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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 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); it will fail and return an error code if the device has a child
439	      which is active and the 'power.ignore_children' flag is unset
440	
441	  bool pm_runtime_active(struct device *dev);
442	    - return true if the device's runtime PM status is 'active' or its
443	      'power.disable_depth' field is not equal to zero, or false otherwise
444	
445	  bool pm_runtime_suspended(struct device *dev);
446	    - return true if the device's runtime PM status is 'suspended' and its
447	      'power.disable_depth' field is equal to zero, or false otherwise
448	
449	  bool pm_runtime_status_suspended(struct device *dev);
450	    - return true if the device's runtime PM status is 'suspended'
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	  bool pm_runtime_is_irq_safe(struct device *dev);
472	    - return true if power.irq_safe flag was set for the device, causing
473	      the runtime-PM callbacks to be invoked with interrupts off
474	
475	  void pm_runtime_mark_last_busy(struct device *dev);
476	    - set the power.last_busy field to the current time
477	
478	  void pm_runtime_use_autosuspend(struct device *dev);
479	    - set the power.use_autosuspend flag, enabling autosuspend delays; call
480	      pm_runtime_get_sync if the flag was previously cleared and
481	      power.autosuspend_delay is negative
482	
483	  void pm_runtime_dont_use_autosuspend(struct device *dev);
484	    - clear the power.use_autosuspend flag, disabling autosuspend delays;
485	      decrement the device's usage counter if the flag was previously set and
486	      power.autosuspend_delay is negative; call pm_runtime_idle
487	
488	  void pm_runtime_set_autosuspend_delay(struct device *dev, int delay);
489	    - set the power.autosuspend_delay value to 'delay' (expressed in
490	      milliseconds); if 'delay' is negative then runtime suspends are
491	      prevented; if power.use_autosuspend is set, pm_runtime_get_sync may be
492	      called or the device's usage counter may be decremented and
493	      pm_runtime_idle called depending on if power.autosuspend_delay is
494	      changed to or from a negative value; if power.use_autosuspend is clear,
495	      pm_runtime_idle is called
496	
497	  unsigned long pm_runtime_autosuspend_expiration(struct device *dev);
498	    - calculate the time when the current autosuspend delay period will expire,
499	      based on power.last_busy and power.autosuspend_delay; if the delay time
500	      is 1000 ms or larger then the expiration time is rounded up to the
501	      nearest second; returns 0 if the delay period has already expired or
502	      power.use_autosuspend isn't set, otherwise returns the expiration time
503	      in jiffies
504	
505	It is safe to execute the following helper functions from interrupt context:
506	
507	pm_request_idle()
508	pm_request_autosuspend()
509	pm_schedule_suspend()
510	pm_request_resume()
511	pm_runtime_get_noresume()
512	pm_runtime_get()
513	pm_runtime_put_noidle()
514	pm_runtime_put()
515	pm_runtime_put_autosuspend()
516	pm_runtime_enable()
517	pm_suspend_ignore_children()
518	pm_runtime_set_active()
519	pm_runtime_set_suspended()
520	pm_runtime_suspended()
521	pm_runtime_mark_last_busy()
522	pm_runtime_autosuspend_expiration()
523	
524	If pm_runtime_irq_safe() has been called for a device then the following helper
525	functions may also be used in interrupt context:
526	
527	pm_runtime_idle()
528	pm_runtime_suspend()
529	pm_runtime_autosuspend()
530	pm_runtime_resume()
531	pm_runtime_get_sync()
532	pm_runtime_put_sync()
533	pm_runtime_put_sync_suspend()
534	pm_runtime_put_sync_autosuspend()
535	
536	5. Runtime PM Initialization, Device Probing and Removal
537	
538	Initially, the runtime PM is disabled for all devices, which means that the
539	majority of the runtime PM helper functions described in Section 4 will return
540	-EAGAIN until pm_runtime_enable() is called for the device.
541	
542	In addition to that, the initial runtime PM status of all devices is
543	'suspended', but it need not reflect the actual physical state of the device.
544	Thus, if the device is initially active (i.e. it is able to process I/O), its
545	runtime PM status must be changed to 'active', with the help of
546	pm_runtime_set_active(), before pm_runtime_enable() is called for the device.
547	
548	However, if the device has a parent and the parent's runtime PM is enabled,
549	calling pm_runtime_set_active() for the device will affect the parent, unless
550	the parent's 'power.ignore_children' flag is set.  Namely, in that case the
551	parent won't be able to suspend at run time, using the PM core's helper
552	functions, as long as the child's status is 'active', even if the child's
553	runtime PM is still disabled (i.e. pm_runtime_enable() hasn't been called for
554	the child yet or pm_runtime_disable() has been called for it).  For this reason,
555	once pm_runtime_set_active() has been called for the device, pm_runtime_enable()
556	should be called for it too as soon as reasonably possible or its runtime PM
557	status should be changed back to 'suspended' with the help of
558	pm_runtime_set_suspended().
