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Based on kernel version 4.7.2. Page generated on 2016-08-22 22:47 EST.

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