linux/Documentation/power/devices.txt
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   1Device Power Management
   2
   3Copyright (c) 2010-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
   4Copyright (c) 2010 Alan Stern <stern@rowland.harvard.edu>
   5
   6
   7Most of the code in Linux is device drivers, so most of the Linux power
   8management (PM) code is also driver-specific.  Most drivers will do very
   9little; others, especially for platforms with small batteries (like cell
  10phones), will do a lot.
  11
  12This writeup gives an overview of how drivers interact with system-wide
  13power management goals, emphasizing the models and interfaces that are
  14shared by everything that hooks up to the driver model core.  Read it as
  15background for the domain-specific work you'd do with any specific driver.
  16
  17
  18Two Models for Device Power Management
  19======================================
  20Drivers will use one or both of these models to put devices into low-power
  21states:
  22
  23    System Sleep model:
  24        Drivers can enter low-power states as part of entering system-wide
  25        low-power states like "suspend" (also known as "suspend-to-RAM"), or
  26        (mostly for systems with disks) "hibernation" (also known as
  27        "suspend-to-disk").
  28
  29        This is something that device, bus, and class drivers collaborate on
  30        by implementing various role-specific suspend and resume methods to
  31        cleanly power down hardware and software subsystems, then reactivate
  32        them without loss of data.
  33
  34        Some drivers can manage hardware wakeup events, which make the system
  35        leave the low-power state.  This feature may be enabled or disabled
  36        using the relevant /sys/devices/.../power/wakeup file (for Ethernet
  37        drivers the ioctl interface used by ethtool may also be used for this
  38        purpose); enabling it may cost some power usage, but let the whole
  39        system enter low-power states more often.
  40
  41    Runtime Power Management model:
  42        Devices may also be put into low-power states while the system is
  43        running, independently of other power management activity in principle.
  44        However, devices are not generally independent of each other (for
  45        example, a parent device cannot be suspended unless all of its child
  46        devices have been suspended).  Moreover, depending on the bus type the
  47        device is on, it may be necessary to carry out some bus-specific
  48        operations on the device for this purpose.  Devices put into low power
  49        states at run time may require special handling during system-wide power
  50        transitions (suspend or hibernation).
  51
  52        For these reasons not only the device driver itself, but also the
  53        appropriate subsystem (bus type, device type or device class) driver and
  54        the PM core are involved in runtime power management.  As in the system
  55        sleep power management case, they need to collaborate by implementing
  56        various role-specific suspend and resume methods, so that the hardware
  57        is cleanly powered down and reactivated without data or service loss.
  58
  59There's not a lot to be said about those low-power states except that they are
  60very system-specific, and often device-specific.  Also, that if enough devices
  61have been put into low-power states (at runtime), the effect may be very similar
  62to entering some system-wide low-power state (system sleep) ... and that
  63synergies exist, so that several drivers using runtime PM might put the system
  64into a state where even deeper power saving options are available.
  65
  66Most suspended devices will have quiesced all I/O: no more DMA or IRQs (except
  67for wakeup events), no more data read or written, and requests from upstream
  68drivers are no longer accepted.  A given bus or platform may have different
  69requirements though.
  70
  71Examples of hardware wakeup events include an alarm from a real time clock,
  72network wake-on-LAN packets, keyboard or mouse activity, and media insertion
  73or removal (for PCMCIA, MMC/SD, USB, and so on).
  74
  75
  76Interfaces for Entering System Sleep States
  77===========================================
  78There are programming interfaces provided for subsystems (bus type, device type,
  79device class) and device drivers to allow them to participate in the power
  80management of devices they are concerned with.  These interfaces cover both
  81system sleep and runtime power management.
