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1 Device Power Management
2
3 Copyright (c) 2010-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
4 Copyright (c) 2010 Alan Stern <stern@rowland.harvard.edu>
5
6
7 Most of the code in Linux is device drivers, so most of the Linux power
8 management (PM) code is also driver-specific. Most drivers will do very
9 little; others, especially for platforms with small batteries (like cell
10 phones), will do a lot.
11
12 This writeup gives an overview of how drivers interact with system-wide
13 power management goals, emphasizing the models and interfaces that are
14 shared by everything that hooks up to the driver model core. Read it as
15 background for the domain-specific work you'd do with any specific driver.
16
17
18 Two Models for Device Power Management
19 ======================================
20 Drivers will use one or both of these models to put devices into low-power
21 states:
22
23 System Sleep model:
24 Drivers can enter low-power states as part of entering system-wide
25 low-power states like "suspend" (also known as "suspend-to-RAM"), or
26 (mostly for systems with disks) "hibernation" (also known as
27 "suspend-to-disk").
28
29 This is something that device, bus, and class drivers collaborate on
30 by implementing various role-specific suspend and resume methods to
31 cleanly power down hardware and software subsystems, then reactivate
32 them without loss of data.
33
34 Some drivers can manage hardware wakeup events, which make the system
35 leave the low-power state. This feature may be enabled or disabled
36 using the relevant /sys/devices/.../power/wakeup file (for Ethernet
37 drivers the ioctl interface used by ethtool may also be used for this
38 purpose); enabling it may cost some power usage, but let the whole
39 system enter low-power states more often.
40
41 Runtime Power Management model:
42 Devices may also be put into low-power states while the system is
43 running, independently of other power management activity in principle.
44 However, devices are not generally independent of each other (for
45 example, a parent device cannot be suspended unless all of its child
46 devices have been suspended). Moreover, depending on the bus type the
47 device is on, it may be necessary to carry out some bus-specific
48 operations on the device for this purpose. Devices put into low power
49 states at run time may require special handling during system-wide power
50 transitions (suspend or hibernation).
51
52 For these reasons not only the device driver itself, but also the
53 appropriate subsystem (bus type, device type or device class) driver and
54 the PM core are involved in runtime power management. As in the system
55 sleep power management case, they need to collaborate by implementing
56 various role-specific suspend and resume methods, so that the hardware
57 is cleanly powered down and reactivated without data or service loss.
58
59 There's not a lot to be said about those low-power states except that they are
60 very system-specific, and often device-specific. Also, that if enough devices
61 have been put into low-power states (at runtime), the effect may be very similar
62 to entering some system-wide low-power state (system sleep) ... and that
63 synergies exist, so that several drivers using runtime PM might put the system
64 into a state where even deeper power saving options are available.
65
66 Most suspended devices will have quiesced all I/O: no more DMA or IRQs (except
67 for wakeup events), no more data read or written, and requests from upstream
68 drivers are no longer accepted. A given bus or platform may have different
69 requirements though.
70
71 Examples of hardware wakeup events include an alarm from a real time clock,
72 network wake-on-LAN packets, keyboard or mouse activity, and media insertion
73 or removal (for PCMCIA, MMC/SD, USB, and so on).
74
75
76 Interfaces for Entering System Sleep States
77 ===========================================
78 There are programming interfaces provided for subsystems (bus type, device type,
79 device class) and device drivers to allow them to participate in the power
80 management of devices they are concerned with. These interfaces cover both
81 system sleep and runtime power management.
82
83
84 Device Power Management Operations
85 ----------------------------------
86 Device power management operations, at the subsystem level as well as at the
87 device driver level, are implemented by defining and populating objects of type
88 struct dev_pm_ops:
89
90 struct dev_pm_ops {
91 int (*prepare)(struct device *dev);
92 void (*complete)(struct device *dev);
93 int (*suspend)(struct device *dev);
94 int (*resume)(struct device *dev);
95 int (*freeze)(struct device *dev);
96 int (*thaw)(struct device *dev);
97 int (*poweroff)(struct device *dev);
98 int (*restore)(struct device *dev);
99 int (*suspend_late)(struct device *dev);
100 int (*resume_early)(struct device *dev);
101 int (*freeze_late)(struct device *dev);
102 int (*thaw_early)(struct device *dev);
103 int (*poweroff_late)(struct device *dev);
104 int (*restore_early)(struct device *dev);
105 int (*suspend_noirq)(struct device *dev);
106 int (*resume_noirq)(struct device *dev);
107 int (*freeze_noirq)(struct device *dev);
108 int (*thaw_noirq)(struct device *dev);
109 int (*poweroff_noirq)(struct device *dev);
110 int (*restore_noirq)(struct device *dev);
111 int (*runtime_suspend)(struct device *dev);
112 int (*runtime_resume)(struct device *dev);
113 int (*runtime_idle)(struct device *dev);
114 };
115
116 This structure is defined in include/linux/pm.h and the methods included in it
117 are also described in that file. Their roles will be explained in what follows.
