Lines Matching +full:low +full:- +full:power
10 Device Power Management Basics
15 Copyright (c) 2010-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
19 Most of the code in Linux is device drivers, so most of the Linux power
20 management (PM) code is also driver-specific. Most drivers will do very
24 This writeup gives an overview of how drivers interact with system-wide
25 power management goals, emphasizing the models and interfaces that are
27 background for the domain-specific work you'd do with any specific driver.
30 Two Models for Device Power Management
33 Drivers will use one or both of these models to put devices into low-power
38 Drivers can enter low-power states as part of entering system-wide
39 low-power states like "suspend" (also known as "suspend-to-RAM"), or
41 "suspend-to-disk").
44 by implementing various role-specific suspend and resume methods to
45 cleanly power down hardware and software subsystems, then reactivate
49 leave the low-power state. This feature may be enabled or disabled
50 using the relevant :file:`/sys/devices/.../power/wakeup` file (for
52 for this purpose); enabling it may cost some power usage, but let the
53 whole system enter low-power states more often.
55 Runtime Power Management model:
57 Devices may also be put into low-power states while the system is
58 running, independently of other power management activity in principle.
62 device is on, it may be necessary to carry out some bus-specific
63 operations on the device for this purpose. Devices put into low power
64 states at run time may require special handling during system-wide power
69 the PM core are involved in runtime power management. As in the system
70 sleep power management case, they need to collaborate by implementing
71 various role-specific suspend and resume methods, so that the hardware
74 There's not a lot to be said about those low-power states except that they are
75 very system-specific, and often device-specific. Also, that if enough devices
76 have been put into low-power states (at runtime), the effect may be very similar
77 to entering some system-wide low-power state (system sleep) ... and that
79 into a state where even deeper power saving options are available.
87 network wake-on-LAN packets, keyboard or mouse activity, and media insertion
94 device class) and device drivers to allow them to participate in the power
96 system sleep and runtime power management.
99 Device Power Management Operations
100 ----------------------------------
102 Device power management operations, at the subsystem level as well as at the
106 sufficient to remember that the last three methods are specific to runtime power
107 management while the remaining ones are used during system-wide power
110 There also is a deprecated "old" or "legacy" interface for power management
113 sleep power management methods in a limited way. Therefore it is not described
118 Subsystem-Level Methods
119 -----------------------
128 bus types) don't provide all power management methods.
132 write subsystem-level drivers; most driver code is a "device driver" that builds
133 on top of bus-specific framework code.
136 they are called in phases for every device, respecting the parent-child
140 :file:`/sys/devices/.../power/wakeup` files
141 -------------------------------------------
149 The :c:member:`power.can_wakeup` flag just records whether the device (and its
152 :c:member:`power.wakeup` field is a pointer to an object of type
155 events signaled by the device. This object is only present for wakeup-capable
161 whether or not a wakeup-capable device should issue wakeup events is a policy
163 :file:`power/wakeup` file. User space can write the "enabled" or "disabled"
166 :c:member:`power.wakeup` object exists for the given device and is created (or
170 The initial value in the :file:`power/wakeup` file is "disabled" for the
171 majority of devices; the major exceptions are power buttons, keyboards, and
172 Ethernet adapters whose WoL (wake-on-LAN) feature has been set up with ethtool.
178 :c:member:`power.wakeup` object exists and the corresponding :file:`power/wakeup`
187 wakeup" used by runtime power management, although it may be supported by the
189 low-power states to trigger specific interrupts to signal conditions in which
190 they should be put into the full-power state. Those interrupts may or may not
193 case, remote wakeup should always be enabled for runtime power management for
197 :file:`/sys/devices/.../power/control` files
198 --------------------------------------------
201 runtime power management. This flag, :c:member:`runtime_auto`, is initialized
203 or :c:func:`pm_runtime_forbid()`; the default is to allow runtime power
207 the device's :file:`power/control` sysfs file. Writing "auto" calls
209 runtime power-managed by its driver. Writing "on" calls
211 power if it was in a low-power state, and preventing the
212 device from being runtime power-managed. User space can check the current value
216 system-wide power transitions. In particular, the device can (and in the
217 majority of cases should and will) be put into a low-power state during a
218 system-wide transition to a sleep state even though its :c:member:`runtime_auto`
221 For more information about the runtime power management framework, refer to
222 :file:`Documentation/power/runtime_pm.txt`.
