial value is
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200254 RPM_SUSPENDED, which means that each device is initially regarded by the
255 PM core as 'suspended', regardless of its real hardware status
256
Rafael J. Wysocki87d1b3e2010-03-06 21:28:17 +0100257 unsigned int runtime_auto;
258 - if set, indicates that the user space has allowed the device driver to
259 power manage the device at run time via the /sys/devices/.../power/control
260 interface; it may only be modified with the help of the pm_runtime_allow()
261 and pm_runtime_forbid() helper functions
262
Alan Stern7490e442010-09-25 23:35:15 +0200263 unsigned int no_callbacks;
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200264 - indicates that the device does not use the runtime PM callbacks (see
Alan Stern7490e442010-09-25 23:35:15 +0200265 Section 8); it may be modified only by the pm_runtime_no_callbacks()
266 helper function
267
Alan Sternc7b61de2010-12-01 00:14:42 +0100268 unsigned int irq_safe;
269 - indicates that the ->runtime_suspend() and ->runtime_resume() callbacks
270 will be invoked with the spinlock held and interrupts disabled
271
Alan Stern15bcb91d2010-09-25 23:35:21 +0200272 unsigned int use_autosuspend;
273 - indicates that the device's driver supports delayed autosuspend (see
274 Section 9); it may be modified only by the
275 pm_runtime{_dont}_use_autosuspend() helper functions
276
277 unsigned int timer_autosuspends;
278 - indicates that the PM core should attempt to carry out an autosuspend
279 when the timer expires rather than a normal suspend
280
281 int autosuspend_delay;
282 - the delay time (in milliseconds) to be used for autosuspend
283
284 unsigned long last_busy;
285 - the time (in jiffies) when the pm_runtime_mark_last_busy() helper
286 function was last called for this device; used in calculating inactivity
287 periods for autosuspend
288
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200289All of the above fields are members of the 'power' member of 'struct device'.
290
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +02002914. Runtime PM Device Helper Functions
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200292
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200293The following runtime PM helper functions are defined in
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200294drivers/base/power/runtime.c and include/linux/pm_runtime.h:
295
296 void pm_runtime_init(struct device *dev);
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200297 - initialize the device runtime PM fields in 'struct dev_pm_info'
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200298
299 void pm_runtime_remove(struct device *dev);
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200300 - make sure that the runtime PM of the device will be disabled after
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200301 removing the device from device hierarchy
302
303 int pm_runtime_idle(struct device *dev);
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +0100304 - execute the subsystem-level idle callback for the device; returns 0 on
305 success or error code on failure, where -EINPROGRESS means that
306 ->runtime_idle() is already being executed
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200307
308 int pm_runtime_suspend(struct device *dev);
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +0100309 - execute the subsystem-level suspend callback for the device; returns 0 on
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200310 success, 1 if the device's runtime PM status was already 'suspended', or
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200311 error code on failure, where -EAGAIN or -EBUSY means it is safe to attempt
Rafael J. Wysocki632e2702011-07-01 22:29:15 +0200312 to suspend the device again in future and -EACCES means that
313 'power.disable_depth' is different from 0
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200314
Alan Stern15bcb91d2010-09-25 23:35:21 +0200315 int pm_runtime_autosuspend(struct device *dev);
316 - same as pm_runtime_suspend() except that the autosuspend delay is taken
317 into account; if pm_runtime_autosuspend_expiration() says the delay has
318 not yet expired then an autosuspend is scheduled for the appropriate time
319 and 0 is returned
320
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200321 int pm_runtime_resume(struct device *dev);
Thadeu Lima de Souza Cascardode8164f2010-01-17 19:22:28 -0200322 - execute the subsystem-level resume callback for the device; returns 0 on
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200323 success, 1 if the device's runtime PM status was already 'active' or
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200324 error code on failure, where -EAGAIN means it may be safe to attempt to
325 resume the device again in future, but 'power.runtime_error' should be
Rafael J. Wysocki632e2702011-07-01 22:29:15 +0200326 checked additionally, and -EACCES means that 'power.disable_depth' is
327 different from 0
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200328
329 int pm_request_idle(struct device *dev);
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +0100330 - submit a request to execute the subsystem-level idle callback for the
