Lines Matching +full:low +full:- +full:power
1 .. SPDX-License-Identifier: GPL-2.0
7 Device Power Management Basics
10 :Copyright: |copy| 2010-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
17 Most of the code in Linux is device drivers, so most of the Linux power
18 management (PM) code is also driver-specific. Most drivers will do very
22 This writeup gives an overview of how drivers interact with system-wide
23 power management goals, emphasizing the models and interfaces that are
25 background for the domain-specific work you'd do with any specific driver.
28 Two Models for Device Power Management
31 Drivers will use one or both of these models to put devices into low-power
36 Drivers can enter low-power states as part of entering system-wide
37 low-power states like "suspend" (also known as "suspend-to-RAM"), or
39 "suspend-to-disk").
42 by implementing various role-specific suspend and resume methods to
43 cleanly power down hardware and software subsystems, then reactivate
47 leave the low-power state. This feature may be enabled or disabled
48 using the relevant :file:`/sys/devices/.../power/wakeup` file (for
50 for this purpose); enabling it may cost some power usage, but let the
51 whole system enter low-power states more often.
53 Runtime Power Management model:
55 Devices may also be put into low-power states while the system is
56 running, independently of other power management activity in principle.
60 device is on, it may be necessary to carry out some bus-specific
61 operations on the device for this purpose. Devices put into low power
62 states at run time may require special handling during system-wide power
67 the PM core are involved in runtime power management. As in the system
68 sleep power management case, they need to collaborate by implementing
69 various role-specific suspend and resume methods, so that the hardware
72 There's not a lot to be said about those low-power states except that they are
73 very system-specific, and often device-specific. Also, that if enough devices
74 have been put into low-power states (at runtime), the effect may be very similar
75 to entering some system-wide low-power state (system sleep) ... and that
77 into a state where even deeper power saving options are available.
85 network wake-on-LAN packets, keyboard or mouse activity, and media insertion
92 device class) and device drivers to allow them to participate in the power
94 system sleep and runtime power management.
97 Device Power Management Operations
98 ----------------------------------
100 Device power management operations, at the subsystem level as well as at the
104 sufficient to remember that the last three methods are specific to runtime power
105 management while the remaining ones are used during system-wide power
108 There also is a deprecated "old" or "legacy" interface for power management
111 sleep power management methods in a limited way. Therefore it is not described
116 Subsystem-Level Methods
117 -----------------------
126 bus types) don't provide all power management methods.
130 write subsystem-level drivers; most driver code is a "device driver" that builds
131 on top of bus-specific framework code.
134 they are called in phases for every device, respecting the parent-child
138 :file:`/sys/devices/.../power/wakeup` files
139 -------------------------------------------
147 The :c:member:`power.can_wakeup` flag just records whether the device (and its
150 :c:member:`power.wakeup` field is a pointer to an object of type
153 events signaled by the device. This object is only present for wakeup-capable
159 whether or not a wakeup-capable device should issue wakeup events is a policy
161 :file:`power/wakeup` file. User space can write the "enabled" or "disabled"
164 :c:member:`power.wakeup` object exists for the given device and is created (or
168 The initial value in the :file:`power/wakeup` file is "disabled" for the
169 majority of devices; the major exceptions are power buttons, keyboards, and
170 Ethernet adapters whose WoL (wake-on-LAN) feature has been set up with ethtool.
176 :c:member:`power.wakeup` object exists and the corresponding :file:`power/wakeup`
185 wakeup" used by runtime power management, although it may be supported by the
187 low-power states to trigger specific interrupts to signal conditions in which
188 they should be put into the full-power state. Those interrupts may or may not
191 case, remote wakeup should always be enabled for runtime power management for
195 :file:`/sys/devices/.../power/control` files
196 --------------------------------------------
199 runtime power management. This flag, :c:member:`runtime_auto`, is initialized
201 or :c:func:`pm_runtime_forbid()`; the default is to allow runtime power
205 the device's :file:`power/control` sysfs file. Writing "auto" calls
207 runtime power-managed by its driver. Writing "on" calls
209 power if it was in a low-power state, and preventing the
210 device from being runtime power-managed. User space can check the current value
214 system-wide power transitions. In particular, the device can (and in the
215 majority of cases should and will) be put into a low-power state during a
216 system-wide transition to a sleep state even though its :c:member:`runtime_auto`
219 For more information about the runtime power management framework, refer to
220 Documentation/power/runtime_pm.rst.
229 system-specific. Also, wakeup-enabled devices will usually stay partly
232 When the system leaves that low-power state, the device's driver is asked to
233 resume it by returning it to full power. The suspend and resume operations
234 always go together, and both are multi-phase operations.
