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Based on kernel version 4.15. Page generated on 2018-01-29 10:00 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_can_wakeup() returns 'false' for the
109	device, then ->runtime_suspend() should return -EBUSY.  On the other hand, if
110	device_can_wakeup() 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 synchronization
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	  enum rpm_status runtime_status;
257	    - the runtime PM status of the device; this field's initial value is
258	      RPM_SUSPENDED, which means that each device is initially regarded by the
259	      PM core as 'suspended', regardless of its real hardware status
260	
261	  unsigned int runtime_auto;
262	    - if set, indicates that the user space has allowed the device driver to
263	      power manage the device at run time via the /sys/devices/.../power/control
264	      interface; it may only be modified with the help of the pm_runtime_allow()
265	      and pm_runtime_forbid() helper functions
266	
267	  unsigned int no_callbacks;
268	    - indicates that the device does not use the runtime PM callbacks (see
269	      Section 8); it may be modified only by the pm_runtime_no_callbacks()
270	      helper function
271	
272	  unsigned int irq_safe;
273	    - indicates that the ->runtime_suspend() and ->runtime_resume() callbacks
274	      will be invoked with the spinlock held and interrupts disabled
275	
276	  unsigned int use_autosuspend;
277	    - indicates that the device's driver supports delayed autosuspend (see
278	      Section 9); it may be modified only by the
279	      pm_runtime{_dont}_use_autosuspend() helper functions
280	
281	  unsigned int timer_autosuspends;
282	    - indicates that the PM core should attempt to carry out an autosuspend
283	      when the timer expires rather than a normal suspend
284	
285	  int autosuspend_delay;
286	    - the delay time (in milliseconds) to be used for autosuspend
287	
288	  unsigned long last_busy;
289	    - the time (in jiffies) when the pm_runtime_mark_last_busy() helper
290	      function was last called for this device; used in calculating inactivity
291	      periods for autosuspend
292	
293	All of the above fields are members of the 'power' member of 'struct device'.
294	
295	4. Runtime PM Device Helper Functions
296	
297	The following runtime PM helper functions are defined in
298	drivers/base/power/runtime.c and include/linux/pm_runtime.h:
299	
300	  void pm_runtime_init(struct device *dev);
301	    - initialize the device runtime PM fields in 'struct dev_pm_info'
302	
303	  void pm_runtime_remove(struct device *dev);
304	    - make sure that the runtime PM of the device will be disabled after
305	      removing the device from device hierarchy
306	
307	  int pm_runtime_idle(struct device *dev);
308	    - execute the subsystem-level idle callback for the device; returns an
309	      error code on failure, where -EINPROGRESS means that ->runtime_idle() is
310	      already being executed; if there is no callback or the callback returns 0
311	      then run pm_runtime_autosuspend(dev) and return its result
312	
313	  int pm_runtime_suspend(struct device *dev);
314	    - execute the subsystem-level suspend callback for the device; returns 0 on
315	      success, 1 if the device's runtime PM status was already 'suspended', or
316	      error code on failure, where -EAGAIN or -EBUSY means it is safe to attempt
317	      to suspend the device again in future and -EACCES means that
318	      'power.disable_depth' is different from 0
319	
320	  int pm_runtime_autosuspend(struct device *dev);
321	    - same as pm_runtime_suspend() except that the autosuspend delay is taken
322	      into account; if pm_runtime_autosuspend_expiration() says the delay has
323	      not yet expired then an autosuspend is scheduled for the appropriate time
324	      and 0 is returned
325	
326	  int pm_runtime_resume(struct device *dev);
327	    - execute the subsystem-level resume callback for the device; returns 0 on
328	      success, 1 if the device's runtime PM status was already 'active' or
329	      error code on failure, where -EAGAIN means it may be safe to attempt to
330	      resume the device again in future, but 'power.runtime_error' should be
331	      checked additionally, and -EACCES means that 'power.disable_depth' is
332	      different from 0
333	
334	  int pm_request_idle(struct device *dev);
335	    - submit a request to execute the subsystem-level idle callback for the