331 device (the request is represented by a work item in pm_wq); returns 0 on
332 success or error code if the request has not been queued up
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200333
Alan Stern15bcb91d2010-09-25 23:35:21 +0200334 int pm_request_autosuspend(struct device *dev);
335 - schedule the execution of the subsystem-level suspend callback for the
336 device when the autosuspend delay has expired; if the delay has already
337 expired then the work item is queued up immediately
338
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200339 int pm_schedule_suspend(struct device *dev, unsigned int delay);
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +0100340 - schedule the execution of the subsystem-level suspend callback for the
341 device in future, where 'delay' is the time to wait before queuing up a
342 suspend work item in pm_wq, in milliseconds (if 'delay' is zero, the work
343 item is queued up immediately); returns 0 on success, 1 if the device's PM
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200344 runtime status was already 'suspended', or error code if the request
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200345 hasn't been scheduled (or queued up if 'delay' is 0); if the execution of
346 ->runtime_suspend() is already scheduled and not yet expired, the new
347 value of 'delay' will be used as the time to wait
348
349 int pm_request_resume(struct device *dev);
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +0100350 - submit a request to execute the subsystem-level resume callback for the
351 device (the request is represented by a work item in pm_wq); returns 0 on
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200352 success, 1 if the device's runtime PM status was already 'active', or
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200353 error code if the request hasn't been queued up
354
355 void pm_runtime_get_noresume(struct device *dev);
356 - increment the device's usage counter
357
358 int pm_runtime_get(struct device *dev);
359 - increment the device's usage counter, run pm_request_resume(dev) and
360 return its result
361
362 int pm_runtime_get_sync(struct device *dev);
363 - increment the device's usage counter, run pm_runtime_resume(dev) and
364 return its result
365
366 void pm_runtime_put_noidle(struct device *dev);
367 - decrement the device's usage counter
368
369 int pm_runtime_put(struct device *dev);
Alan Stern15bcb91d2010-09-25 23:35:21 +0200370 - decrement the device's usage counter; if the result is 0 then run
371 pm_request_idle(dev) and return its result
372
373 int pm_runtime_put_autosuspend(struct device *dev);
374 - decrement the device's usage counter; if the result is 0 then run
375 pm_request_autosuspend(dev) and return its result
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200376
377 int pm_runtime_put_sync(struct device *dev);
Alan Stern15bcb91d2010-09-25 23:35:21 +0200378 - decrement the device's usage counter; if the result is 0 then run
379 pm_runtime_idle(dev) and return its result
380
Alan Sternc7b61de2010-12-01 00:14:42 +0100381 int pm_runtime_put_sync_suspend(struct device *dev);
382 - decrement the device's usage counter; if the result is 0 then run
383 pm_runtime_suspend(dev) and return its result
384
Alan Stern15bcb91d2010-09-25 23:35:21 +0200385 int pm_runtime_put_sync_autosuspend(struct device *dev);
386 - decrement the device's usage counter; if the result is 0 then run
387 pm_runtime_autosuspend(dev) and return its result
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200388
389 void pm_runtime_enable(struct device *dev);
Rafael J. Wysockie358bad2011-07-06 10:52:06 +0200390 - decrement the device's 'power.disable_depth' field; if that field is equal
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200391 to zero, the runtime PM helper functions can execute subsystem-level
Rafael J. Wysockie358bad2011-07-06 10:52:06 +0200392 callbacks described in Section 2 for the device
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200393
394 int pm_runtime_disable(struct device *dev);
Rafael J. Wysockie358bad2011-07-06 10:52:06 +0200395 - increment the device's 'power.disable_depth' field (if the value of that
396 field was previously zero, this prevents subsystem-level runtime PM
397 callbacks from being run for the device), make sure that all of the pending
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200398 runtime PM operations on the device are either completed or canceled;
Rafael J. Wysockia6ab7aa2009-12-22 20:43:17 +0100399 returns 1 if there was a resume request pending and it was necessary to
400 execute the subsystem-level resume callback for the device to satisfy that
401 request, otherwise 0 is returned
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200402
Rafael J. Wysockie358bad2011-07-06 10:52:06 +0200403 int pm_runtime_barrier(struct device *dev);
404 - check if there's a resume request pending for the device and resume it
405 (synchronously) in that case, cancel any other pending runtime PM requests
406 regarding it and wait for all runtime PM operations on it in progress to
407 complete; returns 1 if there was a resume request pending and it was
408 necessary to execute the subsystem-level resume callback for the device to
