TOMOYO Linux Cross Reference
Linux/Documentation/driver-api/pm/devices.rst

   .. SPDX-License-Identifier: GPL-2.0
   .. include:: <isonum.txt>
   
   .. _driverapi_pm_devices:
   
   ==============================
   Device Power Management Basics
   ==============================
   
  :Copyright: |copy| 2010-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
  :Copyright: |copy| 2010 Alan Stern <stern@rowland.harvard.edu>
  :Copyright: |copy| 2016 Intel Corporation
  
  :Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
  
  
  Most of the code in Linux is device drivers, so most of the Linux power
  management (PM) code is also driver-specific.  Most drivers will do very
  little; others, especially for platforms with small batteries (like cell
  phones), will do a lot.
  
  This writeup gives an overview of how drivers interact with system-wide
  power management goals, emphasizing the models and interfaces that are
  shared by everything that hooks up to the driver model core.  Read it as
  background for the domain-specific work you'd do with any specific driver.
  
  
  Two Models for Device Power Management
  ======================================
  
  Drivers will use one or both of these models to put devices into low-power
  states:
  
      System Sleep model:
  
          Drivers can enter low-power states as part of entering system-wide
          low-power states like "suspend" (also known as "suspend-to-RAM"), or
          (mostly for systems with disks) "hibernation" (also known as
          "suspend-to-disk").
  
          This is something that device, bus, and class drivers collaborate on
          by implementing various role-specific suspend and resume methods to
          cleanly power down hardware and software subsystems, then reactivate
          them without loss of data.
  
          Some drivers can manage hardware wakeup events, which make the system
          leave the low-power state.  This feature may be enabled or disabled
          using the relevant :file:`/sys/devices/.../power/wakeup` file (for
          Ethernet drivers the ioctl interface used by ethtool may also be used
          for this purpose); enabling it may cost some power usage, but let the
          whole system enter low-power states more often.
  
      Runtime Power Management model:
  
          Devices may also be put into low-power states while the system is
          running, independently of other power management activity in principle.
          However, devices are not generally independent of each other (for
          example, a parent device cannot be suspended unless all of its child
          devices have been suspended).  Moreover, depending on the bus type the
          device is on, it may be necessary to carry out some bus-specific
          operations on the device for this purpose.  Devices put into low power
          states at run time may require special handling during system-wide power
          transitions (suspend or hibernation).
  
          For these reasons not only the device driver itself, but also the
          appropriate subsystem (bus type, device type or device class) driver and
          the PM core are involved in runtime power management.  As in the system
          sleep power management case, they need to collaborate by implementing
          various role-specific suspend and resume methods, so that the hardware
          is cleanly powered down and reactivated without data or service loss.
  
  There's not a lot to be said about those low-power states except that they are
  very system-specific, and often device-specific.  Also, that if enough devices
  have been put into low-power states (at runtime), the effect may be very similar
  to entering some system-wide low-power state (system sleep) ... and that
  synergies exist, so that several drivers using runtime PM might put the system
  into a state where even deeper power saving options are available.
  
  Most suspended devices will have quiesced all I/O: no more DMA or IRQs (except
  for wakeup events), no more data read or written, and requests from upstream
  drivers are no longer accepted.  A given bus or platform may have different
  requirements though.
  
  Examples of hardware wakeup events include an alarm from a real time clock,
  network wake-on-LAN packets, keyboard or mouse activity, and media insertion
  or removal (for PCMCIA, MMC/SD, USB, and so on).
  
  Interfaces for Entering System Sleep States
  ===========================================
  
  There are programming interfaces provided for subsystems (bus type, device type,
  device class) and device drivers to allow them to participate in the power
  management of devices they are concerned with.  These interfaces cover both
  system sleep and runtime power management.
  
  
  Device Power Management Operations
  ----------------------------------
  
 Device power management operations, at the subsystem level as well as at the
 device driver level, are implemented by defining and populating objects of type
 struct dev_pm_ops defined in :file:`include/linux/pm.h`.  The roles of the
 methods included in it will be explained in what follows.  For now, it should be
 sufficient to remember that the last three methods are specific to runtime power
 management while the remaining ones are used during system-wide power
 transitions.
 
 There also is a deprecated "old" or "legacy" interface for power management
 operations available at least for some subsystems.  This approach does not use
 struct dev_pm_ops objects and it is suitable only for implementing system
 sleep power management methods in a limited way.  Therefore it is not described
 in this document, so please refer directly to the source code for more
 information about it.
 
 
 Subsystem-Level Methods
 -----------------------
 
 The core methods to suspend and resume devices reside in
 struct dev_pm_ops pointed to by the :c:member:`ops` member of
 struct dev_pm_domain, or by the :c:member:`pm` member of struct bus_type,
 struct device_type and struct class.  They are mostly of interest to the
 people writing infrastructure for platforms and buses, like PCI or USB, or
 device type and device class drivers.  They also are relevant to the writers of
 device drivers whose subsystems (PM domains, device types, device classes and
 bus types) don't provide all power management methods.
 
 Bus drivers implement these methods as appropriate for the hardware and the
 drivers using it; PCI works differently from USB, and so on.  Not many people
 write subsystem-level drivers; most driver code is a "device driver" that builds
 on top of bus-specific framework code.
 