336	      device (the request is represented by a work item in pm_wq); returns 0 on
337	      success or error code if the request has not been queued up
338	
339	  int pm_request_autosuspend(struct device *dev);
340	    - schedule the execution of the subsystem-level suspend callback for the
341	      device when the autosuspend delay has expired; if the delay has already
342	      expired then the work item is queued up immediately
343	
344	  int pm_schedule_suspend(struct device *dev, unsigned int delay);
345	    - schedule the execution of the subsystem-level suspend callback for the
346	      device in future, where 'delay' is the time to wait before queuing up a
347	      suspend work item in pm_wq, in milliseconds (if 'delay' is zero, the work
348	      item is queued up immediately); returns 0 on success, 1 if the device's PM
349	      runtime status was already 'suspended', or error code if the request
350	      hasn't been scheduled (or queued up if 'delay' is 0); if the execution of
351	      ->runtime_suspend() is already scheduled and not yet expired, the new
352	      value of 'delay' will be used as the time to wait
353	
354	  int pm_request_resume(struct device *dev);
355	    - submit a request to execute the subsystem-level resume callback for the
356	      device (the request is represented by a work item in pm_wq); returns 0 on
357	      success, 1 if the device's runtime PM status was already 'active', or
358	      error code if the request hasn't been queued up
359	
360	  void pm_runtime_get_noresume(struct device *dev);
361	    - increment the device's usage counter
362	
363	  int pm_runtime_get(struct device *dev);
364	    - increment the device's usage counter, run pm_request_resume(dev) and
365	      return its result
366	
367	  int pm_runtime_get_sync(struct device *dev);
368	    - increment the device's usage counter, run pm_runtime_resume(dev) and
369	      return its result
370	
371	  int pm_runtime_get_if_in_use(struct device *dev);
372	    - return -EINVAL if 'power.disable_depth' is nonzero; otherwise, if the
373	      runtime PM status is RPM_ACTIVE and the runtime PM usage counter is
374	      nonzero, increment the counter and return 1; otherwise return 0 without
375	      changing the counter
376	
377	  void pm_runtime_put_noidle(struct device *dev);
378	    - decrement the device's usage counter
379	
380	  int pm_runtime_put(struct device *dev);
381	    - decrement the device's usage counter; if the result is 0 then run
382	      pm_request_idle(dev) and return its result
383	
384	  int pm_runtime_put_autosuspend(struct device *dev);
385	    - decrement the device's usage counter; if the result is 0 then run
386	      pm_request_autosuspend(dev) and return its result
387	
388	  int pm_runtime_put_sync(struct device *dev);
389	    - decrement the device's usage counter; if the result is 0 then run
390	      pm_runtime_idle(dev) and return its result
391	
392	  int pm_runtime_put_sync_suspend(struct device *dev);
393	    - decrement the device's usage counter; if the result is 0 then run
394	      pm_runtime_suspend(dev) and return its result
395	
396	  int pm_runtime_put_sync_autosuspend(struct device *dev);
397	    - decrement the device's usage counter; if the result is 0 then run
398	      pm_runtime_autosuspend(dev) and return its result
399	
400	  void pm_runtime_enable(struct device *dev);
401	    - decrement the device's 'power.disable_depth' field; if that field is equal
402	      to zero, the runtime PM helper functions can execute subsystem-level
403	      callbacks described in Section 2 for the device
404	
405	  int pm_runtime_disable(struct device *dev);
406	    - increment the device's 'power.disable_depth' field (if the value of that
407	      field was previously zero, this prevents subsystem-level runtime PM
408	      callbacks from being run for the device), make sure that all of the
409	      pending runtime PM operations on the device are either completed or
410	      canceled; returns 1 if there was a resume request pending and it was
411	      necessary to execute the subsystem-level resume callback for the device
412	      to satisfy that request, otherwise 0 is returned
413	
414	  int pm_runtime_barrier(struct device *dev);
415	    - check if there's a resume request pending for the device and resume it
416	      (synchronously) in that case, cancel any other pending runtime PM requests
417	      regarding it and wait for all runtime PM operations on it in progress to