409 satisfy that request, otherwise 0 is returned
410
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200411 void pm_suspend_ignore_children(struct device *dev, bool enable);
412 - set/unset the power.ignore_children flag of the device
413
414 int pm_runtime_set_active(struct device *dev);
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200415 - clear the device's 'power.runtime_error' flag, set the device's runtime
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200416 PM status to 'active' and update its parent's counter of 'active'
417 children as appropriate (it is only valid to use this function if
418 'power.runtime_error' is set or 'power.disable_depth' is greater than
419 zero); it will fail and return error code if the device has a parent
420 which is not active and the 'power.ignore_children' flag of which is unset
421
422 void pm_runtime_set_suspended(struct device *dev);
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200423 - clear the device's 'power.runtime_error' flag, set the device's runtime
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200424 PM status to 'suspended' and update its parent's counter of 'active'
425 children as appropriate (it is only valid to use this function if
426 'power.runtime_error' is set or 'power.disable_depth' is greater than
427 zero)
428
Rafael J. Wysockid690b2c2010-03-06 21:28:37 +0100429 bool pm_runtime_suspended(struct device *dev);
Rafael J. Wysockif08f5a02010-12-16 17:11:58 +0100430 - return true if the device's runtime PM status is 'suspended' and its
431 'power.disable_depth' field is equal to zero, or false otherwise
Rafael J. Wysockid690b2c2010-03-06 21:28:37 +0100432
Kevin Hilmanf3393b62011-07-12 11:17:09 +0200433 bool pm_runtime_status_suspended(struct device *dev);
434 - return true if the device's runtime PM status is 'suspended'
435
Rafael J. Wysocki87d1b3e2010-03-06 21:28:17 +0100436 void pm_runtime_allow(struct device *dev);
437 - set the power.runtime_auto flag for the device and decrease its usage
438 counter (used by the /sys/devices/.../power/control interface to
439 effectively allow the device to be power managed at run time)
440
441 void pm_runtime_forbid(struct device *dev);
442 - unset the power.runtime_auto flag for the device and increase its usage
443 counter (used by the /sys/devices/.../power/control interface to
444 effectively prevent the device from being power managed at run time)
445
Alan Stern7490e442010-09-25 23:35:15 +0200446 void pm_runtime_no_callbacks(struct device *dev);
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200447 - set the power.no_callbacks flag for the device and remove the runtime
Alan Stern7490e442010-09-25 23:35:15 +0200448 PM attributes from /sys/devices/.../power (or prevent them from being
449 added when the device is registered)
450
Alan Sternc7b61de2010-12-01 00:14:42 +0100451 void pm_runtime_irq_safe(struct device *dev);
452 - set the power.irq_safe flag for the device, causing the runtime-PM
Rafael J. Wysocki64584eb2011-08-25 15:31:05 +0200453 callbacks to be invoked with interrupts off
Alan Sternc7b61de2010-12-01 00:14:42 +0100454
Alan Stern15bcb91d2010-09-25 23:35:21 +0200455 void pm_runtime_mark_last_busy(struct device *dev);
456 - set the power.last_busy field to the current time
457
458 void pm_runtime_use_autosuspend(struct device *dev);
459 - set the power.use_autosuspend flag, enabling autosuspend delays
460
461 void pm_runtime_dont_use_autosuspend(struct device *dev);
462 - clear the power.use_autosuspend flag, disabling autosuspend delays
463
464 void pm_runtime_set_autosuspend_delay(struct device *dev, int delay);
465 - set the power.autosuspend_delay value to 'delay' (expressed in
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200466 milliseconds); if 'delay' is negative then runtime suspends are
Alan Stern15bcb91d2010-09-25 23:35:21 +0200467 prevented
468
469 unsigned long pm_runtime_autosuspend_expiration(struct device *dev);
470 - calculate the time when the current autosuspend delay period will expire,
471 based on power.last_busy and power.autosuspend_delay; if the delay time
472 is 1000 ms or larger then the expiration time is rounded up to the
473 nearest second; returns 0 if the delay period has already expired or
474 power.use_autosuspend isn't set, otherwise returns the expiration time
475 in jiffies
476
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200477It is safe to execute the following helper functions from interrupt context:
478
479pm_request_idle()
Alan Stern15bcb91d2010-09-25 23:35:21 +0200480pm_request_autosuspend()
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200481pm_schedule_suspend()
482pm_request_resume()
483pm_runtime_get_noresume()
484pm_runtime_get()
485pm_runtime_put_noidle()
486pm_runtime_put()
Alan Stern15bcb91d2010-09-25 23:35:21 +0200487pm_runtime_put_autosuspend()
488pm_runtime_enable()
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200489pm_suspend_ignore_children()
490pm_runtime_set_active()
491pm_runtime_set_suspended()
Alan Stern15bcb91d2010-09-25 23:35:21 +0200492pm_runtime_suspended()
493pm_runtime_mark_last_busy()
494pm_runtime_autosuspend_expiration()
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200495
