basics
infrastructure
+ pm/index
device-io
dma-buf
device_link
--- /dev/null
+# -*- coding: utf-8; mode: python -*-
+
+project = "Device Power Management"
+
+tags.add("subproject")
+
+latex_documents = [
+ ('index', 'pm.tex', project,
+ 'The kernel development community', 'manual'),
+]
--- /dev/null
+.. |struct| replace:: :c:type:`struct`
+
+==============================
+Device Power Management Basics
+==============================
+
+::
+
+ Copyright (c) 2010-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
+ Copyright (c) 2010 Alan Stern <stern@rowland.harvard.edu>
+ Copyright (c) 2016 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
+
+Most of the code in Linux is device drivers, so most of the Linux power
+management (PM) code is also driver-specific. Most drivers will do very
+little; others, especially for platforms with small batteries (like cell
+phones), will do a lot.
+
+This writeup gives an overview of how drivers interact with system-wide
+power management goals, emphasizing the models and interfaces that are
+shared by everything that hooks up to the driver model core. Read it as
+background for the domain-specific work you'd do with any specific driver.
+
+
+Two Models for Device Power Management
+======================================
+
+Drivers will use one or both of these models to put devices into low-power
+states:
+
+ System Sleep model:
+
+ Drivers can enter low-power states as part of entering system-wide
+ low-power states like "suspend" (also known as "suspend-to-RAM"), or
+ (mostly for systems with disks) "hibernation" (also known as
+ "suspend-to-disk").
+
+ This is something that device, bus, and class drivers collaborate on
+ by implementing various role-specific suspend and resume methods to
+ cleanly power down hardware and software subsystems, then reactivate
+ them without loss of data.
+
+ Some drivers can manage hardware wakeup events, which make the system
+ leave the low-power state. This feature may be enabled or disabled
+ using the relevant :file:`/sys/devices/.../power/wakeup` file (for
+ Ethernet drivers the ioctl interface used by ethtool may also be used
+ for this purpose); enabling it may cost some power usage, but let the
+ whole system enter low-power states more often.
+
+ Runtime Power Management model:
+
+ Devices may also be put into low-power states while the system is
+ running, independently of other power management activity in principle.
+ However, devices are not generally independent of each other (for
+ example, a parent device cannot be suspended unless all of its child
+ devices have been suspended). Moreover, depending on the bus type the
+ device is on, it may be necessary to carry out some bus-specific
+ operations on the device for this purpose. Devices put into low power
+ states at run time may require special handling during system-wide power
+ transitions (suspend or hibernation).
+
+ For these reasons not only the device driver itself, but also the
+ appropriate subsystem (bus type, device type or device class) driver and
+ the PM core are involved in runtime power management. As in the system
+ sleep power management case, they need to collaborate by implementing
+ various role-specific suspend and resume methods, so that the hardware
+ is cleanly powered down and reactivated without data or service loss.
+
+There's not a lot to be said about those low-power states except that they are
+very system-specific, and often device-specific. Also, that if enough devices
+have been put into low-power states (at runtime), the effect may be very similar
+to entering some system-wide low-power state (system sleep) ... and that
+synergies exist, so that several drivers using runtime PM might put the system
+into a state where even deeper power saving options are available.
+
+Most suspended devices will have quiesced all I/O: no more DMA or IRQs (except
+for wakeup events), no more data read or written, and requests from upstream
+drivers are no longer accepted. A given bus or platform may have different
+requirements though.
+
+Examples of hardware wakeup events include an alarm from a real time clock,
+network wake-on-LAN packets, keyboard or mouse activity, and media insertion
+or removal (for PCMCIA, MMC/SD, USB, and so on).
+
+Interfaces for Entering System Sleep States
+===========================================
+
+There are programming interfaces provided for subsystems (bus type, device type,
+device class) and device drivers to allow them to participate in the power
+management of devices they are concerned with. These interfaces cover both
+system sleep and runtime power management.
+
+
+Device Power Management Operations
+----------------------------------
+
+Device power management operations, at the subsystem level as well as at the
+device driver level, are implemented by defining and populating objects of type
+|struct| :c:type:`dev_pm_ops` defined in :file:`include/linux/pm.h`.
