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Based on kernel version 3.16. Page generated on 2014-08-06 21:40 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 funcions (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 funtions 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	  void pm_runtime_put_noidle(struct device *dev);
375	    - decrement the device's usage counter
376	
377	  int pm_runtime_put(struct device *dev);
378	    - decrement the device's usage counter; if the result is 0 then run
379	      pm_request_idle(dev) and return its result
380	
381	  int pm_runtime_put_autosuspend(struct device *dev);
382	    - decrement the device's usage counter; if the result is 0 then run
383	      pm_request_autosuspend(dev) and return its result
384	
385	  int pm_runtime_put_sync(struct device *dev);
386	    - decrement the device's usage counter; if the result is 0 then run
387	      pm_runtime_idle(dev) and return its result
388	
389	  int pm_runtime_put_sync_suspend(struct device *dev);
390	    - decrement the device's usage counter; if the result is 0 then run
391	      pm_runtime_suspend(dev) and return its result
392	
393	  int pm_runtime_put_sync_autosuspend(struct device *dev);
394	    - decrement the device's usage counter; if the result is 0 then run
395	      pm_runtime_autosuspend(dev) and return its result
396	
397	  void pm_runtime_enable(struct device *dev);
398	    - decrement the device's 'power.disable_depth' field; if that field is equal
399	      to zero, the runtime PM helper functions can execute subsystem-level
400	      callbacks described in Section 2 for the device
401	
402	  int pm_runtime_disable(struct device *dev);
403	    - increment the device's 'power.disable_depth' field (if the value of that
404	      field was previously zero, this prevents subsystem-level runtime PM
405	      callbacks from being run for the device), make sure that all of the
406	      pending runtime PM operations on the device are either completed or
407	      canceled; returns 1 if there was a resume request pending and it was
408	      necessary to execute the subsystem-level resume callback for the device
409	      to satisfy that request, otherwise 0 is returned
410	
411	  int pm_runtime_barrier(struct device *dev);
412	    - check if there's a resume request pending for the device and resume it
413	      (synchronously) in that case, cancel any other pending runtime PM requests
414	      regarding it and wait for all runtime PM operations on it in progress to
415	      complete; returns 1 if there was a resume request pending and it was
416	      necessary to execute the subsystem-level resume callback for the device to
417	      satisfy that request, otherwise 0 is returned
418	
419	  void pm_suspend_ignore_children(struct device *dev, bool enable);
420	    - set/unset the power.ignore_children flag of the device
421	
422	  int pm_runtime_set_active(struct device *dev);
423	    - clear the device's 'power.runtime_error' flag, set the device's runtime
424	      PM status to 'active' and update its parent's counter of 'active'
425	      children as appropriate (it is only valid to use this function if
426	      'power.runtime_error' is set or 'power.disable_depth' is greater than
427	      zero); it will fail and return error code if the device has a parent
428	      which is not active and the 'power.ignore_children' flag of which is unset
429	
430	  void pm_runtime_set_suspended(struct device *dev);
431	    - clear the device's 'power.runtime_error' flag, set the device's runtime
432	      PM status to 'suspended' and update its parent's counter of 'active'
433	      children as appropriate (it is only valid to use this function if
434	      'power.runtime_error' is set or 'power.disable_depth' is greater than
435	      zero)
436	
437	  bool pm_runtime_active(struct device *dev);
438	    - return true if the device's runtime PM status is 'active' or its
439	      'power.disable_depth' field is not equal to zero, or false otherwise
440	
441	  bool pm_runtime_suspended(struct device *dev);
442	    - return true if the device's runtime PM status is 'suspended' and its
443	      'power.disable_depth' field is equal to zero, or false otherwise
444	
445	  bool pm_runtime_status_suspended(struct device *dev);
446	    - return true if the device's runtime PM status is 'suspended'
447	
448	  bool pm_runtime_suspended_if_enabled(struct device *dev);
449	    - return true if the device's runtime PM status is 'suspended' and its
450	      'power.disable_depth' field is equal to 1
451	
452	  void pm_runtime_allow(struct device *dev);
453	    - set the power.runtime_auto flag for the device and decrease its usage
454	      counter (used by the /sys/devices/.../power/control interface to
455	      effectively allow the device to be power managed at run time)