559	
560	If the default initial runtime PM status of the device (i.e. 'suspended')
561	reflects the actual state of the device, its bus type's or its driver's
562	->probe() callback will likely need to wake it up using one of the PM core's
563	helper functions described in Section 4.  In that case, pm_runtime_resume()
564	should be used.  Of course, for this purpose the device's runtime PM has to be
565	enabled earlier by calling pm_runtime_enable().
566	
567	Note, if the device may execute pm_runtime calls during the probe (such as
568	if it is registers with a subsystem that may call back in) then the
569	pm_runtime_get_sync() call paired with a pm_runtime_put() call will be
570	appropriate to ensure that the device is not put back to sleep during the
571	probe. This can happen with systems such as the network device layer.
572	
573	It may be desirable to suspend the device once ->probe() has finished.
574	Therefore the driver core uses the asynchronous pm_request_idle() to submit a
575	request to execute the subsystem-level idle callback for the device at that
576	time.  A driver that makes use of the runtime autosuspend feature, may want to
577	update the last busy mark before returning from ->probe().
578	
579	Moreover, the driver core prevents runtime PM callbacks from racing with the bus
580	notifier callback in __device_release_driver(), which is necessary, because the
581	notifier is used by some subsystems to carry out operations affecting the
582	runtime PM functionality.  It does so by calling pm_runtime_get_sync() before
583	driver_sysfs_remove() and the BUS_NOTIFY_UNBIND_DRIVER notifications.  This
584	resumes the device if it's in the suspended state and prevents it from
585	being suspended again while those routines are being executed.
586	
587	To allow bus types and drivers to put devices into the suspended state by
588	calling pm_runtime_suspend() from their ->remove() routines, the driver core
589	executes pm_runtime_put_sync() after running the BUS_NOTIFY_UNBIND_DRIVER
590	notifications in __device_release_driver().  This requires bus types and
591	drivers to make their ->remove() callbacks avoid races with runtime PM directly,
592	but also it allows of more flexibility in the handling of devices during the
593	removal of their drivers.
594	
595	Drivers in ->remove() callback should undo the runtime PM changes done
596	in ->probe(). Usually this means calling pm_runtime_disable(),
597	pm_runtime_dont_use_autosuspend() etc.
598	
599	The user space can effectively disallow the driver of the device to power manage
600	it at run time by changing the value of its /sys/devices/.../power/control
601	attribute to "on", which causes pm_runtime_forbid() to be called.  In principle,
602	this mechanism may also be used by the driver to effectively turn off the
603	runtime power management of the device until the user space turns it on.
604	Namely, during the initialization the driver can make sure that the runtime PM
605	status of the device is 'active' and call pm_runtime_forbid().  It should be
606	noted, however, that if the user space has already intentionally changed the
607	value of /sys/devices/.../power/control to "auto" to allow the driver to power
608	manage the device at run time, the driver may confuse it by using
609	pm_runtime_forbid() this way.
610	
611	6. Runtime PM and System Sleep
612	
613	Runtime PM and system sleep (i.e., system suspend and hibernation, also known
614	as suspend-to-RAM and suspend-to-disk) interact with each other in a couple of
615	ways.  If a device is active when a system sleep starts, everything is
616	straightforward.  But what should happen if the device is already suspended?
617	
618	The device may have different wake-up settings for runtime PM and system sleep.
619	For example, remote wake-up may be enabled for runtime suspend but disallowed
620	for system sleep (device_may_wakeup(dev) returns 'false').  When this happens,
621	the subsystem-level system suspend callback is responsible for changing the
622	device's wake-up setting (it may leave that to the device driver's system
623	suspend routine).  It may be necessary to resume the device and suspend it again
624	in order to do so.  The same is true if the driver uses different power levels
625	or other settings for runtime suspend and system sleep.
626	
627	During system resume, the simplest approach is to bring all devices back to full
628	power, even if they had been suspended before the system suspend began.  There
629	are several reasons for this, including:
630	
631	  * The device might need to switch power levels, wake-up settings, etc.
632	
633	  * Remote wake-up events might have been lost by the firmware.
634	
635	  * The device's children may need the device to be at full power in order
636	    to resume themselves.
637	
638	  * The driver's idea of the device state may not agree with the device's
639	    physical state.  This can happen during resume from hibernation.