Alan Sternc7b61de2010-12-01 00:14:42 +0100496If pm_runtime_irq_safe() has been called for a device then the following helper
497functions may also be used in interrupt context:
498
Ming Lei2e6ba512011-09-21 22:31:33 +0200499pm_runtime_idle()
Alan Sternc7b61de2010-12-01 00:14:42 +0100500pm_runtime_suspend()
501pm_runtime_autosuspend()
502pm_runtime_resume()
503pm_runtime_get_sync()
Kevin Hilman02b26772011-08-05 21:45:20 +0200504pm_runtime_put_sync()
Alan Sternc7b61de2010-12-01 00:14:42 +0100505pm_runtime_put_sync_suspend()
Colin Cross311aab72011-08-08 23:39:36 +0200506pm_runtime_put_sync_autosuspend()
Alan Sternc7b61de2010-12-01 00:14:42 +0100507
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +02005085. Runtime PM Initialization, Device Probing and Removal
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200509
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200510Initially, the runtime PM is disabled for all devices, which means that the
511majority of the runtime PM helper funtions described in Section 4 will return
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200512-EAGAIN until pm_runtime_enable() is called for the device.
513
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200514In addition to that, the initial runtime PM status of all devices is
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200515'suspended', but it need not reflect the actual physical state of the device.
516Thus, if the device is initially active (i.e. it is able to process I/O), its
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200517runtime PM status must be changed to 'active', with the help of
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200518pm_runtime_set_active(), before pm_runtime_enable() is called for the device.
519
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200520However, if the device has a parent and the parent's runtime PM is enabled,
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200521calling pm_runtime_set_active() for the device will affect the parent, unless
522the parent's 'power.ignore_children' flag is set. Namely, in that case the
523parent won't be able to suspend at run time, using the PM core's helper
524functions, as long as the child's status is 'active', even if the child's
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200525runtime PM is still disabled (i.e. pm_runtime_enable() hasn't been called for
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200526the child yet or pm_runtime_disable() has been called for it). For this reason,
527once pm_runtime_set_active() has been called for the device, pm_runtime_enable()
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200528should be called for it too as soon as reasonably possible or its runtime PM
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200529status should be changed back to 'suspended' with the help of
530pm_runtime_set_suspended().
531
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200532If the default initial runtime PM status of the device (i.e. 'suspended')
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200533reflects the actual state of the device, its bus type's or its driver's
534->probe() callback will likely need to wake it up using one of the PM core's
535helper functions described in Section 4. In that case, pm_runtime_resume()
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200536should be used. Of course, for this purpose the device's runtime PM has to be
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200537enabled earlier by calling pm_runtime_enable().
538
Rafael J. Wysockif5da24d2011-07-02 14:27:11 +0200539If the device bus type's or driver's ->probe() callback runs
Rafael J. Wysocki5e928f72009-08-18 23:38:32 +0200540pm_runtime_suspend() or pm_runtime_idle() or their asynchronous counterparts,
541they will fail returning -EAGAIN, because the device's usage counter is
Rafael J. Wysockif5da24d2011-07-02 14:27:11 +0200542incremented by the driver core before executing ->probe(). Still, it may be
543desirable to suspend the device as soon as ->probe() has finished, so the driver
544core uses pm_runtime_put_sync() to invoke the subsystem-level idle callback for
545the device at that time.
546
547Moreover, the driver core prevents runtime PM callbacks from racing with the bus
548notifier callback in __device_release_driver(), which is necessary, because the
549notifier is used by some subsystems to carry out operations affecting the
550runtime PM functionality. It does so by calling pm_runtime_get_sync() before
551driver_sysfs_remove() and the BUS_NOTIFY_UNBIND_DRIVER notifications. This
552resumes the device if it's in the suspended state and prevents it from
553being suspended again while those routines are being executed.
554
555To allow bus types and drivers to put devices into the suspended state by
556calling pm_runtime_suspend() from their ->remove() routines, the driver core
557executes pm_runtime_put_sync() after running the BUS_NOTIFY_UNBIND_DRIVER
558notifications in __device_release_driver(). This requires bus types and
559drivers to make their ->remove() callbacks avoid races with runtime PM directly,
560but also it allows of more flexibility in the handling of devices during the
561removal of their drivers.
