1 ==================================================
2 Runtime Power Management Framework for I/O Devices
3 ==================================================
4
5 (C) 2009-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
6
7 (C) 2010 Alan Stern <stern@rowland.harvard.edu>
8
9 (C) 2014 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
10
11 1. Introduction
12 ===============
13
14 Support for runtime power management (runtime PM) of I/O devices is provided
15 at the power management core (PM core) level by means of:
16
17 * The power management workqueue pm_wq in which bus types and device drivers can
18 put their PM-related work items. It is strongly recommended that pm_wq be
19 used for queuing all work items related to runtime PM, because this allows
20 them to be synchronized with system-wide power transitions (suspend to RAM,
21 hibernation and resume from system sleep states). pm_wq is declared in
22 include/linux/pm_runtime.h and defined in kernel/power/main.c.
23
24 * A number of runtime PM fields in the 'power' member of 'struct device' (which
25 is of the type 'struct dev_pm_info', defined in include/linux/pm.h) that can
26 be used for synchronizing runtime PM operations with one another.
27
28 * Three device runtime PM callbacks in 'struct dev_pm_ops' (defined in
29 include/linux/pm.h).
30
31 * A set of helper functions defined in drivers/base/power/runtime.c that can be
32 used for carrying out runtime PM operations in such a way that the
33 synchronization between them is taken care of by the PM core. Bus types and
34 device drivers are encouraged to use these functions.
35
36 The runtime PM callbacks present in 'struct dev_pm_ops', the device runtime PM
37 fields of 'struct dev_pm_info' and the core helper functions provided for
38 runtime PM are described below.
39
40 2. Device Runtime PM Callbacks
41 ==============================
42
43 There are three device runtime PM callbacks defined in 'struct dev_pm_ops'::
44
45 struct dev_pm_ops {
46 ...
47 int (*runtime_suspend)(struct device *dev);
48 int (*runtime_resume)(struct device *dev);
49 int (*runtime_idle)(struct device *dev);
50 ...
51 };
52
53 The ->runtime_suspend(), ->runtime_resume() and ->runtime_idle() callbacks
54 are executed by the PM core for the device's subsystem that may be either of
55 the following:
56
57 1. PM domain of the device, if the device's PM domain object, dev->pm_domain,
58 is present.
59
60 2. Device type of the device, if both dev->type and dev->type->pm are present.
61
62 3. Device class of the device, if both dev->class and dev->class->pm are
63 present.
64
65 4. Bus type of the device, if both dev->bus and dev->bus->pm are present.
66
67 If the subsystem chosen by applying the above rules doesn't provide the relevant
68 callback, the PM core will invoke the corresponding driver callback stored in
69 dev->driver->pm directly (if present).
70
71 The PM core always checks which callback to use in the order given above, so the
72 priority order of callbacks from high to low is: PM domain, device type, class
73 and bus type. Moreover, the high-priority one will always take precedence over
74 a low-priority one. The PM domain, bus type, device type and class callbacks
75 are referred to as subsystem-level callbacks in what follows.
76
77 By default, the callbacks are always invoked in process context with interrupts
78 enabled. However, the pm_runtime_irq_safe() helper function can be used to tell
79 the PM core that it is safe to run the ->runtime_suspend(), ->runtime_resume()
80 and ->runtime_idle() callbacks for the given device in atomic context with
81 interrupts disabled. This implies that the callback routines in question must
82 not block or sleep, but it also means that the synchronous helper functions
83 listed at the end of Section 4 may be used for that device within an interrupt
84 handler or generally in an atomic context.
85
86 The subsystem-level suspend callback, if present, is _entirely_ _responsible_
87 for handling the suspend of the device as appropriate, which may, but need not
88 include executing the device driver's own ->runtime_suspend() callback (from the
89 PM core's point of view it is not necessary to implement a ->runtime_suspend()
90 callback in a device driver as long as the subsystem-level suspend callback
91 knows what to do to handle the device).
92
93 * Once the subsystem-level suspend callback (or the driver suspend callback,
94 if invoked directly) has completed successfully for the given device, the PM
95 core regards the device as suspended, which need not mean that it has been
96 put into a low power state. It is supposed to mean, however, that the
97 device will not process data and will not communicate with the CPU(s) and
98 RAM until the appropriate resume callback is executed for it. The runtime
99 PM status of a device after successful execution of the suspend callback is
100 'suspended'.
101
102 * If the suspend callback returns -EBUSY or -EAGAIN, the device's runtime PM
103 status remains 'active', which means that the device _must_ be fully
104 operational afterwards.
105
106 * If the suspend callback returns an error code different from -EBUSY and
107 -EAGAIN, the PM core regards this as a fatal error and will refuse to run
108 the helper functions described in Section 4 for the device until its status
109 is directly set to either 'active', or 'suspended' (the PM core provides
110 special helper functions for this purpose).
111
112 In particular, if the driver requires remote wakeup capability (i.e. hardware
113 mechanism allowing the device to request a change of its power state, such as
114 PCI PME) for proper functioning and device_can_wakeup() returns 'false' for the
115 device, then ->runtime_suspend() should return -EBUSY. On the other hand, if
116 device_can_wakeup() returns 'true' for the device and the device is put into a
117 low-power state during the execution of the suspend callback, it is expected
118 that remote wakeup will be enabled for the device. Generally, remote wakeup
119 should be enabled for all input devices put into low-power states at run time.
120
121 The subsystem-level resume callback, if present, is **entirely responsible** for
122 handling the resume of the device as appropriate, which may, but need not
123 include executing the device driver's own ->runtime_resume() callback (from the
124 PM core's point of view it is not necessary to implement a ->runtime_resume()
125 callback in a device driver as long as the subsystem-level resume callback knows
126 what to do to handle the device).