+The roles of the methods included in it will be explained in what follows. For
+now, it should be sufficient to remember that the last three methods are
+specific to runtime power management while the remaining ones are used during
+system-wide power transitions.
+
+There also is a deprecated "old" or "legacy" interface for power management
+operations available at least for some subsystems. This approach does not use
+|struct| :c:type:`dev_pm_ops` objects and it is suitable only for implementing
+system sleep power management methods in a limited way. Therefore it is not
+described in this document, so please refer directly to the source code for more
+information about it.
+
+
+Subsystem-Level Methods
+-----------------------
+
+The core methods to suspend and resume devices reside in
+|struct| :c:type:`dev_pm_ops` pointed to by the :c:member:`ops`
+member of |struct| :c:type:`dev_pm_domain`, or by the :c:member:`pm`
+member of |struct| :c:type:`bus_type`, |struct| :c:type:`device_type` and
+|struct| :c:type:`class`. They are mostly of interest to the people writing
+infrastructure for platforms and buses, like PCI or USB, or device type and
+device class drivers. They also are relevant to the writers of device drivers
+whose subsystems (PM domains, device types, device classes and bus types) don't
+provide all power management methods.
+
+Bus drivers implement these methods as appropriate for the hardware and the
+drivers using it; PCI works differently from USB, and so on. Not many people
+write subsystem-level drivers; most driver code is a "device driver" that builds
+on top of bus-specific framework code.
+
+For more information on these driver calls, see the description later;
+they are called in phases for every device, respecting the parent-child
+sequencing in the driver model tree.
+
+
+:file:`/sys/devices/.../power/wakeup` files
+-------------------------------------------
+
+All device objects in the driver model contain fields that control the handling
+of system wakeup events (hardware signals that can force the system out of a
+sleep state). These fields are initialized by bus or device driver code using
+:c:func:`device_set_wakeup_capable()` and :c:func:`device_set_wakeup_enable()`,
+defined in :file:`include/linux/pm_wakeup.h`.
+
+The :c:member:`power.can_wakeup` flag just records whether the device (and its
+driver) can physically support wakeup events. The
+:c:func:`device_set_wakeup_capable()` routine affects this flag. The
+:c:member:`power.wakeup` field is a pointer to an object of type
+|struct| :c:type:`wakeup_source` used for controlling whether or not
+the device should use its system wakeup mechanism and for notifying the
+PM core of system wakeup events signaled by the device. This object is only
+present for wakeup-capable devices (i.e. devices whose
+:c:member:`can_wakeup` flags are set) and is created (or removed) by
+:c:func:`device_set_wakeup_capable()`.
+
+Whether or not a device is capable of issuing wakeup events is a hardware
+matter, and the kernel is responsible for keeping track of it. By contrast,
+whether or not a wakeup-capable device should issue wakeup events is a policy
+decision, and it is managed by user space through a sysfs attribute: the
+:file:`power/wakeup` file. User space can write the "enabled" or "disabled"
+strings to it to indicate whether or not, respectively, the device is supposed
+to signal system wakeup. This file is only present if the
+:c:member:`power.wakeup` object exists for the given device and is created (or
+removed) along with that object, by :c:func:`device_set_wakeup_capable()`.
+Reads from the file will return the corresponding string.
+
+The initial value in the :file:`power/wakeup` file is "disabled" for the
+majority of devices; the major exceptions are power buttons, keyboards, and
+Ethernet adapters whose WoL (wake-on-LAN) feature has been set up with ethtool.
+It should also default to "enabled" for devices that don't generate wakeup
+requests on their own but merely forward wakeup requests from one bus to another
+(like PCI Express ports).
+
+The :c:func:`device_may_wakeup()` routine returns true only if the
+:c:member:`power.wakeup` object exists and the corresponding :file:`power/wakeup`
+file contains the "enabled" string. This information is used by subsystems,
+like the PCI bus type code, to see whether or not to enable the devices' wakeup
+mechanisms. If device wakeup mechanisms are enabled or disabled directly by
+drivers, they also should use :c:func:`device_may_wakeup()` to decide what to do
+during a system sleep transition. Device drivers, however, are not expected to
+call :c:func:`device_set_wakeup_enable()` directly in any case.
