889 lines
47 KiB
ReStructuredText
889 lines
47 KiB
ReStructuredText
.. 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 the next one invoked after the
|
|
``->prepare`` callback and is entirely responsible for putting the
|
|
device into a consistent state as appropriate.
|
|
|
|
Note that this direct-complete procedure applies even if the device is
|
|
disabled for runtime PM; only the runtime-PM status matters. It follows
|
|
that if a device has system-sleep callbacks but does not support runtime
|
|
PM, then its prepare callback must never return a positive value. This
|
|
is because all such devices are initially set to runtime-suspended with
|
|
runtime PM disabled.
|
|
|
|
This feature also can be controlled by device drivers by using the
|
|
``DPM_FLAG_NO_DIRECT_COMPLETE`` and ``DPM_FLAG_SMART_PREPARE`` driver
|
|
power management flags. [Typically, they are set at the time the driver
|
|
is probed against the device in question by passing them to the
|
|
:c:func:`dev_pm_set_driver_flags` helper function.] If the first of
|
|
these flags is set, the PM core will not apply the direct-complete
|
|
procedure described above to the given device and, consequenty, to any
|
|
of its ancestors. The second flag, when set, informs the middle layer
|
|
code (bus types, device types, PM domains, classes) that it should take
|
|
the return value of the ``->prepare`` callback provided by the driver
|
|
into account and it may only return a positive value from its own
|
|
``->prepare`` callback if the driver's one also has returned a positive
|
|
value.
|
|
|
|
2. The ``->suspend`` methods should quiesce the device to stop it from
|
|
performing I/O. They also may save the device registers and put it into
|
|
the appropriate low-power state, depending on the bus type the device is
|
|
on, and they may enable wakeup events.
|
|
|
|
However, for devices supporting runtime power management, the
|
|
``->suspend`` methods provided by subsystems (bus types and PM domains
|
|
in particular) must follow an additional rule regarding what can be done
|
|
to the devices before their drivers' ``->suspend`` methods are called.
|
|
Namely, they may resume the devices from runtime suspend by
|
|
calling :c:func:`pm_runtime_resume` for them, if that is necessary, but
|
|
they must not update the state of the devices in any other way at that
|
|
time (in case the drivers need to resume the devices from runtime
|
|
suspend in their ``->suspend`` methods). In fact, the PM core prevents
|
|
subsystems or drivers from putting devices into runtime suspend at
|
|
these times by calling :c:func:`pm_runtime_get_noresume` before issuing
|
|
the ``->prepare`` callback (and calling :c:func:`pm_runtime_put` after
|
|
issuing the ``->complete`` callback).
|
|
|
|
3. For a number of devices it is convenient to split suspend into the
|
|
"quiesce device" and "save device state" phases, in which cases
|
|
``suspend_late`` is meant to do the latter. It is always executed after
|
|
runtime power management has been disabled for the device in question.
|
|
|
|
4. The ``suspend_noirq`` phase occurs after IRQ handlers have been disabled,
|
|
which means that the driver's interrupt handler will not be called while
|
|
the callback method is running. The ``->suspend_noirq`` methods should
|
|
save the values of the device's registers that weren't saved previously
|
|
and finally put the device into the appropriate low-power state.
|
|
|
|
The majority of subsystems and device drivers need not implement this
|
|
callback. However, bus types allowing devices to share interrupt
|
|
vectors, like PCI, generally need it; otherwise a driver might encounter
|
|
an error during the suspend phase by fielding a shared interrupt
|
|
generated by some other device after its own device had been set to low
|
|
power.
|
|
|
|
At the end of these phases, drivers should have stopped all I/O transactions
|
|
(DMA, IRQs), saved enough state that they can re-initialize or restore previous
|
|
state (as needed by the hardware), and placed the device into a low-power state.
|
|
On many platforms they will gate off one or more clock sources; sometimes they
|
|
will also switch off power supplies or reduce voltages. [Drivers supporting
|
|
runtime PM may already have performed some or all of these steps.]
|
|
|
|
If :c:func:`device_may_wakeup(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 runs.
|
|
|
|
Moreover, if the preceding ``->prepare`` callback returned a positive
|
|
number, the device may have been left in runtime suspend throughout the
|
|
whole system suspend and resume (its ``->suspend``, ``->suspend_late``,
|
|
``->suspend_noirq``, ``->resume_noirq``,
|
|
``->resume_early``, and ``->resume`` callbacks may have been
|
|
skipped). In that case, the ``->complete`` callback is entirely
|
|
responsible for putting the device into a consistent state after system
|
|
suspend if necessary. [For example, it may need to queue up a runtime
|
|
resume request for the device for this purpose.] To check if that is
|
|
the case, the ``->complete`` callback can consult the device's
|
|
``power.direct_complete`` flag. If that flag is set when the
|
|
``->complete`` callback is being run then the direct-complete mechanism
|
|
was used, and special actions may be required to make the device work
|
|
correctly afterward.