 For more information on these driver calls, see the description later;
 they are called in phases for every device, respecting the parent-child
 sequencing in the driver model tree.
 
 
 :file:`/sys/devices/.../power/wakeup` files
 -------------------------------------------
 
 All device objects in the driver model contain fields that control the handling
 of system wakeup events (hardware signals that can force the system out of a
 sleep state).  These fields are initialized by bus or device driver code using
 :c:func:`device_set_wakeup_capable()` and :c:func:`device_set_wakeup_enable()`,
 defined in :file:`include/linux/pm_wakeup.h`.
 
 The :c:member:`power.can_wakeup` flag just records whether the device (and its
 driver) can physically support wakeup events.  The
 :c:func:`device_set_wakeup_capable()` routine affects this flag.  The
 :c:member:`power.wakeup` field is a pointer to an object of type
 struct wakeup_source used for controlling whether or not the device should use
 its system wakeup mechanism and for notifying the PM core of system wakeup
 events signaled by the device.  This object is only present for wakeup-capable
 devices (i.e. devices whose :c:member:`can_wakeup` flags are set) and is created
 (or removed) by :c:func:`device_set_wakeup_capable()`.
 
 Whether or not a device is capable of issuing wakeup events is a hardware
 matter, and the kernel is responsible for keeping track of it.  By contrast,
 whether or not a wakeup-capable device should issue wakeup events is a policy
 decision, and it is managed by user space through a sysfs attribute: the
 :file:`power/wakeup` file.  User space can write the "enabled" or "disabled"
 strings to it to indicate whether or not, respectively, the device is supposed
 to signal system wakeup.  This file is only present if the
 :c:member:`power.wakeup` object exists for the given device and is created (or
 removed) along with that object, by :c:func:`device_set_wakeup_capable()`.
 Reads from the file will return the corresponding string.
 
 The initial value in the :file:`power/wakeup` file is "disabled" for the
 majority of devices; the major exceptions are power buttons, keyboards, and
 Ethernet adapters whose WoL (wake-on-LAN) feature has been set up with ethtool.
 It should also default to "enabled" for devices that don't generate wakeup
 requests on their own but merely forward wakeup requests from one bus to another
 (like PCI Express ports).
 
 The :c:func:`device_may_wakeup()` routine returns true only if the
 :c:member:`power.wakeup` object exists and the corresponding :file:`power/wakeup`
 file contains the "enabled" string.  This information is used by subsystems,
 like the PCI bus type code, to see whether or not to enable the devices' wakeup
 mechanisms.  If device wakeup mechanisms are enabled or disabled directly by
 drivers, they also should use :c:func:`device_may_wakeup()` to decide what to do
 during a system sleep transition.  Device drivers, however, are not expected to
 call :c:func:`device_set_wakeup_enable()` directly in any case.
 
 It ought to be noted that system wakeup is conceptually different from "remote
 wakeup" used by runtime power management, although it may be supported by the
 same physical mechanism.  Remote wakeup is a feature allowing devices in
 low-power states to trigger specific interrupts to signal conditions in which
 they should be put into the full-power state.  Those interrupts may or may not
 be used to signal system wakeup events, depending on the hardware design.  On
 some systems it is impossible to trigger them from system sleep states.  In any
 case, remote wakeup should always be enabled for runtime power management for
 all devices and drivers that support it.
 
 
 :file:`/sys/devices/.../power/control` files
 --------------------------------------------
 
 Each device in the driver model has a flag to control whether it is subject to
 runtime power management.  This flag, :c:member:`runtime_auto`, is initialized
 by the bus type (or generally subsystem) code using :c:func:`pm_runtime_allow()`
 or :c:func:`pm_runtime_forbid()`; the default is to allow runtime power
 management.
 
 The setting can be adjusted by user space by writing either "on" or "auto" to
 the device's :file:`power/control` sysfs file.  Writing "auto" calls
 :c:func:`pm_runtime_allow()`, setting the flag and allowing the device to be
 runtime power-managed by its driver.  Writing "on" calls
 :c:func:`pm_runtime_forbid()`, clearing the flag, returning the device to full
 power if it was in a low-power state, and preventing the
 device from being runtime power-managed.  User space can check the current value
 of the :c:member:`runtime_auto` flag by reading that file.
 
 The device's :c:member:`runtime_auto` flag has no effect on the handling of
 system-wide power transitions.  In particular, the device can (and in the
 majority of cases should and will) be put into a low-power state during a
 system-wide transition to a sleep state even though its :c:member:`runtime_auto`
 flag is clear.
 
 For more information about the runtime power management framework, refer to
 Documentation/power/runtime_pm.rst.
 
 
 Calling Drivers to Enter and Leave System Sleep States
 ======================================================
 
 When the system goes into a sleep state, each device's driver is asked to
 suspend the device by putting it into a state compatible with the target
 system state.  That's usually some version of "off", but the details are
 system-specific.  Also, wakeup-enabled devices will usually stay partly
 functional in order to wake the system.
 
 When the system leaves that low-power state, the device's driver is asked to
 resume it by returning it to full power.  The suspend and resume operations
 always go together, and both are multi-phase operations.
 
 For simple drivers, suspend might quiesce the device using class code
 and then turn its hardware as "off" as possible during suspend_noirq.  The
 matching resume calls would then completely reinitialize the hardware
 before reactivating its class I/O queues.
 