118 For now, it should be sufficient to remember that the last three methods are
119 specific to runtime power management while the remaining ones are used during
120 system-wide power transitions.
121
122 There also is a deprecated "old" or "legacy" interface for power management
123 operations available at least for some subsystems. This approach does not use
124 struct dev_pm_ops objects and it is suitable only for implementing system sleep
125 power management methods. Therefore it is not described in this document, so
126 please refer directly to the source code for more information about it.
127
128
129 Subsystem-Level Methods
130 -----------------------
131 The core methods to suspend and resume devices reside in struct dev_pm_ops
132 pointed to by the ops member of struct dev_pm_domain, or by the pm member of
133 struct bus_type, struct device_type and struct class. They are mostly of
134 interest to the people writing infrastructure for platforms and buses, like PCI
135 or USB, or device type and device class drivers. They also are relevant to the
136 writers of device drivers whose subsystems (PM domains, device types, device
137 classes and bus types) don't provide all power management methods.
138
139 Bus drivers implement these methods as appropriate for the hardware and the
140 drivers using it; PCI works differently from USB, and so on. Not many people
141 write subsystem-level drivers; most driver code is a "device driver" that builds
142 on top of bus-specific framework code.
143
144 For more information on these driver calls, see the description later;
145 they are called in phases for every device, respecting the parent-child
146 sequencing in the driver model tree.
147
148
149 /sys/devices/.../power/wakeup files
150 -----------------------------------
151 All device objects in the driver model contain fields that control the handling
152 of system wakeup events (hardware signals that can force the system out of a
153 sleep state). These fields are initialized by bus or device driver code using
154 device_set_wakeup_capable() and device_set_wakeup_enable(), defined in
155 include/linux/pm_wakeup.h.
156
157 The "power.can_wakeup" flag just records whether the device (and its driver) can
158 physically support wakeup events. The device_set_wakeup_capable() routine
159 affects this flag. The "power.wakeup" field is a pointer to an object of type
160 struct wakeup_source used for controlling whether or not the device should use
161 its system wakeup mechanism and for notifying the PM core of system wakeup
162 events signaled by the device. This object is only present for wakeup-capable
163 devices (i.e. devices whose "can_wakeup" flags are set) and is created (or
164 removed) by device_set_wakeup_capable().
165
166 Whether or not a device is capable of issuing wakeup events is a hardware
167 matter, and the kernel is responsible for keeping track of it. By contrast,
168 whether or not a wakeup-capable device should issue wakeup events is a policy
169 decision, and it is managed by user space through a sysfs attribute: the
170 "power/wakeup" file. User space can write the strings "enabled" or "disabled"
171 to it to indicate whether or not, respectively, the device is supposed to signal
172 system wakeup. This file is only present if the "power.wakeup" object exists
173 for the given device and is created (or removed) along with that object, by
174 device_set_wakeup_capable(). Reads from the file will return the corresponding
175 string.
176
177 The "power/wakeup" file is supposed to contain the "disabled" string initially
178 for the majority of devices; the major exceptions are power buttons, keyboards,
179 and Ethernet adapters whose WoL (wake-on-LAN) feature has been set up with
180 ethtool. It should also default to "enabled" for devices that don't generate
181 wakeup requests on their own but merely forward wakeup requests from one bus to
182 another (like PCI Express ports).
183
184 The device_may_wakeup() routine returns true only if the "power.wakeup" object
185 exists and the corresponding "power/wakeup" file contains the string "enabled".
186 This information is used by subsystems, like the PCI bus type code, to see
187 whether or not to enable the devices' wakeup mechanisms. If device wakeup
188 mechanisms are enabled or disabled directly by drivers, they also should use
189 device_may_wakeup() to decide what to do during a system sleep transition.