Alan Sternf1212ae2009-12-22 20:43:40 +0100562
Rafael J. Wysocki87d1b3e2010-03-06 21:28:17 +0100563The user space can effectively disallow the driver of the device to power manage
564it at run time by changing the value of its /sys/devices/.../power/control
565attribute to "on", which causes pm_runtime_forbid() to be called. In principle,
566this mechanism may also be used by the driver to effectively turn off the
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200567runtime power management of the device until the user space turns it on.
568Namely, during the initialization the driver can make sure that the runtime PM
Rafael J. Wysocki87d1b3e2010-03-06 21:28:17 +0100569status of the device is 'active' and call pm_runtime_forbid(). It should be
570noted, however, that if the user space has already intentionally changed the
571value of /sys/devices/.../power/control to "auto" to allow the driver to power
572manage the device at run time, the driver may confuse it by using
573pm_runtime_forbid() this way.
574
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +02005756. Runtime PM and System Sleep
Alan Sternf1212ae2009-12-22 20:43:40 +0100576
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200577Runtime PM and system sleep (i.e., system suspend and hibernation, also known
Alan Sternf1212ae2009-12-22 20:43:40 +0100578as suspend-to-RAM and suspend-to-disk) interact with each other in a couple of
579ways. If a device is active when a system sleep starts, everything is
580straightforward. But what should happen if the device is already suspended?
581
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200582The device may have different wake-up settings for runtime PM and system sleep.
583For example, remote wake-up may be enabled for runtime suspend but disallowed
Alan Sternf1212ae2009-12-22 20:43:40 +0100584for system sleep (device_may_wakeup(dev) returns 'false'). When this happens,
585the subsystem-level system suspend callback is responsible for changing the
586device's wake-up setting (it may leave that to the device driver's system
587suspend routine). It may be necessary to resume the device and suspend it again
588in order to do so. The same is true if the driver uses different power levels
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200589or other settings for runtime suspend and system sleep.
Alan Sternf1212ae2009-12-22 20:43:40 +0100590
Rafael J. Wysocki455716e2011-07-01 22:29:05 +0200591During system resume, the simplest approach is to bring all devices back to full
592power, even if they had been suspended before the system suspend began. There
593are several reasons for this, including:
Alan Sternf1212ae2009-12-22 20:43:40 +0100594
595 * The device might need to switch power levels, wake-up settings, etc.
596
597 * Remote wake-up events might have been lost by the firmware.
598
599 * The device's children may need the device to be at full power in order
600 to resume themselves.
601
602 * The driver's idea of the device state may not agree with the device's
603 physical state. This can happen during resume from hibernation.
604
605 * The device might need to be reset.
606
607 * Even though the device was suspended, if its usage counter was > 0 then most
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200608 likely it would need a runtime resume in the near future anyway.
Alan Sternf1212ae2009-12-22 20:43:40 +0100609
Rafael J. Wysocki455716e2011-07-01 22:29:05 +0200610If the device had been suspended before the system suspend began and it's
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200611brought back to full power during resume, then its runtime PM status will have
Rafael J. Wysocki455716e2011-07-01 22:29:05 +0200612to be updated to reflect the actual post-system sleep status. The way to do
613this is:
Alan Sternf1212ae2009-12-22 20:43:40 +0100614
615 pm_runtime_disable(dev);
616 pm_runtime_set_active(dev);
617 pm_runtime_enable(dev);
618
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200619The PM core always increments the runtime usage counter before calling the
Rafael J. Wysocki1e2ef052011-07-06 10:51:58 +0200620->suspend() callback and decrements it after calling the ->resume() callback.
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200621Hence disabling runtime PM temporarily like this will not cause any runtime
Rafael J. Wysocki1e2ef052011-07-06 10:51:58 +0200622suspend attempts to be permanently lost. If the usage count goes to zero
623following the return of the ->resume() callback, the ->runtime_idle() callback
624will be invoked as usual.
625
Rafael J. Wysocki455716e2011-07-01 22:29:05 +0200626On some systems, however, system sleep is not entered through a global firmware
627or hardware operation. Instead, all hardware components are put into low-power
628states directly by the kernel in a coordinated way. Then, the system sleep
629state effectively follows from the states the hardware components end up in
630and the system is woken up from that state by a hardware interrupt or a similar
631mechanism entirely under the kernel's control. As a result, the kernel never
632gives control away and the states of all devices during resume are precisely
633known to it. If that is the case and none of the situations listed above takes
634place (in particular, if the system is not waking up from hibernation), it may
635be more efficient to leave the devices that had been suspended before the system
636suspend began in the suspended state.