640	
641	  * The device might need to be reset.
642	
643	  * Even though the device was suspended, if its usage counter was > 0 then most
644	    likely it would need a runtime resume in the near future anyway.
645	
646	If the device had been suspended before the system suspend began and it's
647	brought back to full power during resume, then its runtime PM status will have
648	to be updated to reflect the actual post-system sleep status.  The way to do
649	this is:
650	
651		pm_runtime_disable(dev);
652		pm_runtime_set_active(dev);
653		pm_runtime_enable(dev);
654	
655	The PM core always increments the runtime usage counter before calling the
656	->suspend() callback and decrements it after calling the ->resume() callback.
657	Hence disabling runtime PM temporarily like this will not cause any runtime
658	suspend attempts to be permanently lost.  If the usage count goes to zero
659	following the return of the ->resume() callback, the ->runtime_idle() callback
660	will be invoked as usual.
661	
662	On some systems, however, system sleep is not entered through a global firmware
663	or hardware operation.  Instead, all hardware components are put into low-power
664	states directly by the kernel in a coordinated way.  Then, the system sleep
665	state effectively follows from the states the hardware components end up in
666	and the system is woken up from that state by a hardware interrupt or a similar
667	mechanism entirely under the kernel's control.  As a result, the kernel never
668	gives control away and the states of all devices during resume are precisely
669	known to it.  If that is the case and none of the situations listed above takes
670	place (in particular, if the system is not waking up from hibernation), it may
671	be more efficient to leave the devices that had been suspended before the system
672	suspend began in the suspended state.
673	
674	To this end, the PM core provides a mechanism allowing some coordination between
675	different levels of device hierarchy.  Namely, if a system suspend .prepare()
676	callback returns a positive number for a device, that indicates to the PM core
677	that the device appears to be runtime-suspended and its state is fine, so it
678	may be left in runtime suspend provided that all of its descendants are also
679	left in runtime suspend.  If that happens, the PM core will not execute any
680	system suspend and resume callbacks for all of those devices, except for the
681	complete callback, which is then entirely responsible for handling the device
682	as appropriate.  This only applies to system suspend transitions that are not
683	related to hibernation (see Documentation/power/admin-guide/devices.rst for more
684	information).
685	
686	The PM core does its best to reduce the probability of race conditions between
687	the runtime PM and system suspend/resume (and hibernation) callbacks by carrying
688	out the following operations:
689	
690	  * During system suspend pm_runtime_get_noresume() is called for every device
691	    right before executing the subsystem-level .prepare() callback for it and
692	    pm_runtime_barrier() is called for every device right before executing the
693	    subsystem-level .suspend() callback for it.  In addition to that the PM core
694	    calls  __pm_runtime_disable() with 'false' as the second argument for every
695	    device right before executing the subsystem-level .suspend_late() callback
696	    for it.
697	
698	  * During system resume pm_runtime_enable() and pm_runtime_put() are called for
699	    every device right after executing the subsystem-level .resume_early()
700	    callback and right after executing the subsystem-level .complete() callback
701	    for it, respectively.
702	
703	7. Generic subsystem callbacks
704	
705	Subsystems may wish to conserve code space by using the set of generic power
706	management callbacks provided by the PM core, defined in
707	driver/base/power/generic_ops.c:
708	
709	  int pm_generic_runtime_suspend(struct device *dev);
710	    - invoke the ->runtime_suspend() callback provided by the driver of this
711	      device and return its result, or return 0 if not defined
712	
713	  int pm_generic_runtime_resume(struct device *dev);
714	    - invoke the ->runtime_resume() callback provided by the driver of this
715	      device and return its result, or return 0 if not defined
716	
717	  int pm_generic_suspend(struct device *dev);
718	    - if the device has not been suspended at run time, invoke the ->suspend()
719	      callback provided by its driver and return its result, or return 0 if not
720	      defined
721	
722	  int pm_generic_suspend_noirq(struct device *dev);
723	    - if pm_runtime_suspended(dev) returns "false", invoke the ->suspend_noirq()
724	      callback provided by the device's driver and return its result, or return
725	      0 if not defined
726	
727	  int pm_generic_resume(struct device *dev);
728	    - invoke the ->resume() callback provided by the driver of this device and,
729	      if successful, change the device's runtime PM status to 'active'
730	
731	  int pm_generic_resume_noirq(struct device *dev);
732	    - invoke the ->resume_noirq() callback provided by the driver of this device
