Device Power Management Basics

device power management의 두 모델

Linux code의 대부분이 device driver이므로 power management code 역시 상당 부분이 driver-specific입니다. 이 문서는 driver model core에 연결되는 모든 subsystem이 공유하는 PM model과 interface를 설명하며, 각 domain-specific driver 작업의 배경이 됩니다.

System Sleep model에서는 system 전체가 suspend-to-RAM 또는 hibernation으로 들어갈 때 device, bus, class driver가 역할별 suspend/resume method로 협력합니다. hardware와 software subsystem을 data 손실 없이 정지하고 다시 활성화하며, wakeup-capable device는 `/sys/devices/.../power/wakeup` 또는 일부 Ethernet의 ethtool interface로 system wakeup 사용 여부를 제어합니다.

Runtime Power Management model에서는 system이 실행 중이어도 사용하지 않는 device를 독립적으로 저전력 상태에 넣습니다. 다만 parent는 모든 child가 suspend되기 전에 suspend할 수 없고 bus-specific operation이 필요할 수 있으므로 device driver뿐 아니라 bus type·device type·device class 같은 subsystem driver와 PM core가 함께 관여합니다. runtime-suspended device는 system suspend나 hibernation 전환에서 특별히 처리해야 할 수도 있습니다.

대부분의 suspended device는 wakeup event를 제외한 DMA와 IRQ, data I/O, upstream request를 모두 quiesce합니다. hardware wakeup 예에는 RTC alarm, Wake-on-LAN packet, keyboard·mouse activity, PCMCIA/MMC/USB media insertion과 removal이 있습니다.

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

.. _driverapi_pm_devices:

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

:Copyright: |copy| 2010-2011 Rafael J. Wysocki <[email protected]>, Novell Inc.
:Copyright: |copy| 2010 Alan Stern <[email protected]>
:Copyright: |copy| 2016 Intel Corporation

:Author: Rafael J. Wysocki <[email protected]>


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).

dev_pm_ops와 subsystem-level method

bus type, device type, device class와 device driver는 system sleep과 runtime PM에 참여하는 programming interface를 제공합니다. subsystem과 driver level의 PM operation은 `include/linux/pm.h`에 정의된 `struct dev_pm_ops` object를 채워 구현합니다. 마지막 세 method는 runtime PM 전용이고 나머지는 system-wide power transition에 사용됩니다.

일부 subsystem에는 `struct dev_pm_ops`를 사용하지 않는 deprecated legacy PM interface도 있지만 system sleep method를 제한적으로만 구현하므로 이 문서에서는 다루지 않습니다.

core suspend/resume method는 `struct dev_pm_domain`의 `ops`, 또는 `struct bus_type`, `struct device_type`, `struct class`의 `pm` member가 가리키는 `struct dev_pm_ops`에 있습니다. PCI·USB 같은 infrastructure와 type/class driver가 hardware 특성에 맞게 이를 구현하며, 각 phase에서 driver-model tree의 parent-child 순서를 지켜 호출됩니다.

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.

power/wakeup capability와 policy

모든 device object에는 system sleep에서 system을 깨울 수 있는 hardware event를 제어하는 field가 있습니다. bus 또는 device driver는 `include/linux/pm_wakeup.h`의 `device_set_wakeup_capable()`과 `device_set_wakeup_enable()`로 이를 초기화합니다.

`power.can_wakeup`은 device와 driver가 wakeup event를 물리적으로 지원하는지 기록하는 capability입니다. `power.wakeup`은 `struct wakeup_source` pointer로, wakeup mechanism 사용 여부 제어와 device가 signal한 system wakeup event를 PM core에 알리는 데 사용합니다. wakeup-capable device에만 존재하며 `device_set_wakeup_capable()`이 object와 sysfs file을 만들거나 제거합니다.

capability는 hardware 사실이라 kernel이 추적하지만 실제 사용 여부는 user-space policy입니다. user space는 `power/wakeup`에 `enabled` 또는 `disabled`를 쓰고 읽을 수 있습니다. 기본값은 대부분 disabled이지만 power button, keyboard, ethtool로 WoL을 설정한 Ethernet adapter, 다른 bus의 wakeup request를 전달하는 PCIe port 같은 device는 enabled가 적절합니다.

`device_may_wakeup()`은 wakeup object가 존재하고 sysfs 값이 enabled일 때만 true입니다. subsystem과 driver는 system sleep에서 wake mechanism 설정 여부를 이 값으로 결정해야 하며 device driver가 `device_set_wakeup_enable()`을 직접 호출해서는 안 됩니다. system wakeup과 runtime PM의 remote wakeup은 개념적으로 다르며, remote wakeup을 지원하는 runtime-PM device에서는 항상 enable해야 합니다.