637
Rafael J. Wysocki1e2ef052011-07-06 10:51:58 +0200638The PM core does its best to reduce the probability of race conditions between
639the runtime PM and system suspend/resume (and hibernation) callbacks by carrying
640out the following operations:
641
642 * During system suspend it calls pm_runtime_get_noresume() and
643 pm_runtime_barrier() for every device right before executing the
644 subsystem-level .suspend() callback for it. In addition to that it calls
645 pm_runtime_disable() for every device right after executing the
646 subsystem-level .suspend() callback for it.
647
648 * During system resume it calls pm_runtime_enable() and pm_runtime_put_sync()
649 for every device right before and right after executing the subsystem-level
650 .resume() callback for it, respectively.
651
Rafael J. Wysockid690b2c2010-03-06 21:28:37 +01006527. Generic subsystem callbacks
653
654Subsystems may wish to conserve code space by using the set of generic power
655management callbacks provided by the PM core, defined in
656driver/base/power/generic_ops.c:
657
658 int pm_generic_runtime_idle(struct device *dev);
659 - invoke the ->runtime_idle() callback provided by the driver of this
660 device, if defined, and call pm_runtime_suspend() for this device if the
661 return value is 0 or the callback is not defined
662
663 int pm_generic_runtime_suspend(struct device *dev);
664 - invoke the ->runtime_suspend() callback provided by the driver of this
665 device and return its result, or return -EINVAL if not defined
666
667 int pm_generic_runtime_resume(struct device *dev);
668 - invoke the ->runtime_resume() callback provided by the driver of this
669 device and return its result, or return -EINVAL if not defined
670
671 int pm_generic_suspend(struct device *dev);
672 - if the device has not been suspended at run time, invoke the ->suspend()
673 callback provided by its driver and return its result, or return 0 if not
674 defined
675
Rafael J. Wysockie5291922011-07-01 22:12:59 +0200676 int pm_generic_suspend_noirq(struct device *dev);
677 - if pm_runtime_suspended(dev) returns "false", invoke the ->suspend_noirq()
678 callback provided by the device's driver and return its result, or return
679 0 if not defined
680
Rafael J. Wysockid690b2c2010-03-06 21:28:37 +0100681 int pm_generic_resume(struct device *dev);
682 - invoke the ->resume() callback provided by the driver of this device and,
683 if successful, change the device's runtime PM status to 'active'
684
Rafael J. Wysockie5291922011-07-01 22:12:59 +0200685 int pm_generic_resume_noirq(struct device *dev);
686 - invoke the ->resume_noirq() callback provided by the driver of this device
687
Rafael J. Wysockid690b2c2010-03-06 21:28:37 +0100688 int pm_generic_freeze(struct device *dev);
689 - if the device has not been suspended at run time, invoke the ->freeze()
690 callback provided by its driver and return its result, or return 0 if not
691 defined
692
Rafael J. Wysockie5291922011-07-01 22:12:59 +0200693 int pm_generic_freeze_noirq(struct device *dev);
694 - if pm_runtime_suspended(dev) returns "false", invoke the ->freeze_noirq()
695 callback provided by the device's driver and return its result, or return
696 0 if not defined
697
Rafael J. Wysockid690b2c2010-03-06 21:28:37 +0100698 int pm_generic_thaw(struct device *dev);
699 - if the device has not been suspended at run time, invoke the ->thaw()
700 callback provided by its driver and return its result, or return 0 if not
701 defined
702
Rafael J. Wysockie5291922011-07-01 22:12:59 +0200703 int pm_generic_thaw_noirq(struct device *dev);
704 - if pm_runtime_suspended(dev) returns "false", invoke the ->thaw_noirq()
705 callback provided by the device's driver and return its result, or return
706 0 if not defined
707
Rafael J. Wysockid690b2c2010-03-06 21:28:37 +0100708 int pm_generic_poweroff(struct device *dev);
709 - if the device has not been suspended at run time, invoke the ->poweroff()
710 callback provided by its driver and return its result, or return 0 if not
711 defined
712
Rafael J. Wysockie5291922011-07-01 22:12:59 +0200713 int pm_generic_poweroff_noirq(struct device *dev);
714 - if pm_runtime_suspended(dev) returns "false", run the ->poweroff_noirq()
715 callback provided by the device's driver and return its result, or return
716 0 if not defined
717
Rafael J. Wysockid690b2c2010-03-06 21:28:37 +0100718 int pm_generic_restore(struct device *dev);
719 - invoke the ->restore() callback provided by the driver of this device and,
720 if successful, change the device's runtime PM status to 'active'
721
Rafael J. Wysockie5291922011-07-01 22:12:59 +0200722 int pm_generic_restore_noirq(struct device *dev);
723 - invoke the ->restore_noirq() callback provided by the device's driver
724
Rafael J. Wysockid690b2c2010-03-06 21:28:37 +0100725These functions can be assigned to the ->runtime_idle(), ->runtime_suspend(),
Rafael J. Wysockie5291922011-07-01 22:12:59 +0200726->runtime_resume(), ->suspend(), ->suspend_noirq(), ->resume(),
727->resume_noirq(), ->freeze(), ->freeze_noirq(), ->thaw(), ->thaw_noirq(),
728->poweroff(), ->poweroff_noirq(), ->restore(), ->restore_noirq() callback
729pointers in the subsystem-level dev_pm_ops structures.