190 Device drivers, however, are not supposed to call device_set_wakeup_enable()
191 directly in any case.
192
193 It ought to be noted that system wakeup is conceptually different from "remote
194 wakeup" used by runtime power management, although it may be supported by the
195 same physical mechanism. Remote wakeup is a feature allowing devices in
196 low-power states to trigger specific interrupts to signal conditions in which
197 they should be put into the full-power state. Those interrupts may or may not
198 be used to signal system wakeup events, depending on the hardware design. On
199 some systems it is impossible to trigger them from system sleep states. In any
200 case, remote wakeup should always be enabled for runtime power management for
201 all devices and drivers that support it.
202
203 /sys/devices/.../power/control files
204 ------------------------------------
205 Each device in the driver model has a flag to control whether it is subject to
206 runtime power management. This flag, called runtime_auto, is initialized by the
207 bus type (or generally subsystem) code using pm_runtime_allow() or
208 pm_runtime_forbid(); the default is to allow runtime power management.
209
210 The setting can be adjusted by user space by writing either "on" or "auto" to
211 the device's power/control sysfs file. Writing "auto" calls pm_runtime_allow(),
212 setting the flag and allowing the device to be runtime power-managed by its
213 driver. Writing "on" calls pm_runtime_forbid(), clearing the flag, returning
214 the device to full power if it was in a low-power state, and preventing the
215 device from being runtime power-managed. User space can check the current value
216 of the runtime_auto flag by reading the file.
217
218 The device's runtime_auto flag has no effect on the handling of system-wide
219 power transitions. In particular, the device can (and in the majority of cases
220 should and will) be put into a low-power state during a system-wide transition
221 to a sleep state even though its runtime_auto flag is clear.
222
223 For more information about the runtime power management framework, refer to
224 Documentation/power/runtime_pm.txt.
225
226
227 Calling Drivers to Enter and Leave System Sleep States
228 ======================================================
229 When the system goes into a sleep state, each device's driver is asked to
230 suspend the device by putting it into a state compatible with the target
231 system state. That's usually some version of "off", but the details are
232 system-specific. Also, wakeup-enabled devices will usually stay partly
233 functional in order to wake the system.
234
235 When the system leaves that low-power state, the device's driver is asked to
236 resume it by returning it to full power. The suspend and resume operations
237 always go together, and both are multi-phase operations.
238
239 For simple drivers, suspend might quiesce the device using class code
240 and then turn its hardware as "off" as possible during suspend_noirq. The
241 matching resume calls would then completely reinitialize the hardware
242 before reactivating its class I/O queues.
243
244 More power-aware drivers might prepare the devices for triggering system wakeup
245 events.
246
247
248 Call Sequence Guarantees
249 ------------------------
250 To ensure that bridges and similar links needing to talk to a device are
251 available when the device is suspended or resumed, the device tree is
252 walked in a bottom-up order to suspend devices. A top-down order is
253 used to resume those devices.
254
255 The ordering of the device tree is defined by the order in which devices
256 get registered: a child can never be registered, probed or resumed before
257 its parent; and can't be removed or suspended after that parent.
258
259 The policy is that the device tree should match hardware bus topology.
260 (Or at least the control bus, for devices which use multiple busses.)
261 In particular, this means that a device registration may fail if the parent of
262 the device is suspending (i.e. has been chosen by the PM core as the next
263 device to suspend) or has already suspended, as well as after all of the other
264 devices have been suspended. Device drivers must be prepared to cope with such
265 situations.
266
267
268 System Power Management Phases
269 ------------------------------
270 Suspending or resuming the system is done in several phases. Different phases
271 are used for standby or memory sleep states ("suspend-to-RAM") and the
272 hibernation state ("suspend-to-disk"). Each phase involves executing callbacks
273 for every device before the next phase begins. Not all busses or classes
274 support all these callbacks and not all drivers use all the callbacks. The
275 various phases always run after tasks have been frozen and before they are
276 unfrozen. Furthermore, the *_noirq phases run at a time when IRQ handlers have
277 been disabled (except for those marked with the IRQF_NO_SUSPEND flag).