+
+It ought to be noted that system wakeup is conceptually different from "remote
+wakeup" used by runtime power management, although it may be supported by the
+same physical mechanism. Remote wakeup is a feature allowing devices in
+low-power states to trigger specific interrupts to signal conditions in which
+they should be put into the full-power state. Those interrupts may or may not
+be used to signal system wakeup events, depending on the hardware design. On
+some systems it is impossible to trigger them from system sleep states. In any
+case, remote wakeup should always be enabled for runtime power management for
+all devices and drivers that support it.
+
+
+:file:`/sys/devices/.../power/control` files
+--------------------------------------------
+
+Each device in the driver model has a flag to control whether it is subject to
+runtime power management. This flag, :c:member:`runtime_auto`, is initialized
+by the bus type (or generally subsystem) code using :c:func:`pm_runtime_allow()`
+or :c:func:`pm_runtime_forbid()`; the default is to allow runtime power
+management.
+
+The setting can be adjusted by user space by writing either "on" or "auto" to
+the device's :file:`power/control` sysfs file. Writing "auto" calls
+:c:func:`pm_runtime_allow()`, setting the flag and allowing the device to be
+runtime power-managed by its driver. Writing "on" calls
+:c:func:`pm_runtime_forbid()`, clearing the flag, returning the device to full
+power if it was in a low-power state, and preventing the
+device from being runtime power-managed. User space can check the current value
+of the :c:member:`runtime_auto` flag by reading that file.
+
+The device's :c:member:`runtime_auto` flag has no effect on the handling of
+system-wide power transitions. In particular, the device can (and in the
+majority of cases should and will) be put into a low-power state during a
+system-wide transition to a sleep state even though its :c:member:`runtime_auto`
+flag is clear.
+
+For more information about the runtime power management framework, refer to
+:file:`Documentation/power/runtime_pm.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 hierarchy is
+walked in a bottom-up order to suspend devices. A top-down order is
+used to resume those devices.
+
+The ordering of the device hierarchy is defined by the order in which devices
+get registered: a child can never be registered, probed or resumed before
+its parent; and can't be removed or suspended after that parent.
+
+The policy is that the device hierarchy should match hardware bus topology.
+[Or at least the control bus, for devices which use multiple busses.]
+In particular, this means that a device registration may fail if the parent of
+the device is suspending (i.e. has been chosen by the PM core as the next
+device to suspend) or has already suspended, as well as after all of the other
+devices have been suspended. Device drivers must be prepared to cope with such
+situations.
+
+
+System Power Management Phases
+------------------------------
+
+Suspending or resuming the system is done in several phases. Different phases
+are used for suspend-to-idle, shallow (standby), and deep ("suspend-to-RAM")
+sleep states and the hibernation state ("suspend-to-disk"). Each phase involves
+executing callbacks for every device before the next phase begins. Not all
+buses or classes support all these callbacks and not all drivers use all the
+callbacks. The various phases always run after tasks have been frozen and
+before they are unfrozen. Furthermore, the ``*_noirq phases`` run at a time
+when IRQ handlers have been disabled (except for those marked with the
+IRQF_NO_SUSPEND flag).
+
+All phases use PM domain, bus, type, class or driver callbacks (that is, methods
+defined in ``dev->pm_domain->ops``, ``dev->bus->pm``, ``dev->type->pm``,
+``dev->class->pm`` or ``dev->driver->pm``). These callbacks are regarded by the
+PM core as mutually exclusive. Moreover, PM domain callbacks always take
+precedence over all of the other callbacks and, for example, type callbacks take
+precedence over bus, class and driver callbacks. To be precise, the following
+rules are used to determine which callback to execute in the given phase:
+
+ 1. If ``dev->pm_domain`` is present, the PM core will choose the callback
+ provided by ``dev->pm_domain->ops`` for execution.
+
+ 2. Otherwise, if both ``dev->type`` and ``dev->type->pm`` are present, the
+ callback provided by ``dev->type->pm`` will be chosen for execution.
+
+ 3. Otherwise, if both ``dev->class`` and ``dev->class->pm`` are present,
+ the callback provided by ``dev->class->pm`` will be chosen for
+ execution.