 More power-aware drivers might prepare the devices for triggering system wakeup
 events.
 
 
 Call Sequence Guarantees
 ------------------------
 
 To ensure that bridges and similar links needing to talk to a device are
 available when the device is suspended or resumed, the device hierarchy is
 walked in a bottom-up order to suspend devices.  A top-down order is
 used to resume those devices.
 
 The ordering of the device hierarchy is defined by the order in which devices
 get registered:  a child can never be registered, probed or resumed before
 its parent; and can't be removed or suspended after that parent.
 
 The policy is that the device hierarchy should match hardware bus topology.
 [Or at least the control bus, for devices which use multiple busses.]
 In particular, this means that a device registration may fail if the parent of
 the device is suspending (i.e. has been chosen by the PM core as the next
 device to suspend) or has already suspended, as well as after all of the other
 devices have been suspended.  Device drivers must be prepared to cope with such
 situations.
 
 
 System Power Management Phases
 ------------------------------
 
 Suspending or resuming the system is done in several phases.  Different phases
 are used for suspend-to-idle, shallow (standby), and deep ("suspend-to-RAM")
 sleep states and the hibernation state ("suspend-to-disk").  Each phase involves
 executing callbacks for every device before the next phase begins.  Not all
 buses or classes support all these callbacks and not all drivers use all the
 callbacks.  The various phases always run after tasks have been frozen and
 before they are unfrozen.  Furthermore, the ``*_noirq`` phases run at a time
 when IRQ handlers have been disabled (except for those marked with the
 IRQF_NO_SUSPEND flag).
 
 All phases use PM domain, bus, type, class or driver callbacks (that is, methods
 defined in ``dev->pm_domain->ops``, ``dev->bus->pm``, ``dev->type->pm``,
 ``dev->class->pm`` or ``dev->driver->pm``).  These callbacks are regarded by the
 PM core as mutually exclusive.  Moreover, PM domain callbacks always take
 precedence over all of the other callbacks and, for example, type callbacks take
 precedence over bus, class and driver callbacks.  To be precise, the following
 rules are used to determine which callback to execute in the given phase:
 
     1.  If ``dev->pm_domain`` is present, the PM core will choose the callback
         provided by ``dev->pm_domain->ops`` for execution.
 
     2.  Otherwise, if both ``dev->type`` and ``dev->type->pm`` are present, the
         callback provided by ``dev->type->pm`` will be chosen for execution.
 
     3.  Otherwise, if both ``dev->class`` and ``dev->class->pm`` are present,
         the callback provided by ``dev->class->pm`` will be chosen for
         execution.
 
     4.  Otherwise, if both ``dev->bus`` and ``dev->bus->pm`` are present, the
         callback provided by ``dev->bus->pm`` will be chosen for execution.
 
 This allows PM domains and device types to override callbacks provided by bus
 types or device classes if necessary.
 
 The PM domain, type, class and bus callbacks may in turn invoke device- or
 driver-specific methods stored in ``dev->driver->pm``, but they don't have to do
 that.
 
 If the subsystem callback chosen for execution is not present, the PM core will
 execute the corresponding method from the ``dev->driver->pm`` set instead if
 there is one.
 
 
 Entering System Suspend
 -----------------------
 
 When the system goes into the freeze, standby or memory sleep state,
 the phases are: ``prepare``, ``suspend``, ``suspend_late``, ``suspend_noirq``.
 
     1.  The ``prepare`` phase is meant to prevent races by preventing new
         devices from being registered; the PM core would never know that all the
         children of a device had been suspended if new children could be
         registered at will.  [By contrast, from the PM core's perspective,
         devices may be unregistered at any time.]  Unlike the other
         suspend-related phases, during the ``prepare`` phase the device
         hierarchy is traversed top-down.
 
         After the ``->prepare`` callback method returns, no new children may be
         registered below the device.  The method may also prepare the device or
         driver in some way for the upcoming system power transition, but it
         should not put the device into a low-power state.  Moreover, if the
         device supports runtime power management, the ``->prepare`` callback
         method must not update its state in case it is necessary to resume it
         from runtime suspend later on.
 
         For devices supporting runtime power management, the return value of the
         prepare callback can be used to indicate to the PM core that it may
         safely leave the device in runtime suspend (if runtime-suspended
         already), provided that all of the device's descendants are also left in
         runtime suspend.  Namely, if the prepare callback returns a positive
         number and that happens for all of the descendants of the device too,
         and all of them (including the device itself) are runtime-suspended, the
         PM core will skip the ``suspend``, ``suspend_late`` and
         ``suspend_noirq`` phases as well as all of the corresponding phases of
         the subsequent device resume for all of these devices.  In that case,
         the ``->complete`` callback will be the next one invoked after the
         ``->prepare`` callback and is entirely responsible for putting the
         device into a consistent state as appropriate.
 
         Note that this direct-complete procedure applies even if the device is
         disabled for runtime PM; only the runtime-PM status matters.  It follows
         that if a device has system-sleep callbacks but does not support runtime
         PM, then its prepare callback must never return a positive value.  This
         is because all such devices are initially set to runtime-suspended with
         runtime PM disabled.
 