127
128 * Once the subsystem-level resume callback (or the driver resume callback, if
129 invoked directly) has completed successfully, the PM core regards the device
130 as fully operational, which means that the device _must_ be able to complete
131 I/O operations as needed. The runtime PM status of the device is then
132 'active'.
133
134 * If the resume callback returns an error code, the PM core regards this as a
135 fatal error and will refuse to run the helper functions described in Section
136 4 for the device, until its status is directly set to either 'active', or
137 'suspended' (by means of special helper functions provided by the PM core
138 for this purpose).
139
140 The idle callback (a subsystem-level one, if present, or the driver one) is
141 executed by the PM core whenever the device appears to be idle, which is
142 indicated to the PM core by two counters, the device's usage counter and the
143 counter of 'active' children of the device.
144
145 * If any of these counters is decreased using a helper function provided by
146 the PM core and it turns out to be equal to zero, the other counter is
147 checked. If that counter also is equal to zero, the PM core executes the
148 idle callback with the device as its argument.
149
150 The action performed by the idle callback is totally dependent on the subsystem
151 (or driver) in question, but the expected and recommended action is to check
152 if the device can be suspended (i.e. if all of the conditions necessary for
153 suspending the device are satisfied) and to queue up a suspend request for the
154 device in that case. If there is no idle callback, or if the callback returns
155 0, then the PM core will attempt to carry out a runtime suspend of the device,
156 also respecting devices configured for autosuspend. In essence this means a
157 call to __pm_runtime_autosuspend() (do note that drivers needs to update the
158 device last busy mark, pm_runtime_mark_last_busy(), to control the delay under
159 this circumstance). To prevent this (for example, if the callback routine has
160 started a delayed suspend), the routine must return a non-zero value. Negative
161 error return codes are ignored by the PM core.
162
163 The helper functions provided by the PM core, described in Section 4, guarantee
164 that the following constraints are met with respect to runtime PM callbacks for
165 one device:
166
167 (1) The callbacks are mutually exclusive (e.g. it is forbidden to execute
168 ->runtime_suspend() in parallel with ->runtime_resume() or with another
169 instance of ->runtime_suspend() for the same device) with the exception that
170 ->runtime_suspend() or ->runtime_resume() can be executed in parallel with
171 ->runtime_idle() (although ->runtime_idle() will not be started while any
172 of the other callbacks is being executed for the same device).
173
174 (2) ->runtime_idle() and ->runtime_suspend() can only be executed for 'active'
175 devices (i.e. the PM core will only execute ->runtime_idle() or
176 ->runtime_suspend() for the devices the runtime PM status of which is
177 'active').
178
179 (3) ->runtime_idle() and ->runtime_suspend() can only be executed for a device
180 the usage counter of which is equal to zero _and_ either the counter of
181 'active' children of which is equal to zero, or the 'power.ignore_children'
182 flag of which is set.
183
184 (4) ->runtime_resume() can only be executed for 'suspended' devices (i.e. the
185 PM core will only execute ->runtime_resume() for the devices the runtime
186 PM status of which is 'suspended').
187
188 Additionally, the helper functions provided by the PM core obey the following
189 rules:
190
191 * If ->runtime_suspend() is about to be executed or there's a pending request
192 to execute it, ->runtime_idle() will not be executed for the same device.
193
194 * A request to execute or to schedule the execution of ->runtime_suspend()
195 will cancel any pending requests to execute ->runtime_idle() for the same
196 device.
197
198 * If ->runtime_resume() is about to be executed or there's a pending request
199 to execute it, the other callbacks will not be executed for the same device.
200
201 * A request to execute ->runtime_resume() will cancel any pending or
202 scheduled requests to execute the other callbacks for the same device,
203 except for scheduled autosuspends.
204
205 3. Runtime PM Device Fields
206 ===========================
207
208 The following device runtime PM fields are present in 'struct dev_pm_info', as
209 defined in include/linux/pm.h:
210
211 `struct timer_list suspend_timer;`
212 - timer used for scheduling (delayed) suspend and autosuspend requests
213
214 `unsigned long timer_expires;`
215 - timer expiration time, in jiffies (if this is different from zero, the
216 timer is running and will expire at that time, otherwise the timer is not
217 running)
218
219 `struct work_struct work;`
220 - work structure used for queuing up requests (i.e. work items in pm_wq)
221
222 `wait_queue_head_t wait_queue;`
223 - wait queue used if any of the helper functions needs to wait for another
224 one to complete
225
226 `spinlock_t lock;`
227 - lock used for synchronization
228
229 `atomic_t usage_count;`
230 - the usage counter of the device
231
232 `atomic_t child_count;`
233 - the count of 'active' children of the device
234
235 `unsigned int ignore_children;`
236 - if set, the value of child_count is ignored (but still updated)
237
238 `unsigned int disable_depth;`
239 - used for disabling the helper functions (they work normally if this is
240 equal to zero); the initial value of it is 1 (i.e. runtime PM is
241 initially disabled for all devices)
242
243 `int runtime_error;`
244 - if set, there was a fatal error (one of the callbacks returned error code
245 as described in Section 2), so the helper functions will not work until
246 this flag is cleared; this is the error code returned by the failing
247 callback
248
249 `unsigned int idle_notification;`
250 - if set, ->runtime_idle() is being executed
251
252 `unsigned int request_pending;`
253 - if set, there's a pending request (i.e. a work item queued up into pm_wq)
254
255 `enum rpm_request request;`
256 - type of request that's pending (valid if request_pending is set)
257
258 `unsigned int deferred_resume;`
259 - set if ->runtime_resume() is about to be run while ->runtime_suspend() is
260 being executed for that device and it is not practical to wait for the
261 suspend to complete; means "start a resume as soon as you've suspended"