278
279 All phases use PM domain, bus, type, class or driver callbacks (that is, methods
280 defined in dev->pm_domain->ops, dev->bus->pm, dev->type->pm, dev->class->pm or
281 dev->driver->pm). These callbacks are regarded by the PM core as mutually
282 exclusive. Moreover, PM domain callbacks always take precedence over all of the
283 other callbacks and, for example, type callbacks take precedence over bus, class
284 and driver callbacks. To be precise, the following rules are used to determine
285 which callback to execute in the given phase:
286
287 1. If dev->pm_domain is present, the PM core will choose the callback
288 included in dev->pm_domain->ops for execution
289
290 2. Otherwise, if both dev->type and dev->type->pm are present, the callback
291 included in dev->type->pm will be chosen for execution.
292
293 3. Otherwise, if both dev->class and dev->class->pm are present, the
294 callback included in dev->class->pm will be chosen for execution.
295
296 4. Otherwise, if both dev->bus and dev->bus->pm are present, the callback
297 included in dev->bus->pm will be chosen for execution.
298
299 This allows PM domains and device types to override callbacks provided by bus
300 types or device classes if necessary.
301
302 The PM domain, type, class and bus callbacks may in turn invoke device- or
303 driver-specific methods stored in dev->driver->pm, but they don't have to do
304 that.
305
306 If the subsystem callback chosen for execution is not present, the PM core will
307 execute the corresponding method from dev->driver->pm instead if there is one.
308
309
310 Entering System Suspend
311 -----------------------
312 When the system goes into the standby or memory sleep state, the phases are:
313
314 prepare, suspend, suspend_late, suspend_noirq.
315
316 1. The prepare phase is meant to prevent races by preventing new devices
317 from being registered; the PM core would never know that all the
318 children of a device had been suspended if new children could be
319 registered at will. (By contrast, devices may be unregistered at any
320 time.) Unlike the other suspend-related phases, during the prepare
321 phase the device tree is traversed top-down.
322
323 After the prepare callback method returns, no new children may be
324 registered below the device. The method may also prepare the device or
325 driver in some way for the upcoming system power transition, but it
326 should not put the device into a low-power state.
327
328 2. The suspend methods should quiesce the device to stop it from performing
329 I/O. They also may save the device registers and put it into the
330 appropriate low-power state, depending on the bus type the device is on,
331 and they may enable wakeup events.
332
333 3 For a number of devices it is convenient to split suspend into the
334 "quiesce device" and "save device state" phases, in which cases
335 suspend_late is meant to do the latter. It is always executed after
336 runtime power management has been disabled for all devices.
337
338 4. The suspend_noirq phase occurs after IRQ handlers have been disabled,
339 which means that the driver's interrupt handler will not be called while
340 the callback method is running. The methods should save the values of
341 the device's registers that weren't saved previously and finally put the
342 device into the appropriate low-power state.
343
344 The majority of subsystems and device drivers need not implement this
345 callback. However, bus types allowing devices to share interrupt
346 vectors, like PCI, generally need it; otherwise a driver might encounter
347 an error during the suspend phase by fielding a shared interrupt
348 generated by some other device after its own device had been set to low
349 power.
350
351 At the end of these phases, drivers should have stopped all I/O transactions
352 (DMA, IRQs), saved enough state that they can re-initialize or restore previous
353 state (as needed by the hardware), and placed the device into a low-power state.
354 On many platforms they will gate off one or more clock sources; sometimes they
355 will also switch off power supplies or reduce voltages. (Drivers supporting
356 runtime PM may already have performed some or all of these steps.)
357
358 If device_may_wakeup(dev) returns true, the device should be prepared for
359 generating hardware wakeup signals to trigger a system wakeup event when the
360 system is in the sleep state. For example, enable_irq_wake() might identify
361 GPIO signals hooked up to a switch or other external hardware, and
362 pci_enable_wake() does something similar for the PCI PME signal.
363
364 If any of these callbacks returns an error, the system won't enter the desired
365 low-power state. Instead the PM core will unwind its actions by resuming all
366 the devices that were suspended.
367
368
369 Leaving System Suspend
370 ----------------------
371 When resuming from standby or memory sleep, the phases are:
372
373 resume_noirq, resume_early, resume, complete.
374
375 1. The resume_noirq callback methods should perform any actions needed
376 before the driver's interrupt handlers are invoked. This generally
377 means undoing the actions of the suspend_noirq phase. If the bus type
378 permits devices to share interrupt vectors, like PCI, the method should
379 bring the device and its driver into a state in which the driver can
380 recognize if the device is the source of incoming interrupts, if any,
381 and handle them correctly.