         This feature also can be controlled by device drivers by using the
         ``DPM_FLAG_NO_DIRECT_COMPLETE`` and ``DPM_FLAG_SMART_PREPARE`` driver
         power management flags.  [Typically, they are set at the time the driver
         is probed against the device in question by passing them to the
         :c:func:`dev_pm_set_driver_flags` helper function.]  If the first of
         these flags is set, the PM core will not apply the direct-complete
         procedure described above to the given device and, consequenty, to any
         of its ancestors.  The second flag, when set, informs the middle layer
         code (bus types, device types, PM domains, classes) that it should take
         the return value of the ``->prepare`` callback provided by the driver
         into account and it may only return a positive value from its own
         ``->prepare`` callback if the driver's one also has returned a positive
         value.
 
     2.  The ``->suspend`` methods should quiesce the device to stop it from
         performing I/O.  They also may save the device registers and put it into
         the appropriate low-power state, depending on the bus type the device is
         on, and they may enable wakeup events.
 
         However, for devices supporting runtime power management, the
         ``->suspend`` methods provided by subsystems (bus types and PM domains
         in particular) must follow an additional rule regarding what can be done
         to the devices before their drivers' ``->suspend`` methods are called.
         Namely, they may resume the devices from runtime suspend by
         calling :c:func:`pm_runtime_resume` for them, if that is necessary, but
         they must not update the state of the devices in any other way at that
         time (in case the drivers need to resume the devices from runtime
         suspend in their ``->suspend`` methods).  In fact, the PM core prevents
         subsystems or drivers from putting devices into runtime suspend at
         these times by calling :c:func:`pm_runtime_get_noresume` before issuing
         the ``->prepare`` callback (and calling :c:func:`pm_runtime_put` after
         issuing the ``->complete`` callback).
 
     3.  For a number of devices it is convenient to split suspend into the
         "quiesce device" and "save device state" phases, in which cases
         ``suspend_late`` is meant to do the latter.  It is always executed after
         runtime power management has been disabled for the device in question.
 
     4.  The ``suspend_noirq`` phase occurs after IRQ handlers have been disabled,
         which means that the driver's interrupt handler will not be called while
         the callback method is running.  The ``->suspend_noirq`` methods should
         save the values of the device's registers that weren't saved previously
         and finally put the device into the appropriate low-power state.
 
         The majority of subsystems and device drivers need not implement this
         callback.  However, bus types allowing devices to share interrupt
         vectors, like PCI, generally need it; otherwise a driver might encounter
         an error during the suspend phase by fielding a shared interrupt
         generated by some other device after its own device had been set to low
         power.
 
 At the end of these phases, drivers should have stopped all I/O transactions
 (DMA, IRQs), saved enough state that they can re-initialize or restore previous
 state (as needed by the hardware), and placed the device into a low-power state.
 On many platforms they will gate off one or more clock sources; sometimes they
 will also switch off power supplies or reduce voltages.  [Drivers supporting
 runtime PM may already have performed some or all of these steps.]
 
 If :c:func:`device_may_wakeup()` returns ``true``, the device should be
 prepared for generating hardware wakeup signals to trigger a system wakeup event
 when the system is in the sleep state.  For example, :c:func:`enable_irq_wake()`
 might identify GPIO signals hooked up to a switch or other external hardware,
 and :c:func:`pci_enable_wake()` does something similar for the PCI PME signal.
 
 If any of these callbacks returns an error, the system won't enter the desired
 low-power state.  Instead, the PM core will unwind its actions by resuming all
 the devices that were suspended.
 
 
 Leaving System Suspend
 ----------------------
 
 When resuming from freeze, standby or memory sleep, the phases are:
 ``resume_noirq``, ``resume_early``, ``resume``, ``complete``.
 
     1.  The ``->resume_noirq`` callback methods should perform any actions
         needed before the driver's interrupt handlers are invoked.  This
         generally means undoing the actions of the ``suspend_noirq`` phase.  If
         the bus type permits devices to share interrupt vectors, like PCI, the
         method should bring the device and its driver into a state in which the
         driver can recognize if the device is the source of incoming interrupts,
         if any, and handle them correctly.
 
         For example, the PCI bus type's ``->pm.resume_noirq()`` puts the device
         into the full-power state (D0 in the PCI terminology) and restores the
         standard configuration registers of the device.  Then it calls the
         device driver's ``->pm.resume_noirq()`` method to perform device-specific
         actions.
 
     2.  The ``->resume_early`` methods should prepare devices for the execution
         of the resume methods.  This generally involves undoing the actions of
         the preceding ``suspend_late`` phase.
 
     3.  The ``->resume`` methods should bring the device back to its operating
         state, so that it can perform normal I/O.  This generally involves
         undoing the actions of the ``suspend`` phase.
 
     4.  The ``complete`` phase should undo the actions of the ``prepare`` phase.
         For this reason, unlike the other resume-related phases, during the
         ``complete`` phase the device hierarchy is traversed bottom-up.
 
         Note, however, that new children may be registered below the device as
         soon as the ``->resume`` callbacks occur; it's not necessary to wait
         until the ``complete`` phase runs.
 