418	      complete; returns 1 if there was a resume request pending and it was
419	      necessary to execute the subsystem-level resume callback for the device to
420	      satisfy that request, otherwise 0 is returned
421	
422	  void pm_suspend_ignore_children(struct device *dev, bool enable);
423	    - set/unset the power.ignore_children flag of the device
424	
425	  int pm_runtime_set_active(struct device *dev);
426	    - clear the device's 'power.runtime_error' flag, set the device's runtime
427	      PM status to 'active' and update its parent's counter of 'active'
428	      children as appropriate (it is only valid to use this function if
429	      'power.runtime_error' is set or 'power.disable_depth' is greater than
430	      zero); it will fail and return error code if the device has a parent
431	      which is not active and the 'power.ignore_children' flag of which is unset
432	
433	  void pm_runtime_set_suspended(struct device *dev);
434	    - clear the device's 'power.runtime_error' flag, set the device's runtime
435	      PM status to 'suspended' and update its parent's counter of 'active'
436	      children as appropriate (it is only valid to use this function if
437	      'power.runtime_error' is set or 'power.disable_depth' is greater than
438	      zero)
439	
440	  bool pm_runtime_active(struct device *dev);
441	    - return true if the device's runtime PM status is 'active' or its
442	      'power.disable_depth' field is not equal to zero, or false otherwise
443	
444	  bool pm_runtime_suspended(struct device *dev);
445	    - return true if the device's runtime PM status is 'suspended' and its
446	      'power.disable_depth' field is equal to zero, or false otherwise
447	
448	  bool pm_runtime_status_suspended(struct device *dev);
449	    - return true if the device's runtime PM status is 'suspended'
450	
451	  void pm_runtime_allow(struct device *dev);
452	    - set the power.runtime_auto flag for the device and decrease its usage
453	      counter (used by the /sys/devices/.../power/control interface to
454	      effectively allow the device to be power managed at run time)
455	
456	  void pm_runtime_forbid(struct device *dev);
457	    - unset the power.runtime_auto flag for the device and increase its usage
458	      counter (used by the /sys/devices/.../power/control interface to
459	      effectively prevent the device from being power managed at run time)
460	
461	  void pm_runtime_no_callbacks(struct device *dev);
462	    - set the power.no_callbacks flag for the device and remove the runtime
463	      PM attributes from /sys/devices/.../power (or prevent them from being
464	      added when the device is registered)
465	
466	  void pm_runtime_irq_safe(struct device *dev);
467	    - set the power.irq_safe flag for the device, causing the runtime-PM
468	      callbacks to be invoked with interrupts off
469	
470	  bool pm_runtime_is_irq_safe(struct device *dev);
471	    - return true if power.irq_safe flag was set for the device, causing
472	      the runtime-PM callbacks to be invoked with interrupts off
473	
474	  void pm_runtime_mark_last_busy(struct device *dev);
475	    - set the power.last_busy field to the current time
476	
477	  void pm_runtime_use_autosuspend(struct device *dev);
478	    - set the power.use_autosuspend flag, enabling autosuspend delays; call
479	      pm_runtime_get_sync if the flag was previously cleared and
480	      power.autosuspend_delay is negative
481	
482	  void pm_runtime_dont_use_autosuspend(struct device *dev);
483	    - clear the power.use_autosuspend flag, disabling autosuspend delays;
484	      decrement the device's usage counter if the flag was previously set and
485	      power.autosuspend_delay is negative; call pm_runtime_idle
486	
487	  void pm_runtime_set_autosuspend_delay(struct device *dev, int delay);
488	    - set the power.autosuspend_delay value to 'delay' (expressed in
489	      milliseconds); if 'delay' is negative then runtime suspends are
490	      prevented; if power.use_autosuspend is set, pm_runtime_get_sync may be
491	      called or the device's usage counter may be decremented and
492	      pm_runtime_idle called depending on if power.autosuspend_delay is
493	      changed to or from a negative value; if power.use_autosuspend is clear,
494	      pm_runtime_idle is called
495	
496	  unsigned long pm_runtime_autosuspend_expiration(struct device *dev);
497	    - calculate the time when the current autosuspend delay period will expire,