262
263 `enum rpm_status runtime_status;`
264 - the runtime PM status of the device; this field's initial value is
265 RPM_SUSPENDED, which means that each device is initially regarded by the
266 PM core as 'suspended', regardless of its real hardware status
267
268 `enum rpm_status last_status;`
269 - the last runtime PM status of the device captured before disabling runtime
270 PM for it (invalid initially and when disable_depth is 0)
271
272 `unsigned int runtime_auto;`
273 - if set, indicates that the user space has allowed the device driver to
274 power manage the device at run time via the /sys/devices/.../power/control
275 `interface;` it may only be modified with the help of the
276 pm_runtime_allow() and pm_runtime_forbid() helper functions
277
278 `unsigned int no_callbacks;`
279 - indicates that the device does not use the runtime PM callbacks (see
280 Section 8); it may be modified only by the pm_runtime_no_callbacks()
281 helper function
282
283 `unsigned int irq_safe;`
284 - indicates that the ->runtime_suspend() and ->runtime_resume() callbacks
285 will be invoked with the spinlock held and interrupts disabled
286
287 `unsigned int use_autosuspend;`
288 - indicates that the device's driver supports delayed autosuspend (see
289 Section 9); it may be modified only by the
290 pm_runtime{_dont}_use_autosuspend() helper functions
291
292 `unsigned int timer_autosuspends;`
293 - indicates that the PM core should attempt to carry out an autosuspend
294 when the timer expires rather than a normal suspend
295
296 `int autosuspend_delay;`
297 - the delay time (in milliseconds) to be used for autosuspend
298
299 `unsigned long last_busy;`
300 - the time (in jiffies) when the pm_runtime_mark_last_busy() helper
301 function was last called for this device; used in calculating inactivity
302 periods for autosuspend
303
304 All of the above fields are members of the 'power' member of 'struct device'.
305
306 4. Runtime PM Device Helper Functions
307 =====================================
308
309 The following runtime PM helper functions are defined in
310 drivers/base/power/runtime.c and include/linux/pm_runtime.h:
311
312 `void pm_runtime_init(struct device *dev);`
313 - initialize the device runtime PM fields in 'struct dev_pm_info'
314
315 `void pm_runtime_remove(struct device *dev);`
316 - make sure that the runtime PM of the device will be disabled after
317 removing the device from device hierarchy
318
319 `int pm_runtime_idle(struct device *dev);`
320 - execute the subsystem-level idle callback for the device; returns an
321 error code on failure, where -EINPROGRESS means that ->runtime_idle() is
322 already being executed; if there is no callback or the callback returns 0
323 then run pm_runtime_autosuspend(dev) and return its result
324
325 `int pm_runtime_suspend(struct device *dev);`
326 - execute the subsystem-level suspend callback for the device; returns 0 on
327 success, 1 if the device's runtime PM status was already 'suspended', or
328 error code on failure, where -EAGAIN or -EBUSY means it is safe to attempt
329 to suspend the device again in future and -EACCES means that
330 'power.disable_depth' is different from 0
331
332 `int pm_runtime_autosuspend(struct device *dev);`
333 - same as pm_runtime_suspend() except that the autosuspend delay is taken
334 `into account;` if pm_runtime_autosuspend_expiration() says the delay has
335 not yet expired then an autosuspend is scheduled for the appropriate time
336 and 0 is returned
337
338 `int pm_runtime_resume(struct device *dev);`
339 - execute the subsystem-level resume callback for the device; returns 0 on
340 success, 1 if the device's runtime PM status is already 'active' (also if
341 'power.disable_depth' is nonzero, but the status was 'active' when it was
342 changing from 0 to 1) or error code on failure, where -EAGAIN means it may
343 be safe to attempt to resume the device again in future, but
344 'power.runtime_error' should be checked additionally, and -EACCES means
345 that the callback could not be run, because 'power.disable_depth' was
346 different from 0
347
348 `int pm_runtime_resume_and_get(struct device *dev);`
349 - run pm_runtime_resume(dev) and if successful, increment the device's
350 usage counter; return the result of pm_runtime_resume
351
352 `int pm_request_idle(struct device *dev);`
353 - submit a request to execute the subsystem-level idle callback for the
354 device (the request is represented by a work item in pm_wq); returns 0 on
355 success or error code if the request has not been queued up
356
357 `int pm_request_autosuspend(struct device *dev);`
358 - schedule the execution of the subsystem-level suspend callback for the
359 device when the autosuspend delay has expired; if the delay has already
360 expired then the work item is queued up immediately
361
362 `int pm_schedule_suspend(struct device *dev, unsigned int delay);`
363 - schedule the execution of the subsystem-level suspend callback for the
364 device in future, where 'delay' is the time to wait before queuing up a
365 suspend work item in pm_wq, in milliseconds (if 'delay' is zero, the work
366 item is queued up immediately); returns 0 on success, 1 if the device's PM
367 runtime status was already 'suspended', or error code if the request
368 hasn't been scheduled (or queued up if 'delay' is 0); if the execution of
369 ->runtime_suspend() is already scheduled and not yet expired, the new
370 value of 'delay' will be used as the time to wait
371
372 `int pm_request_resume(struct device *dev);`
373 - submit a request to execute the subsystem-level resume callback for the
374 device (the request is represented by a work item in pm_wq); returns 0 on
375 success, 1 if the device's runtime PM status was already 'active', or
376 error code if the request hasn't been queued up
377
378 `void pm_runtime_get_noresume(struct device *dev);`
379 - increment the device's usage counter
380
381 `int pm_runtime_get(struct device *dev);`
382 - increment the device's usage counter, run pm_request_resume(dev) and
383 return its result
384
385 `int pm_runtime_get_sync(struct device *dev);`
386 - increment the device's usage counter, run pm_runtime_resume(dev) and
387 return its result;
388 note that it does not drop the device's usage counter on errors, so
389 consider using pm_runtime_resume_and_get() instead of it, especially
390 if its return value is checked by the caller, as this is likely to
391 result in cleaner code.