382
383 For example, the PCI bus type's ->pm.resume_noirq() puts the device into
384 the full-power state (D0 in the PCI terminology) and restores the
385 standard configuration registers of the device. Then it calls the
386 device driver's ->pm.resume_noirq() method to perform device-specific
387 actions.
388
389 2. The resume_early methods should prepare devices for the execution of
390 the resume methods. This generally involves undoing the actions of the
391 preceding suspend_late phase.
392
393 3 The resume methods should bring the the device back to its operating
394 state, so that it can perform normal I/O. This generally involves
395 undoing the actions of the suspend phase.
396
397 4. The complete phase should undo the actions of the prepare phase. Note,
398 however, that new children may be registered below the device as soon as
399 the resume callbacks occur; it's not necessary to wait until the
400 complete phase.
401
402 At the end of these phases, drivers should be as functional as they were before
403 suspending: I/O can be performed using DMA and IRQs, and the relevant clocks are
404 gated on. Even if the device was in a low-power state before the system sleep
405 because of runtime power management, afterwards it should be back in its
406 full-power state. There are multiple reasons why it's best to do this; they are
407 discussed in more detail in Documentation/power/runtime_pm.txt.
408
409 However, the details here may again be platform-specific. For example,
410 some systems support multiple "run" states, and the mode in effect at
411 the end of resume might not be the one which preceded suspension.
412 That means availability of certain clocks or power supplies changed,
413 which could easily affect how a driver works.
414
415 Drivers need to be able to handle hardware which has been reset since the
416 suspend methods were called, for example by complete reinitialization.
417 This may be the hardest part, and the one most protected by NDA'd documents
418 and chip errata. It's simplest if the hardware state hasn't changed since
419 the suspend was carried out, but that can't be guaranteed (in fact, it usually
420 is not the case).
421
422 Drivers must also be prepared to notice that the device has been removed
423 while the system was powered down, whenever that's physically possible.
424 PCMCIA, MMC, USB, Firewire, SCSI, and even IDE are common examples of busses
425 where common Linux platforms will see such removal. Details of how drivers
426 will notice and handle such removals are currently bus-specific, and often
427 involve a separate thread.
428
429 These callbacks may return an error value, but the PM core will ignore such
430 errors since there's nothing it can do about them other than printing them in
431 the system log.
432
433
434 Entering Hibernation
435 --------------------
436 Hibernating the system is more complicated than putting it into the standby or
437 memory sleep state, because it involves creating and saving a system image.
438 Therefore there are more phases for hibernation, with a different set of
439 callbacks. These phases always run after tasks have been frozen and memory has
440 been freed.
441
442 The general procedure for hibernation is to quiesce all devices (freeze), create
443 an image of the system memory while everything is stable, reactivate all
444 devices (thaw), write the image to permanent storage, and finally shut down the
445 system (poweroff). The phases used to accomplish this are:
446
447 prepare, freeze, freeze_late, freeze_noirq, thaw_noirq, thaw_early,
448 thaw, complete, prepare, poweroff, poweroff_late, poweroff_noirq
449
450 1. The prepare phase is discussed in the "Entering System Suspend" section
451 above.
452
453 2. The freeze methods should quiesce the device so that it doesn't generate
454 IRQs or DMA, and they may need to save the values of device registers.
455 However the device does not have to be put in a low-power state, and to
456 save time it's best not to do so. Also, the device should not be
457 prepared to generate wakeup events.
458
459 3. The freeze_late phase is analogous to the suspend_late phase described
460 above, except that the device should not be put in a low-power state and
461 should not be allowed to generate wakeup events by it.
462
463 4. The freeze_noirq phase is analogous to the suspend_noirq phase discussed
464 above, except again that the device should not be put in a low-power
465 state and should not be allowed to generate wakeup events.
466
467 At this point the system image is created. All devices should be inactive and
468 the contents of memory should remain undisturbed while this happens, so that the
469 image forms an atomic snapshot of the system state.
470
471 5. The thaw_noirq phase is analogous to the resume_noirq phase discussed
472 above. The main difference is that its methods can assume the device is
473 in the same state as at the end of the freeze_noirq phase.
474
475 6. The thaw_early phase is analogous to the resume_early phase described
476 above. Its methods should undo the actions of the preceding
477 freeze_late, if necessary.
478
479 7. The thaw phase is analogous to the resume phase discussed above. Its
480 methods should bring the device back to an operating state, so that it
481 can be used for saving the image if necessary.