  82
  83
  84Device Power Management Operations
  85----------------------------------
  86Device power management operations, at the subsystem level as well as at the
  87device driver level, are implemented by defining and populating objects of type
  88struct dev_pm_ops:
  89
  90struct dev_pm_ops {
  91        int (*prepare)(struct device *dev);
  92        void (*complete)(struct device *dev);
  93        int (*suspend)(struct device *dev);
  94        int (*resume)(struct device *dev);
  95        int (*freeze)(struct device *dev);
  96        int (*thaw)(struct device *dev);
  97        int (*poweroff)(struct device *dev);
  98        int (*restore)(struct device *dev);
  99        int (*suspend_late)(struct device *dev);
 100        int (*resume_early)(struct device *dev);
 101        int (*freeze_late)(struct device *dev);
 102        int (*thaw_early)(struct device *dev);
 103        int (*poweroff_late)(struct device *dev);
 104        int (*restore_early)(struct device *dev);
 105        int (*suspend_noirq)(struct device *dev);
 106        int (*resume_noirq)(struct device *dev);
 107        int (*freeze_noirq)(struct device *dev);
 108        int (*thaw_noirq)(struct device *dev);
 109        int (*poweroff_noirq)(struct device *dev);
 110        int (*restore_noirq)(struct device *dev);
 111        int (*runtime_suspend)(struct device *dev);
 112        int (*runtime_resume)(struct device *dev);
 113        int (*runtime_idle)(struct device *dev);
 114};
 115
 116This structure is defined in include/linux/pm.h and the methods included in it
 117are also described in that file.  Their roles will be explained in what follows.
 118For now, it should be sufficient to remember that the last three methods are
 119specific to runtime power management while the remaining ones are used during
 120system-wide power transitions.
 121
 122There also is a deprecated "old" or "legacy" interface for power management
 123operations available at least for some subsystems.  This approach does not use
 124struct dev_pm_ops objects and it is suitable only for implementing system sleep
 125power management methods.  Therefore it is not described in this document, so
 126please refer directly to the source code for more information about it.
 127
 128
 129Subsystem-Level Methods
 130-----------------------
 131The core methods to suspend and resume devices reside in struct dev_pm_ops
 132pointed to by the ops member of struct dev_pm_domain, or by the pm member of
 133struct bus_type, struct device_type and struct class.  They are mostly of
 134interest to the people writing infrastructure for platforms and buses, like PCI
 135or USB, or device type and device class drivers.  They also are relevant to the
 136writers of device drivers whose subsystems (PM domains, device types, device
 137classes and bus types) don't provide all power management methods.
 138
 139Bus drivers implement these methods as appropriate for the hardware and the
 140drivers using it; PCI works differently from USB, and so on.  Not many people
 141write subsystem-level drivers; most driver code is a "device driver" that builds
 142on top of bus-specific framework code.
 143
 144For more information on these driver calls, see the description later;
 145they are called in phases for every device, respecting the parent-child
 146sequencing in the driver model tree.
 147
 148
 149/sys/devices/.../power/wakeup files
 150-----------------------------------
 151All device objects in the driver model contain fields that control the handling
 152of system wakeup events (hardware signals that can force the system out of a
 153sleep state).  These fields are initialized by bus or device driver code using
 154device_set_wakeup_capable() and device_set_wakeup_enable(), defined in
 155include/linux/pm_wakeup.h.
 156
 157The "power.can_wakeup" flag just records whether the device (and its driver) can
 158physically support wakeup events.  The device_set_wakeup_capable() routine
 159affects this flag.  The "power.wakeup" field is a pointer to an object of type
 160struct wakeup_source used for controlling whether or not the device should use
 161its system wakeup mechanism and for notifying the PM core of system wakeup
 162events signaled by the device.  This object is only present for wakeup-capable
 163devices (i.e. devices whose "can_wakeup" flags are set) and is created (or
 164removed) by device_set_wakeup_capable().
 165
 166Whether or not a device is capable of issuing wakeup events is a hardware
 167matter, and the kernel is responsible for keeping track of it.  By contrast,
 168whether or not a wakeup-capable device should issue wakeup events is a policy
 169decision, and it is managed by user space through a sysfs attribute: the
 170"power/wakeup" file.  User space can write the strings "enabled" or "disabled"
 171to it to indicate whether or not, respectively, the device is supposed to signal
 172system wakeup.  This file is only present if the "power.wakeup" object exists
 173for the given device and is created (or removed) along with that object, by
 174device_set_wakeup_capable().  Reads from the file will return the corresponding
 175string.
