path: root/Documentation/driver-api/pm/devices.rst

.. SPDX-License-Identifier: GPL-2.0
.. include:: <isonum.txt>

.. |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>`

.. _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
:file:`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 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