241 More power-aware drivers might prepare the devices for triggering system wakeup
246 ------------------------
250 walked in a bottom-up order to suspend devices. A top-down order is
266 System Power Management Phases
267 ------------------------------
270 are used for suspend-to-idle, shallow (standby), and deep ("suspend-to-RAM")
271 sleep states and the hibernation state ("suspend-to-disk"). Each phase involves
280 defined in ``dev->pm_domain->ops``, ``dev->bus->pm``, ``dev->type->pm``,
281 ``dev->class->pm`` or ``dev->driver->pm``). These callbacks are regarded by the
287 1. If ``dev->pm_domain`` is present, the PM core will choose the callback
288 provided by ``dev->pm_domain->ops`` for execution.
290 2. Otherwise, if both ``dev->type`` and ``dev->type->pm`` are present, the
291 callback provided by ``dev->type->pm`` will be chosen for execution.
293 3. Otherwise, if both ``dev->class`` and ``dev->class->pm`` are present,
294 the callback provided by ``dev->class->pm`` will be chosen for
297 4. Otherwise, if both ``dev->bus`` and ``dev->bus->pm`` are present, the
298 callback provided by ``dev->bus->pm`` will be chosen for execution.
303 The PM domain, type, class and bus callbacks may in turn invoke device- or
304 driver-specific methods stored in ``dev->driver->pm``, but they don't have to do
308 execute the corresponding method from the ``dev->driver->pm`` set instead if
313 -----------------------
323 suspend-related phases, during the ``prepare`` phase the device
324 hierarchy is traversed top-down.
326 After the ``->prepare`` callback method returns, no new children may be
328 driver in some way for the upcoming system power transition, but it
329 should not put the device into a low-power state. Moreover, if the
330 device supports runtime power management, the ``->prepare`` callback
334 For devices supporting runtime power management, the return value of the
336 safely leave the device in runtime suspend (if runtime-suspended
340 and all of them (including the device itself) are runtime-suspended, the
344 the ``->complete`` callback will be the next one invoked after the
345 ``->prepare`` callback and is entirely responsible for putting the
348 Note that this direct-complete procedure applies even if the device is
349 disabled for runtime PM; only the runtime-PM status matters. It follows
350 that if a device has system-sleep callbacks but does not support runtime
352 is because all such devices are initially set to runtime-suspended with
357 power management flags. [Typically, they are set at the time the driver
360 these flags is set, the PM core will not apply the direct-complete
364 the return value of the ``->prepare`` callback provided by the driver
366 ``->prepare`` callback if the driver's one also has returned a positive
369 2. The ``->suspend`` methods should quiesce the device to stop it from
371 the appropriate low-power state, depending on the bus type the device is
374 However, for devices supporting runtime power management, the
375 ``->suspend`` methods provided by subsystems (bus types and PM domains
377 to the devices before their drivers' ``->suspend`` methods are called.
382 suspend in their ``->suspend`` methods). In fact, the PM core prevents
385 the ``->prepare`` callback (and calling :c:func:`pm_runtime_put` after
386 issuing the ``->complete`` callback).
391 runtime power management has been disabled for the device in question.
395 the callback method is running. The ``->suspend_noirq`` methods should
397 and finally put the device into the appropriate low-power state.
403 generated by some other device after its own device had been set to low
404 power.
407 (DMA, IRQs), saved enough state that they can re-initialize or restore previous
408 state (as needed by the hardware), and placed the device into a low-power state.
410 will also switch off power supplies or reduce voltages. [Drivers supporting
420 low-power state. Instead, the PM core will unwind its actions by resuming all
425 ----------------------
430 1. The ``->resume_noirq`` callback methods should perform any actions
438 For example, the PCI bus type's ``->pm.resume_noirq()`` puts the device
439 into the full-power state (D0 in the PCI terminology) and restores the
441 device driver's ``->pm.resume_noirq()`` method to perform device-specific
444 2. The ``->resume_early`` methods should prepare devices for the execution
448 3. The ``->resume`` methods should bring the device back to its operating
453 For this reason, unlike the other resume-related phases, during the
454 ``complete`` phase the device hierarchy is traversed bottom-up.