 176
 177The "power/wakeup" file is supposed to contain the "disabled" string initially
 178for the majority of devices; the major exceptions are power buttons, keyboards,
 179and Ethernet adapters whose WoL (wake-on-LAN) feature has been set up with
 180ethtool.  It should also default to "enabled" for devices that don't generate
 181wakeup requests on their own but merely forward wakeup requests from one bus to
 182another (like PCI Express ports).
 183
 184The device_may_wakeup() routine returns true only if the "power.wakeup" object
 185exists and the corresponding "power/wakeup" file contains the string "enabled".
 186This information is used by subsystems, like the PCI bus type code, to see
 187whether or not to enable the devices' wakeup mechanisms.  If device wakeup
 188mechanisms are enabled or disabled directly by drivers, they also should use
 189device_may_wakeup() to decide what to do during a system sleep transition.
 190Device drivers, however, are not supposed to call device_set_wakeup_enable()
 191directly in any case.
 192
 193It ought to be noted that system wakeup is conceptually different from "remote
 194wakeup" used by runtime power management, although it may be supported by the
 195same physical mechanism.  Remote wakeup is a feature allowing devices in
 196low-power states to trigger specific interrupts to signal conditions in which
 197they should be put into the full-power state.  Those interrupts may or may not
 198be used to signal system wakeup events, depending on the hardware design.  On
 199some systems it is impossible to trigger them from system sleep states.  In any
 200case, remote wakeup should always be enabled for runtime power management for
 201all devices and drivers that support it.
 202
 203/sys/devices/.../power/control files
 204------------------------------------
 205Each device in the driver model has a flag to control whether it is subject to
 206runtime power management.  This flag, called runtime_auto, is initialized by the
 207bus type (or generally subsystem) code using pm_runtime_allow() or
 208pm_runtime_forbid(); the default is to allow runtime power management.
 209
 210The setting can be adjusted by user space by writing either "on" or "auto" to
 211the device's power/control sysfs file.  Writing "auto" calls pm_runtime_allow(),
 212setting the flag and allowing the device to be runtime power-managed by its
 213driver.  Writing "on" calls pm_runtime_forbid(), clearing the flag, returning
 214the device to full power if it was in a low-power state, and preventing the
 215device from being runtime power-managed.  User space can check the current value
 216of the runtime_auto flag by reading the file.
 217
 218The device's runtime_auto flag has no effect on the handling of system-wide
 219power transitions.  In particular, the device can (and in the majority of cases
 220should and will) be put into a low-power state during a system-wide transition
 221to a sleep state even though its runtime_auto flag is clear.
 222
 223For more information about the runtime power management framework, refer to
 224Documentation/power/runtime_pm.txt.
 225
 226
 227Calling Drivers to Enter and Leave System Sleep States
 228======================================================
 229When the system goes into a sleep state, each device's driver is asked to
 230suspend the device by putting it into a state compatible with the target
 231system state.  That's usually some version of "off", but the details are
 232system-specific.  Also, wakeup-enabled devices will usually stay partly
 233functional in order to wake the system.
 234
 235When the system leaves that low-power state, the device's driver is asked to
 236resume it by returning it to full power.  The suspend and resume operations
 237always go together, and both are multi-phase operations.
 238
 239For simple drivers, suspend might quiesce the device using class code
 240and then turn its hardware as "off" as possible during suspend_noirq.  The
 241matching resume calls would then completely reinitialize the hardware
 242before reactivating its class I/O queues.
 243
 244More power-aware drivers might prepare the devices for triggering system wakeup
 245events.
 246
 247
 248Call Sequence Guarantees
 249------------------------
 250To ensure that bridges and similar links needing to talk to a device are
 251available when the device is suspended or resumed, the device tree is
 252walked in a bottom-up order to suspend devices.  A top-down order is
 253used to resume those devices.