: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.

power/control과 runtime_auto

driver-model device의 `runtime_auto` flag는 runtime PM 대상인지 제어합니다. bus type 또는 subsystem code가 `pm_runtime_allow()`나 `pm_runtime_forbid()`로 초기화하며 기본은 runtime PM 허용입니다.

user space가 `power/control`에 `auto`를 쓰면 `pm_runtime_allow()`가 flag를 설정해 driver의 runtime PM을 허용합니다. `on`을 쓰면 `pm_runtime_forbid()`가 flag를 지우고, device가 저전력 상태였다면 full power로 복귀시킨 뒤 runtime PM을 막습니다. file을 읽으면 현재 flag 상태를 확인할 수 있습니다.

`runtime_auto`는 system-wide transition 처리에는 영향을 주지 않습니다. flag가 clear여도 system sleep으로 갈 때 device는 대부분 저전력 상태로 전환되어야 합니다. runtime PM framework의 상세 내용은 `Documentation/power/runtime_pm.rst`에 있습니다.

: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.

system sleep driver 호출과 hierarchy ordering

system이 sleep state로 들어갈 때 각 driver는 target system state와 호환되는 상태로 device를 suspend합니다. 보통 off에 가깝지만 system-specific이며 wakeup-enabled device는 system을 깨우기 위해 일부 기능을 유지합니다. 복귀할 때 driver가 full power로 resume하며 suspend와 resume은 항상 짝을 이루는 multi-phase operation입니다.

단순 driver는 suspend에서 class I/O를 quiesce하고 `suspend_noirq`에서 hardware를 가능한 한 끈 뒤, resume에서 hardware를 완전히 재초기화하고 class queue를 다시 활성화할 수 있습니다. wakeup을 지원하는 driver는 hardware event 생성도 준비합니다.

bridge와 control link가 child suspend/resume 중 사용 가능하도록 suspend는 device hierarchy를 bottom-up으로, resume은 top-down으로 순회합니다. child는 parent보다 먼저 register·probe·resume할 수 없고 parent보다 나중에 remove·suspend할 수 없습니다.

device tree는 hardware control-bus topology와 맞아야 합니다. parent가 suspend 대상으로 선택되었거나 이미 suspended인 동안, 또는 다른 모든 device가 suspend된 뒤에는 child registration이 실패할 수 있으므로 driver는 이를 처리해야 합니다.

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 buses.]
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 PM phase와 callback 우선순위

suspend-to-idle, standby, suspend-to-RAM, hibernation은 여러 phase로 수행되며 각 phase에서 모든 device callback이 끝나야 다음 phase가 시작됩니다. phase는 task freeze 뒤, thaw 전에 실행됩니다. `*_noirq` phase에서는 `IRQF_NO_SUSPEND` handler를 제외한 IRQ handler가 disabled 상태입니다.

각 phase의 PM domain, type, class, bus, driver callback은 core 관점에서 mutually exclusive입니다. 선택 우선순위는 `dev->pm_domain->ops`, `dev->type->pm`, `dev->class->pm`, `dev->bus->pm` 순입니다. 이를 통해 PM domain과 device type이 bus 또는 class callback을 override할 수 있습니다.

선택된 domain/type/class/bus callback은 필요하면 `dev->driver->pm` method를 직접 호출할 수 있지만 의무는 아닙니다. 선택할 subsystem callback이 없으면 PM core가 대응하는 driver method를 대신 실행합니다.

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.

system suspend 진입과 direct-complete

freeze, standby, memory sleep 진입 phase는 `prepare`, `suspend`, `suspend_late`, `suspend_noirq`입니다. `prepare`만 hierarchy를 top-down으로 순회하며 새 child 등록 race를 막습니다. callback 뒤에는 device 아래에 child를 등록할 수 없습니다. transition 준비는 할 수 있지만 저전력 상태로 넣거나 runtime-PM state를 변경해서는 안 됩니다.

runtime-suspended device와 모든 descendant의 `prepare`가 positive를 반환하면 PM core는 direct-complete를 적용할 수 있습니다. 해당 subtree의 suspend·late·noirq와 대응 resume phase를 모두 건너뛰고 다음 호출은 `complete`가 됩니다. runtime PM이 disabled여도 status만 suspended면 적용될 수 있으므로 runtime PM을 지원하지 않는 system-sleep driver는 prepare에서 positive를 반환하면 안 됩니다.