457 soon as the ``->resume`` callbacks occur; it's not necessary to wait
460 Moreover, if the preceding ``->prepare`` callback returned a positive
462 whole system suspend and resume (its ``->suspend``, ``->suspend_late``,
463 ``->suspend_noirq``, ``->resume_noirq``,
464 ``->resume_early``, and ``->resume`` callbacks may have been
465 skipped). In that case, the ``->complete`` callback is entirely
469 the case, the ``->complete`` callback can consult the device's
470 ``power.direct_complete`` flag. If that flag is set when the
471 ``->complete`` callback is being run then the direct-complete mechanism
479 However, the details here may again be platform-specific. For example,
482 That means availability of certain clocks or power supplies changed,
490 system sleep entered was suspend-to-idle. For the other system sleep states
491 that may not be the case (and usually isn't for ACPI-defined system sleep
498 will notice and handle such removals are currently bus-specific, and often
507 --------------------
517 the system ("power off"). The phases used to accomplish this are: ``prepare``,
525 2. The ``->freeze`` methods should quiesce the device so that it doesn't
527 registers. However the device does not have to be put in a low-power
533 low-power state and should not be allowed to generate wakeup events.
537 a low-power state and should not be allowed to generate wakeup events.
561 before putting the system into the suspend-to-idle, shallow or deep sleep state,
572 The ``->poweroff``, ``->poweroff_late`` and ``->poweroff_noirq`` callbacks
573 should do essentially the same things as the ``->suspend``, ``->suspend_late``
574 and ``->suspend_noirq`` callbacks, respectively. A notable difference is
577 ``freeze_noirq`` phases. Also, on many machines the firmware will power-down
579 a low-power state.
583 -------------------
587 a system image to be loaded into memory and the pre-hibernation memory contents
591 pre-hibernation memory contents restored by the boot loader, in practice this
596 reads the system image, restores the pre-hibernation memory contents, and passes
614 Should the restoration of the pre-hibernation memory contents fail, the restore
618 pre-hibernation memory contents are restored successfully and control is passed
622 To achieve this, the image kernel must restore the devices' pre-hibernation
640 reset and completely re-initialized. In many cases this difference doesn't
641 matter, so the ``->resume[_early|_noirq]`` and ``->restore[_early|_norq]``
647 Power Management Notifiers
650 There are some operations that cannot be carried out by the power management
652 To handle these cases, subsystems and device drivers may register power
658 For details refer to Documentation/driver-api/pm/notifiers.rst.
661 Device Low-Power (suspend) States
664 Device low-power states aren't standard. One device might only handle
670 gives one example: after the suspend sequence completes, a non-legacy
673 several PCI-standard device states, some of which are optional.
675 In contrast, integrated system-on-chip processors often use IRQs as the
680 Some details here may be platform-specific. Systems may have devices that
683 its frame buffer might even be updated by a DSP or other non-Linux CPU while
688 another might require a hard shut down with re-initialization on resume.
694 Device Power Management Domains
697 Sometimes devices share reference clocks or other power resources. In those
698 cases it generally is not possible to put devices into low-power states
699 individually. Instead, a set of devices sharing a power resource can be put
700 into a low-power state together at the same time by turning off the shared
701 power resource. Of course, they also need to be put into the full-power state
702 together, by turning the shared power resource on. A set of devices with this
703 property is often referred to as a power domain. A power domain may also be
704 nested inside another power domain. The nested domain is referred to as the
705 sub-domain of the parent domain.
707 Support for power domains is provided through the :c:member:`pm_domain` field of
710 of power management callbacks analogous to the subsystem-level and device driver
711 callbacks that are executed for the given device during all power transitions,
712 instead of the respective subsystem-level callbacks. Specifically, if a
713 device's :c:member:`pm_domain` pointer is not NULL, the ``->suspend()`` callback
715 (e.g. bus type's) ``->suspend()`` callback and analogously for all of the
716 remaining callbacks. In other words, power management domain callbacks, if
720 The support for device power management domains is only relevant to platforms
721 needing to use the same device driver power management callbacks in many
722 different power domain configurations and wanting to avoid incorporating the
723 support for power domains into subsystem-level callbacks, for example by
727 Devices may be defined as IRQ-safe which indicates to the PM core that their
729 Documentation/power/runtime_pm.rst for more information). If an
730 IRQ-safe device belongs to a PM domain, the runtime PM of the domain will be
731 disallowed, unless the domain itself is defined as IRQ-safe. However, it
732 makes sense to define a PM domain as IRQ-safe only if all the devices in it
733 are IRQ-safe. Moreover, if an IRQ-safe domain has a parent domain, the runtime
734 PM of the parent is only allowed if the parent itself is IRQ-safe too with the
735 additional restriction that all child domains of an IRQ-safe parent must also
736 be IRQ-safe.