498	      based on power.last_busy and power.autosuspend_delay; if the delay time
499	      is 1000 ms or larger then the expiration time is rounded up to the
500	      nearest second; returns 0 if the delay period has already expired or
501	      power.use_autosuspend isn't set, otherwise returns the expiration time
502	      in jiffies
503	
504	It is safe to execute the following helper functions from interrupt context:
505	
506	pm_request_idle()
507	pm_request_autosuspend()
508	pm_schedule_suspend()
509	pm_request_resume()
510	pm_runtime_get_noresume()
511	pm_runtime_get()
512	pm_runtime_put_noidle()
513	pm_runtime_put()
514	pm_runtime_put_autosuspend()
515	pm_runtime_enable()
516	pm_suspend_ignore_children()
517	pm_runtime_set_active()
518	pm_runtime_set_suspended()
519	pm_runtime_suspended()
520	pm_runtime_mark_last_busy()
521	pm_runtime_autosuspend_expiration()
522	
523	If pm_runtime_irq_safe() has been called for a device then the following helper
524	functions may also be used in interrupt context:
525	
526	pm_runtime_idle()
527	pm_runtime_suspend()
528	pm_runtime_autosuspend()
529	pm_runtime_resume()
530	pm_runtime_get_sync()
531	pm_runtime_put_sync()
532	pm_runtime_put_sync_suspend()
533	pm_runtime_put_sync_autosuspend()
534	
535	5. Runtime PM Initialization, Device Probing and Removal
536	
537	Initially, the runtime PM is disabled for all devices, which means that the
538	majority of the runtime PM helper functions described in Section 4 will return
539	-EAGAIN until pm_runtime_enable() is called for the device.
540	
541	In addition to that, the initial runtime PM status of all devices is
542	'suspended', but it need not reflect the actual physical state of the device.
543	Thus, if the device is initially active (i.e. it is able to process I/O), its
544	runtime PM status must be changed to 'active', with the help of
545	pm_runtime_set_active(), before pm_runtime_enable() is called for the device.
546	
547	However, if the device has a parent and the parent's runtime PM is enabled,
548	calling pm_runtime_set_active() for the device will affect the parent, unless
549	the parent's 'power.ignore_children' flag is set.  Namely, in that case the
550	parent won't be able to suspend at run time, using the PM core's helper
551	functions, as long as the child's status is 'active', even if the child's
552	runtime PM is still disabled (i.e. pm_runtime_enable() hasn't been called for
553	the child yet or pm_runtime_disable() has been called for it).  For this reason,
554	once pm_runtime_set_active() has been called for the device, pm_runtime_enable()
555	should be called for it too as soon as reasonably possible or its runtime PM
556	status should be changed back to 'suspended' with the help of
557	pm_runtime_set_suspended().
558	
559	If the default initial runtime PM status of the device (i.e. 'suspended')
560	reflects the actual state of the device, its bus type's or its driver's
561	->probe() callback will likely need to wake it up using one of the PM core's
562	helper functions described in Section 4.  In that case, pm_runtime_resume()
563	should be used.  Of course, for this purpose the device's runtime PM has to be
564	enabled earlier by calling pm_runtime_enable().
565	
566	Note, if the device may execute pm_runtime calls during the probe (such as
567	if it is registers with a subsystem that may call back in) then the
568	pm_runtime_get_sync() call paired with a pm_runtime_put() call will be
569	appropriate to ensure that the device is not put back to sleep during the
570	probe. This can happen with systems such as the network device layer.
571	
572	It may be desirable to suspend the device once ->probe() has finished.
573	Therefore the driver core uses the asynchronous pm_request_idle() to submit a
574	request to execute the subsystem-level idle callback for the device at that
575	time.  A driver that makes use of the runtime autosuspend feature, may want to
576	update the last busy mark before returning from ->probe().
577	
578	Moreover, the driver core prevents runtime PM callbacks from racing with the bus
579	notifier callback in __device_release_driver(), which is necessary, because the
580	notifier is used by some subsystems to carry out operations affecting the
581	runtime PM functionality.  It does so by calling pm_runtime_get_sync() before
582	driver_sysfs_remove() and the BUS_NOTIFY_UNBIND_DRIVER notifications.  This
583	resumes the device if it's in the suspended state and prevents it from
584	being suspended again while those routines are being executed.