392
393 `int pm_runtime_get_if_in_use(struct device *dev);`
394 - return -EINVAL if 'power.disable_depth' is nonzero; otherwise, if the
395 runtime PM status is RPM_ACTIVE and the runtime PM usage counter is
396 nonzero, increment the counter and return 1; otherwise return 0 without
397 changing the counter
398
399 `int pm_runtime_get_if_active(struct device *dev);`
400 - return -EINVAL if 'power.disable_depth' is nonzero; otherwise, if the
401 runtime PM status is RPM_ACTIVE, increment the counter and
402 return 1; otherwise return 0 without changing the counter
403
404 `void pm_runtime_put_noidle(struct device *dev);`
405 - decrement the device's usage counter
406
407 `int pm_runtime_put(struct device *dev);`
408 - decrement the device's usage counter; if the result is 0 then run
409 pm_request_idle(dev) and return its result
410
411 `int pm_runtime_put_autosuspend(struct device *dev);`
412 - does the same as __pm_runtime_put_autosuspend() for now, but in the
413 future, will also call pm_runtime_mark_last_busy() as well, DO NOT USE!
414
415 `int __pm_runtime_put_autosuspend(struct device *dev);`
416 - decrement the device's usage counter; if the result is 0 then run
417 pm_request_autosuspend(dev) and return its result
418
419 `int pm_runtime_put_sync(struct device *dev);`
420 - decrement the device's usage counter; if the result is 0 then run
421 pm_runtime_idle(dev) and return its result
422
423 `int pm_runtime_put_sync_suspend(struct device *dev);`
424 - decrement the device's usage counter; if the result is 0 then run
425 pm_runtime_suspend(dev) and return its result
426
427 `int pm_runtime_put_sync_autosuspend(struct device *dev);`
428 - decrement the device's usage counter; if the result is 0 then run
429 pm_runtime_autosuspend(dev) and return its result
430
431 `void pm_runtime_enable(struct device *dev);`
432 - decrement the device's 'power.disable_depth' field; if that field is equal
433 to zero, the runtime PM helper functions can execute subsystem-level
434 callbacks described in Section 2 for the device
435
436 `int pm_runtime_disable(struct device *dev);`
437 - increment the device's 'power.disable_depth' field (if the value of that
438 field was previously zero, this prevents subsystem-level runtime PM
439 callbacks from being run for the device), make sure that all of the
440 pending runtime PM operations on the device are either completed or
441 canceled; returns 1 if there was a resume request pending and it was
442 necessary to execute the subsystem-level resume callback for the device
443 to satisfy that request, otherwise 0 is returned
444
445 `int pm_runtime_barrier(struct device *dev);`
446 - check if there's a resume request pending for the device and resume it
447 (synchronously) in that case, cancel any other pending runtime PM requests
448 regarding it and wait for all runtime PM operations on it in progress to
449 complete; returns 1 if there was a resume request pending and it was
450 necessary to execute the subsystem-level resume callback for the device to
451 satisfy that request, otherwise 0 is returned
452
453 `void pm_suspend_ignore_children(struct device *dev, bool enable);`
454 - set/unset the power.ignore_children flag of the device
455
456 `int pm_runtime_set_active(struct device *dev);`
457 - clear the device's 'power.runtime_error' flag, set the device's runtime
458 PM status to 'active' and update its parent's counter of 'active'
459 children as appropriate (it is only valid to use this function if
460 'power.runtime_error' is set or 'power.disable_depth' is greater than
461 zero); it will fail and return error code if the device has a parent
462 which is not active and the 'power.ignore_children' flag of which is unset
463
464 `void pm_runtime_set_suspended(struct device *dev);`
465 - clear the device's 'power.runtime_error' flag, set the device's runtime
466 PM status to 'suspended' and update its parent's counter of 'active'
467 children as appropriate (it is only valid to use this function if
468 'power.runtime_error' is set or 'power.disable_depth' is greater than
469 zero)
470
471 `bool pm_runtime_active(struct device *dev);`
472 - return true if the device's runtime PM status is 'active' or its
473 'power.disable_depth' field is not equal to zero, or false otherwise
474
475 `bool pm_runtime_suspended(struct device *dev);`
476 - return true if the device's runtime PM status is 'suspended' and its
477 'power.disable_depth' field is equal to zero, or false otherwise
478
479 `bool pm_runtime_status_suspended(struct device *dev);`
480 - return true if the device's runtime PM status is 'suspended'
481
482 `void pm_runtime_allow(struct device *dev);`
483 - set the power.runtime_auto flag for the device and decrease its usage
484 counter (used by the /sys/devices/.../power/control interface to
485 effectively allow the device to be power managed at run time)
486
487 `void pm_runtime_forbid(struct device *dev);`
488 - unset the power.runtime_auto flag for the device and increase its usage
489 counter (used by the /sys/devices/.../power/control interface to
490 effectively prevent the device from being power managed at run time)
491
492 `void pm_runtime_no_callbacks(struct device *dev);`
493 - set the power.no_callbacks flag for the device and remove the runtime
494 PM attributes from /sys/devices/.../power (or prevent them from being
495 added when the device is registered)
496
497 `void pm_runtime_irq_safe(struct device *dev);`
498 - set the power.irq_safe flag for the device, causing the runtime-PM
499 callbacks to be invoked with interrupts off
500
