.. |struct dev_pm_ops| replace:: :c:type:`struct dev_pm_ops <dev_pm_ops>`
.. |struct dev_pm_domain| replace:: :c:type:`struct dev_pm_domain <dev_pm_domain>`
.. |struct bus_type| replace:: :c:type:`struct bus_type <bus_type>`
.. |struct device_type| replace:: :c:type:`struct device_type <device_type>`
.. |struct class| replace:: :c:type:`struct class <class>`
.. |struct wakeup_source| replace:: :c:type:`struct wakeup_source <wakeup_source>`
.. |struct device| replace:: :c:type:`struct device <device>`

==============================
Device Power Management Basics
==============================

::

 Copyright (c) 2010-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
 Copyright (c) 2010 Alan Stern <stern@rowland.harvard.edu>
 Copyright (c) 2016 Intel Corp., 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
:file:`Documentation/power/runtime_pm.txt`.


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 invoked directly 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_NEVER_SKIP`` 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 can only resume the devices from runtime suspend by
	calling :c:func:`pm_runtime_resume` for them, if that is necessary, and
	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).

    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(dev)` 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 with that.

	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 (the ``suspend``, ``suspend_late``,
	``suspend_noirq`` phases of system suspend and the ``resume_noirq``,
	``resume_early``, ``resume`` phases of system resume may have been
	skipped for it).  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.  Namely, if that flag is set when the
	``->complete`` callback is being run, it has been called directly after
	the preceding ``->prepare`` 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.  The only 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.


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 :doc:`notifiers`.


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
:file:`Documentation/power/runtime_pm.txt` 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` for it from the ``->suspend`` callback (or its
couterpart for transitions related to hibernation) of either the device's driver
or a subsystem responsible for it (for example, a bus type or a PM domain).
That is guaranteed to work by the requirement that subsystems must not change
the state of devices (possibly except for resuming them from runtime suspend)
from their ``->prepare`` and ``->suspend`` callbacks (or equivalent) *before*
invoking device drivers' ``->suspend`` callbacks (or equivalent).

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 driver of the device can cope with runtime-suspended devices.
The driver can indicate that by setting ``DPM_FLAG_SMART_SUSPEND`` in
:c:member:`power.driver_flags` at the probe time, by passing it to the
:c:func:`dev_pm_set_driver_flags` helper.  That also may cause 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 at the beginning of the ``suspend_late`` phase of system-wide
suspend (or in the ``poweroff_late`` phase of hibernation), when runtime PM
has been disabled for it, under the assumption that its state should not change
after that point until the system-wide transition is over.  If that happens, the
driver's system-wide resume callbacks, if present, may still be invoked during
the subsequent system-wide resume transition and the device's runtime power
management status may be set to "active" before enabling runtime PM for it,
so 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 so on) and the final state of the device
must reflect the "active" status for runtime PM in that case.

During system-wide resume from a sleep state it's easiest to put devices into
the full-power state, as explained in :file:`Documentation/power/runtime_pm.txt`.
[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.  Device drivers can use the ``DPM_FLAG_LEAVE_SUSPENDED`` flag to
indicate to the PM core (and middle-layer code) that they prefer the specific
devices handled by them to be left suspended and they have no problems with
skipping their system-wide resume callbacks for this reason.  Whether or not the
devices will actually be left in suspend may depend on their state before the
given system suspend-resume cycle and on the type of the system transition under
way.  In particular, devices are not left suspended if that transition is a
restore from hibernation, as device states are not guaranteed to be reflected
by the information stored in the hibernation image in that case.

The middle-layer code involved in the handling of the device is expected to
indicate to the PM core if the device may be left in suspend by setting its
:c:member:`power.may_skip_resume` status bit which is checked by the PM core
during the "noirq" phase of the preceding system-wide suspend (or analogous)
transition.  The middle layer is then responsible for handling the device as
appropriate in its "noirq" resume callback, which is executed regardless of
whether or not the device is left suspended, but the other resume callbacks
(except for ``->complete``) will be skipped automatically by the PM core if the
device really can be left in suspend.