+
+ 4. Otherwise, if both ``dev->bus`` and ``dev->bus->pm`` are present, the
+ callback provided by ``dev->bus->pm`` will be chosen for execution.
+
+This allows PM domains and device types to override callbacks provided by bus
+types or device classes if necessary.
+
+The PM domain, type, class and bus callbacks may in turn invoke device- or
+driver-specific methods stored in ``dev->driver->pm``, but they don't have to do
+that.
+
+If the subsystem callback chosen for execution is not present, the PM core will
+execute the corresponding method from the ``dev->driver->pm`` set instead if
+there is one.
+
+
+Entering System Suspend
+-----------------------
+
+When the system goes into the freeze, standby or memory sleep state,
+the phases are: ``prepare``, ``suspend``, ``suspend_late``, ``suspend_noirq``.
+
+ 1. The ``prepare`` phase is meant to prevent races by preventing new
+ devices from being registered; the PM core would never know that all the
+ children of a device had been suspended if new children could be
+ registered at will. [By contrast, from the PM core's perspective,
+ devices may be unregistered at any time.] Unlike the other
+ suspend-related phases, during the ``prepare`` phase the device
+ hierarchy is traversed top-down.
+
+ After the ``->prepare`` callback method returns, no new children may be
+ registered below the device. The method may also prepare the device or
+ driver in some way for the upcoming system power transition, but it
+ should not put the device into a low-power state.
+
+ For devices supporting runtime power management, the return value of the
+ prepare callback can be used to indicate to the PM core that it may
+ safely leave the device in runtime suspend (if runtime-suspended
+ already), provided that all of the device's descendants are also left in
+ runtime suspend. Namely, if the prepare callback returns a positive
+ number and that happens for all of the descendants of the device too,
+ and all of them (including the device itself) are runtime-suspended, the
+ PM core will skip the ``suspend``, ``suspend_late`` and
+ ``suspend_noirq`` phases as well as all of the corresponding phases of
+ the subsequent device resume for all of these devices. In that case,
+ the ``->complete`` callback will be invoked directly after the
+ ``->prepare`` callback and is entirely responsible for putting the
+ device into a consistent state as appropriate.
+
+ Note that this direct-complete procedure applies even if the device is
+ disabled for runtime PM; only the runtime-PM status matters. It follows
+ that if a device has system-sleep callbacks but does not support runtime
+ PM, then its prepare callback must never return a positive value. This
+ is because all such devices are initially set to runtime-suspended with
+ runtime PM disabled.
+
+ 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 the device in question.
+
+ 4. The ``suspend_noirq`` phase occurs after IRQ handlers have been disabled,
+ which means that the driver's interrupt handler will not be called while
+ the callback method is running. The ``->suspend_noirq`` methods should
+ save the values of the device's registers that weren't saved previously
+ and finally put the device into the appropriate low-power state.
+
+ The majority of subsystems and device drivers need not implement this
+ callback. However, bus types allowing devices to share interrupt
+ vectors, like PCI, generally need it; otherwise a driver might encounter
+ an error during the suspend phase by fielding a shared interrupt
+ generated by some other device after its own device had been set to low
+ power.
+
+At the end of these phases, drivers should have stopped all I/O transactions
+(DMA, IRQs), saved enough state that they can re-initialize or restore previous
+state (as needed by the hardware), and placed the device into a low-power state.
+On many platforms they will gate off one or more clock sources; sometimes they
+will also switch off power supplies or reduce voltages. [Drivers supporting
+runtime PM may already have performed some or all of these steps.]
+
+If :c:func:`device_may_wakeup(dev)` returns ``true``, the device should be
+prepared for generating hardware wakeup signals to trigger a system wakeup event
+when the system is in the sleep state. For example, :c:func:`enable_irq_wake()`
+might identify GPIO signals hooked up to a switch or other external hardware,
+and :c:func:`pci_enable_wake()` does something similar for the PCI PME signal.
+
+If any of these callbacks returns an error, the system won't enter the desired
+low-power state. Instead, the PM core will unwind its actions by resuming all
+the devices that were suspended.