`DPM_FLAG_NO_DIRECT_COMPLETE`는 device와 ancestor에 direct-complete를 금지합니다. `DPM_FLAG_SMART_PREPARE`는 middle layer가 driver의 prepare return을 고려하게 하며, driver도 positive를 반환한 경우에만 middle-layer prepare가 positive를 반환할 수 있습니다. flag는 probe 때 `dev_pm_set_driver_flags()`로 설정합니다.

`suspend`는 I/O를 quiesce하고 필요하면 register 저장, 저전력 진입, wakeup enable을 수행합니다. subsystem은 driver suspend 전에 `pm_runtime_resume()`로 runtime-suspended device를 깨울 수 있지만 그 밖의 state 변경은 하면 안 됩니다. core는 prepare 전에 `pm_runtime_get_noresume()`, complete 뒤 `pm_runtime_put()`을 호출해 새 runtime suspend를 막습니다.

`suspend_late`는 quiesce와 state save를 분리할 때 후자를 수행하며 runtime PM disabled 뒤 실행됩니다. IRQ handler가 disabled된 `suspend_noirq`는 남은 register를 저장하고 최종 저전력 상태로 전환합니다. shared interrupt bus에서는 다른 device interrupt를 잘못 처리하지 않도록 특히 필요합니다.

모든 I/O·DMA·IRQ를 멈추고 충분한 state를 저장한 뒤 clock, supply, voltage를 줄입니다. `device_may_wakeup()`이 true면 `enable_irq_wake()`나 `pci_enable_wake()`로 wake signal을 준비합니다. 어느 callback이든 error를 반환하면 PM core가 이미 suspend한 device를 resume해 전체 작업을 unwind합니다.

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, consequently, 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.

system suspend 복귀

freeze, standby, memory sleep에서 복귀하는 phase는 `resume_noirq`, `resume_early`, `resume`, `complete`입니다. `resume_noirq`는 interrupt handler가 다시 호출되기 전에 `suspend_noirq`의 동작을 되돌립니다. shared vector bus에서는 driver가 interrupt source를 판별·처리할 수 있는 상태를 만들어야 합니다. PCI는 device를 D0로 놓고 standard configuration register를 복원한 뒤 driver callback을 호출합니다.

`resume_early`는 `suspend_late`를 되돌려 일반 resume을 준비하고, `resume`은 `suspend`를 되돌려 normal I/O가 가능한 operating state로 복귀합니다. `complete`는 prepare를 되돌리며 다른 resume phase와 달리 hierarchy를 bottom-up으로 순회합니다. 새 child는 resume callback이 실행된 뒤부터 등록할 수 있습니다.

direct-complete를 사용했다면 중간 suspend/resume callback이 모두 생략되었으므로 `complete`가 device consistency를 전적으로 책임집니다. `power.direct_complete` flag를 확인해 runtime resume request 같은 특별 조치를 할 수 있습니다.

복귀 뒤 DMA·IRQ·clock이 정상이어야 하지만 platform의 run state가 이전과 다르거나 hardware가 reset되었을 수 있으므로 driver는 complete reinitialization을 지원해야 합니다. removable bus에서는 sleep 중 device 제거도 감지해야 합니다. resume callback error는 PM core가 복구할 방법이 없어 log만 남기고 무시합니다.

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 buses
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.

hibernation image 생성과 poweroff

hibernation은 system image를 만들고 저장하므로 memory를 보존하는 sleep보다 복잡합니다. task를 freeze하고 충분한 memory를 확보한 뒤 device를 freeze해 안정된 memory image를 만들고, thaw해 image를 permanent storage에 저장한 다음 다시 poweroff phase로 system을 종료합니다.

전체 순서는 `prepare`, `freeze`, `freeze_late`, `freeze_noirq`, image creation, `thaw_noirq`, `thaw_early`, `thaw`, `complete`, image save, `prepare`, `poweroff`, `poweroff_late`, `poweroff_noirq`입니다.

`freeze`는 IRQ·DMA를 quiesce하고 필요하면 register를 저장하지만 저전력 상태로 넣거나 wakeup event를 준비할 필요가 없습니다. `freeze_late`와 `freeze_noirq`도 대응 suspend phase와 유사하지만 low power와 wakeup을 설정하지 않습니다. 이 상태에서 모든 device가 inactive여야 memory의 atomic snapshot을 만들 수 있습니다.