456	
457	  void pm_runtime_forbid(struct device *dev);
458	    - unset the power.runtime_auto flag for the device and increase its usage
459	      counter (used by the /sys/devices/.../power/control interface to
460	      effectively prevent the device from being power managed at run time)
461	
462	  void pm_runtime_no_callbacks(struct device *dev);
463	    - set the power.no_callbacks flag for the device and remove the runtime
464	      PM attributes from /sys/devices/.../power (or prevent them from being
465	      added when the device is registered)
466	
467	  void pm_runtime_irq_safe(struct device *dev);
468	    - set the power.irq_safe flag for the device, causing the runtime-PM
469	      callbacks to be invoked with interrupts off
470	
471	  void pm_runtime_mark_last_busy(struct device *dev);
472	    - set the power.last_busy field to the current time
473	
474	  void pm_runtime_use_autosuspend(struct device *dev);
475	    - set the power.use_autosuspend flag, enabling autosuspend delays
476	
477	  void pm_runtime_dont_use_autosuspend(struct device *dev);
478	    - clear the power.use_autosuspend flag, disabling autosuspend delays
479	
480	  void pm_runtime_set_autosuspend_delay(struct device *dev, int delay);
481	    - set the power.autosuspend_delay value to 'delay' (expressed in
482	      milliseconds); if 'delay' is negative then runtime suspends are
483	      prevented
484	
485	  unsigned long pm_runtime_autosuspend_expiration(struct device *dev);
486	    - calculate the time when the current autosuspend delay period will expire,
487	      based on power.last_busy and power.autosuspend_delay; if the delay time
488	      is 1000 ms or larger then the expiration time is rounded up to the
489	      nearest second; returns 0 if the delay period has already expired or
490	      power.use_autosuspend isn't set, otherwise returns the expiration time
491	      in jiffies
492	
493	It is safe to execute the following helper functions from interrupt context:
494	
495	pm_request_idle()
496	pm_request_autosuspend()
497	pm_schedule_suspend()
498	pm_request_resume()
499	pm_runtime_get_noresume()
500	pm_runtime_get()
501	pm_runtime_put_noidle()
502	pm_runtime_put()
503	pm_runtime_put_autosuspend()
504	pm_runtime_enable()
505	pm_suspend_ignore_children()
506	pm_runtime_set_active()
507	pm_runtime_set_suspended()
508	pm_runtime_suspended()
509	pm_runtime_mark_last_busy()
510	pm_runtime_autosuspend_expiration()
511	
512	If pm_runtime_irq_safe() has been called for a device then the following helper
513	functions may also be used in interrupt context:
514	
515	pm_runtime_idle()
516	pm_runtime_suspend()
517	pm_runtime_autosuspend()
518	pm_runtime_resume()
519	pm_runtime_get_sync()
520	pm_runtime_put_sync()
521	pm_runtime_put_sync_suspend()
522	pm_runtime_put_sync_autosuspend()
523	
524	5. Runtime PM Initialization, Device Probing and Removal
525	
526	Initially, the runtime PM is disabled for all devices, which means that the
527	majority of the runtime PM helper funtions described in Section 4 will return
528	-EAGAIN until pm_runtime_enable() is called for the device.
529	
530	In addition to that, the initial runtime PM status of all devices is
531	'suspended', but it need not reflect the actual physical state of the device.
532	Thus, if the device is initially active (i.e. it is able to process I/O), its
533	runtime PM status must be changed to 'active', with the help of
534	pm_runtime_set_active(), before pm_runtime_enable() is called for the device.
535	
536	However, if the device has a parent and the parent's runtime PM is enabled,
537	calling pm_runtime_set_active() for the device will affect the parent, unless
538	the parent's 'power.ignore_children' flag is set.  Namely, in that case the
539	parent won't be able to suspend at run time, using the PM core's helper
540	functions, as long as the child's status is 'active', even if the child's
541	runtime PM is still disabled (i.e. pm_runtime_enable() hasn't been called for
542	the child yet or pm_runtime_disable() has been called for it).  For this reason,
543	once pm_runtime_set_active() has been called for the device, pm_runtime_enable()
544	should be called for it too as soon as reasonably possible or its runtime PM
545	status should be changed back to 'suspended' with the help of
546	pm_runtime_set_suspended().
547	
548	If the default initial runtime PM status of the device (i.e. 'suspended')
549	reflects the actual state of the device, its bus type's or its driver's
550	->probe() callback will likely need to wake it up using one of the PM core's
551	helper functions described in Section 4.  In that case, pm_runtime_resume()
552	should be used.  Of course, for this purpose the device's runtime PM has to be
553	enabled earlier by calling pm_runtime_enable().