 254
 255The ordering of the device tree is defined by the order in which devices
 256get registered:  a child can never be registered, probed or resumed before
 257its parent; and can't be removed or suspended after that parent.
 258
 259The policy is that the device tree should match hardware bus topology.
 260(Or at least the control bus, for devices which use multiple busses.)
 261In particular, this means that a device registration may fail if the parent of
 262the device is suspending (i.e. has been chosen by the PM core as the next
 263device to suspend) or has already suspended, as well as after all of the other
 264devices have been suspended.  Device drivers must be prepared to cope with such
 265situations.
 266
 267
 268System Power Management Phases
 269------------------------------
 270Suspending or resuming the system is done in several phases.  Different phases
 271are used for freeze, standby, and memory sleep states ("suspend-to-RAM") and the
 272hibernation state ("suspend-to-disk").  Each phase involves executing callbacks
 273for every device before the next phase begins.  Not all busses or classes
 274support all these callbacks and not all drivers use all the callbacks.  The
 275various phases always run after tasks have been frozen and before they are
 276unfrozen.  Furthermore, the *_noirq phases run at a time when IRQ handlers have
 277been disabled (except for those marked with the IRQF_NO_SUSPEND flag).
 278
 279All phases use PM domain, bus, type, class or driver callbacks (that is, methods
 280defined in dev->pm_domain->ops, dev->bus->pm, dev->type->pm, dev->class->pm or
 281dev->driver->pm).  These callbacks are regarded by the PM core as mutually
 282exclusive.  Moreover, PM domain callbacks always take precedence over all of the
 283other callbacks and, for example, type callbacks take precedence over bus, class
 284and driver callbacks.  To be precise, the following rules are used to determine
 285which callback to execute in the given phase:
 286
 287    1.  If dev->pm_domain is present, the PM core will choose the callback
 288        included in dev->pm_domain->ops for execution
 289
 290    2.  Otherwise, if both dev->type and dev->type->pm are present, the callback
 291        included in dev->type->pm will be chosen for execution.
 292
 293    3.  Otherwise, if both dev->class and dev->class->pm are present, the
 294        callback included in dev->class->pm will be chosen for execution.
 295
 296    4.  Otherwise, if both dev->bus and dev->bus->pm are present, the callback
 297        included in dev->bus->pm will be chosen for execution.
 298
 299This allows PM domains and device types to override callbacks provided by bus
 300types or device classes if necessary.
 301
 302The PM domain, type, class and bus callbacks may in turn invoke device- or
 303driver-specific methods stored in dev->driver->pm, but they don't have to do
 304that.
 305
 306If the subsystem callback chosen for execution is not present, the PM core will
 307execute the corresponding method from dev->driver->pm instead if there is one.
 308
 309
 310Entering System Suspend
 311-----------------------
 312When the system goes into the freeze, standby or memory sleep state,
 313the phases are:
 314
 315                prepare, suspend, suspend_late, suspend_noirq.
 316
 317    1.  The prepare phase is meant to prevent races by preventing new devices
 318        from being registered; the PM core would never know that all the
 319        children of a device had been suspended if new children could be
 320        registered at will.  (By contrast, devices may be unregistered at any
 321        time.)  Unlike the other suspend-related phases, during the prepare
 322        phase the device tree is traversed top-down.
 323
 324        After the prepare callback method returns, no new children may be
 325        registered below the device.  The method may also prepare the device or
 326        driver in some way for the upcoming system power transition, but it
 327        should not put the device into a low-power state.
 328
 329    2.  The suspend methods should quiesce the device to stop it from performing
 330        I/O.  They also may save the device registers and put it into the
 331        appropriate low-power state, depending on the bus type the device is on,
 332        and they may enable wakeup events.