231 system-specific. Also, wakeup-enabled devices will usually stay partly
234 When the system leaves that low-power state, the device's driver is asked to
235 resume it by returning it to full power. The suspend and resume operations
236 always go together, and both are multi-phase operations.
243 More power-aware drivers might prepare the devices for triggering system wakeup
248 ------------------------
252 walked in a bottom-up order to suspend devices. A top-down order is
268 System Power Management Phases
269 ------------------------------
272 are used for suspend-to-idle, shallow (standby), and deep ("suspend-to-RAM")
273 sleep states and the hibernation state ("suspend-to-disk"). Each phase involves
282 defined in ``dev->pm_domain->ops``, ``dev->bus->pm``, ``dev->type->pm``,
283 ``dev->class->pm`` or ``dev->driver->pm``). These callbacks are regarded by the
289 1. If ``dev->pm_domain`` is present, the PM core will choose the callback
290 provided by ``dev->pm_domain->ops`` for execution.
292 2. Otherwise, if both ``dev->type`` and ``dev->type->pm`` are present, the
293 callback provided by ``dev->type->pm`` will be chosen for execution.
295 3. Otherwise, if both ``dev->class`` and ``dev->class->pm`` are present,
296 the callback provided by ``dev->class->pm`` will be chosen for
299 4. Otherwise, if both ``dev->bus`` and ``dev->bus->pm`` are present, the
300 callback provided by ``dev->bus->pm`` will be chosen for execution.
305 The PM domain, type, class and bus callbacks may in turn invoke device- or
306 driver-specific methods stored in ``dev->driver->pm``, but they don't have to do
310 execute the corresponding method from the ``dev->driver->pm`` set instead if
315 -----------------------
325 suspend-related phases, during the ``prepare`` phase the device
326 hierarchy is traversed top-down.
328 After the ``->prepare`` callback method returns, no new children may be
330 driver in some way for the upcoming system power transition, but it
331 should not put the device into a low-power state. Moreover, if the
332 device supports runtime power management, the ``->prepare`` callback
336 For devices supporting runtime power management, the return value of the
338 safely leave the device in runtime suspend (if runtime-suspended
342 and all of them (including the device itself) are runtime-suspended, the
346 the ``->complete`` callback will be invoked directly after the
347 ``->prepare`` callback and is entirely responsible for putting the
350 Note that this direct-complete procedure applies even if the device is
351 disabled for runtime PM; only the runtime-PM status matters. It follows
352 that if a device has system-sleep callbacks but does not support runtime
354 is because all such devices are initially set to runtime-suspended with
358 ``DPM_FLAG_NEVER_SKIP`` and ``DPM_FLAG_SMART_PREPARE`` driver power
362 these flags is set, the PM core will not apply the direct-complete
366 the return value of the ``->prepare`` callback provided by the driver
368 ``->prepare`` callback if the driver's one also has returned a positive
371 2. The ``->suspend`` methods should quiesce the device to stop it from
373 the appropriate low-power state, depending on the bus type the device is
376 However, for devices supporting runtime power management, the
377 ``->suspend`` methods provided by subsystems (bus types and PM domains
379 to the devices before their drivers' ``->suspend`` methods are called.
384 suspend in their ``->suspend`` methods).
389 runtime power management has been disabled for the device in question.
393 the callback method is running. The ``->suspend_noirq`` methods should
395 and finally put the device into the appropriate low-power state.
401 generated by some other device after its own device had been set to low
402 power.
405 (DMA, IRQs), saved enough state that they can re-initialize or restore previous
406 state (as needed by the hardware), and placed the device into a low-power state.