733	
734	  int pm_generic_freeze(struct device *dev);
735	    - if the device has not been suspended at run time, invoke the ->freeze()
736	      callback provided by its driver and return its result, or return 0 if not
737	      defined
738	
739	  int pm_generic_freeze_noirq(struct device *dev);
740	    - if pm_runtime_suspended(dev) returns "false", invoke the ->freeze_noirq()
741	      callback provided by the device's driver and return its result, or return
742	      0 if not defined
743	
744	  int pm_generic_thaw(struct device *dev);
745	    - if the device has not been suspended at run time, invoke the ->thaw()
746	      callback provided by its driver and return its result, or return 0 if not
747	      defined
748	
749	  int pm_generic_thaw_noirq(struct device *dev);
750	    - if pm_runtime_suspended(dev) returns "false", invoke the ->thaw_noirq()
751	      callback provided by the device's driver and return its result, or return
752	      0 if not defined
753	
754	  int pm_generic_poweroff(struct device *dev);
755	    - if the device has not been suspended at run time, invoke the ->poweroff()
756	      callback provided by its driver and return its result, or return 0 if not
757	      defined
758	
759	  int pm_generic_poweroff_noirq(struct device *dev);
760	    - if pm_runtime_suspended(dev) returns "false", run the ->poweroff_noirq()
761	      callback provided by the device's driver and return its result, or return
762	      0 if not defined
763	
764	  int pm_generic_restore(struct device *dev);
765	    - invoke the ->restore() callback provided by the driver of this device and,
766	      if successful, change the device's runtime PM status to 'active'
767	
768	  int pm_generic_restore_noirq(struct device *dev);
769	    - invoke the ->restore_noirq() callback provided by the device's driver
770	
771	These functions are the defaults used by the PM core, if a subsystem doesn't
772	provide its own callbacks for ->runtime_idle(), ->runtime_suspend(),
773	->runtime_resume(), ->suspend(), ->suspend_noirq(), ->resume(),
774	->resume_noirq(), ->freeze(), ->freeze_noirq(), ->thaw(), ->thaw_noirq(),
775	->poweroff(), ->poweroff_noirq(), ->restore(), ->restore_noirq() in the
776	subsystem-level dev_pm_ops structure.
777	
778	Device drivers that wish to use the same function as a system suspend, freeze,
779	poweroff and runtime suspend callback, and similarly for system resume, thaw,
780	restore, and runtime resume, can achieve this with the help of the
781	UNIVERSAL_DEV_PM_OPS macro defined in include/linux/pm.h (possibly setting its
782	last argument to NULL).
783	
784	8. "No-Callback" Devices
785	
786	Some "devices" are only logical sub-devices of their parent and cannot be
787	power-managed on their own.  (The prototype example is a USB interface.  Entire
788	USB devices can go into low-power mode or send wake-up requests, but neither is
789	possible for individual interfaces.)  The drivers for these devices have no
790	need of runtime PM callbacks; if the callbacks did exist, ->runtime_suspend()
791	and ->runtime_resume() would always return 0 without doing anything else and
792	->runtime_idle() would always call pm_runtime_suspend().
793	
794	Subsystems can tell the PM core about these devices by calling
795	pm_runtime_no_callbacks().  This should be done after the device structure is
796	initialized and before it is registered (although after device registration is
797	also okay).  The routine will set the device's power.no_callbacks flag and
798	prevent the non-debugging runtime PM sysfs attributes from being created.
799	
800	When power.no_callbacks is set, the PM core will not invoke the
801	->runtime_idle(), ->runtime_suspend(), or ->runtime_resume() callbacks.
802	Instead it will assume that suspends and resumes always succeed and that idle
803	devices should be suspended.
804	
805	As a consequence, the PM core will never directly inform the device's subsystem
806	or driver about runtime power changes.  Instead, the driver for the device's
807	parent must take responsibility for telling the device's driver when the
808	parent's power state changes.
809	
810	9. Autosuspend, or automatically-delayed suspends
811	
812	Changing a device's power state isn't free; it requires both time and energy.
813	A device should be put in a low-power state only when there's some reason to
814	think it will remain in that state for a substantial time.  A common heuristic
815	says that a device which hasn't been used for a while is liable to remain
816	unused; following this advice, drivers should not allow devices to be suspended
817	at runtime until they have been inactive for some minimum period.  Even when
818	the heuristic ends up being non-optimal, it will still prevent devices from
819	"bouncing" too rapidly between low-power and full-power states.
820	
821	The term "autosuspend" is an historical remnant.  It doesn't mean that the
822	device is automatically suspended (the subsystem or driver still has to call
823	the appropriate PM routines); rather it means that runtime suspends will
824	automatically be delayed until the desired period of inactivity has elapsed.