585	
586	To allow bus types and drivers to put devices into the suspended state by
587	calling pm_runtime_suspend() from their ->remove() routines, the driver core
588	executes pm_runtime_put_sync() after running the BUS_NOTIFY_UNBIND_DRIVER
589	notifications in __device_release_driver().  This requires bus types and
590	drivers to make their ->remove() callbacks avoid races with runtime PM directly,
591	but also it allows of more flexibility in the handling of devices during the
592	removal of their drivers.
593	
594	Drivers in ->remove() callback should undo the runtime PM changes done
595	in ->probe(). Usually this means calling pm_runtime_disable(),
596	pm_runtime_dont_use_autosuspend() etc.
597	
598	The user space can effectively disallow the driver of the device to power manage
599	it at run time by changing the value of its /sys/devices/.../power/control
600	attribute to "on", which causes pm_runtime_forbid() to be called.  In principle,
601	this mechanism may also be used by the driver to effectively turn off the
602	runtime power management of the device until the user space turns it on.
603	Namely, during the initialization the driver can make sure that the runtime PM
604	status of the device is 'active' and call pm_runtime_forbid().  It should be
605	noted, however, that if the user space has already intentionally changed the
606	value of /sys/devices/.../power/control to "auto" to allow the driver to power
607	manage the device at run time, the driver may confuse it by using
608	pm_runtime_forbid() this way.
609	
610	6. Runtime PM and System Sleep
611	
612	Runtime PM and system sleep (i.e., system suspend and hibernation, also known
613	as suspend-to-RAM and suspend-to-disk) interact with each other in a couple of
614	ways.  If a device is active when a system sleep starts, everything is
615	straightforward.  But what should happen if the device is already suspended?
616	
617	The device may have different wake-up settings for runtime PM and system sleep.
618	For example, remote wake-up may be enabled for runtime suspend but disallowed
619	for system sleep (device_may_wakeup(dev) returns 'false').  When this happens,
620	the subsystem-level system suspend callback is responsible for changing the
621	device's wake-up setting (it may leave that to the device driver's system
622	suspend routine).  It may be necessary to resume the device and suspend it again
623	in order to do so.  The same is true if the driver uses different power levels
624	or other settings for runtime suspend and system sleep.
625	
626	During system resume, the simplest approach is to bring all devices back to full
627	power, even if they had been suspended before the system suspend began.  There
628	are several reasons for this, including:
629	
630	  * The device might need to switch power levels, wake-up settings, etc.
631	
632	  * Remote wake-up events might have been lost by the firmware.
633	
634	  * The device's children may need the device to be at full power in order
635	    to resume themselves.
636	
637	  * The driver's idea of the device state may not agree with the device's
638	    physical state.  This can happen during resume from hibernation.
639	
640	  * The device might need to be reset.
641	
642	  * Even though the device was suspended, if its usage counter was > 0 then most
643	    likely it would need a runtime resume in the near future anyway.
644	
645	If the device had been suspended before the system suspend began and it's
646	brought back to full power during resume, then its runtime PM status will have
647	to be updated to reflect the actual post-system sleep status.  The way to do
648	this is:
649	
650		pm_runtime_disable(dev);
651		pm_runtime_set_active(dev);
652		pm_runtime_enable(dev);
653	
654	The PM core always increments the runtime usage counter before calling the
655	->suspend() callback and decrements it after calling the ->resume() callback.
656	Hence disabling runtime PM temporarily like this will not cause any runtime
657	suspend attempts to be permanently lost.  If the usage count goes to zero
658	following the return of the ->resume() callback, the ->runtime_idle() callback
659	will be invoked as usual.
660	
661	On some systems, however, system sleep is not entered through a global firmware
662	or hardware operation.  Instead, all hardware components are put into low-power
663	states directly by the kernel in a coordinated way.  Then, the system sleep
664	state effectively follows from the states the hardware components end up in
665	and the system is woken up from that state by a hardware interrupt or a similar
666	mechanism entirely under the kernel's control.  As a result, the kernel never
667	gives control away and the states of all devices during resume are precisely
668	known to it.  If that is the case and none of the situations listed above takes
669	place (in particular, if the system is not waking up from hibernation), it may
670	be more efficient to leave the devices that had been suspended before the system
671	suspend began in the suspended state.