554	
555	It may be desirable to suspend the device once ->probe() has finished.
556	Therefore the driver core uses the asyncronous pm_request_idle() to submit a
557	request to execute the subsystem-level idle callback for the device at that
558	time.  A driver that makes use of the runtime autosuspend feature, may want to
559	update the last busy mark before returning from ->probe().
560	
561	Moreover, the driver core prevents runtime PM callbacks from racing with the bus
562	notifier callback in __device_release_driver(), which is necessary, because the
563	notifier is used by some subsystems to carry out operations affecting the
564	runtime PM functionality.  It does so by calling pm_runtime_get_sync() before
565	driver_sysfs_remove() and the BUS_NOTIFY_UNBIND_DRIVER notifications.  This
566	resumes the device if it's in the suspended state and prevents it from
567	being suspended again while those routines are being executed.
568	
569	To allow bus types and drivers to put devices into the suspended state by
570	calling pm_runtime_suspend() from their ->remove() routines, the driver core
571	executes pm_runtime_put_sync() after running the BUS_NOTIFY_UNBIND_DRIVER
572	notifications in __device_release_driver().  This requires bus types and
573	drivers to make their ->remove() callbacks avoid races with runtime PM directly,
574	but also it allows of more flexibility in the handling of devices during the
575	removal of their drivers.
576	
577	The user space can effectively disallow the driver of the device to power manage
578	it at run time by changing the value of its /sys/devices/.../power/control
579	attribute to "on", which causes pm_runtime_forbid() to be called.  In principle,
580	this mechanism may also be used by the driver to effectively turn off the
581	runtime power management of the device until the user space turns it on.
582	Namely, during the initialization the driver can make sure that the runtime PM
583	status of the device is 'active' and call pm_runtime_forbid().  It should be
584	noted, however, that if the user space has already intentionally changed the
585	value of /sys/devices/.../power/control to "auto" to allow the driver to power
586	manage the device at run time, the driver may confuse it by using
587	pm_runtime_forbid() this way.
588	
589	6. Runtime PM and System Sleep
590	
591	Runtime PM and system sleep (i.e., system suspend and hibernation, also known
592	as suspend-to-RAM and suspend-to-disk) interact with each other in a couple of
593	ways.  If a device is active when a system sleep starts, everything is
594	straightforward.  But what should happen if the device is already suspended?
595	
596	The device may have different wake-up settings for runtime PM and system sleep.
597	For example, remote wake-up may be enabled for runtime suspend but disallowed
598	for system sleep (device_may_wakeup(dev) returns 'false').  When this happens,
599	the subsystem-level system suspend callback is responsible for changing the
600	device's wake-up setting (it may leave that to the device driver's system
601	suspend routine).  It may be necessary to resume the device and suspend it again
602	in order to do so.  The same is true if the driver uses different power levels
603	or other settings for runtime suspend and system sleep.
604	
605	During system resume, the simplest approach is to bring all devices back to full
606	power, even if they had been suspended before the system suspend began.  There
607	are several reasons for this, including:
608	
609	  * The device might need to switch power levels, wake-up settings, etc.
610	
611	  * Remote wake-up events might have been lost by the firmware.
612	
613	  * The device's children may need the device to be at full power in order
614	    to resume themselves.
615	
616	  * The driver's idea of the device state may not agree with the device's
617	    physical state.  This can happen during resume from hibernation.
618	
619	  * The device might need to be reset.
620	
621	  * Even though the device was suspended, if its usage counter was > 0 then most
622	    likely it would need a runtime resume in the near future anyway.
623	
624	If the device had been suspended before the system suspend began and it's
625	brought back to full power during resume, then its runtime PM status will have
626	to be updated to reflect the actual post-system sleep status.  The way to do
627	this is:
628	
629		pm_runtime_disable(dev);
630		pm_runtime_set_active(dev);
631		pm_runtime_enable(dev);
632	
633	The PM core always increments the runtime usage counter before calling the
634	->suspend() callback and decrements it after calling the ->resume() callback.
635	Hence disabling runtime PM temporarily like this will not cause any runtime
636	suspend attempts to be permanently lost.  If the usage count goes to zero
637	following the return of the ->resume() callback, the ->runtime_idle() callback
638	will be invoked as usual.
