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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
+ 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
+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
+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
+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
+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
+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
+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
+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
+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.