 333
 334    3   For a number of devices it is convenient to split suspend into the
 335        "quiesce device" and "save device state" phases, in which cases
 336        suspend_late is meant to do the latter.  It is always executed after
 337        runtime power management has been disabled for all devices.
 338
 339    4.  The suspend_noirq phase occurs after IRQ handlers have been disabled,
 340        which means that the driver's interrupt handler will not be called while
 341        the callback method is running.  The methods should save the values of
 342        the device's registers that weren't saved previously and finally put the
 343        device into the appropriate low-power state.
 344
 345        The majority of subsystems and device drivers need not implement this
 346        callback.  However, bus types allowing devices to share interrupt
 347        vectors, like PCI, generally need it; otherwise a driver might encounter
 348        an error during the suspend phase by fielding a shared interrupt
 349        generated by some other device after its own device had been set to low
 350        power.
 351
 352At the end of these phases, drivers should have stopped all I/O transactions
 353(DMA, IRQs), saved enough state that they can re-initialize or restore previous
 354state (as needed by the hardware), and placed the device into a low-power state.
 355On many platforms they will gate off one or more clock sources; sometimes they
 356will also switch off power supplies or reduce voltages.  (Drivers supporting
 357runtime PM may already have performed some or all of these steps.)
 358
 359If device_may_wakeup(dev) returns true, the device should be prepared for
 360generating hardware wakeup signals to trigger a system wakeup event when the
 361system is in the sleep state.  For example, enable_irq_wake() might identify
 362GPIO signals hooked up to a switch or other external hardware, and
 363pci_enable_wake() does something similar for the PCI PME signal.
 364
 365If any of these callbacks returns an error, the system won't enter the desired
 366low-power state.  Instead the PM core will unwind its actions by resuming all
 367the devices that were suspended.
 368
 369
 370Leaving System Suspend
 371----------------------
 372When resuming from freeze, standby or memory sleep, the phases are:
 373
 374                resume_noirq, resume_early, resume, complete.
 375
 376    1.  The resume_noirq callback methods should perform any actions needed
 377        before the driver's interrupt handlers are invoked.  This generally
 378        means undoing the actions of the suspend_noirq phase.  If the bus type
 379        permits devices to share interrupt vectors, like PCI, the method should
 380        bring the device and its driver into a state in which the driver can
 381        recognize if the device is the source of incoming interrupts, if any,
 382        and handle them correctly.
 383
 384        For example, the PCI bus type's ->pm.resume_noirq() puts the device into
 385        the full-power state (D0 in the PCI terminology) and restores the
 386        standard configuration registers of the device.  Then it calls the
 387        device driver's ->pm.resume_noirq() method to perform device-specific
 388        actions.
 389
 390    2.  The resume_early methods should prepare devices for the execution of
 391        the resume methods.  This generally involves undoing the actions of the
 392        preceding suspend_late phase.
 393
 394    3   The resume methods should bring the the device back to its operating
 395        state, so that it can perform normal I/O.  This generally involves
 396        undoing the actions of the suspend phase.
 397
 398    4.  The complete phase should undo the actions of the prepare phase.  Note,
 399        however, that new children may be registered below the device as soon as
 400        the resume callbacks occur; it's not necessary to wait until the
 401        complete phase.
 402
 403At the end of these phases, drivers should be as functional as they were before
 404suspending: I/O can be performed using DMA and IRQs, and the relevant clocks are
 405gated on.  Even if the device was in a low-power state before the system sleep
 406because of runtime power management, afterwards it should be back in its
 407full-power state.  There are multiple reasons why it's best to do this; they are
 408discussed in more detail in Documentation/power/runtime_pm.txt.
 409
 410However, the details here may again be platform-specific.  For example,
 411some systems support multiple "run" states, and the mode in effect at
 412the end of resume might not be the one which preceded suspension.
 413That means availability of certain clocks or power supplies changed,
 414which could easily affect how a driver works.
 415
 416Drivers need to be able to handle hardware which has been reset since the
 417suspend methods were called, for example by complete reinitialization.