739 Runtime Power Management
742 Many devices are able to dynamically power down while the system is still
744 can offer significant power savings on a running system. These devices
745 often support a range of runtime power states, which might use names such
750 A system-wide power transition can be started while some devices are in low
751 power states due to runtime power management. The system sleep PM callbacks
753 necessary actions are subsystem-specific.
757 desirable to leave a suspended device in that state during a system-wide power
758 transition, but in other cases the device must be put back into the full-power
763 If it is necessary to resume a device from runtime suspend during a system-wide
765 :c:func:`pm_runtime_resume` from the ``->suspend`` callback (or the ``->freeze``
766 or ``->poweroff`` callback for transitions related to hibernation) of either the
769 from their ``->prepare`` and ``->suspend`` callbacks (or equivalent) *before*
770 invoking device drivers' ``->suspend`` callbacks (or equivalent).
775 ------------------------------------------
778 suspend upfront in their ``->suspend`` callbacks, but that may not be really
779 necessary if the device's driver can cope with runtime-suspended devices.
781 :c:member:`power.driver_flags` at probe time, with the assistance of the
784 Setting that flag causes the PM core and middle-layer code
785 (bus types, PM domains etc.) to skip the ``->suspend_late`` and
786 ``->suspend_noirq`` callbacks provided by the driver if the device remains in
787 runtime suspend throughout those phases of the system-wide suspend (and
791 be valid in general.] If the middle-layer system-wide PM callbacks are present
797 In addition, with ``DPM_FLAG_SMART_SUSPEND`` set, the driver's ``->thaw_noirq``
798 and ``->thaw_early`` callbacks are skipped in hibernation if the device remained
800 middle-layer callbacks are present for the device, they are responsible for
805 --------------------------------------------
807 During system-wide resume from a sleep state it's easiest to put devices into
808 the full-power state, as explained in Documentation/power/runtime_pm.rst.
810 well as for information on the device runtime power management framework in
813 runtime suspend before the preceding system-wide suspend (or analogous)
817 indicate to the PM core and middle-layer code that they allow their "noirq" and
819 after system-wide PM transitions to the working state. Whether or not that is
821 suspend-resume cycle and on the type of the system transition under way.
829 children will be returned to full power.]
832 the :c:member:`power.may_skip_resume` status bit set by the PM core during the
833 "suspend" phase of suspend-type transitions. If the driver or the middle layer
836 clear :c:member:`power.may_skip_resume` in its ``->suspend``, ``->suspend_late``
837 or ``->suspend_noirq`` callback. [Note that the drivers setting
838 ``DPM_FLAG_SMART_SUSPEND`` need to clear :c:member:`power.may_skip_resume` in
839 their ``->suspend`` callback in case the other two are skipped.]
841 Setting the :c:member:`power.may_skip_resume` status bit along with the
849 "suspended" by the PM core. Otherwise, if the device was runtime-suspended
850 during the preceding system-wide suspend transition and its
855 system-wide resume-type transitions.]
859 callbacks are skipped, its system-wide "noirq" and "early" resume callbacks, if
862 driver must be prepared to cope with the invocation of its system-wide resume
863 callbacks back-to-back with its ``->runtime_suspend`` one (without the
864 intervening ``->runtime_resume`` and system-wide suspend callbacks) and the
867 ``->suspend_late`` callback pointer points to the same function as its
868 ``->runtime_suspend`` one and its ``->resume_early`` callback pointer points to
869 the same function as the ``->runtime_resume`` one, while none of the other
870 system-wide suspend-resume callbacks of the driver are present, for example.]
873 system-wide "noirq" and "early" resume callbacks may be skipped while its "late"
876 needs to be able to cope with the invocation of its ``->runtime_resume``
877 callback back-to-back with its "late" and "noirq" suspend ones. [For instance,
880 functions for runtime PM and system-wide suspend/resume.]