Root/Documentation/power/devices.txt

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

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