501 `bool pm_runtime_is_irq_safe(struct device *dev);`
502 - return true if power.irq_safe flag was set for the device, causing
503 the runtime-PM callbacks to be invoked with interrupts off
504
505 `void pm_runtime_mark_last_busy(struct device *dev);`
506 - set the power.last_busy field to the current time
507
508 `void pm_runtime_use_autosuspend(struct device *dev);`
509 - set the power.use_autosuspend flag, enabling autosuspend delays; call
510 pm_runtime_get_sync if the flag was previously cleared and
511 power.autosuspend_delay is negative
512
513 `void pm_runtime_dont_use_autosuspend(struct device *dev);`
514 - clear the power.use_autosuspend flag, disabling autosuspend delays;
515 decrement the device's usage counter if the flag was previously set and
516 power.autosuspend_delay is negative; call pm_runtime_idle
517
518 `void pm_runtime_set_autosuspend_delay(struct device *dev, int delay);`
519 - set the power.autosuspend_delay value to 'delay' (expressed in
520 milliseconds); if 'delay' is negative then runtime suspends are
521 prevented; if power.use_autosuspend is set, pm_runtime_get_sync may be
522 called or the device's usage counter may be decremented and
523 pm_runtime_idle called depending on if power.autosuspend_delay is
524 changed to or from a negative value; if power.use_autosuspend is clear,
525 pm_runtime_idle is called
526
527 `unsigned long pm_runtime_autosuspend_expiration(struct device *dev);`
528 - calculate the time when the current autosuspend delay period will expire,
529 based on power.last_busy and power.autosuspend_delay; if the delay time
530 is 1000 ms or larger then the expiration time is rounded up to the
531 nearest second; returns 0 if the delay period has already expired or
532 power.use_autosuspend isn't set, otherwise returns the expiration time
533 in jiffies
534
535 It is safe to execute the following helper functions from interrupt context:
536
537 - pm_request_idle()
538 - pm_request_autosuspend()
539 - pm_schedule_suspend()
540 - pm_request_resume()
541 - pm_runtime_get_noresume()
542 - pm_runtime_get()
543 - pm_runtime_put_noidle()
544 - pm_runtime_put()
545 - pm_runtime_put_autosuspend()
546 - __pm_runtime_put_autosuspend()
547 - pm_runtime_enable()
548 - pm_suspend_ignore_children()
549 - pm_runtime_set_active()
550 - pm_runtime_set_suspended()
551 - pm_runtime_suspended()
552 - pm_runtime_mark_last_busy()
553 - pm_runtime_autosuspend_expiration()
554
555 If pm_runtime_irq_safe() has been called for a device then the following helper
556 functions may also be used in interrupt context:
557
558 - pm_runtime_idle()
559 - pm_runtime_suspend()
560 - pm_runtime_autosuspend()
561 - pm_runtime_resume()
562 - pm_runtime_get_sync()
563 - pm_runtime_put_sync()
564 - pm_runtime_put_sync_suspend()
565 - pm_runtime_put_sync_autosuspend()
566
567 5. Runtime PM Initialization, Device Probing and Removal
568 ========================================================
569
570 Initially, the runtime PM is disabled for all devices, which means that the
571 majority of the runtime PM helper functions described in Section 4 will return
572 -EAGAIN until pm_runtime_enable() is called for the device.
573
574 In addition to that, the initial runtime PM status of all devices is
575 'suspended', but it need not reflect the actual physical state of the device.
576 Thus, if the device is initially active (i.e. it is able to process I/O), its
577 runtime PM status must be changed to 'active', with the help of
578 pm_runtime_set_active(), before pm_runtime_enable() is called for the device.
579
580 However, if the device has a parent and the parent's runtime PM is enabled,
581 calling pm_runtime_set_active() for the device will affect the parent, unless
582 the parent's 'power.ignore_children' flag is set. Namely, in that case the
583 parent won't be able to suspend at run time, using the PM core's helper
584 functions, as long as the child's status is 'active', even if the child's
585 runtime PM is still disabled (i.e. pm_runtime_enable() hasn't been called for
586 the child yet or pm_runtime_disable() has been called for it). For this reason,
587 once pm_runtime_set_active() has been called for the device, pm_runtime_enable()
588 should be called for it too as soon as reasonably possible or its runtime PM
589 status should be changed back to 'suspended' with the help of
590 pm_runtime_set_suspended().
591
592 If the default initial runtime PM status of the device (i.e. 'suspended')
593 reflects the actual state of the device, its bus type's or its driver's
594 ->probe() callback will likely need to wake it up using one of the PM core's
595 helper functions described in Section 4. In that case, pm_runtime_resume()
596 should be used. Of course, for this purpose the device's runtime PM has to be
597 enabled earlier by calling pm_runtime_enable().
598
599 Note, if the device may execute pm_runtime calls during the probe (such as
600 if it is registered with a subsystem that may call back in) then the
601 pm_runtime_get_sync() call paired with a pm_runtime_put() call will be
602 appropriate to ensure that the device is not put back to sleep during the
603 probe. This can happen with systems such as the network device layer.
604
605 It may be desirable to suspend the device once ->probe() has finished.
606 Therefore the driver core uses the asynchronous pm_request_idle() to submit a
607 request to execute the subsystem-level idle callback for the device at that
608 time. A driver that makes use of the runtime autosuspend feature may want to
609 update the last busy mark before returning from ->probe().