408 will also switch off power supplies or reduce voltages. [Drivers supporting
418 low-power state. Instead, the PM core will unwind its actions by resuming all
423 ----------------------
428 1. The ``->resume_noirq`` callback methods should perform any actions
436 For example, the PCI bus type's ``->pm.resume_noirq()`` puts the device
437 into the full-power state (D0 in the PCI terminology) and restores the
439 device driver's ``->pm.resume_noirq()`` method to perform device-specific
442 2. The ``->resume_early`` methods should prepare devices for the execution
446 3. The ``->resume`` methods should bring the device back to its operating
451 For this reason, unlike the other resume-related phases, during the
452 ``complete`` phase the device hierarchy is traversed bottom-up.
455 soon as the ``->resume`` callbacks occur; it's not necessary to wait
458 Moreover, if the preceding ``->prepare`` callback returned a positive
463 skipped for it). In that case, the ``->complete`` callback is entirely
467 the case, the ``->complete`` callback can consult the device's
468 ``power.direct_complete`` flag. Namely, if that flag is set when the
469 ``->complete`` callback is being run, it has been called directly after
470 the preceding ``->prepare`` and special actions may be required
477 However, the details here may again be platform-specific. For example,
480 That means availability of certain clocks or power supplies changed,
488 system sleep entered was suspend-to-idle. For the other system sleep states
489 that may not be the case (and usually isn't for ACPI-defined system sleep
496 will notice and handle such removals are currently bus-specific, and often
505 --------------------
515 the system ("power off"). The phases used to accomplish this are: ``prepare``,
523 2. The ``->freeze`` methods should quiesce the device so that it doesn't
525 registers. However the device does not have to be put in a low-power
531 low-power state and should not be allowed to generate wakeup events.
535 a low-power state and should not be allowed to generate wakeup events.
559 before putting the system into the suspend-to-idle, shallow or deep sleep state,
570 The ``->poweroff``, ``->poweroff_late`` and ``->poweroff_noirq`` callbacks
571 should do essentially the same things as the ``->suspend``, ``->suspend_late``
572 and ``->suspend_noirq`` callbacks, respectively. The only notable difference is
579 -------------------
583 a system image to be loaded into memory and the pre-hibernation memory contents
587 pre-hibernation memory contents restored by the boot loader, in practice this
592 reads the system image, restores the pre-hibernation memory contents, and passes
610 Should the restoration of the pre-hibernation memory contents fail, the restore
614 pre-hibernation memory contents are restored successfully and control is passed
618 To achieve this, the image kernel must restore the devices' pre-hibernation
636 reset and completely re-initialized. In many cases this difference doesn't
637 matter, so the ``->resume[_early|_noirq]`` and ``->restore[_early|_norq]``
643 Power Management Notifiers
646 There are some operations that cannot be carried out by the power management
648 To handle these cases, subsystems and device drivers may register power
657 Device Low-Power (suspend) States
660 Device low-power states aren't standard. One device might only handle
666 gives one example: after the suspend sequence completes, a non-legacy
669 several PCI-standard device states, some of which are optional.
671 In contrast, integrated system-on-chip processors often use IRQs as the
676 Some details here may be platform-specific. Systems may have devices that
679 its frame buffer might even be updated by a DSP or other non-Linux CPU while
684 another might require a hard shut down with re-initialization on resume.
690 Device Power Management Domains
693 Sometimes devices share reference clocks or other power resources. In those
694 cases it generally is not possible to put devices into low-power states
695 individually. Instead, a set of devices sharing a power resource can be put
696 into a low-power state together at the same time by turning off the shared
697 power resource. Of course, they also need to be put into the full-power state
698 together, by turning the shared power resource on. A set of devices with this
699 property is often referred to as a power domain. A power domain may also be
700 nested inside another power domain. The nested domain is referred to as the
701 sub-domain of the parent domain.