`thaw_noirq`, `thaw_early`, `thaw`는 freeze phase를 되돌려 image 저장에 필요한 device를 operating state로 만듭니다. image를 저장한 뒤 poweroff 계열은 suspend 계열과 비슷하지만 register는 이미 freeze 때 저장했으므로 다시 저장하지 않아도 되고 firmware가 system 전체를 끄면 device별 low-power 진입도 불필요할 수 있습니다.

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.

restore kernel과 image kernel의 hibernation 복귀

hibernation 복귀는 system image를 memory에 load하고 pre-hibernation contents를 복원해야 합니다. boot loader만으로는 필요한 protocol과 기능이 부족하므로 fresh `restore kernel`을 부팅합니다. restore kernel이 image를 읽고 memory를 복원한 뒤 `image kernel`로 control을 넘기며, 두 kernel은 configuration이나 version이 다를 수도 있습니다.

restore kernel에는 image storage 접근에 필요한 driver subset만 있으면 됩니다. image를 load한 뒤 restore-kernel이 관리한 device를 `prepare`, `freeze`, `freeze_noirq`로 정리해 image kernel에 넘깁니다. 다른 device는 boot loader가 남긴 상태일 수 있습니다.

memory restore에 실패하면 restore kernel이 `thaw_noirq`, `thaw_early`, `thaw`, `complete`로 복귀해 계속 실행합니다. 성공하면 image kernel이 `restore_noirq`, `restore_early`, `restore`, `complete`로 pre-hibernation 기능을 회복합니다.

restore callback은 device가 boot loader나 restore kernel에 의해 access·reconfigure되었다고 가정해야 합니다. freeze 때 기억한 상태와 다를 수 있어 reset과 완전한 재초기화가 필요할 수 있습니다. 차이가 중요하지 않으면 resume과 restore callback pointer를 같은 routine으로 지정할 수 있지만 이를 구분할 상황을 위해 별도 callback이 존재합니다.

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 notifier

device PM callback은 task freeze보다 늦거나 thaw보다 이른 시점에 실행되므로 user space가 필요한 일부 작업에는 맞지 않습니다. subsystem과 driver는 task freeze 전과 thaw 후에 호출되는 power management notifier를 등록할 수 있습니다.

PM notifier는 user space가 사용 가능해야 하거나 user space 동작을 방해하지 않는 작업에 적합합니다. 자세한 내용은 `Documentation/driver-api/pm/notifiers.rst`를 참조합니다.

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 state의 platform 차이

device low-power state는 표준화되어 있지 않습니다. 어떤 device는 on/off만 제공하고 다른 device는 활성 engine 수에 따른 여러 on 변형과 full off보다 빠른 복귀 state를 제공할 수 있습니다.

bus가 suspend 의미를 정의하기도 합니다. PCI device는 suspend 뒤 DMA와 IRQ를 내지 않고 PME#로 wakeup을 signal하며 표준 state 중 일부는 optional입니다. 반면 integrated SoC는 IRQ를 wake source로 써 `enable_irq_wake()`를 호출하고 DMA completion을 wakeup으로 취급하거나 DMA를 계속 동작시킬 수도 있습니다.

platform에 따라 LCD가 DMA refresh를 계속하거나 DSP가 framebuffer를 갱신하는 동안 Linux CPU만 idle일 수 있습니다. 동일 device도 target system state와 product 설계에 따라 active에 가깝게 유지되거나 hard shutdown과 resume reinitialization이 필요합니다.

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 domain

여러 device가 reference clock이나 power resource를 공유하면 개별적으로 저전력 상태에 넣을 수 없습니다. 공유 resource를 끄고 켜며 함께 suspend/resume하는 집합을 power domain이라 하며 domain은 parent domain 안에 sub-domain으로 중첩될 수 있습니다.

`struct device`의 `pm_domain`은 `include/linux/pm.h`의 `struct dev_pm_domain`을 가리킵니다. 이 object는 subsystem/driver와 유사한 PM callback set을 제공하며 모든 transition에서 해당 subsystem callback 대신 실행됩니다. 즉 device에 PM domain callback이 있으면 bus type 등 subsystem callback보다 항상 우선합니다.

PM domain은 같은 device driver callback을 여러 power-domain 구성에서 재사용하면서 platform bus 같은 subsystem code에 domain support를 넣지 않으려는 platform에 유용합니다. 필요 없는 platform은 구현하거나 고려하지 않아도 됩니다.

IRQ-safe device의 runtime callback은 interrupt-disabled 상태에서 호출될 수 있습니다. 이런 device가 PM domain에 속하면 domain 자체도 IRQ-safe가 아니면 domain runtime PM은 금지됩니다. IRQ-safe domain에는 모든 device가 IRQ-safe여야 하고, parent domain이 있으면 parent와 그 모든 child domain도 IRQ-safe여야 runtime PM이 허용됩니다.