|
|
|
|
At the end of these phases, drivers should be as functional as they were before
|
|
suspending: I/O can be performed using DMA and IRQs, and the relevant clocks are
|
|
gated on.
|
|
|
|
However, the details here may again be platform-specific. For example,
|
|
some systems support multiple "run" states, and the mode in effect at
|
|
the end of resume might not be the one which preceded suspension.
|
|
That means availability of certain clocks or power supplies changed,
|
|
which could easily affect how a driver works.
|
|
|
|
Drivers need to be able to handle hardware which has been reset since all of the
|
|
suspend methods were called, for example by complete reinitialization.
|
|
This may be the hardest part, and the one most protected by NDA'd documents
|
|
and chip errata. It's simplest if the hardware state hasn't changed since
|
|
the suspend was carried out, but that can only be guaranteed if the target
|
|
system sleep entered was suspend-to-idle. For the other system sleep states
|
|
that may not be the case (and usually isn't for ACPI-defined system sleep
|
|
states, like S3).
|
|
|
|
Drivers must also be prepared to notice that the device has been removed
|
|
while the system was powered down, whenever that's physically possible.
|
|
PCMCIA, MMC, USB, Firewire, SCSI, and even IDE are common examples of busses
|
|
where common Linux platforms will see such removal. Details of how drivers
|
|
will notice and handle such removals are currently bus-specific, and often
|
|
involve a separate thread.
|
|
|
|
These callbacks may return an error value, but the PM core will ignore such
|
|
errors since there's nothing it can do about them other than printing them in
|
|
the system log.
|
|
|
|
|
|
Entering Hibernation
|
|
--------------------
|
|
|
|
Hibernating the system is more complicated than putting it into sleep states,
|
|
because it involves creating and saving a system image. Therefore there are
|
|
more phases for hibernation, with a different set of callbacks. These phases
|
|
always run after tasks have been frozen and enough memory has been freed.
|
|
|
|
The general procedure for hibernation is to quiesce all devices ("freeze"),
|
|
create an image of the system memory while everything is stable, reactivate all
|
|
devices ("thaw"), write the image to permanent storage, and finally shut down
|
|
the system ("power off"). The phases used to accomplish this are: ``prepare``,
|
|
``freeze``, ``freeze_late``, ``freeze_noirq``, ``thaw_noirq``, ``thaw_early``,
|
|
``thaw``, ``complete``, ``prepare``, ``poweroff``, ``poweroff_late``,
|
|
``poweroff_noirq``.
|
|
|
|
1. The ``prepare`` phase is discussed in the "Entering System Suspend"
|
|
section above.
|
|
|
|
2. The ``->freeze`` methods should quiesce the device so that it doesn't
|
|
generate IRQs or DMA, and they may need to save the values of device
|
|
registers. However the device does not have to be put in a low-power
|
|
state, and to save time it's best not to do so. Also, the device should
|
|
not be prepared to generate wakeup events.
|
|
|
|
3. The ``freeze_late`` phase is analogous to the ``suspend_late`` phase
|
|
described earlier, except that the device should not be put into a
|
|
low-power state and should not be allowed to generate wakeup events.
|
|
|
|
4. The ``freeze_noirq`` phase is analogous to the ``suspend_noirq`` phase
|
|
discussed earlier, except again that the device should not be put into
|
|
a low-power state and should not be allowed to generate wakeup events.
|
|
|
|
At this point the system image is created. All devices should be inactive and
|
|
the contents of memory should remain undisturbed while this happens, so that the
|
|
image forms an atomic snapshot of the system state.
|
|
|
|
5. The ``thaw_noirq`` phase is analogous to the ``resume_noirq`` phase
|
|
discussed earlier. The main difference is that its methods can assume
|
|
the device is in the same state as at the end of the ``freeze_noirq``
|
|
phase.
|
|
|
|
6. The ``thaw_early`` phase is analogous to the ``resume_early`` phase
|
|
described above. Its methods should undo the actions of the preceding
|
|
``freeze_late``, if necessary.
|
|
|
|
7. The ``thaw`` phase is analogous to the ``resume`` phase discussed
|
|
earlier. Its methods should bring the device back to an operating
|
|
state, so that it can be used for saving the image if necessary.
|
|
|
|
8. The ``complete`` phase is discussed in the "Leaving System Suspend"
|
|
section above.
|
|
|
|
At this point the system image is saved, and the devices then need to be
|
|
prepared for the upcoming system shutdown. This is much like suspending them
|
|
before putting the system into the suspend-to-idle, shallow or deep sleep state,
|
|
and the phases are similar.