672	
673	To this end, the PM core provides a mechanism allowing some coordination between
674	different levels of device hierarchy.  Namely, if a system suspend .prepare()
675	callback returns a positive number for a device, that indicates to the PM core
676	that the device appears to be runtime-suspended and its state is fine, so it
677	may be left in runtime suspend provided that all of its descendants are also
678	left in runtime suspend.  If that happens, the PM core will not execute any
679	system suspend and resume callbacks for all of those devices, except for the
680	complete callback, which is then entirely responsible for handling the device
681	as appropriate.  This only applies to system suspend transitions that are not
682	related to hibernation (see Documentation/driver-api/pm/devices.rst for more
683	information).
684	
685	The PM core does its best to reduce the probability of race conditions between
686	the runtime PM and system suspend/resume (and hibernation) callbacks by carrying
687	out the following operations:
688	
689	  * During system suspend pm_runtime_get_noresume() is called for every device
690	    right before executing the subsystem-level .prepare() callback for it and
691	    pm_runtime_barrier() is called for every device right before executing the
692	    subsystem-level .suspend() callback for it.  In addition to that the PM core
693	    calls  __pm_runtime_disable() with 'false' as the second argument for every
694	    device right before executing the subsystem-level .suspend_late() callback
695	    for it.
696	
697	  * During system resume pm_runtime_enable() and pm_runtime_put() are called for
698	    every device right after executing the subsystem-level .resume_early()
699	    callback and right after executing the subsystem-level .complete() callback
700	    for it, respectively.
701	
702	7. Generic subsystem callbacks
703	
704	Subsystems may wish to conserve code space by using the set of generic power
705	management callbacks provided by the PM core, defined in
706	driver/base/power/generic_ops.c:
707	
708	  int pm_generic_runtime_suspend(struct device *dev);
709	    - invoke the ->runtime_suspend() callback provided by the driver of this
710	      device and return its result, or return 0 if not defined
711	
712	  int pm_generic_runtime_resume(struct device *dev);
713	    - invoke the ->runtime_resume() callback provided by the driver of this
714	      device and return its result, or return 0 if not defined
715	
716	  int pm_generic_suspend(struct device *dev);
717	    - if the device has not been suspended at run time, invoke the ->suspend()
718	      callback provided by its driver and return its result, or return 0 if not
719	      defined
720	
721	  int pm_generic_suspend_noirq(struct device *dev);
722	    - if pm_runtime_suspended(dev) returns "false", invoke the ->suspend_noirq()
723	      callback provided by the device's driver and return its result, or return
724	      0 if not defined
725	
726	  int pm_generic_resume(struct device *dev);
727	    - invoke the ->resume() callback provided by the driver of this device and,
728	      if successful, change the device's runtime PM status to 'active'
729	
730	  int pm_generic_resume_noirq(struct device *dev);
731	    - invoke the ->resume_noirq() callback provided by the driver of this device
732	
733	  int pm_generic_freeze(struct device *dev);
734	    - if the device has not been suspended at run time, invoke the ->freeze()
735	      callback provided by its driver and return its result, or return 0 if not
736	      defined
737	
738	  int pm_generic_freeze_noirq(struct device *dev);
739	    - if pm_runtime_suspended(dev) returns "false", invoke the ->freeze_noirq()
740	      callback provided by the device's driver and return its result, or return
741	      0 if not defined
742	
743	  int pm_generic_thaw(struct device *dev);
744	    - if the device has not been suspended at run time, invoke the ->thaw()
745	      callback provided by its driver and return its result, or return 0 if not
746	      defined
747	
748	  int pm_generic_thaw_noirq(struct device *dev);
749	    - if pm_runtime_suspended(dev) returns "false", invoke the ->thaw_noirq()
750	      callback provided by the device's driver and return its result, or return
751	      0 if not defined
752	
753	  int pm_generic_poweroff(struct device *dev);
754	    - if the device has not been suspended at run time, invoke the ->poweroff()
755	      callback provided by its driver and return its result, or return 0 if not
756	      defined
757	
758	  int pm_generic_poweroff_noirq(struct device *dev);
759	    - if pm_runtime_suspended(dev) returns "false", run the ->poweroff_noirq()
760	      callback provided by the device's driver and return its result, or return
761	      0 if not defined
762	
763	  int pm_generic_restore(struct device *dev);
764	    - invoke the ->restore() callback provided by the driver of this device and,
765	      if successful, change the device's runtime PM status to 'active'
766	
767	  int pm_generic_restore_noirq(struct device *dev);
768	    - invoke the ->restore_noirq() callback provided by the device's driver
769	
770	These functions are the defaults used by the PM core, if a subsystem doesn't
771	provide its own callbacks for ->runtime_idle(), ->runtime_suspend(),
772	->runtime_resume(), ->suspend(), ->suspend_noirq(), ->resume(),
773	->resume_noirq(), ->freeze(), ->freeze_noirq(), ->thaw(), ->thaw_noirq(),
774	->poweroff(), ->poweroff_noirq(), ->restore(), ->restore_noirq() in the
775	subsystem-level dev_pm_ops structure.