610
611 Moreover, the driver core prevents runtime PM callbacks from racing with the bus
612 notifier callback in __device_release_driver(), which is necessary because the
613 notifier is used by some subsystems to carry out operations affecting the
614 runtime PM functionality. It does so by calling pm_runtime_get_sync() before
615 driver_sysfs_remove() and the BUS_NOTIFY_UNBIND_DRIVER notifications. This
616 resumes the device if it's in the suspended state and prevents it from
617 being suspended again while those routines are being executed.
618
619 To allow bus types and drivers to put devices into the suspended state by
620 calling pm_runtime_suspend() from their ->remove() routines, the driver core
621 executes pm_runtime_put_sync() after running the BUS_NOTIFY_UNBIND_DRIVER
622 notifications in __device_release_driver(). This requires bus types and
623 drivers to make their ->remove() callbacks avoid races with runtime PM directly,
624 but it also allows more flexibility in the handling of devices during the
625 removal of their drivers.
626
627 Drivers in ->remove() callback should undo the runtime PM changes done
628 in ->probe(). Usually this means calling pm_runtime_disable(),
629 pm_runtime_dont_use_autosuspend() etc.
630
631 The user space can effectively disallow the driver of the device to power manage
632 it at run time by changing the value of its /sys/devices/.../power/control
633 attribute to "on", which causes pm_runtime_forbid() to be called. In principle,
634 this mechanism may also be used by the driver to effectively turn off the
635 runtime power management of the device until the user space turns it on.
636 Namely, during the initialization the driver can make sure that the runtime PM
637 status of the device is 'active' and call pm_runtime_forbid(). It should be
638 noted, however, that if the user space has already intentionally changed the
639 value of /sys/devices/.../power/control to "auto" to allow the driver to power
640 manage the device at run time, the driver may confuse it by using
641 pm_runtime_forbid() this way.
642
643 6. Runtime PM and System Sleep
644 ==============================
645
646 Runtime PM and system sleep (i.e., system suspend and hibernation, also known
647 as suspend-to-RAM and suspend-to-disk) interact with each other in a couple of
648 ways. If a device is active when a system sleep starts, everything is
649 straightforward. But what should happen if the device is already suspended?
650
651 The device may have different wake-up settings for runtime PM and system sleep.
652 For example, remote wake-up may be enabled for runtime suspend but disallowed
653 for system sleep (device_may_wakeup(dev) returns 'false'). When this happens,
654 the subsystem-level system suspend callback is responsible for changing the
655 device's wake-up setting (it may leave that to the device driver's system
656 suspend routine). It may be necessary to resume the device and suspend it again
657 in order to do so. The same is true if the driver uses different power levels
658 or other settings for runtime suspend and system sleep.
659
660 During system resume, the simplest approach is to bring all devices back to full
661 power, even if they had been suspended before the system suspend began. There
662 are several reasons for this, including:
663
664 * The device might need to switch power levels, wake-up settings, etc.
665
666 * Remote wake-up events might have been lost by the firmware.
667
668 * The device's children may need the device to be at full power in order
669 to resume themselves.
670
671 * The driver's idea of the device state may not agree with the device's
672 physical state. This can happen during resume from hibernation.
673
674 * The device might need to be reset.
675
676 * Even though the device was suspended, if its usage counter was > 0 then most
677 likely it would need a runtime resume in the near future anyway.
678
679 If the device had been suspended before the system suspend began and it's
680 brought back to full power during resume, then its runtime PM status will have
681 to be updated to reflect the actual post-system sleep status. The way to do
682 this is:
683
684 - pm_runtime_disable(dev);
685 - pm_runtime_set_active(dev);
686 - pm_runtime_enable(dev);
687
688 The PM core always increments the runtime usage counter before calling the
689 ->suspend() callback and decrements it after calling the ->resume() callback.
690 Hence disabling runtime PM temporarily like this will not cause any runtime
691 suspend attempts to be permanently lost. If the usage count goes to zero
692 following the return of the ->resume() callback, the ->runtime_idle() callback
693 will be invoked as usual.
694
695 On some systems, however, system sleep is not entered through a global firmware
696 or hardware operation. Instead, all hardware components are put into low-power
697 states directly by the kernel in a coordinated way. Then, the system sleep
698 state effectively follows from the states the hardware components end up in
699 and the system is woken up from that state by a hardware interrupt or a similar
700 mechanism entirely under the kernel's control. As a result, the kernel never
701 gives control away and the states of all devices during resume are precisely
702 known to it. If that is the case and none of the situations listed above takes
703 place (in particular, if the system is not waking up from hibernation), it may
704 be more efficient to leave the devices that had been suspended before the system
705 suspend began in the suspended state.
706
707 To this end, the PM core provides a mechanism allowing some coordination between
708 different levels of device hierarchy. Namely, if a system suspend .prepare()
709 callback returns a positive number for a device, that indicates to the PM core
710 that the device appears to be runtime-suspended and its state is fine, so it
711 may be left in runtime suspend provided that all of its descendants are also
712 left in runtime suspend. If that happens, the PM core will not execute any
713 system suspend and resume callbacks for all of those devices, except for the
714 .complete() callback, which is then entirely responsible for handling the device
715 as appropriate. This only applies to system suspend transitions that are not
716 related to hibernation (see Documentation/driver-api/pm/devices.rst for more
717 information).
718
719 The PM core does its best to reduce the probability of race conditions between
720 the runtime PM and system suspend/resume (and hibernation) callbacks by carrying
721 out the following operations:
722
723 * During system suspend pm_runtime_get_noresume() is called for every device
724 right before executing the subsystem-level .prepare() callback for it and
725 pm_runtime_barrier() is called for every device right before executing the
726 subsystem-level .suspend() callback for it. In addition to that the PM core
727 calls __pm_runtime_disable() with 'false' as the second argument for every
728 device right before executing the subsystem-level .suspend_late() callback
729 for it.