482
483 8. The complete phase is discussed in the "Leaving System Suspend" section
484 above.
485
486 At this point the system image is saved, and the devices then need to be
487 prepared for the upcoming system shutdown. This is much like suspending them
488 before putting the system into the standby or memory sleep state, and the phases
489 are similar.
490
491 9. The prepare phase is discussed above.
492
493 10. The poweroff phase is analogous to the suspend phase.
494
495 11. The poweroff_late phase is analogous to the suspend_late phase.
496
497 12. The poweroff_noirq phase is analogous to the suspend_noirq phase.
498
499 The poweroff, poweroff_late and poweroff_noirq callbacks should do essentially
500 the same things as the suspend, suspend_late and suspend_noirq callbacks,
501 respectively. The only notable difference is that they need not store the
502 device register values, because the registers should already have been stored
503 during the freeze, freeze_late or freeze_noirq phases.
504
505
506 Leaving Hibernation
507 -------------------
508 Resuming from hibernation is, again, more complicated than resuming from a sleep
509 state in which the contents of main memory are preserved, because it requires
510 a system image to be loaded into memory and the pre-hibernation memory contents
511 to be restored before control can be passed back to the image kernel.
512
513 Although in principle, the image might be loaded into memory and the
514 pre-hibernation memory contents restored by the boot loader, in practice this
515 can't be done because boot loaders aren't smart enough and there is no
516 established protocol for passing the necessary information. So instead, the
517 boot loader loads a fresh instance of the kernel, called the boot kernel, into
518 memory and passes control to it in the usual way. Then the boot kernel reads
519 the system image, restores the pre-hibernation memory contents, and passes
520 control to the image kernel. Thus two different kernels are involved in
521 resuming from hibernation. In fact, the boot kernel may be completely different
522 from the image kernel: a different configuration and even a different version.
523 This has important consequences for device drivers and their subsystems.
524
525 To be able to load the system image into memory, the boot kernel needs to
526 include at least a subset of device drivers allowing it to access the storage
527 medium containing the image, although it doesn't need to include all of the
528 drivers present in the image kernel. After the image has been loaded, the
529 devices managed by the boot kernel need to be prepared for passing control back
530 to the image kernel. This is very similar to the initial steps involved in
531 creating a system image, and it is accomplished in the same way, using prepare,
532 freeze, and freeze_noirq phases. However the devices affected by these phases
533 are only those having drivers in the boot kernel; other devices will still be in
534 whatever state the boot loader left them.
535
536 Should the restoration of the pre-hibernation memory contents fail, the boot
537 kernel would go through the "thawing" procedure described above, using the
538 thaw_noirq, thaw, and complete phases, and then continue running normally. This
539 happens only rarely. Most often the pre-hibernation memory contents are
540 restored successfully and control is passed to the image kernel, which then
541 becomes responsible for bringing the system back to the working state.
542
543 To achieve this, the image kernel must restore the devices' pre-hibernation
544 functionality. The operation is much like waking up from the memory sleep
545 state, although it involves different phases:
546
547 restore_noirq, restore_early, restore, complete
548
549 1. The restore_noirq phase is analogous to the resume_noirq phase.
550
551 2. The restore_early phase is analogous to the resume_early phase.
552
553 3. The restore phase is analogous to the resume phase.
554
555 4. The complete phase is discussed above.
556
557 The main difference from resume[_early|_noirq] is that restore[_early|_noirq]
558 must assume the device has been accessed and reconfigured by the boot loader or
559 the boot kernel. Consequently the state of the device may be different from the
560 state remembered from the freeze, freeze_late and freeze_noirq phases. The
561 device may even need to be reset and completely re-initialized. In many cases
562 this difference doesn't matter, so the resume[_early|_noirq] and
563 restore[_early|_norq] method pointers can be set to the same routines.
564 Nevertheless, different callback pointers are used in case there is a situation
565 where it actually does matter.
566
567
568 Device Power Management Domains
569 -------------------------------
570 Sometimes devices share reference clocks or other power resources. In those
571 cases it generally is not possible to put devices into low-power states
572 individually. Instead, a set of devices sharing a power resource can be put
573 into a low-power state together at the same time by turning off the shared
574 power resource. Of course, they also need to be put into the full-power state
575 together, by turning the shared power resource on. A set of devices with this
576 property is often referred to as a power domain.