         Moreover, if the preceding ``->prepare`` callback returned a positive
         number, the device may have been left in runtime suspend throughout the
         whole system suspend and resume (its ``->suspend``, ``->suspend_late``,
         ``->suspend_noirq``, ``->resume_noirq``,
         ``->resume_early``, and ``->resume`` callbacks may have been
         skipped).  In that case, the ``->complete`` callback is entirely
         responsible for putting the device into a consistent state after system
         suspend if necessary.  [For example, it may need to queue up a runtime
         resume request for the device for this purpose.]  To check if that is
         the case, the ``->complete`` callback can consult the device's
         ``power.direct_complete`` flag.  If that flag is set when the
         ``->complete`` callback is being run then the direct-complete mechanism
         was used, and special actions may be required to make the device work
         correctly afterward.
 
 At the end of these phases, drivers should be as functional as they were before
 suspending: I/O can be performed using DMA and IRQs, and the relevant clocks are
 gated on.
 
 However, the details here may again be platform-specific.  For example,
 some systems support multiple "run" states, and the mode in effect at
 the end of resume might not be the one which preceded suspension.
 That means availability of certain clocks or power supplies changed,
 which could easily affect how a driver works.
 
 Drivers need to be able to handle hardware which has been reset since all of the
 suspend methods were called, for example by complete reinitialization.
 This may be the hardest part, and the one most protected by NDA'd documents
 and chip errata.  It's simplest if the hardware state hasn't changed since
 the suspend was carried out, but that can only be guaranteed if the target
 system sleep entered was suspend-to-idle.  For the other system sleep states
 that may not be the case (and usually isn't for ACPI-defined system sleep
 states, like S3).
 
 Drivers must also be prepared to notice that the device has been removed
 while the system was powered down, whenever that's physically possible.
 PCMCIA, MMC, USB, Firewire, SCSI, and even IDE are common examples of busses
 where common Linux platforms will see such removal.  Details of how drivers
 will notice and handle such removals are currently bus-specific, and often
 involve a separate thread.
 
 These callbacks may return an error value, but the PM core will ignore such
 errors since there's nothing it can do about them other than printing them in
 the system log.
 
 
 Entering Hibernation
 --------------------
 
 Hibernating the system is more complicated than putting it into sleep states,
 because it involves creating and saving a system image.  Therefore there are
 more phases for hibernation, with a different set of callbacks.  These phases
 always run after tasks have been frozen and enough memory has been freed.
 
 The general procedure for hibernation is to quiesce all devices ("freeze"),
 create an image of the system memory while everything is stable, reactivate all
 devices ("thaw"), write the image to permanent storage, and finally shut down
 the system ("power off").  The phases used to accomplish this are: ``prepare``,
 ``freeze``, ``freeze_late``, ``freeze_noirq``, ``thaw_noirq``, ``thaw_early``,
 ``thaw``, ``complete``, ``prepare``, ``poweroff``, ``poweroff_late``,
 ``poweroff_noirq``.
 
     1.  The ``prepare`` phase is discussed in the "Entering System Suspend"
         section above.
 
     2.  The ``->freeze`` methods should quiesce the device so that it doesn't
         generate IRQs or DMA, and they may need to save the values of device
         registers.  However the device does not have to be put in a low-power
         state, and to save time it's best not to do so.  Also, the device should
         not be prepared to generate wakeup events.
 
     3.  The ``freeze_late`` phase is analogous to the ``suspend_late`` phase
         described earlier, except that the device should not be put into a
         low-power state and should not be allowed to generate wakeup events.
 
     4.  The ``freeze_noirq`` phase is analogous to the ``suspend_noirq`` phase
         discussed earlier, except again that the device should not be put into
         a low-power state and should not be allowed to generate wakeup events.
 
 At this point the system image is created.  All devices should be inactive and
 the contents of memory should remain undisturbed while this happens, so that the
 image forms an atomic snapshot of the system state.
 
     5.  The ``thaw_noirq`` phase is analogous to the ``resume_noirq`` phase
         discussed earlier.  The main difference is that its methods can assume
         the device is in the same state as at the end of the ``freeze_noirq``
         phase.
 
     6.  The ``thaw_early`` phase is analogous to the ``resume_early`` phase
         described above.  Its methods should undo the actions of the preceding
         ``freeze_late``, if necessary.
 
     7.  The ``thaw`` phase is analogous to the ``resume`` phase discussed
         earlier.  Its methods should bring the device back to an operating
         state, so that it can be used for saving the image if necessary.
 
     8.  The ``complete`` phase is discussed in the "Leaving System Suspend"
         section above.
 
 At this point the system image is saved, and the devices then need to be
 prepared for the upcoming system shutdown.  This is much like suspending them
 before putting the system into the suspend-to-idle, shallow or deep sleep state,
 and the phases are similar.
 
     9.  The ``prepare`` phase is discussed above.
 
     10. The ``poweroff`` phase is analogous to the ``suspend`` phase.
 
     11. The ``poweroff_late`` phase is analogous to the ``suspend_late`` phase.
 
     12. The ``poweroff_noirq`` phase is analogous to the ``suspend_noirq`` phase.
 
 The ``->poweroff``, ``->poweroff_late`` and ``->poweroff_noirq`` callbacks
 should do essentially the same things as the ``->suspend``, ``->suspend_late``
 and ``->suspend_noirq`` callbacks, respectively.  A notable difference is
 that they need not store the device register values, because the registers
 should already have been stored during the ``freeze``, ``freeze_late`` or
 ``freeze_noirq`` phases.  Also, on many machines the firmware will power-down
 the entire system, so it is not necessary for the callback to put the device in
 a low-power state.
 