825	
826	Inactivity is determined based on the power.last_busy field.  Drivers should
827	call pm_runtime_mark_last_busy() to update this field after carrying out I/O,
828	typically just before calling pm_runtime_put_autosuspend().  The desired length
829	of the inactivity period is a matter of policy.  Subsystems can set this length
830	initially by calling pm_runtime_set_autosuspend_delay(), but after device
831	registration the length should be controlled by user space, using the
832	/sys/devices/.../power/autosuspend_delay_ms attribute.
833	
834	In order to use autosuspend, subsystems or drivers must call
835	pm_runtime_use_autosuspend() (preferably before registering the device), and
836	thereafter they should use the various *_autosuspend() helper functions instead
837	of the non-autosuspend counterparts:
838	
839		Instead of: pm_runtime_suspend    use: pm_runtime_autosuspend;
840		Instead of: pm_schedule_suspend   use: pm_request_autosuspend;
841		Instead of: pm_runtime_put        use: pm_runtime_put_autosuspend;
842		Instead of: pm_runtime_put_sync   use: pm_runtime_put_sync_autosuspend.
843	
844	Drivers may also continue to use the non-autosuspend helper functions; they
845	will behave normally, which means sometimes taking the autosuspend delay into
846	account (see pm_runtime_idle).
847	
848	Under some circumstances a driver or subsystem may want to prevent a device
849	from autosuspending immediately, even though the usage counter is zero and the
850	autosuspend delay time has expired.  If the ->runtime_suspend() callback
851	returns -EAGAIN or -EBUSY, and if the next autosuspend delay expiration time is
852	in the future (as it normally would be if the callback invoked
853	pm_runtime_mark_last_busy()), the PM core will automatically reschedule the
854	autosuspend.  The ->runtime_suspend() callback can't do this rescheduling
855	itself because no suspend requests of any kind are accepted while the device is
856	suspending (i.e., while the callback is running).
857	
858	The implementation is well suited for asynchronous use in interrupt contexts.
859	However such use inevitably involves races, because the PM core can't
860	synchronize ->runtime_suspend() callbacks with the arrival of I/O requests.
861	This synchronization must be handled by the driver, using its private lock.
862	Here is a schematic pseudo-code example:
863	
864		foo_read_or_write(struct foo_priv *foo, void *data)
865		{
866			lock(&foo->private_lock);
867			add_request_to_io_queue(foo, data);
868			if (foo->num_pending_requests++ == 0)
869				pm_runtime_get(&foo->dev);
870			if (!foo->is_suspended)
871				foo_process_next_request(foo);
872			unlock(&foo->private_lock);
873		}
874	
875		foo_io_completion(struct foo_priv *foo, void *req)
876		{
877			lock(&foo->private_lock);
878			if (--foo->num_pending_requests == 0) {
879				pm_runtime_mark_last_busy(&foo->dev);
880				pm_runtime_put_autosuspend(&foo->dev);
881			} else {
882				foo_process_next_request(foo);
883			}
884			unlock(&foo->private_lock);
885			/* Send req result back to the user ... */
886		}
887	
888		int foo_runtime_suspend(struct device *dev)
889		{
890			struct foo_priv foo = container_of(dev, ...);
891			int ret = 0;
892	
893			lock(&foo->private_lock);
894			if (foo->num_pending_requests > 0) {
895				ret = -EBUSY;
896			} else {
897				/* ... suspend the device ... */
898				foo->is_suspended = 1;
899			}
900			unlock(&foo->private_lock);
901			return ret;
902		}
903	
904		int foo_runtime_resume(struct device *dev)
905		{
906			struct foo_priv foo = container_of(dev, ...);
907	
908			lock(&foo->private_lock);
909			/* ... resume the device ... */
910			foo->is_suspended = 0;
911			pm_runtime_mark_last_busy(&foo->dev);
912			if (foo->num_pending_requests > 0)
913				foo_process_next_request(foo);
914			unlock(&foo->private_lock);
915			return 0;
916		}
917	
918	The important point is that after foo_io_completion() asks for an autosuspend,
919	the foo_runtime_suspend() callback may race with foo_read_or_write().
920	Therefore foo_runtime_suspend() has to check whether there are any pending I/O
921	requests (while holding the private lock) before allowing the suspend to
922	proceed.
923	
924	In addition, the power.autosuspend_delay field can be changed by user space at
925	any time.  If a driver cares about this, it can call
926	pm_runtime_autosuspend_expiration() from within the ->runtime_suspend()
927	callback while holding its private lock.  If the function returns a nonzero
928	value then the delay has not yet expired and the callback should return
929	-EAGAIN.
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