 418This may be the hardest part, and the one most protected by NDA'd documents
 419and chip errata.  It's simplest if the hardware state hasn't changed since
 420the suspend was carried out, but that can't be guaranteed (in fact, it usually
 421is not the case).
 422
 423Drivers must also be prepared to notice that the device has been removed
 424while the system was powered down, whenever that's physically possible.
 425PCMCIA, MMC, USB, Firewire, SCSI, and even IDE are common examples of busses
 426where common Linux platforms will see such removal.  Details of how drivers
 427will notice and handle such removals are currently bus-specific, and often
 428involve a separate thread.
 429
 430These callbacks may return an error value, but the PM core will ignore such
 431errors since there's nothing it can do about them other than printing them in
 432the system log.
 433
 434
 435Entering Hibernation
 436--------------------
 437Hibernating the system is more complicated than putting it into the other
 438sleep states, because it involves creating and saving a system image.
 439Therefore there are more phases for hibernation, with a different set of
 440callbacks.  These phases always run after tasks have been frozen and memory has
 441been freed.
 442
 443The general procedure for hibernation is to quiesce all devices (freeze), create
 444an image of the system memory while everything is stable, reactivate all
 445devices (thaw), write the image to permanent storage, and finally shut down the
 446system (poweroff).  The phases used to accomplish this are:
 447
 448        prepare, freeze, freeze_late, freeze_noirq, thaw_noirq, thaw_early,
 449        thaw, complete, prepare, poweroff, poweroff_late, poweroff_noirq
 450
 451    1.  The prepare phase is discussed in the "Entering System Suspend" section
 452        above.
 453
 454    2.  The freeze methods should quiesce the device so that it doesn't generate
 455        IRQs or DMA, and they may need to save the values of device registers.
 456        However the device does not have to be put in a low-power state, and to
 457        save time it's best not to do so.  Also, the device should not be
 458        prepared to generate wakeup events.
 459
 460    3.  The freeze_late phase is analogous to the suspend_late phase described
 461        above, except that the device should not be put in a low-power state and
 462        should not be allowed to generate wakeup events by it.
 463
 464    4.  The freeze_noirq phase is analogous to the suspend_noirq phase discussed
 465        above, except again that the device should not be put in a low-power
 466        state and should not be allowed to generate wakeup events.
 467
 468At this point the system image is created.  All devices should be inactive and
 469the contents of memory should remain undisturbed while this happens, so that the
 470image forms an atomic snapshot of the system state.
 471
 472    5.  The thaw_noirq phase is analogous to the resume_noirq phase discussed
 473        above.  The main difference is that its methods can assume the device is
 474        in the same state as at the end of the freeze_noirq phase.
 475
 476    6.  The thaw_early phase is analogous to the resume_early phase described
 477        above.  Its methods should undo the actions of the preceding
 478        freeze_late, if necessary.
 479
 480    7.  The thaw phase is analogous to the resume phase discussed above.  Its
 481        methods should bring the device back to an operating state, so that it
 482        can be used for saving the image if necessary.
 483
 484    8.  The complete phase is discussed in the "Leaving System Suspend" section
 485        above.
 486
 487At this point the system image is saved, and the devices then need to be
 488prepared for the upcoming system shutdown.  This is much like suspending them
 489before putting the system into the freeze, standby or memory sleep state,
 490and the phases are similar.
 491
 492    9.  The prepare phase is discussed above.
 493
 494    10. The poweroff phase is analogous to the suspend phase.
 495
 496    11. The poweroff_late phase is analogous to the suspend_late phase.
 497
 498    12. The poweroff_noirq phase is analogous to the suspend_noirq phase.
 499
 500The poweroff, poweroff_late and poweroff_noirq callbacks should do essentially
 501the same things as the suspend, suspend_late and suspend_noirq callbacks,
 502respectively.  The only notable difference is that they need not store the
 503device register values, because the registers should already have been stored
 504during the freeze, freeze_late or freeze_noirq phases.
