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-Device Power Management
-
-Copyright (c) 2010-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
-Copyright (c) 2010 Alan Stern <stern@rowland.harvard.edu>
-
-
-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 /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:
-
-struct dev_pm_ops {
- int (*prepare)(struct device *dev);
- void (*complete)(struct device *dev);
- int (*suspend)(struct device *dev);
- int (*resume)(struct device *dev);
- int (*freeze)(struct device *dev);
- int (*thaw)(struct device *dev);
- int (*poweroff)(struct device *dev);
- int (*restore)(struct device *dev);
- int (*suspend_late)(struct device *dev);
- int (*resume_early)(struct device *dev);
- int (*freeze_late)(struct device *dev);
- int (*thaw_early)(struct device *dev);
- int (*poweroff_late)(struct device *dev);
- int (*restore_early)(struct device *dev);
- int (*suspend_noirq)(struct device *dev);
- int (*resume_noirq)(struct device *dev);
- int (*freeze_noirq)(struct device *dev);
- int (*thaw_noirq)(struct device *dev);
- int (*poweroff_noirq)(struct device *dev);
- int (*restore_noirq)(struct device *dev);
- int (*runtime_suspend)(struct device *dev);
- int (*runtime_resume)(struct device *dev);
- int (*runtime_idle)(struct device *dev);
-};
-
-This structure is defined in include/linux/pm.h and the methods included in it
-are also described in that file. Their roles 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. 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 ops member of struct dev_pm_domain, or by the 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.
-
-
-/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
-device_set_wakeup_capable() and device_set_wakeup_enable(), defined in
-include/linux/pm_wakeup.h.
-
-The "power.can_wakeup" flag just records whether the device (and its driver) can
-physically support wakeup events. The device_set_wakeup_capable() routine
-affects this flag. The "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 "can_wakeup" flags are set) and is created (or
-removed) by 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
-"power/wakeup" file. User space can write the strings "enabled" or "disabled"
-to it to indicate whether or not, respectively, the device is supposed to signal
-system wakeup. This file is only present if the "power.wakeup" object exists
-for the given device and is created (or removed) along with that object, by
-device_set_wakeup_capable(). Reads from the file will return the corresponding
-string.
-
-The "power/wakeup" file is supposed to contain the "disabled" string initially
-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 device_may_wakeup() routine returns true only if the "power.wakeup" object
-exists and the corresponding "power/wakeup" file contains the string "enabled".
-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
-device_may_wakeup() to decide what to do during a system sleep transition.
-Device drivers, however, are not supposed to call 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.
-
-/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, called runtime_auto, is initialized by the
-bus type (or generally subsystem) code using pm_runtime_allow() or
-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 power/control sysfs file. Writing "auto" calls pm_runtime_allow(),
-setting the flag and allowing the device to be runtime power-managed by its
-driver. Writing "on" calls 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 runtime_auto flag by reading the file.
-
-The device's 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 runtime_auto flag is clear.
-
-For more information about the runtime power management framework, refer to
-Documentation/power/runtime_pm.txt.
-
-
-Calling Drivers to Enter and Leave System Sleep States
-======================================================
-When the system goes into a sleep state, each device's driver is asked to
-suspend the device by putting it into a state compatible with the target
-system state. That's usually some version of "off", but the details are
-system-specific. Also, wakeup-enabled devices will usually stay partly
-functional in order to wake the system.
-
-When the system leaves that low-power state, the device's driver is asked to
-resume it by returning it to full power. The suspend and resume operations
-always go together, and both are multi-phase operations.
-
-For simple drivers, suspend might quiesce the device using class code
-and then turn its hardware as "off" as possible during suspend_noirq. The
-matching resume calls would then completely reinitialize the hardware
-before reactivating its class I/O queues.
-
-More power-aware drivers might prepare the devices for triggering system wakeup
-events.