730
731 * During system resume pm_runtime_enable() and pm_runtime_put() are called for
732 every device right after executing the subsystem-level .resume_early()
733 callback and right after executing the subsystem-level .complete() callback
734 for it, respectively.
735
736 7. Generic subsystem callbacks
737 ==============================
738
739 Subsystems may wish to conserve code space by using the set of generic power
740 management callbacks provided by the PM core, defined in
741 driver/base/power/generic_ops.c:
742
743 `int pm_generic_runtime_suspend(struct device *dev);`
744 - invoke the ->runtime_suspend() callback provided by the driver of this
745 device and return its result, or return 0 if not defined
746
747 `int pm_generic_runtime_resume(struct device *dev);`
748 - invoke the ->runtime_resume() callback provided by the driver of this
749 device and return its result, or return 0 if not defined
750
751 `int pm_generic_suspend(struct device *dev);`
752 - if the device has not been suspended at run time, invoke the ->suspend()
753 callback provided by its driver and return its result, or return 0 if not
754 defined
755
756 `int pm_generic_suspend_noirq(struct device *dev);`
757 - if pm_runtime_suspended(dev) returns "false", invoke the ->suspend_noirq()
758 callback provided by the device's driver and return its result, or return
759 0 if not defined
760
761 `int pm_generic_resume(struct device *dev);`
762 - invoke the ->resume() callback provided by the driver of this device and,
763 if successful, change the device's runtime PM status to 'active'
764
765 `int pm_generic_resume_noirq(struct device *dev);`
766 - invoke the ->resume_noirq() callback provided by the driver of this device
767
768 `int pm_generic_freeze(struct device *dev);`
769 - if the device has not been suspended at run time, invoke the ->freeze()
770 callback provided by its driver and return its result, or return 0 if not
771 defined
772
773 `int pm_generic_freeze_noirq(struct device *dev);`
774 - if pm_runtime_suspended(dev) returns "false", invoke the ->freeze_noirq()
775 callback provided by the device's driver and return its result, or return
776 0 if not defined
777
778 `int pm_generic_thaw(struct device *dev);`
779 - if the device has not been suspended at run time, invoke the ->thaw()
780 callback provided by its driver and return its result, or return 0 if not
781 defined
782
783 `int pm_generic_thaw_noirq(struct device *dev);`
784 - if pm_runtime_suspended(dev) returns "false", invoke the ->thaw_noirq()
785 callback provided by the device's driver and return its result, or return
786 0 if not defined
787
788 `int pm_generic_poweroff(struct device *dev);`
789 - if the device has not been suspended at run time, invoke the ->poweroff()
790 callback provided by its driver and return its result, or return 0 if not
791 defined
792
793 `int pm_generic_poweroff_noirq(struct device *dev);`
794 - if pm_runtime_suspended(dev) returns "false", run the ->poweroff_noirq()
795 callback provided by the device's driver and return its result, or return
796 0 if not defined
797
798 `int pm_generic_restore(struct device *dev);`
799 - invoke the ->restore() callback provided by the driver of this device and,
800 if successful, change the device's runtime PM status to 'active'
801
802 `int pm_generic_restore_noirq(struct device *dev);`
803 - invoke the ->restore_noirq() callback provided by the device's driver
804
805 These functions are the defaults used by the PM core if a subsystem doesn't
806 provide its own callbacks for ->runtime_idle(), ->runtime_suspend(),
807 ->runtime_resume(), ->suspend(), ->suspend_noirq(), ->resume(),
808 ->resume_noirq(), ->freeze(), ->freeze_noirq(), ->thaw(), ->thaw_noirq(),
809 ->poweroff(), ->poweroff_noirq(), ->restore(), ->restore_noirq() in the
810 subsystem-level dev_pm_ops structure.
811
812 Device drivers that wish to use the same function as a system suspend, freeze,
813 poweroff and runtime suspend callback, and similarly for system resume, thaw,
814 restore, and runtime resume, can achieve this with the help of the
815 UNIVERSAL_DEV_PM_OPS macro defined in include/linux/pm.h (possibly setting its
816 last argument to NULL).
817
818 8. "No-Callback" Devices
819 ========================
820
821 Some "devices" are only logical sub-devices of their parent and cannot be
822 power-managed on their own. (The prototype example is a USB interface. Entire
823 USB devices can go into low-power mode or send wake-up requests, but neither is
824 possible for individual interfaces.) The drivers for these devices have no
825 need of runtime PM callbacks; if the callbacks did exist, ->runtime_suspend()
826 and ->runtime_resume() would always return 0 without doing anything else and
827 ->runtime_idle() would always call pm_runtime_suspend().
828
829 Subsystems can tell the PM core about these devices by calling
830 pm_runtime_no_callbacks(). This should be done after the device structure is
831 initialized and before it is registered (although after device registration is
832 also okay). The routine will set the device's power.no_callbacks flag and
833 prevent the non-debugging runtime PM sysfs attributes from being created.
834
835 When power.no_callbacks is set, the PM core will not invoke the
836 ->runtime_idle(), ->runtime_suspend(), or ->runtime_resume() callbacks.
837 Instead it will assume that suspends and resumes always succeed and that idle
838 devices should be suspended.
839
840 As a consequence, the PM core will never directly inform the device's subsystem
841 or driver about runtime power changes. Instead, the driver for the device's
842 parent must take responsibility for telling the device's driver when the
843 parent's power state changes.