+
+
+Leaving System Suspend
+----------------------
+
+When resuming from freeze, standby or memory sleep, the phases are:
+``resume_noirq``, ``resume_early``, ``resume``, ``complete``.
+
+ 1. The ``->resume_noirq`` callback methods should perform any actions
+ needed before the driver's interrupt handlers are invoked. This
+ generally means undoing the actions of the ``suspend_noirq`` phase. If
+ the bus type permits devices to share interrupt vectors, like PCI, the
+ method should bring the device and its driver into a state in which the
+ driver can recognize if the device is the source of incoming interrupts,
+ if any, and handle them correctly.
+
+ For example, the PCI bus type's ``->pm.resume_noirq()`` puts the device
+ into the full-power state (D0 in the PCI terminology) and restores the
+ standard configuration registers of the device. Then it calls the
+ device driver's ``->pm.resume_noirq()`` method to perform device-specific
+ actions.
+
+ 2. The ``->resume_early`` methods should prepare devices for the execution
+ of the resume methods. This generally involves undoing the actions of
+ the preceding ``suspend_late`` phase.
+
+ 3. The ``->resume`` methods should bring the device back to its operating
+ state, so that it can perform normal I/O. This generally involves
+ undoing the actions of the ``suspend`` phase.
+
+ 4. The ``complete`` phase should undo the actions of the ``prepare`` phase.
+ For this reason, unlike the other resume-related phases, during the
+ ``complete`` phase the device hierarchy is traversed bottom-up.
+
+ Note, however, that new children may be registered below the device as
+ soon as the ``->resume`` callbacks occur; it's not necessary to wait
+ until the ``complete`` phase with that.
+
+ Moreover, if the preceding ``->prepare`` callback returned a positive
+ number, the device may have been left in runtime suspend throughout the
+ whole system suspend and resume (the ``suspend``, ``suspend_late``,
+ ``suspend_noirq`` phases of system suspend and the ``resume_noirq``,
+ ``resume_early``, ``resume`` phases of system resume may have been
+ skipped for it). In that case, the ``->complete`` callback is entirely
+ responsible for putting the device into a consistent state after system
+ suspend if necessary. [For example, it may need to queue up a runtime
+ resume request for the device for this purpose.] To check if that is
+ the case, the ``->complete`` callback can consult the device's
+ ``power.direct_complete`` flag. Namely, if that flag is set when the
+ ``->complete`` callback is being run, it has been called directly after
+ the preceding ``->prepare`` and special actions may be required
+ to make the device work correctly afterward.
+
+At the end of these phases, drivers should be as functional as they were before
+suspending: I/O can be performed using DMA and IRQs, and the relevant clocks are
+gated on.
+
+However, the details here may again be platform-specific. For example,
+some systems support multiple "run" states, and the mode in effect at
+the end of resume might not be the one which preceded suspension.
+That means availability of certain clocks or power supplies changed,
+which could easily affect how a driver works.
+
+Drivers need to be able to handle hardware which has been reset since all of the
+suspend methods were called, for example by complete reinitialization.
+This may be the hardest part, and the one most protected by NDA'd documents
+and chip errata. It's simplest if the hardware state hasn't changed since
+the suspend was carried out, but that can only be guaranteed if the target
+system sleep entered was suspend-to-idle. For the other system sleep states
+that may not be the case (and usually isn't for ACPI-defined system sleep
+states, like S3).
+
+Drivers must also be prepared to notice that the device has been removed
+while the system was powered down, whenever that's physically possible.
+PCMCIA, MMC, USB, Firewire, SCSI, and even IDE are common examples of busses
+where common Linux platforms will see such removal. Details of how drivers
+will notice and handle such removals are currently bus-specific, and often
+involve a separate thread.
+
+These callbacks may return an error value, but the PM core will ignore such
+errors since there's nothing it can do about them other than printing them in
+the system log.
+
+
+Entering Hibernation
+--------------------
+
+Hibernating the system is more complicated than putting it into sleep states,
+because it involves creating and saving a system image. Therefore there are
+more phases for hibernation, with a different set of callbacks. These phases
+always run after tasks have been frozen and enough memory has been freed.