|
|
|
|
9. The ``prepare`` phase is discussed above.
|
|
|
|
10. The ``poweroff`` phase is analogous to the ``suspend`` phase.
|
|
|
|
11. The ``poweroff_late`` phase is analogous to the ``suspend_late`` phase.
|
|
|
|
12. The ``poweroff_noirq`` phase is analogous to the ``suspend_noirq`` phase.
|
|
|
|
The ``->poweroff``, ``->poweroff_late`` and ``->poweroff_noirq`` callbacks
|
|
should do essentially the same things as the ``->suspend``, ``->suspend_late``
|
|
and ``->suspend_noirq`` callbacks, respectively. A notable difference is
|
|
that they need not store the device register values, because the registers
|
|
should already have been stored during the ``freeze``, ``freeze_late`` or
|
|
``freeze_noirq`` phases. Also, on many machines the firmware will power-down
|
|
the entire system, so it is not necessary for the callback to put the device in
|
|
a low-power state.
|
|
|
|
|
|
Leaving Hibernation
|
|
-------------------
|
|
|
|
Resuming from hibernation is, again, more complicated than resuming from a sleep
|
|
state in which the contents of main memory are preserved, because it requires
|
|
a system image to be loaded into memory and the pre-hibernation memory contents
|
|
to be restored before control can be passed back to the image kernel.
|
|
|
|
Although in principle the image might be loaded into memory and the
|
|
pre-hibernation memory contents restored by the boot loader, in practice this
|
|
can't be done because boot loaders aren't smart enough and there is no
|
|
established protocol for passing the necessary information. So instead, the
|
|
boot loader loads a fresh instance of the kernel, called "the restore kernel",
|
|
into memory and passes control to it in the usual way. Then the restore kernel
|
|
reads the system image, restores the pre-hibernation memory contents, and passes
|
|
control to the image kernel. Thus two different kernel instances are involved
|
|
in resuming from hibernation. In fact, the restore kernel may be completely
|
|
different from the image kernel: a different configuration and even a different
|
|
version. This has important consequences for device drivers and their
|
|
subsystems.
|
|
|
|
To be able to load the system image into memory, the restore kernel needs to
|
|
include at least a subset of device drivers allowing it to access the storage
|
|
medium containing the image, although it doesn't need to include all of the
|
|
drivers present in the image kernel. After the image has been loaded, the
|
|
devices managed by the boot kernel need to be prepared for passing control back
|
|
to the image kernel. This is very similar to the initial steps involved in
|
|
creating a system image, and it is accomplished in the same way, using
|
|
``prepare``, ``freeze``, and ``freeze_noirq`` phases. However, the devices
|
|
affected by these phases are only those having drivers in the restore kernel;
|
|
other devices will still be in whatever state the boot loader left them.
|
|
|
|
Should the restoration of the pre-hibernation memory contents fail, the restore
|
|
kernel would go through the "thawing" procedure described above, using the
|
|
``thaw_noirq``, ``thaw_early``, ``thaw``, and ``complete`` phases, and then
|
|
continue running normally. This happens only rarely. Most often the
|
|
pre-hibernation memory contents are restored successfully and control is passed
|
|
to the image kernel, which then becomes responsible for bringing the system back
|
|
to the working state.
|
|
|
|
To achieve this, the image kernel must restore the devices' pre-hibernation
|
|
functionality. The operation is much like waking up from a sleep state (with
|
|
the memory contents preserved), although it involves different phases:
|
|
``restore_noirq``, ``restore_early``, ``restore``, ``complete``.
|
|
|
|
1. The ``restore_noirq`` phase is analogous to the ``resume_noirq`` phase.
|
|
|
|
2. The ``restore_early`` phase is analogous to the ``resume_early`` phase.
|
|
|
|
3. The ``restore`` phase is analogous to the ``resume`` phase.
|
|
|
|
4. The ``complete`` phase is discussed above.
|
|
|
|
The main difference from ``resume[_early|_noirq]`` is that
|
|
``restore[_early|_noirq]`` must assume the device has been accessed and
|
|
reconfigured by the boot loader or the restore kernel. Consequently, the state
|
|
of the device may be different from the state remembered from the ``freeze``,
|
|
``freeze_late`` and ``freeze_noirq`` phases. The device may even need to be
|
|
reset and completely re-initialized. In many cases this difference doesn't
|
|
matter, so the ``->resume[_early|_noirq]`` and ``->restore[_early|_norq]``
|
|
method pointers can be set to the same routines. Nevertheless, different
|
|
callback pointers are used in case there is a situation where it actually does
|
|
matter.
|
|
|
|
|
|
Power Management Notifiers
|
|
==========================
|
|
|
|
There are some operations that cannot be carried out by the power management
|
|
callbacks discussed above, because the callbacks occur too late or too early.