639	
640	On some systems, however, system sleep is not entered through a global firmware
641	or hardware operation.  Instead, all hardware components are put into low-power
642	states directly by the kernel in a coordinated way.  Then, the system sleep
643	state effectively follows from the states the hardware components end up in
644	and the system is woken up from that state by a hardware interrupt or a similar
645	mechanism entirely under the kernel's control.  As a result, the kernel never
646	gives control away and the states of all devices during resume are precisely
647	known to it.  If that is the case and none of the situations listed above takes
648	place (in particular, if the system is not waking up from hibernation), it may
649	be more efficient to leave the devices that had been suspended before the system
650	suspend began in the suspended state.
651	
652	To this end, the PM core provides a mechanism allowing some coordination between
653	different levels of device hierarchy.  Namely, if a system suspend .prepare()
654	callback returns a positive number for a device, that indicates to the PM core
655	that the device appears to be runtime-suspended and its state is fine, so it
656	may be left in runtime suspend provided that all of its descendants are also
657	left in runtime suspend.  If that happens, the PM core will not execute any
658	system suspend and resume callbacks for all of those devices, except for the
659	complete callback, which is then entirely responsible for handling the device
660	as appropriate.  This only applies to system suspend transitions that are not
661	related to hibernation (see Documentation/power/devices.txt for more
662	information).
663	
664	The PM core does its best to reduce the probability of race conditions between
665	the runtime PM and system suspend/resume (and hibernation) callbacks by carrying
666	out the following operations:
667	
668	  * During system suspend pm_runtime_get_noresume() is called for every device
669	    right before executing the subsystem-level .prepare() callback for it and
670	    pm_runtime_barrier() is called for every device right before executing the
671	    subsystem-level .suspend() callback for it.  In addition to that the PM core
672	    calls  __pm_runtime_disable() with 'false' as the second argument for every
673	    device right before executing the subsystem-level .suspend_late() callback
674	    for it.
675	
676	  * During system resume pm_runtime_enable() and pm_runtime_put() are called for
677	    every device right after executing the subsystem-level .resume_early()
678	    callback and right after executing the subsystem-level .complete() callback
679	    for it, respectively.
680	
681	7. Generic subsystem callbacks
682	
683	Subsystems may wish to conserve code space by using the set of generic power
684	management callbacks provided by the PM core, defined in
685	driver/base/power/generic_ops.c:
686	
687	  int pm_generic_runtime_suspend(struct device *dev);
688	    - invoke the ->runtime_suspend() callback provided by the driver of this
689	      device and return its result, or return 0 if not defined
690	
691	  int pm_generic_runtime_resume(struct device *dev);
692	    - invoke the ->runtime_resume() callback provided by the driver of this
693	      device and return its result, or return 0 if not defined
694	
695	  int pm_generic_suspend(struct device *dev);
696	    - if the device has not been suspended at run time, invoke the ->suspend()
697	      callback provided by its driver and return its result, or return 0 if not
698	      defined
699	
700	  int pm_generic_suspend_noirq(struct device *dev);
701	    - if pm_runtime_suspended(dev) returns "false", invoke the ->suspend_noirq()
702	      callback provided by the device's driver and return its result, or return
703	      0 if not defined
704	
705	  int pm_generic_resume(struct device *dev);
706	    - invoke the ->resume() callback provided by the driver of this device and,
707	      if successful, change the device's runtime PM status to 'active'
708	
709	  int pm_generic_resume_noirq(struct device *dev);
710	    - invoke the ->resume_noirq() callback provided by the driver of this device
711	
712	  int pm_generic_freeze(struct device *dev);
713	    - if the device has not been suspended at run time, invoke the ->freeze()
714	      callback provided by its driver and return its result, or return 0 if not
715	      defined
716	
717	  int pm_generic_freeze_noirq(struct device *dev);
718	    - if pm_runtime_suspended(dev) returns "false", invoke the ->freeze_noirq()
719	      callback provided by the device's driver and return its result, or return
720	      0 if not defined
721	
722	  int pm_generic_thaw(struct device *dev);
723	    - if the device has not been suspended at run time, invoke the ->thaw()
724	      callback provided by its driver and return its result, or return 0 if not
725	      defined
726	