 
 Leaving Hibernation
 -------------------
 
 Resuming from hibernation is, again, more complicated than resuming from a sleep
 state in which the contents of main memory are preserved, because it requires
 a system image to be loaded into memory and the pre-hibernation memory contents
 to be restored before control can be passed back to the image kernel.
 
 Although in principle the image might be loaded into memory and the
 pre-hibernation memory contents restored by the boot loader, in practice this
 can't be done because boot loaders aren't smart enough and there is no
 established protocol for passing the necessary information.  So instead, the
 boot loader loads a fresh instance of the kernel, called "the restore kernel",
 into memory and passes control to it in the usual way.  Then the restore kernel
 reads the system image, restores the pre-hibernation memory contents, and passes
 control to the image kernel.  Thus two different kernel instances are involved
 in resuming from hibernation.  In fact, the restore kernel may be completely
 different from the image kernel: a different configuration and even a different
 version.  This has important consequences for device drivers and their
 subsystems.
 
 To be able to load the system image into memory, the restore kernel needs to
 include at least a subset of device drivers allowing it to access the storage
 medium containing the image, although it doesn't need to include all of the
 drivers present in the image kernel.  After the image has been loaded, the
 devices managed by the boot kernel need to be prepared for passing control back
 to the image kernel.  This is very similar to the initial steps involved in
 creating a system image, and it is accomplished in the same way, using
 ``prepare``, ``freeze``, and ``freeze_noirq`` phases.  However, the devices
 affected by these phases are only those having drivers in the restore kernel;
 other devices will still be in whatever state the boot loader left them.
 
 Should the restoration of the pre-hibernation memory contents fail, the restore
 kernel would go through the "thawing" procedure described above, using the
 ``thaw_noirq``, ``thaw_early``, ``thaw``, and ``complete`` phases, and then
 continue running normally.  This happens only rarely.  Most often the
 pre-hibernation memory contents are restored successfully and control is passed
 to the image kernel, which then becomes responsible for bringing the system back
 to the working state.
 
 To achieve this, the image kernel must restore the devices' pre-hibernation
 functionality.  The operation is much like waking up from a sleep state (with
 the memory contents preserved), although it involves different phases:
 ``restore_noirq``, ``restore_early``, ``restore``, ``complete``.
 
     1.  The ``restore_noirq`` phase is analogous to the ``resume_noirq`` phase.
 
     2.  The ``restore_early`` phase is analogous to the ``resume_early`` phase.
 
     3.  The ``restore`` phase is analogous to the ``resume`` phase.
 
     4.  The ``complete`` phase is discussed above.
 
 The main difference from ``resume[_early|_noirq]`` is that
 ``restore[_early|_noirq]`` must assume the device has been accessed and
 reconfigured by the boot loader or the restore kernel.  Consequently, the state
 of the device may be different from the state remembered from the ``freeze``,
 ``freeze_late`` and ``freeze_noirq`` phases.  The device may even need to be
 reset and completely re-initialized.  In many cases this difference doesn't
 matter, so the ``->resume[_early|_noirq]`` and ``->restore[_early|_norq]``
 method pointers can be set to the same routines.  Nevertheless, different
 callback pointers are used in case there is a situation where it actually does
 matter.
 
 
 Power Management Notifiers
 ==========================
 
 There are some operations that cannot be carried out by the power management
 callbacks discussed above, because the callbacks occur too late or too early.
 To handle these cases, subsystems and device drivers may register power
 management notifiers that are called before tasks are frozen and after they have
 been thawed.  Generally speaking, the PM notifiers are suitable for performing
 actions that either require user space to be available, or at least won't
 interfere with user space.
 
 For details refer to Documentation/driver-api/pm/notifiers.rst.
 
 
 Device Low-Power (suspend) States
 =================================
 
 Device low-power states aren't standard.  One device might only handle
 "on" and "off", while another might support a dozen different versions of
 "on" (how many engines are active?), plus a state that gets back to "on"
 faster than from a full "off".
 
 Some buses define rules about what different suspend states mean.  PCI
 gives one example: after the suspend sequence completes, a non-legacy
 PCI device may not perform DMA or issue IRQs, and any wakeup events it
 issues would be issued through the PME# bus signal.  Plus, there are
 several PCI-standard device states, some of which are optional.
 
 In contrast, integrated system-on-chip processors often use IRQs as the
 wakeup event sources (so drivers would call :c:func:`enable_irq_wake`) and
 might be able to treat DMA completion as a wakeup event (sometimes DMA can stay
 active too, it'd only be the CPU and some peripherals that sleep).
 
 Some details here may be platform-specific.  Systems may have devices that
 can be fully active in certain sleep states, such as an LCD display that's
 refreshed using DMA while most of the system is sleeping lightly ... and
 its frame buffer might even be updated by a DSP or other non-Linux CPU while
 the Linux control processor stays idle.
 
 Moreover, the specific actions taken may depend on the target system state.
 One target system state might allow a given device to be very operational;
 another might require a hard shut down with re-initialization on resume.
 And two different target systems might use the same device in different
 ways; the aforementioned LCD might be active in one product's "standby",
 but a different product using the same SOC might work differently.
 