|
|
To handle these cases, subsystems and device drivers may register power
|
|
management notifiers that are called before tasks are frozen and after they have
|
|
been thawed. Generally speaking, the PM notifiers are suitable for performing
|
|
actions that either require user space to be available, or at least won't
|
|
interfere with user space.
|
|
|
|
For details refer to :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.rst` for more information). If an
|
|
IRQ-safe device belongs to a PM domain, the runtime PM of the domain will be
|
|
disallowed, unless the domain itself is defined as IRQ-safe. However, it
|
|
makes sense to define a PM domain as IRQ-safe only if all the devices in it
|
|
are IRQ-safe. Moreover, if an IRQ-safe domain has a parent domain, the runtime
|
|
PM of the parent is only allowed if the parent itself is IRQ-safe too with the
|
|
additional restriction that all child domains of an IRQ-safe parent must also
|
|
be IRQ-safe.
|
|
|
|
|
|
Runtime Power Management
|
|
========================
|
|
|
|
Many devices are able to dynamically power down while the system is still
|
|
running. This feature is useful for devices that are not being used, and
|
|
can offer significant power savings on a running system. These devices
|
|
often support a range of runtime power states, which might use names such
|
|
as "off", "sleep", "idle", "active", and so on. Those states will in some
|
|
cases (like PCI) be partially constrained by the bus the device uses, and will
|
|
usually include hardware states that are also used in system sleep states.
|
|
|
|
A system-wide power transition can be started while some devices are in low
|
|
power states due to runtime power management. The system sleep PM callbacks
|
|
should recognize such situations and react to them appropriately, but the
|
|
necessary actions are subsystem-specific.
|
|
|
|
In some cases the decision may be made at the subsystem level while in other
|
|
cases the device driver may be left to decide. In some cases it may be
|
|
desirable to leave a suspended device in that state during a system-wide power
|
|
transition, but in other cases the device must be put back into the full-power
|
|
state temporarily, for example so that its system wakeup capability can be
|
|
disabled. This all depends on the hardware and the design of the subsystem and
|
|
device driver in question.
|
|
|
|
If it is necessary to resume a device from runtime suspend during a system-wide
|
|
transition into a sleep state, that can be done by calling
|
|
:c:func:`pm_runtime_resume` from the ``->suspend`` callback (or the ``->freeze``
|
|
or ``->poweroff`` callback for transitions related to hibernation) of either the
|
|
device's driver or its subsystem (for example, a bus type or a PM domain).
|
|
However, subsystems must not otherwise change the runtime status of devices
|
|
from their ``->prepare`` and ``->suspend`` callbacks (or equivalent) *before*
|
|
invoking device drivers' ``->suspend`` callbacks (or equivalent).
|
|
|
|
.. _smart_suspend_flag:
|
|
|
|
The ``DPM_FLAG_SMART_SUSPEND`` Driver Flag
|
|
------------------------------------------
|
|
|
|
Some bus types and PM domains have a policy to resume all devices from runtime
|
|
suspend upfront in their ``->suspend`` callbacks, but that may not be really
|
|
necessary if the device's driver can cope with runtime-suspended devices.
|
|
The driver can indicate this by setting ``DPM_FLAG_SMART_SUSPEND`` in
|
|
:c:member:`power.driver_flags` at probe time, with the assistance of the
|
|
:c:func:`dev_pm_set_driver_flags` helper routine.
|
|
|
|
Setting that flag causes the PM core and middle-layer code
|
|
(bus types, PM domains etc.) to skip the ``->suspend_late`` and
|
|
``->suspend_noirq`` callbacks provided by the driver if the device remains in
|
|
runtime suspend throughout those phases of the system-wide suspend (and
|
|
similarly for the "freeze" and "poweroff" parts of system hibernation).
|
|
[Otherwise the same driver
|
|
callback might be executed twice in a row for the same device, which would not
|
|
be valid in general.] If the middle-layer system-wide PM callbacks are present
|
|
for the device then they are responsible for skipping these driver callbacks;
|
|
if not then the PM core skips them. The subsystem callback routines can
|
|
determine whether they need to skip the driver callbacks by testing the return
|
|
value from the :c:func:`dev_pm_skip_suspend` helper function.
|
|
|
|
In addition, with ``DPM_FLAG_SMART_SUSPEND`` set, the driver's ``->thaw_noirq``
|
|
and ``->thaw_early`` callbacks are skipped in hibernation if the device remained
|
|
in runtime suspend throughout the preceding "freeze" transition. Again, if the
|
|
middle-layer callbacks are present for the device, they are responsible for
|
|
doing this, otherwise the PM core takes care of it.
|
|
|
|
|
|
The ``DPM_FLAG_MAY_SKIP_RESUME`` Driver Flag
|
|
--------------------------------------------
|
|
|
|
During system-wide resume from a sleep state it's easiest to put devices into
|
|
the full-power state, as explained in :file:`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.]
|