727	  int pm_generic_thaw_noirq(struct device *dev);
728	    - if pm_runtime_suspended(dev) returns "false", invoke the ->thaw_noirq()
729	      callback provided by the device's driver and return its result, or return
730	      0 if not defined
731	
732	  int pm_generic_poweroff(struct device *dev);
733	    - if the device has not been suspended at run time, invoke the ->poweroff()
734	      callback provided by its driver and return its result, or return 0 if not
735	      defined
736	
737	  int pm_generic_poweroff_noirq(struct device *dev);
738	    - if pm_runtime_suspended(dev) returns "false", run the ->poweroff_noirq()
739	      callback provided by the device's driver and return its result, or return
740	      0 if not defined
741	
742	  int pm_generic_restore(struct device *dev);
743	    - invoke the ->restore() callback provided by the driver of this device and,
744	      if successful, change the device's runtime PM status to 'active'
745	
746	  int pm_generic_restore_noirq(struct device *dev);
747	    - invoke the ->restore_noirq() callback provided by the device's driver
748	
749	These functions are the defaults used by the PM core, if a subsystem doesn't
750	provide its own callbacks for ->runtime_idle(), ->runtime_suspend(),
751	->runtime_resume(), ->suspend(), ->suspend_noirq(), ->resume(),
752	->resume_noirq(), ->freeze(), ->freeze_noirq(), ->thaw(), ->thaw_noirq(),
753	->poweroff(), ->poweroff_noirq(), ->restore(), ->restore_noirq() in the
754	subsystem-level dev_pm_ops structure.
755	
756	Device drivers that wish to use the same function as a system suspend, freeze,
757	poweroff and runtime suspend callback, and similarly for system resume, thaw,
758	restore, and runtime resume, can achieve this with the help of the
759	UNIVERSAL_DEV_PM_OPS macro defined in include/linux/pm.h (possibly setting its
760	last argument to NULL).
761	
762	8. "No-Callback" Devices
763	
764	Some "devices" are only logical sub-devices of their parent and cannot be
765	power-managed on their own.  (The prototype example is a USB interface.  Entire
766	USB devices can go into low-power mode or send wake-up requests, but neither is
767	possible for individual interfaces.)  The drivers for these devices have no
768	need of runtime PM callbacks; if the callbacks did exist, ->runtime_suspend()
769	and ->runtime_resume() would always return 0 without doing anything else and
770	->runtime_idle() would always call pm_runtime_suspend().
771	
772	Subsystems can tell the PM core about these devices by calling
773	pm_runtime_no_callbacks().  This should be done after the device structure is
774	initialized and before it is registered (although after device registration is
775	also okay).  The routine will set the device's power.no_callbacks flag and
776	prevent the non-debugging runtime PM sysfs attributes from being created.
777	
778	When power.no_callbacks is set, the PM core will not invoke the
779	->runtime_idle(), ->runtime_suspend(), or ->runtime_resume() callbacks.
780	Instead it will assume that suspends and resumes always succeed and that idle
781	devices should be suspended.
782	
783	As a consequence, the PM core will never directly inform the device's subsystem
784	or driver about runtime power changes.  Instead, the driver for the device's
785	parent must take responsibility for telling the device's driver when the
786	parent's power state changes.
787	
788	9. Autosuspend, or automatically-delayed suspends
789	
790	Changing a device's power state isn't free; it requires both time and energy.
791	A device should be put in a low-power state only when there's some reason to
792	think it will remain in that state for a substantial time.  A common heuristic
793	says that a device which hasn't been used for a while is liable to remain
794	unused; following this advice, drivers should not allow devices to be suspended
795	at runtime until they have been inactive for some minimum period.  Even when
796	the heuristic ends up being non-optimal, it will still prevent devices from
797	"bouncing" too rapidly between low-power and full-power states.
798	
799	The term "autosuspend" is an historical remnant.  It doesn't mean that the
800	device is automatically suspended (the subsystem or driver still has to call
801	the appropriate PM routines); rather it means that runtime suspends will
802	automatically be delayed until the desired period of inactivity has elapsed.
803	
804	Inactivity is determined based on the power.last_busy field.  Drivers should
805	call pm_runtime_mark_last_busy() to update this field after carrying out I/O,
806	typically just before calling pm_runtime_put_autosuspend().  The desired length
807	of the inactivity period is a matter of policy.  Subsystems can set this length
808	initially by calling pm_runtime_set_autosuspend_delay(), but after device
809	registration the length should be controlled by user space, using the
810	/sys/devices/.../power/autosuspend_delay_ms attribute.
