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_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_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.	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, 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 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.

      3.	The complete phase uses only a bus callback.  The method 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_noirq, thaw_noirq, thaw, complete,
  	prepare, poweroff, 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_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.
  
      4.	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.
  
      5.	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.
  
      6.	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.
  
      7.	The prepare phase is discussed above.
  
      8.	The poweroff phase is analogous to the suspend phase.
  
      9.	The poweroff_noirq phase is analogous to the suspend_noirq phase.
  
  The poweroff and poweroff_noirq callbacks should do essentially the same things
  as the suspend and suspend_noirq callbacks.  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 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, complete

      1.	The restore_noirq phase is analogous to the resume_noirq phase.

      2.	The restore phase is analogous to the resume phase.

      3.	The complete phase is discussed above.

  The main difference from resume[_noirq] is that restore[_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 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[_noirq] and restore[_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 matters.

  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.