 505
 506
 507Leaving Hibernation
 508-------------------
 509Resuming from hibernation is, again, more complicated than resuming from a sleep
 510state in which the contents of main memory are preserved, because it requires
 511a system image to be loaded into memory and the pre-hibernation memory contents
 512to be restored before control can be passed back to the image kernel.
 513
 514Although in principle, the image might be loaded into memory and the
 515pre-hibernation memory contents restored by the boot loader, in practice this
 516can't be done because boot loaders aren't smart enough and there is no
 517established protocol for passing the necessary information.  So instead, the
 518boot loader loads a fresh instance of the kernel, called the boot kernel, into
 519memory and passes control to it in the usual way.  Then the boot kernel reads
 520the system image, restores the pre-hibernation memory contents, and passes
 521control to the image kernel.  Thus two different kernels are involved in
 522resuming from hibernation.  In fact, the boot kernel may be completely different
 523from the image kernel: a different configuration and even a different version.
 524This has important consequences for device drivers and their subsystems.
 525
 526To be able to load the system image into memory, the boot kernel needs to
 527include at least a subset of device drivers allowing it to access the storage
 528medium containing the image, although it doesn't need to include all of the
 529drivers present in the image kernel.  After the image has been loaded, the
 530devices managed by the boot kernel need to be prepared for passing control back
 531to the image kernel.  This is very similar to the initial steps involved in
 532creating a system image, and it is accomplished in the same way, using prepare,
 533freeze, and freeze_noirq phases.  However the devices affected by these phases
 534are only those having drivers in the boot kernel; other devices will still be in
 535whatever state the boot loader left them.
 536
 537Should the restoration of the pre-hibernation memory contents fail, the boot
 538kernel would go through the "thawing" procedure described above, using the
 539thaw_noirq, thaw, and complete phases, and then continue running normally.  This
 540happens only rarely.  Most often the pre-hibernation memory contents are
 541restored successfully and control is passed to the image kernel, which then
 542becomes responsible for bringing the system back to the working state.
 543
 544To achieve this, the image kernel must restore the devices' pre-hibernation
 545functionality.  The operation is much like waking up from the memory sleep
 546state, although it involves different phases:
 547
 548        restore_noirq, restore_early, restore, complete
 549
 550    1.  The restore_noirq phase is analogous to the resume_noirq phase.
 551
 552    2.  The restore_early phase is analogous to the resume_early phase.
 553
 554    3.  The restore phase is analogous to the resume phase.
 555
 556    4.  The complete phase is discussed above.
 557
 558The main difference from resume[_early|_noirq] is that restore[_early|_noirq]
 559must assume the device has been accessed and reconfigured by the boot loader or
 560the boot kernel.  Consequently the state of the device may be different from the
 561state remembered from the freeze, freeze_late and freeze_noirq phases.  The
 562device may even need to be reset and completely re-initialized.  In many cases
 563this difference doesn't matter, so the resume[_early|_noirq] and
 564restore[_early|_norq] method pointers can be set to the same routines.
 565Nevertheless, different callback pointers are used in case there is a situation
 566where it actually does matter.
 567
 568
 569Device Power Management Domains
 570-------------------------------
 571Sometimes devices share reference clocks or other power resources.  In those
 572cases it generally is not possible to put devices into low-power states
 573individually.  Instead, a set of devices sharing a power resource can be put
 574into a low-power state together at the same time by turning off the shared
 575power resource.  Of course, they also need to be put into the full-power state
 576together, by turning the shared power resource on.  A set of devices with this
 577property is often referred to as a power domain.
 578
 579Support for power domains is provided through the pm_domain field of struct
 580device.  This field is a pointer to an object of type struct dev_pm_domain,
 581defined in include/linux/pm.h, providing a set of power management callbacks
 582analogous to the subsystem-level and device driver callbacks that are executed
 583for the given device during all power transitions, instead of the respective
 584subsystem-level callbacks.  Specifically, if a device's pm_domain pointer is
 585not NULL, the ->suspend() callback from the object pointed to by it will be
 586executed instead of its subsystem's (e.g. bus type's) ->suspend() callback and
 587analogously for all of the remaining callbacks.  In other words, power
 588management domain callbacks, if defined for the given device, always take
 589precedence over the callbacks provided by the device's subsystem (e.g. bus
 590type).