577
578 Support for power domains is provided through the pm_domain field of struct
579 device. This field is a pointer to an object of type struct dev_pm_domain,
580 defined in include/linux/pm.h, providing a set of power management callbacks
581 analogous to the subsystem-level and device driver callbacks that are executed
582 for the given device during all power transitions, instead of the respective
583 subsystem-level callbacks. Specifically, if a device's pm_domain pointer is
584 not NULL, the ->suspend() callback from the object pointed to by it will be
585 executed instead of its subsystem's (e.g. bus type's) ->suspend() callback and
586 analogously for all of the remaining callbacks. In other words, power
587 management domain callbacks, if defined for the given device, always take
588 precedence over the callbacks provided by the device's subsystem (e.g. bus
589 type).
590
591 The support for device power management domains is only relevant to platforms
592 needing to use the same device driver power management callbacks in many
593 different power domain configurations and wanting to avoid incorporating the
594 support for power domains into subsystem-level callbacks, for example by
595 modifying the platform bus type. Other platforms need not implement it or take
596 it into account in any way.
597
598
599 Device Low Power (suspend) States
600 ---------------------------------
601 Device low-power states aren't standard. One device might only handle
602 "on" and "off", while another might support a dozen different versions of
603 "on" (how many engines are active?), plus a state that gets back to "on"
604 faster than from a full "off".
605
606 Some busses define rules about what different suspend states mean. PCI
607 gives one example: after the suspend sequence completes, a non-legacy
608 PCI device may not perform DMA or issue IRQs, and any wakeup events it
609 issues would be issued through the PME# bus signal. Plus, there are
610 several PCI-standard device states, some of which are optional.
611
612 In contrast, integrated system-on-chip processors often use IRQs as the
613 wakeup event sources (so drivers would call enable_irq_wake) and might
614 be able to treat DMA completion as a wakeup event (sometimes DMA can stay
615 active too, it'd only be the CPU and some peripherals that sleep).
616
617 Some details here may be platform-specific. Systems may have devices that
618 can be fully active in certain sleep states, such as an LCD display that's
619 refreshed using DMA while most of the system is sleeping lightly ... and
620 its frame buffer might even be updated by a DSP or other non-Linux CPU while
621 the Linux control processor stays idle.
622
623 Moreover, the specific actions taken may depend on the target system state.
624 One target system state might allow a given device to be very operational;
625 another might require a hard shut down with re-initialization on resume.
626 And two different target systems might use the same device in different
627 ways; the aforementioned LCD might be active in one product's "standby",
628 but a different product using the same SOC might work differently.
629
630
631 Power Management Notifiers
632 --------------------------
633 There are some operations that cannot be carried out by the power management
634 callbacks discussed above, because the callbacks occur too late or too early.
635 To handle these cases, subsystems and device drivers may register power
636 management notifiers that are called before tasks are frozen and after they have
637 been thawed. Generally speaking, the PM notifiers are suitable for performing
638 actions that either require user space to be available, or at least won't
639 interfere with user space.
640
641 For details refer to Documentation/power/notifiers.txt.
642
643
644 Runtime Power Management
645 ========================
646 Many devices are able to dynamically power down while the system is still
647 running. This feature is useful for devices that are not being used, and
648 can offer significant power savings on a running system. These devices
649 often support a range of runtime power states, which might use names such
650 as "off", "sleep", "idle", "active", and so on. Those states will in some
651 cases (like PCI) be partially constrained by the bus the device uses, and will
652 usually include hardware states that are also used in system sleep states.
653
654 A system-wide power transition can be started while some devices are in low
655 power states due to runtime power management. The system sleep PM callbacks
656 should recognize such situations and react to them appropriately, but the
657 necessary actions are subsystem-specific.
658
659 In some cases the decision may be made at the subsystem level while in other
660 cases the device driver may be left to decide. In some cases it may be
661 desirable to leave a suspended device in that state during a system-wide power
662 transition, but in other cases the device must be put back into the full-power
663 state temporarily, for example so that its system wakeup capability can be
664 disabled. This all depends on the hardware and the design of the subsystem and
665 device driver in question.
666
667 During system-wide resume from a sleep state it's easiest to put devices into
668 the full-power state, as explained in Documentation/power/runtime_pm.txt. Refer
669 to that document for more information regarding this particular issue as well as
670 for information on the device runtime power management framework in general.