811	
812	In order to use autosuspend, subsystems or drivers must call
813	pm_runtime_use_autosuspend() (preferably before registering the device), and
814	thereafter they should use the various *_autosuspend() helper functions instead
815	of the non-autosuspend counterparts:
816	
817		Instead of: pm_runtime_suspend    use: pm_runtime_autosuspend;
818		Instead of: pm_schedule_suspend   use: pm_request_autosuspend;
819		Instead of: pm_runtime_put        use: pm_runtime_put_autosuspend;
820		Instead of: pm_runtime_put_sync   use: pm_runtime_put_sync_autosuspend.
821	
822	Drivers may also continue to use the non-autosuspend helper functions; they
823	will behave normally, not taking the autosuspend delay into account.
824	Similarly, if the power.use_autosuspend field isn't set then the autosuspend
825	helper functions will behave just like the non-autosuspend counterparts.
826	
827	Under some circumstances a driver or subsystem may want to prevent a device
828	from autosuspending immediately, even though the usage counter is zero and the
829	autosuspend delay time has expired.  If the ->runtime_suspend() callback
830	returns -EAGAIN or -EBUSY, and if the next autosuspend delay expiration time is
831	in the future (as it normally would be if the callback invoked
832	pm_runtime_mark_last_busy()), the PM core will automatically reschedule the
833	autosuspend.  The ->runtime_suspend() callback can't do this rescheduling
834	itself because no suspend requests of any kind are accepted while the device is
835	suspending (i.e., while the callback is running).
836	
837	The implementation is well suited for asynchronous use in interrupt contexts.
838	However such use inevitably involves races, because the PM core can't
839	synchronize ->runtime_suspend() callbacks with the arrival of I/O requests.
840	This synchronization must be handled by the driver, using its private lock.
841	Here is a schematic pseudo-code example:
842	
843		foo_read_or_write(struct foo_priv *foo, void *data)
844		{
845			lock(&foo->private_lock);
846			add_request_to_io_queue(foo, data);
847			if (foo->num_pending_requests++ == 0)
848				pm_runtime_get(&foo->dev);
849			if (!foo->is_suspended)
850				foo_process_next_request(foo);
851			unlock(&foo->private_lock);
852		}
853	
854		foo_io_completion(struct foo_priv *foo, void *req)
855		{
856			lock(&foo->private_lock);
857			if (--foo->num_pending_requests == 0) {
858				pm_runtime_mark_last_busy(&foo->dev);
859				pm_runtime_put_autosuspend(&foo->dev);
860			} else {
861				foo_process_next_request(foo);
862			}
863			unlock(&foo->private_lock);
864			/* Send req result back to the user ... */
865		}
866	
867		int foo_runtime_suspend(struct device *dev)
868		{
869			struct foo_priv foo = container_of(dev, ...);
870			int ret = 0;
871	
872			lock(&foo->private_lock);
873			if (foo->num_pending_requests > 0) {
874				ret = -EBUSY;
875			} else {
876				/* ... suspend the device ... */
877				foo->is_suspended = 1;
878			}
879			unlock(&foo->private_lock);
880			return ret;
881		}
882	
883		int foo_runtime_resume(struct device *dev)
884		{
885			struct foo_priv foo = container_of(dev, ...);
886	
887			lock(&foo->private_lock);
888			/* ... resume the device ... */
889			foo->is_suspended = 0;
890			pm_runtime_mark_last_busy(&foo->dev);
891			if (foo->num_pending_requests > 0)
892				foo_process_next_request(foo);
893			unlock(&foo->private_lock);
894			return 0;
895		}
896	
897	The important point is that after foo_io_completion() asks for an autosuspend,
898	the foo_runtime_suspend() callback may race with foo_read_or_write().
899	Therefore foo_runtime_suspend() has to check whether there are any pending I/O
900	requests (while holding the private lock) before allowing the suspend to
901	proceed.
902	
903	In addition, the power.autosuspend_delay field can be changed by user space at
904	any time.  If a driver cares about this, it can call
905	pm_runtime_autosuspend_expiration() from within the ->runtime_suspend()
906	callback while holding its private lock.  If the function returns a nonzero
907	value then the delay has not yet expired and the callback should return
908	-EAGAIN.
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