844
845 Note that, in some cases it may not be desirable for subsystems/drivers to call
846 pm_runtime_no_callbacks() for their devices. This could be because a subset of
847 the runtime PM callbacks needs to be implemented, a platform dependent PM
848 domain could get attached to the device or that the device is power managed
849 through a supplier device link. For these reasons and to avoid boilerplate code
850 in subsystems/drivers, the PM core allows runtime PM callbacks to be
851 unassigned. More precisely, if a callback pointer is NULL, the PM core will act
852 as though there was a callback and it returned 0.
853
854 9. Autosuspend, or automatically-delayed suspends
855 =================================================
856
857 Changing a device's power state isn't free; it requires both time and energy.
858 A device should be put in a low-power state only when there's some reason to
859 think it will remain in that state for a substantial time. A common heuristic
860 says that a device which hasn't been used for a while is liable to remain
861 unused; following this advice, drivers should not allow devices to be suspended
862 at runtime until they have been inactive for some minimum period. Even when
863 the heuristic ends up being non-optimal, it will still prevent devices from
864 "bouncing" too rapidly between low-power and full-power states.
865
866 The term "autosuspend" is an historical remnant. It doesn't mean that the
867 device is automatically suspended (the subsystem or driver still has to call
868 the appropriate PM routines); rather it means that runtime suspends will
869 automatically be delayed until the desired period of inactivity has elapsed.
870
871 Inactivity is determined based on the power.last_busy field. Drivers should
872 call pm_runtime_mark_last_busy() to update this field after carrying out I/O,
873 typically just before calling __pm_runtime_put_autosuspend(). The desired
874 length of the inactivity period is a matter of policy. Subsystems can set this
875 length initially by calling pm_runtime_set_autosuspend_delay(), but after device
876 registration the length should be controlled by user space, using the
877 /sys/devices/.../power/autosuspend_delay_ms attribute.
878
879 In order to use autosuspend, subsystems or drivers must call
880 pm_runtime_use_autosuspend() (preferably before registering the device), and
881 thereafter they should use the various `*_autosuspend()` helper functions
882 instead of the non-autosuspend counterparts::
883
884 Instead of: pm_runtime_suspend use: pm_runtime_autosuspend;
885 Instead of: pm_schedule_suspend use: pm_request_autosuspend;
886 Instead of: pm_runtime_put use: __pm_runtime_put_autosuspend;
887 Instead of: pm_runtime_put_sync use: pm_runtime_put_sync_autosuspend.
888
889 Drivers may also continue to use the non-autosuspend helper functions; they
890 will behave normally, which means sometimes taking the autosuspend delay into
891 account (see pm_runtime_idle).
892
893 Under some circumstances a driver or subsystem may want to prevent a device
894 from autosuspending immediately, even though the usage counter is zero and the
895 autosuspend delay time has expired. If the ->runtime_suspend() callback
896 returns -EAGAIN or -EBUSY, and if the next autosuspend delay expiration time is
897 in the future (as it normally would be if the callback invoked
898 pm_runtime_mark_last_busy()), the PM core will automatically reschedule the
899 autosuspend. The ->runtime_suspend() callback can't do this rescheduling
900 itself because no suspend requests of any kind are accepted while the device is
901 suspending (i.e., while the callback is running).
902
903 The implementation is well suited for asynchronous use in interrupt contexts.
904 However such use inevitably involves races, because the PM core can't
905 synchronize ->runtime_suspend() callbacks with the arrival of I/O requests.
906 This synchronization must be handled by the driver, using its private lock.
907 Here is a schematic pseudo-code example::
908
909 foo_read_or_write(struct foo_priv *foo, void *data)
910 {
911 lock(&foo->private_lock);
912 add_request_to_io_queue(foo, data);
913 if (foo->num_pending_requests++ == 0)
914 pm_runtime_get(&foo->dev);
915 if (!foo->is_suspended)
916 foo_process_next_request(foo);
917 unlock(&foo->private_lock);
918 }
919
920 foo_io_completion(struct foo_priv *foo, void *req)
921 {
922 lock(&foo->private_lock);
923 if (--foo->num_pending_requests == 0) {
924 pm_runtime_mark_last_busy(&foo->dev);
925 __pm_runtime_put_autosuspend(&foo->dev);
926 } else {
927 foo_process_next_request(foo);
928 }
929 unlock(&foo->private_lock);
930 /* Send req result back to the user ... */
931 }
932
933 int foo_runtime_suspend(struct device *dev)
934 {
935 struct foo_priv foo = container_of(dev, ...);
936 int ret = 0;
937
938 lock(&foo->private_lock);
939 if (foo->num_pending_requests > 0) {
940 ret = -EBUSY;
941 } else {
942 /* ... suspend the device ... */
943 foo->is_suspended = 1;
944 }
945 unlock(&foo->private_lock);
946 return ret;
947 }
948
949 int foo_runtime_resume(struct device *dev)
950 {
951 struct foo_priv foo = container_of(dev, ...);
952
953 lock(&foo->private_lock);
954 /* ... resume the device ... */
955 foo->is_suspended = 0;
956 pm_runtime_mark_last_busy(&foo->dev);
957 if (foo->num_pending_requests > 0)
958 foo_process_next_request(foo);
959 unlock(&foo->private_lock);
960 return 0;
961 }
962
963 The important point is that after foo_io_completion() asks for an autosuspend,
964 the foo_runtime_suspend() callback may race with foo_read_or_write().
965 Therefore foo_runtime_suspend() has to check whether there are any pending I/O
966 requests (while holding the private lock) before allowing the suspend to
967 proceed.
968
969 In addition, the power.autosuspend_delay field can be changed by user space at
970 any time. If a driver cares about this, it can call
971 pm_runtime_autosuspend_expiration() from within the ->runtime_suspend()
972 callback while holding its private lock. If the function returns a nonzero
973 value then the delay has not yet expired and the callback should return
974 -EAGAIN.
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