 
 Device Power Management Domains
 ===============================
 
 Sometimes devices share reference clocks or other power resources.  In those
 cases it generally is not possible to put devices into low-power states
 individually.  Instead, a set of devices sharing a power resource can be put
 into a low-power state together at the same time by turning off the shared
 power resource.  Of course, they also need to be put into the full-power state
 together, by turning the shared power resource on.  A set of devices with this
 property is often referred to as a power domain. A power domain may also be
 nested inside another power domain. The nested domain is referred to as the
 sub-domain of the parent domain.
 
 Support for power domains is provided through the :c:member:`pm_domain` field of
 struct device.  This field is a pointer to an object of type
 struct dev_pm_domain, defined in :file:`include/linux/pm.h`, providing a set
 of power management callbacks analogous to the subsystem-level and device driver
 callbacks that are executed for the given device during all power transitions,
 instead of the respective subsystem-level callbacks.  Specifically, if a
 device's :c:member:`pm_domain` pointer is not NULL, the ``->suspend()`` callback
 from the object pointed to by it will be executed instead of its subsystem's
 (e.g. bus type's) ``->suspend()`` callback and analogously for all of the
 remaining callbacks.  In other words, power management domain callbacks, if
 defined for the given device, always take precedence over the callbacks provided
 by the device's subsystem (e.g. bus type).
 
 The support for device power management domains is only relevant to platforms
 needing to use the same device driver power management callbacks in many
 different power domain configurations and wanting to avoid incorporating the
 support for power domains into subsystem-level callbacks, for example by
 modifying the platform bus type.  Other platforms need not implement it or take
 it into account in any way.
 
 Devices may be defined as IRQ-safe which indicates to the PM core that their
 runtime PM callbacks may be invoked with disabled interrupts (see
 Documentation/power/runtime_pm.rst for more information).  If an
 IRQ-safe device belongs to a PM domain, the runtime PM of the domain will be
 disallowed, unless the domain itself is defined as IRQ-safe. However, it
 makes sense to define a PM domain as IRQ-safe only if all the devices in it
 are IRQ-safe. Moreover, if an IRQ-safe domain has a parent domain, the runtime
 PM of the parent is only allowed if the parent itself is IRQ-safe too with the
 additional restriction that all child domains of an IRQ-safe parent must also
 be IRQ-safe.
 
 
 Runtime Power Management
 ========================
 
 Many devices are able to dynamically power down while the system is still
 running. This feature is useful for devices that are not being used, and
 can offer significant power savings on a running system.  These devices
 often support a range of runtime power states, which might use names such
 as "off", "sleep", "idle", "active", and so on.  Those states will in some
 cases (like PCI) be partially constrained by the bus the device uses, and will
 usually include hardware states that are also used in system sleep states.
 
 A system-wide power transition can be started while some devices are in low
 power states due to runtime power management.  The system sleep PM callbacks
 should recognize such situations and react to them appropriately, but the
 necessary actions are subsystem-specific.
 
 In some cases the decision may be made at the subsystem level while in other
 cases the device driver may be left to decide.  In some cases it may be
 desirable to leave a suspended device in that state during a system-wide power
 transition, but in other cases the device must be put back into the full-power
 state temporarily, for example so that its system wakeup capability can be
 disabled.  This all depends on the hardware and the design of the subsystem and
 device driver in question.
 
 If it is necessary to resume a device from runtime suspend during a system-wide
 transition into a sleep state, that can be done by calling
 :c:func:`pm_runtime_resume` from the ``->suspend`` callback (or the ``->freeze``
 or ``->poweroff`` callback for transitions related to hibernation) of either the
 device's driver or its subsystem (for example, a bus type or a PM domain).
 However, subsystems must not otherwise change the runtime status of devices
 from their ``->prepare`` and ``->suspend`` callbacks (or equivalent) *before*
 invoking device drivers' ``->suspend`` callbacks (or equivalent).
 
 .. _smart_suspend_flag:
 
 The ``DPM_FLAG_SMART_SUSPEND`` Driver Flag
 ------------------------------------------
 
 Some bus types and PM domains have a policy to resume all devices from runtime
 suspend upfront in their ``->suspend`` callbacks, but that may not be really
 necessary if the device's driver can cope with runtime-suspended devices.
 The driver can indicate this by setting ``DPM_FLAG_SMART_SUSPEND`` in
 :c:member:`power.driver_flags` at probe time, with the assistance of the
 :c:func:`dev_pm_set_driver_flags` helper routine.
 
 Setting that flag causes the PM core and middle-layer code
 (bus types, PM domains etc.) to skip the ``->suspend_late`` and
 ``->suspend_noirq`` callbacks provided by the driver if the device remains in
 runtime suspend throughout those phases of the system-wide suspend (and
 similarly for the "freeze" and "poweroff" parts of system hibernation).
 [Otherwise the same driver
 callback might be executed twice in a row for the same device, which would not
 be valid in general.]  If the middle-layer system-wide PM callbacks are present
 for the device then they are responsible for skipping these driver callbacks;
 if not then the PM core skips them.  The subsystem callback routines can
 determine whether they need to skip the driver callbacks by testing the return
 value from the :c:func:`dev_pm_skip_suspend` helper function.
 