703 Support for power domains is provided through the :c:member:`pm_domain` field of
706 of power management callbacks analogous to the subsystem-level and device driver
707 callbacks that are executed for the given device during all power transitions,
708 instead of the respective subsystem-level callbacks. Specifically, if a
709 device's :c:member:`pm_domain` pointer is not NULL, the ``->suspend()`` callback
711 (e.g. bus type's) ``->suspend()`` callback and analogously for all of the
712 remaining callbacks. In other words, power management domain callbacks, if
716 The support for device power management domains is only relevant to platforms
717 needing to use the same device driver power management callbacks in many
718 different power domain configurations and wanting to avoid incorporating the
719 support for power domains into subsystem-level callbacks, for example by
723 Devices may be defined as IRQ-safe which indicates to the PM core that their
725 :file:`Documentation/power/runtime_pm.txt` for more information). If an
726 IRQ-safe device belongs to a PM domain, the runtime PM of the domain will be
727 disallowed, unless the domain itself is defined as IRQ-safe. However, it
728 makes sense to define a PM domain as IRQ-safe only if all the devices in it
729 are IRQ-safe. Moreover, if an IRQ-safe domain has a parent domain, the runtime
730 PM of the parent is only allowed if the parent itself is IRQ-safe too with the
731 additional restriction that all child domains of an IRQ-safe parent must also
732 be IRQ-safe.
735 Runtime Power Management
738 Many devices are able to dynamically power down while the system is still
740 can offer significant power savings on a running system. These devices
741 often support a range of runtime power states, which might use names such
746 A system-wide power transition can be started while some devices are in low
747 power states due to runtime power management. The system sleep PM callbacks
749 necessary actions are subsystem-specific.
753 desirable to leave a suspended device in that state during a system-wide power
754 transition, but in other cases the device must be put back into the full-power
759 If it is necessary to resume a device from runtime suspend during a system-wide
761 :c:func:`pm_runtime_resume` for it from the ``->suspend`` callback (or its
766 from their ``->prepare`` and ``->suspend`` callbacks (or equivalent) *before*
767 invoking device drivers' ``->suspend`` callbacks (or equivalent).
770 suspend upfront in their ``->suspend`` callbacks, but that may not be really
771 necessary if the driver of the device can cope with runtime-suspended devices.
773 :c:member:`power.driver_flags` at the probe time, by passing it to the
774 :c:func:`dev_pm_set_driver_flags` helper. That also may cause middle-layer code
775 (bus types, PM domains etc.) to skip the ``->suspend_late`` and
776 ``->suspend_noirq`` callbacks provided by the driver if the device remains in
777 runtime suspend at the beginning of the ``suspend_late`` phase of system-wide
780 after that point until the system-wide transition is over (the PM core itself
781 does that for devices whose "noirq", "late" and "early" system-wide PM callbacks
782 are executed directly by it). If that happens, the driver's system-wide resume
783 callbacks, if present, may still be invoked during the subsequent system-wide
784 resume transition and the device's runtime power management status may be set
786 cope with the invocation of its system-wide resume callbacks back-to-back with
787 its ``->runtime_suspend`` one (without the intervening ``->runtime_resume`` and
791 During system-wide resume from a sleep state it's easiest to put devices into
792 the full-power state, as explained in :file:`Documentation/power/runtime_pm.txt`.
794 well as for information on the device runtime power management framework in
799 runtime suspend before the preceding system-wide suspend (or analogous)
801 indicate to the PM core (and middle-layer code) that they prefer the specific
803 skipping their system-wide resume callbacks for this reason. Whether or not the
805 given system suspend-resume cycle and on the type of the system transition under
810 The middle-layer code involved in the handling of the device is expected to
812 :c:member:`power.may_skip_resume` status bit which is checked by the PM core
813 during the "noirq" phase of the preceding system-wide suspend (or analogous)
817 (except for ``->complete``) will be skipped automatically by the PM core if the
821 directly by the PM core, all of the system-wide resume callbacks are skipped if