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 PM과 system-wide transition의 교차

많은 device는 system 실행 중 사용되지 않을 때 off, sleep, idle, active 같은 runtime power state로 내려가 큰 energy를 절약합니다. bus가 일부 state를 제한할 수 있고 system sleep과 같은 hardware state를 공유하기도 합니다.

runtime-suspended device가 있는 동안 system-wide transition이 시작될 수 있습니다. hardware와 subsystem 설계에 따라 그대로 둘 수도 있고 system wakeup capability를 끄기 위해 잠시 full power로 복귀해야 할 수도 있습니다.

system sleep 진입 중 runtime resume이 필요하면 device driver 또는 bus/PM domain의 `suspend` callback에서 `pm_runtime_resume()`을 호출합니다. hibernation 관련 transition에서는 대응 `freeze` 또는 `poweroff` callback에서 호출합니다. subsystem은 driver callback을 호출하기 전 `prepare`나 `suspend`에서 이 resume 외의 runtime status 변경을 해서는 안 됩니다.

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:

DPM_FLAG_SMART_SUSPEND

일부 bus와 PM domain은 `suspend`에서 모든 runtime-suspended device를 미리 resume하지만 driver가 suspended 상태를 처리할 수 있다면 불필요합니다. driver는 probe 때 `dev_pm_set_driver_flags()`로 `power.driver_flags`에 `DPM_FLAG_SMART_SUSPEND`를 설정해 이를 알립니다.

flag가 설정되고 device가 system suspend의 late/noirq phase까지 계속 runtime suspend라면 PM core 또는 middle layer는 driver의 `suspend_late`와 `suspend_noirq`를 건너뜁니다. hibernation의 freeze와 poweroff 계열에도 같은 원칙이 적용되어 같은 callback이 연속 두 번 실행되는 문제를 막습니다.

middle-layer callback이 있으면 그 계층이 `dev_pm_skip_suspend()` 결과로 driver callback 생략을 결정하고, 없으면 PM core가 처리합니다. hibernation freeze 전체에서 runtime suspend를 유지했다면 `thaw_noirq`와 `thaw_early`도 건너뜁니다.

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.

DPM_FLAG_MAY_SKIP_RESUME

system-wide resume에서는 device를 full power로 올리는 방식이 가장 단순하지만, transition 전 runtime suspend였던 device는 working state로 돌아온 뒤에도 suspend 상태로 남기는 것이 바람직할 수 있습니다. driver는 `DPM_FLAG_MAY_SKIP_RESUME`로 noirq와 early resume callback 생략을 허용합니다.

실제 생략 여부는 이전 device state와 transition type에 따라 달라집니다. hibernation의 restore는 flag와 무관하게 모든 callback을 호출하고 thaw에서의 생략은 `DPM_FLAG_SMART_SUSPEND`에 좌우됩니다. child가 full power로 돌아오면 parent는 runtime suspend에 남을 수 없습니다.

flag는 suspend-type transition의 suspend phase에서 PM core가 설정한 `power.may_skip_resume` status bit와 함께 평가됩니다. driver나 middle layer가 noirq/early resume을 반드시 실행해야 한다면 `suspend`, `suspend_late`, `suspend_noirq` 중 해당 bit를 clear해야 합니다. SMART_SUSPEND driver는 뒤 두 callback이 생략될 수 있으므로 `suspend`에서 clear해야 합니다.

flag와 status bit가 모두 있어도 충분하지 않으며 `dev_pm_skip_resume()`가 true인지 평가합니다. true이면 noirq/early resume을 건너뛰고 core가 runtime status를 suspended로 설정합니다. false이고 이전 transition 동안 runtime-suspended였으며 SMART_SUSPEND가 있으면 core가 active로 설정합니다.

MAY_SKIP_RESUME 없이 SMART_SUSPEND로 late/noirq suspend만 생략된 경우 resume_noirq와 resume_early는 정상 호출되고 core는 runtime PM enable 전에 status를 active로 바꿉니다. driver는 `runtime_suspend` 직후 system resume callback이 연속 호출되는 상황을 처리해야 합니다.

반대로 MAY_SKIP_RESUME가 있으면 late/noirq suspend가 실행되었어도 noirq/early resume이 생략될 수 있어 이후 `runtime_resume`이 그 suspend callback들과 연속 호출될 수 있습니다. runtime과 system-wide PM에 같은 suspend/resume function pair를 쓰면 이러한 ordering을 단순화할 수 있습니다.

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.]