+
+The general procedure for hibernation is to quiesce all devices ("freeze"),
+create an image of the system memory while everything is stable, reactivate all
+devices ("thaw"), write the image to permanent storage, and finally shut down
+the system ("power off"). The phases used to accomplish this are: ``prepare``,
+``freeze``, ``freeze_late``, ``freeze_noirq``, ``thaw_noirq``, ``thaw_early``,
+``thaw``, ``complete``, ``prepare``, ``poweroff``, ``poweroff_late``,
+``poweroff_noirq``.
+
+ 1. The ``prepare`` phase is discussed in the "Entering System Suspend"
+ section above.
+
+ 2. The ``->freeze`` methods should quiesce the device so that it doesn't
+ generate IRQs or DMA, and they may need to save the values of device
+ registers. However the device does not have to be put in a low-power
+ state, and to save time it's best not to do so. Also, the device should
+ not be prepared to generate wakeup events.
+
+ 3. The ``freeze_late`` phase is analogous to the ``suspend_late`` phase
+ described earlier, except that the device should not be put into a
+ low-power state and should not be allowed to generate wakeup events.
+
+ 4. The ``freeze_noirq`` phase is analogous to the ``suspend_noirq`` phase
+ discussed earlier, except again that the device should not be put into
+ a low-power state and should not be allowed to generate wakeup events.
+
+At this point the system image is created. All devices should be inactive and
+the contents of memory should remain undisturbed while this happens, so that the
+image forms an atomic snapshot of the system state.
+
+ 5. The ``thaw_noirq`` phase is analogous to the ``resume_noirq`` phase
+ discussed earlier. The main difference is that its methods can assume
+ the device is in the same state as at the end of the ``freeze_noirq``
+ phase.
+
+ 6. The ``thaw_early`` phase is analogous to the ``resume_early`` phase
+ described above. Its methods should undo the actions of the preceding
+ ``freeze_late``, if necessary.
+
+ 7. The ``thaw`` phase is analogous to the ``resume`` phase discussed
+ earlier. Its methods should bring the device back to an operating
+ state, so that it can be used for saving the image if necessary.
+
+ 8. The ``complete`` phase is discussed in the "Leaving System Suspend"
+ section above.
+
+At this point the system image is saved, and the devices then need to be
+prepared for the upcoming system shutdown. This is much like suspending them
+before putting the system into the suspend-to-idle, shallow or deep sleep state,
+and the phases are similar.
+
+ 9. The ``prepare`` phase is discussed above.
+
+ 10. The ``poweroff`` phase is analogous to the ``suspend`` phase.
+
+ 11. The ``poweroff_late`` phase is analogous to the ``suspend_late`` phase.
+
+ 12. The ``poweroff_noirq`` phase is analogous to the ``suspend_noirq`` phase.
+
+The ``->poweroff``, ``->poweroff_late`` and ``->poweroff_noirq`` callbacks
+should do essentially the same things as the ``->suspend``, ``->suspend_late``
+and ``->suspend_noirq`` callbacks, respectively. 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 restore kernel",
+into memory and passes control to it in the usual way. Then the restore kernel
+reads the system image, restores the pre-hibernation memory contents, and passes
+control to the image kernel. Thus two different kernel instances are involved
+in resuming from hibernation. In fact, the restore kernel may be completely
+different from the image kernel: a different configuration and even a different
+version. This has important consequences for device drivers and their
+subsystems.
+
+To be able to load the system image into memory, the restore kernel needs to
+include at least a subset of device drivers allowing it to access the storage
+medium containing the image, although it doesn't need to include all of the
+drivers present in the image kernel. After the image has been loaded, the
+devices managed by the boot kernel need to be prepared for passing control back
+to the image kernel. This is very similar to the initial steps involved in
+creating a system image, and it is accomplished in the same way, using
+``prepare``, ``freeze``, and ``freeze_noirq`` phases. However, the devices
+affected by these phases are only those having drivers in the restore kernel;
+other devices will still be in whatever state the boot loader left them.
+
+Should the restoration of the pre-hibernation memory contents fail, the restore
+kernel would go through the "thawing" procedure described above, using the
+``thaw_noirq``, ``thaw_early``, ``thaw``, and ``complete`` phases, and then
+continue running normally. This happens only rarely. Most often the
+pre-hibernation memory contents are restored successfully and control is passed
+to the image kernel, which then becomes responsible for bringing the system back
+to the working state.