Rafael J. Wysockid690b2c2010-03-06 21:28:37 +0100730
731If a subsystem wishes to use all of them at the same time, it can simply assign
732the GENERIC_SUBSYS_PM_OPS macro, defined in include/linux/pm.h, to its
733dev_pm_ops structure pointer.
734
735Device drivers that wish to use the same function as a system suspend, freeze,
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200736poweroff and runtime suspend callback, and similarly for system resume, thaw,
737restore, and runtime resume, can achieve this with the help of the
Rafael J. Wysockid690b2c2010-03-06 21:28:37 +0100738UNIVERSAL_DEV_PM_OPS macro defined in include/linux/pm.h (possibly setting its
739last argument to NULL).
Alan Stern7490e442010-09-25 23:35:15 +0200740
7418. "No-Callback" Devices
742
743Some "devices" are only logical sub-devices of their parent and cannot be
744power-managed on their own. (The prototype example is a USB interface. Entire
745USB devices can go into low-power mode or send wake-up requests, but neither is
746possible for individual interfaces.) The drivers for these devices have no
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200747need of runtime PM callbacks; if the callbacks did exist, ->runtime_suspend()
Alan Stern7490e442010-09-25 23:35:15 +0200748and ->runtime_resume() would always return 0 without doing anything else and
749->runtime_idle() would always call pm_runtime_suspend().
750
751Subsystems can tell the PM core about these devices by calling
752pm_runtime_no_callbacks(). This should be done after the device structure is
753initialized and before it is registered (although after device registration is
754also okay). The routine will set the device's power.no_callbacks flag and
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200755prevent the non-debugging runtime PM sysfs attributes from being created.
Alan Stern7490e442010-09-25 23:35:15 +0200756
757When power.no_callbacks is set, the PM core will not invoke the
758->runtime_idle(), ->runtime_suspend(), or ->runtime_resume() callbacks.
759Instead it will assume that suspends and resumes always succeed and that idle
760devices should be suspended.
761
762As a consequence, the PM core will never directly inform the device's subsystem
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200763or driver about runtime power changes. Instead, the driver for the device's
Alan Stern7490e442010-09-25 23:35:15 +0200764parent must take responsibility for telling the device's driver when the
765parent's power state changes.
Alan Stern15bcb91d2010-09-25 23:35:21 +0200766
7679. Autosuspend, or automatically-delayed suspends
768
769Changing a device's power state isn't free; it requires both time and energy.
770A device should be put in a low-power state only when there's some reason to
771think it will remain in that state for a substantial time. A common heuristic
772says that a device which hasn't been used for a while is liable to remain
773unused; following this advice, drivers should not allow devices to be suspended
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200774at runtime until they have been inactive for some minimum period. Even when
Alan Stern15bcb91d2010-09-25 23:35:21 +0200775the heuristic ends up being non-optimal, it will still prevent devices from
776"bouncing" too rapidly between low-power and full-power states.
777
778The term "autosuspend" is an historical remnant. It doesn't mean that the
779device is automatically suspended (the subsystem or driver still has to call
Rafael J. Wysocki62052ab2011-07-06 10:52:13 +0200780the appropriate PM routines); rather it means that runtime suspends will
Alan Stern15bcb91d2010-09-25 23:35:21 +0200781automatically be delayed until the desired period of inactivity has elapsed.