776	
777	Device drivers that wish to use the same function as a system suspend, freeze,
778	poweroff and runtime suspend callback, and similarly for system resume, thaw,
779	restore, and runtime resume, can achieve this with the help of the
780	UNIVERSAL_DEV_PM_OPS macro defined in include/linux/pm.h (possibly setting its
781	last argument to NULL).
782	
783	8. "No-Callback" Devices
784	
785	Some "devices" are only logical sub-devices of their parent and cannot be
786	power-managed on their own.  (The prototype example is a USB interface.  Entire
787	USB devices can go into low-power mode or send wake-up requests, but neither is
788	possible for individual interfaces.)  The drivers for these devices have no
789	need of runtime PM callbacks; if the callbacks did exist, ->runtime_suspend()
790	and ->runtime_resume() would always return 0 without doing anything else and
791	->runtime_idle() would always call pm_runtime_suspend().
792	
793	Subsystems can tell the PM core about these devices by calling
794	pm_runtime_no_callbacks().  This should be done after the device structure is
795	initialized and before it is registered (although after device registration is
796	also okay).  The routine will set the device's power.no_callbacks flag and
797	prevent the non-debugging runtime PM sysfs attributes from being created.
798	
799	When power.no_callbacks is set, the PM core will not invoke the
800	->runtime_idle(), ->runtime_suspend(), or ->runtime_resume() callbacks.
801	Instead it will assume that suspends and resumes always succeed and that idle
802	devices should be suspended.
803	
804	As a consequence, the PM core will never directly inform the device's subsystem
805	or driver about runtime power changes.  Instead, the driver for the device's
806	parent must take responsibility for telling the device's driver when the
807	parent's power state changes.
808	
809	9. Autosuspend, or automatically-delayed suspends
810	
811	Changing a device's power state isn't free; it requires both time and energy.
812	A device should be put in a low-power state only when there's some reason to
813	think it will remain in that state for a substantial time.  A common heuristic
814	says that a device which hasn't been used for a while is liable to remain
815	unused; following this advice, drivers should not allow devices to be suspended
816	at runtime until they have been inactive for some minimum period.  Even when
817	the heuristic ends up being non-optimal, it will still prevent devices from
818	"bouncing" too rapidly between low-power and full-power states.
819	
820	The term "autosuspend" is an historical remnant.  It doesn't mean that the
821	device is automatically suspended (the subsystem or driver still has to call
822	the appropriate PM routines); rather it means that runtime suspends will
823	automatically be delayed until the desired period of inactivity has elapsed.
824	
825	Inactivity is determined based on the power.last_busy field.  Drivers should
826	call pm_runtime_mark_last_busy() to update this field after carrying out I/O,
827	typically just before calling pm_runtime_put_autosuspend().  The desired length
828	of the inactivity period is a matter of policy.  Subsystems can set this length
829	initially by calling pm_runtime_set_autosuspend_delay(), but after device
830	registration the length should be controlled by user space, using the
831	/sys/devices/.../power/autosuspend_delay_ms attribute.