 In addition, with ``DPM_FLAG_SMART_SUSPEND`` set, the driver's ``->thaw_noirq``
 and ``->thaw_early`` callbacks are skipped in hibernation if the device remained
 in runtime suspend throughout the preceding "freeze" transition.  Again, if the
 middle-layer callbacks are present for the device, they are responsible for
 doing this, otherwise the PM core takes care of it.
 
 
 The ``DPM_FLAG_MAY_SKIP_RESUME`` Driver Flag
 --------------------------------------------
 
 During system-wide resume from a sleep state it's easiest to put devices into
 the full-power state, as explained in Documentation/power/runtime_pm.rst.
 [Refer to that document for more information regarding this particular issue as
 well as for information on the device runtime power management framework in
 general.]  However, it often is desirable to leave devices in suspend after
 system transitions to the working state, especially if those devices had been in
 runtime suspend before the preceding system-wide suspend (or analogous)
 transition.
 
 To that end, device drivers can use the ``DPM_FLAG_MAY_SKIP_RESUME`` flag to
 indicate to the PM core and middle-layer code that they allow their "noirq" and
 "early" resume callbacks to be skipped if the device can be left in suspend
 after system-wide PM transitions to the working state.  Whether or not that is
 the case generally depends on the state of the device before the given system
 suspend-resume cycle and on the type of the system transition under way.
 In particular, the "thaw" and "restore" transitions related to hibernation are
 not affected by ``DPM_FLAG_MAY_SKIP_RESUME`` at all.  [All callbacks are
 issued during the "restore" transition regardless of the flag settings,
 and whether or not any driver callbacks
 are skipped during the "thaw" transition depends whether or not the
 ``DPM_FLAG_SMART_SUSPEND`` flag is set (see `above <smart_suspend_flag_>`_).
 In addition, a device is not allowed to remain in runtime suspend if any of its
 children will be returned to full power.]
 
 The ``DPM_FLAG_MAY_SKIP_RESUME`` flag is taken into account in combination with
 the :c:member:`power.may_skip_resume` status bit set by the PM core during the
 "suspend" phase of suspend-type transitions.  If the driver or the middle layer
 has a reason to prevent the driver's "noirq" and "early" resume callbacks from
 being skipped during the subsequent system resume transition, it should
 clear :c:member:`power.may_skip_resume` in its ``->suspend``, ``->suspend_late``
 or ``->suspend_noirq`` callback.  [Note that the drivers setting
 ``DPM_FLAG_SMART_SUSPEND`` need to clear :c:member:`power.may_skip_resume` in
 their ``->suspend`` callback in case the other two are skipped.]
 
 Setting the :c:member:`power.may_skip_resume` status bit along with the
 ``DPM_FLAG_MAY_SKIP_RESUME`` flag is necessary, but generally not sufficient,
 for the driver's "noirq" and "early" resume callbacks to be skipped.  Whether or
 not they should be skipped can be determined by evaluating the
 :c:func:`dev_pm_skip_resume` helper function.
 
 If that function returns ``true``, the driver's "noirq" and "early" resume
 callbacks should be skipped and the device's runtime PM status will be set to
 "suspended" by the PM core.  Otherwise, if the device was runtime-suspended
 during the preceding system-wide suspend transition and its
 ``DPM_FLAG_SMART_SUSPEND`` is set, its runtime PM status will be set to
 "active" by the PM core.  [Hence, the drivers that do not set
 ``DPM_FLAG_SMART_SUSPEND`` should not expect the runtime PM status of their
 devices to be changed from "suspended" to "active" by the PM core during
 system-wide resume-type transitions.]
 
 If the ``DPM_FLAG_MAY_SKIP_RESUME`` flag is not set for a device, but
 ``DPM_FLAG_SMART_SUSPEND`` is set and the driver's "late" and "noirq" suspend
 callbacks are skipped, its system-wide "noirq" and "early" resume callbacks, if
 present, are invoked as usual and the device's runtime PM status is set to
 "active" by the PM core before enabling runtime PM for it.  In that case, the
 driver must be prepared to cope with the invocation of its system-wide resume
 callbacks back-to-back with its ``->runtime_suspend`` one (without the
 intervening ``->runtime_resume`` and system-wide suspend callbacks) and the
 final state of the device must reflect the "active" runtime PM status in that
 case.  [Note that this is not a problem at all if the driver's
 ``->suspend_late`` callback pointer points to the same function as its
 ``->runtime_suspend`` one and its ``->resume_early`` callback pointer points to
 the same function as the ``->runtime_resume`` one, while none of the other
 system-wide suspend-resume callbacks of the driver are present, for example.]
 
 Likewise, if ``DPM_FLAG_MAY_SKIP_RESUME`` is set for a device, its driver's
 system-wide "noirq" and "early" resume callbacks may be skipped while its "late"
 and "noirq" suspend callbacks may have been executed (in principle, regardless
 of whether or not ``DPM_FLAG_SMART_SUSPEND`` is set).  In that case, the driver
 needs to be able to cope with the invocation of its ``->runtime_resume``
 callback back-to-back with its "late" and "noirq" suspend ones.  [For instance,
 that is not a concern if the driver sets both ``DPM_FLAG_SMART_SUSPEND`` and
 ``DPM_FLAG_MAY_SKIP_RESUME`` and uses the same pair of suspend/resume callback
 functions for runtime PM and system-wide suspend/resume.]

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