782
783Inactivity is determined based on the power.last_busy field. Drivers should
784call pm_runtime_mark_last_busy() to update this field after carrying out I/O,
785typically just before calling pm_runtime_put_autosuspend(). The desired length
786of the inactivity period is a matter of policy. Subsystems can set this length
787initially by calling pm_runtime_set_autosuspend_delay(), but after device
788registration the length should be controlled by user space, using the
789/sys/devices/.../power/autosuspend_delay_ms attribute.
790
791In order to use autosuspend, subsystems or drivers must call
792pm_runtime_use_autosuspend() (preferably before registering the device), and
793thereafter they should use the various *_autosuspend() helper functions instead
794of the non-autosuspend counterparts:
795
796 Instead of: pm_runtime_suspend use: pm_runtime_autosuspend;
797 Instead of: pm_schedule_suspend use: pm_request_autosuspend;
798 Instead of: pm_runtime_put use: pm_runtime_put_autosuspend;
799 Instead of: pm_runtime_put_sync use: pm_runtime_put_sync_autosuspend.
800
801Drivers may also continue to use the non-autosuspend helper functions; they
802will behave normally, not taking the autosuspend delay into account.
803Similarly, if the power.use_autosuspend field isn't set then the autosuspend
804helper functions will behave just like the non-autosuspend counterparts.
805
Alan Stern886486b2011-11-03 23:39:18 +0100806Under some circumstances a driver or subsystem may want to prevent a device
807from autosuspending immediately, even though the usage counter is zero and the
808autosuspend delay time has expired. If the ->runtime_suspend() callback
809returns -EAGAIN or -EBUSY, and if the next autosuspend delay expiration time is
810in the future (as it normally would be if the callback invoked
811pm_runtime_mark_last_busy()), the PM core will automatically reschedule the
812autosuspend. The ->runtime_suspend() callback can't do this rescheduling
813itself because no suspend requests of any kind are accepted while the device is
814suspending (i.e., while the callback is running).
815
Alan Stern15bcb91d2010-09-25 23:35:21 +0200816The implementation is well suited for asynchronous use in interrupt contexts.
817However such use inevitably involves races, because the PM core can't
818synchronize ->runtime_suspend() callbacks with the arrival of I/O requests.
819This synchronization must be handled by the driver, using its private lock.
820Here is a schematic pseudo-code example:
821
822 foo_read_or_write(struct foo_priv *foo, void *data)
823 {
824 lock(&foo->private_lock);
825 add_request_to_io_queue(foo, data);
826 if (foo->num_pending_requests++ == 0)
827 pm_runtime_get(&foo->dev);
828 if (!foo->is_suspended)
829 foo_process_next_request(foo);
830 unlock(&foo->private_lock);
831 }
832
833 foo_io_completion(struct foo_priv *foo, void *req)
834 {
835 lock(&foo->private_lock);
836 if (--foo->num_pending_requests == 0) {
837 pm_runtime_mark_last_busy(&foo->dev);
838 pm_runtime_put_autosuspend(&foo->dev);
839 } else {
840 foo_process_next_request(foo);
841 }
842 unlock(&foo->private_lock);
843 /* Send req result back to the user ... */
844 }
845
846 int foo_runtime_suspend(struct device *dev)
847 {
848 struct foo_priv foo = container_of(dev, ...);
849 int ret = 0;
850
851 lock(&foo->private_lock);
852 if (foo->num_pending_requests > 0) {
853 ret = -EBUSY;
854 } else {
855 /* ... suspend the device ... */
856 foo->is_suspended = 1;
857 }
858 unlock(&foo->private_lock);
859 return ret;
860 }
861
862 int foo_runtime_resume(struct device *dev)
863 {
864 struct foo_priv foo = container_of(dev, ...);
865
866 lock(&foo->private_lock);
867 /* ... resume the device ... */
868 foo->is_suspended = 0;
869 pm_runtime_mark_last_busy(&foo->dev);
870 if (foo->num_pending_requests > 0)
871 foo_process_requests(foo);
872 unlock(&foo->private_lock);
873 return 0;
874 }
875
876The important point is that after foo_io_completion() asks for an autosuspend,
877the foo_runtime_suspend() callback may race with foo_read_or_write().
878Therefore foo_runtime_suspend() has to check whether there are any pending I/O
879requests (while holding the private lock) before allowing the suspend to
880proceed.
881
882In addition, the power.autosuspend_delay field can be changed by user space at
883any time. If a driver cares about this, it can call
884pm_runtime_autosuspend_expiration() from within the ->runtime_suspend()
885callback while holding its private lock. If the function returns a nonzero
886value then the delay has not yet expired and the callback should return
887-EAGAIN.