832	
833	In order to use autosuspend, subsystems or drivers must call
834	pm_runtime_use_autosuspend() (preferably before registering the device), and
835	thereafter they should use the various *_autosuspend() helper functions instead
836	of the non-autosuspend counterparts:
837	
838		Instead of: pm_runtime_suspend    use: pm_runtime_autosuspend;
839		Instead of: pm_schedule_suspend   use: pm_request_autosuspend;
840		Instead of: pm_runtime_put        use: pm_runtime_put_autosuspend;
841		Instead of: pm_runtime_put_sync   use: pm_runtime_put_sync_autosuspend.
842	
843	Drivers may also continue to use the non-autosuspend helper functions; they
844	will behave normally, which means sometimes taking the autosuspend delay into
845	account (see pm_runtime_idle).
846	
847	Under some circumstances a driver or subsystem may want to prevent a device
848	from autosuspending immediately, even though the usage counter is zero and the
849	autosuspend delay time has expired.  If the ->runtime_suspend() callback
850	returns -EAGAIN or -EBUSY, and if the next autosuspend delay expiration time is
851	in the future (as it normally would be if the callback invoked
852	pm_runtime_mark_last_busy()), the PM core will automatically reschedule the
853	autosuspend.  The ->runtime_suspend() callback can't do this rescheduling
854	itself because no suspend requests of any kind are accepted while the device is
855	suspending (i.e., while the callback is running).
856	
857	The implementation is well suited for asynchronous use in interrupt contexts.
858	However such use inevitably involves races, because the PM core can't
859	synchronize ->runtime_suspend() callbacks with the arrival of I/O requests.
860	This synchronization must be handled by the driver, using its private lock.
861	Here is a schematic pseudo-code example:
862	
863		foo_read_or_write(struct foo_priv *foo, void *data)
864		{
865			lock(&foo->private_lock);
866			add_request_to_io_queue(foo, data);
867			if (foo->num_pending_requests++ == 0)
868				pm_runtime_get(&foo->dev);
869			if (!foo->is_suspended)
870				foo_process_next_request(foo);
871			unlock(&foo->private_lock);
872		}
873	
874		foo_io_completion(struct foo_priv *foo, void *req)
875		{
876			lock(&foo->private_lock);
877			if (--foo->num_pending_requests == 0) {
878				pm_runtime_mark_last_busy(&foo->dev);
879				pm_runtime_put_autosuspend(&foo->dev);
880			} else {
881				foo_process_next_request(foo);
882			}
883			unlock(&foo->private_lock);
884			/* Send req result back to the user ... */
885		}
886	
887		int foo_runtime_suspend(struct device *dev)
888		{
889			struct foo_priv foo = container_of(dev, ...);
890			int ret = 0;
891	
892			lock(&foo->private_lock);
893			if (foo->num_pending_requests > 0) {
894				ret = -EBUSY;
895			} else {
896				/* ... suspend the device ... */
897				foo->is_suspended = 1;
898			}
899			unlock(&foo->private_lock);
900			return ret;
901		}
902	
903		int foo_runtime_resume(struct device *dev)
904		{
905			struct foo_priv foo = container_of(dev, ...);
906	
907			lock(&foo->private_lock);
908			/* ... resume the device ... */
909			foo->is_suspended = 0;
910			pm_runtime_mark_last_busy(&foo->dev);
911			if (foo->num_pending_requests > 0)
912				foo_process_next_request(foo);
913			unlock(&foo->private_lock);
914			return 0;
915		}
916	
917	The important point is that after foo_io_completion() asks for an autosuspend,
918	the foo_runtime_suspend() callback may race with foo_read_or_write().
919	Therefore foo_runtime_suspend() has to check whether there are any pending I/O
920	requests (while holding the private lock) before allowing the suspend to
921	proceed.
922	
923	In addition, the power.autosuspend_delay field can be changed by user space at
924	any time.  If a driver cares about this, it can call
925	pm_runtime_autosuspend_expiration() from within the ->runtime_suspend()
926	callback while holding its private lock.  If the function returns a nonzero
927	value then the delay has not yet expired and the callback should return
928	-EAGAIN.
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