 591
 592The support for device power management domains is only relevant to platforms
 593needing to use the same device driver power management callbacks in many
 594different power domain configurations and wanting to avoid incorporating the
 595support for power domains into subsystem-level callbacks, for example by
 596modifying the platform bus type.  Other platforms need not implement it or take
 597it into account in any way.
 598
 599
 600Device Low Power (suspend) States
 601---------------------------------
 602Device low-power states aren't standard.  One device might only handle
 603"on" and "off", while another might support a dozen different versions of
 604"on" (how many engines are active?), plus a state that gets back to "on"
 605faster than from a full "off".
 606
 607Some busses define rules about what different suspend states mean.  PCI
 608gives one example:  after the suspend sequence completes, a non-legacy
 609PCI device may not perform DMA or issue IRQs, and any wakeup events it
 610issues would be issued through the PME# bus signal.  Plus, there are
 611several PCI-standard device states, some of which are optional.
 612
 613In contrast, integrated system-on-chip processors often use IRQs as the
 614wakeup event sources (so drivers would call enable_irq_wake) and might
 615be able to treat DMA completion as a wakeup event (sometimes DMA can stay
 616active too, it'd only be the CPU and some peripherals that sleep).
 617
 618Some details here may be platform-specific.  Systems may have devices that
 619can be fully active in certain sleep states, such as an LCD display that's
 620refreshed using DMA while most of the system is sleeping lightly ... and
 621its frame buffer might even be updated by a DSP or other non-Linux CPU while
 622the Linux control processor stays idle.
 623
 624Moreover, the specific actions taken may depend on the target system state.
 625One target system state might allow a given device to be very operational;
 626another might require a hard shut down with re-initialization on resume.
 627And two different target systems might use the same device in different
 628ways; the aforementioned LCD might be active in one product's "standby",
 629but a different product using the same SOC might work differently.
 630
 631
 632Power Management Notifiers
 633--------------------------
 634There are some operations that cannot be carried out by the power management
 635callbacks discussed above, because the callbacks occur too late or too early.
 636To handle these cases, subsystems and device drivers may register power
 637management notifiers that are called before tasks are frozen and after they have
 638been thawed.  Generally speaking, the PM notifiers are suitable for performing
 639actions that either require user space to be available, or at least won't
 640interfere with user space.
 641
 642For details refer to Documentation/power/notifiers.txt.
 643
 644
 645Runtime Power Management
 646========================
 647Many devices are able to dynamically power down while the system is still
 648running. This feature is useful for devices that are not being used, and
 649can offer significant power savings on a running system.  These devices
 650often support a range of runtime power states, which might use names such
 651as "off", "sleep", "idle", "active", and so on.  Those states will in some
 652cases (like PCI) be partially constrained by the bus the device uses, and will
 653usually include hardware states that are also used in system sleep states.
 654
 655A system-wide power transition can be started while some devices are in low
 656power states due to runtime power management.  The system sleep PM callbacks
 657should recognize such situations and react to them appropriately, but the
 658necessary actions are subsystem-specific.
 659
 660In some cases the decision may be made at the subsystem level while in other
 661cases the device driver may be left to decide.  In some cases it may be
 662desirable to leave a suspended device in that state during a system-wide power
 663transition, but in other cases the device must be put back into the full-power
 664state temporarily, for example so that its system wakeup capability can be
 665disabled.  This all depends on the hardware and the design of the subsystem and
 666device driver in question.
 667
 668During system-wide resume from a sleep state it's easiest to put devices into
 669the full-power state, as explained in Documentation/power/runtime_pm.txt.  Refer
 670to that document for more information regarding this particular issue as well as
 671for information on the device runtime power management framework in general.
 672
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