-
-
-Call Sequence Guarantees
-------------------------
-To ensure that bridges and similar links needing to talk to a device are
-available when the device is suspended or resumed, the device tree 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 tree 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 tree 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 standby or memory sleep states ("suspend-to-RAM") and the
-hibernation state ("suspend-to-disk"). Each phase involves executing callbacks
-for every device before the next phase begins. Not all busses 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
- included in dev->pm_domain->ops for execution
-
- 2. Otherwise, if both dev->type and dev->type->pm are present, the callback
- included in dev->type->pm will be chosen for execution.
-
- 3. Otherwise, if both dev->class and dev->class->pm are present, the
- callback included in dev->class->pm will be chosen for execution.
-
- 4. Otherwise, if both dev->bus and dev->bus->pm are present, the callback
- included in 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 dev->driver->pm instead if there is one.
-
-
-Entering System Suspend
------------------------
-When the system goes into the 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, devices may be unregistered at any
- time.) Unlike the other suspend-related phases, during the prepare
- phase the device tree 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.
-
- 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.
-
- 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 all devices.
-
- 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 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 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, enable_irq_wake() might identify
-GPIO signals hooked up to a switch or other external hardware, and
-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 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 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. 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.
-
-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. Even if the device was in a low-power state before the system sleep
-because of runtime power management, afterwards it should be back in its
-full-power state. There are multiple reasons why it's best to do this; they are
-discussed in more detail in Documentation/power/runtime_pm.txt.
-
-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 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't be guaranteed (in fact, it usually
-is not the case).
-
-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 the standby or
-memory sleep state, 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 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 (poweroff). 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
- above, except that the device should not be put in a low-power state and
- should not be allowed to generate wakeup events by it.
-
- 4. The freeze_noirq phase is analogous to the suspend_noirq phase discussed
- above, except again that the device should not be put in 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
- above. 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 above. 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 standby or memory sleep state, and the phases
-are similar.
-
- 9. The prepare phase is discussed above.
-
- 10. The poweroff phase is analogous to the suspend phase.
-
- 11. The poweroff_late phase is analogous to the suspend_late phase.
-
- 12. The poweroff_noirq phase is analogous to the suspend_noirq phase.
-
-The poweroff, poweroff_late and poweroff_noirq callbacks should do essentially
-the same things as the suspend, suspend_late and suspend_noirq callbacks,
-respectively. The only notable difference is that they need not store the
-device register values, because the registers should already have been stored
-during the freeze, freeze_late or freeze_noirq phases.
-
-
-Leaving Hibernation
--------------------
-Resuming from hibernation is, again, more complicated than resuming from a sleep
-state in which the contents of main memory are preserved, because it requires
-a system image to be loaded into memory and the pre-hibernation memory contents
-to be restored before control can be passed back to the image kernel.
-
-Although in principle, the image might be loaded into memory and the
-pre-hibernation memory contents restored by the boot loader, in practice this
-can't be done because boot loaders aren't smart enough and there is no
-established protocol for passing the necessary information. So instead, the
-boot loader loads a fresh instance of the kernel, called the boot kernel, into
-memory and passes control to it in the usual way. Then the boot kernel reads
-the system image, restores the pre-hibernation memory contents, and passes
-control to the image kernel. Thus two different kernels are involved in
-resuming from hibernation. In fact, the boot 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 boot 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 boot 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 boot
-kernel would go through the "thawing" procedure described above, using the
-thaw_noirq, 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 the memory sleep
-state, 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 boot 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.
-
-
-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.
-
-Support for power domains is provided through the pm_domain field of struct
-device. This field is a pointer to an object of type struct dev_pm_domain,
-defined in 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 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
-anlogously 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.
-
-
-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 busses 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 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.
-
-
-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/power/notifiers.txt.
-
-
-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.
-
-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.txt. Refer
-to that document for more information regarding this particular issue as well as
-for information on the device runtime power management framework in general.