+
+To achieve this, the image kernel must restore the devices' pre-hibernation
+functionality. The operation is much like waking up from a sleep state (with
+the memory contents preserved), although it involves different phases:
+``restore_noirq``, ``restore_early``, ``restore``, ``complete``.
+
+ 1. The ``restore_noirq`` phase is analogous to the ``resume_noirq`` phase.
+
+ 2. The ``restore_early`` phase is analogous to the ``resume_early`` phase.
+
+ 3. The ``restore`` phase is analogous to the ``resume`` phase.
+
+ 4. The ``complete`` phase is discussed above.
+
+The main difference from ``resume[_early|_noirq]`` is that
+``restore[_early|_noirq]`` must assume the device has been accessed and
+reconfigured by the boot loader or the restore kernel. Consequently, the state
+of the device may be different from the state remembered from the ``freeze``,
+``freeze_late`` and ``freeze_noirq`` phases. The device may even need to be
+reset and completely re-initialized. In many cases this difference doesn't
+matter, so the ``->resume[_early|_noirq]`` and ``->restore[_early|_norq]``
+method pointers can be set to the same routines. Nevertheless, different
+callback pointers are used in case there is a situation where it actually does
+matter.
+
+
+Power Management Notifiers
+==========================
+
+There are some operations that cannot be carried out by the power management
+callbacks discussed above, because the callbacks occur too late or too early.
+To handle these cases, subsystems and device drivers may register power
+management notifiers that are called before tasks are frozen and after they have
+been thawed. Generally speaking, the PM notifiers are suitable for performing
+actions that either require user space to be available, or at least won't
+interfere with user space.
+
+For details refer to :file:`Documentation/power/notifiers.txt`.
+
+
+Device Low-Power (suspend) States
+=================================
+
+Device low-power states aren't standard. One device might only handle
+"on" and "off", while another might support a dozen different versions of
+"on" (how many engines are active?), plus a state that gets back to "on"
+faster than from a full "off".
+
+Some buses define rules about what different suspend states mean. PCI
+gives one example: after the suspend sequence completes, a non-legacy
+PCI device may not perform DMA or issue IRQs, and any wakeup events it
+issues would be issued through the PME# bus signal. Plus, there are
+several PCI-standard device states, some of which are optional.
+
+In contrast, integrated system-on-chip processors often use IRQs as the
+wakeup event sources (so drivers would call :c:func:`enable_irq_wake`) and
+might be able to treat DMA completion as a wakeup event (sometimes DMA can stay
+active too, it'd only be the CPU and some peripherals that sleep).
+
+Some details here may be platform-specific. Systems may have devices that
+can be fully active in certain sleep states, such as an LCD display that's
+refreshed using DMA while most of the system is sleeping lightly ... and
+its frame buffer might even be updated by a DSP or other non-Linux CPU while
+the Linux control processor stays idle.
+
+Moreover, the specific actions taken may depend on the target system state.
+One target system state might allow a given device to be very operational;
+another might require a hard shut down with re-initialization on resume.
+And two different target systems might use the same device in different
+ways; the aforementioned LCD might be active in one product's "standby",
+but a different product using the same SOC might work differently.
+
+
+Device Power Management Domains
+===============================
+
+Sometimes devices share reference clocks or other power resources. In those
+cases it generally is not possible to put devices into low-power states
+individually. Instead, a set of devices sharing a power resource can be put
+into a low-power state together at the same time by turning off the shared
+power resource. Of course, they also need to be put into the full-power state
+together, by turning the shared power resource on. A set of devices with this
+property is often referred to as a power domain. A power domain may also be
+nested inside another power domain. The nested domain is referred to as the
+sub-domain of the parent domain.
+
+Support for power domains is provided through the :c:member:`pm_domain` field of
+|struct| :c:type:`device`. This field is a pointer to an object of
+type |struct| :c:type:`dev_pm_domain`, defined in :file:`include/linux/pm.h``,
+providing a set of power management callbacks analogous to the subsystem-level
+and device driver callbacks that are executed for the given device during all
+power transitions, instead of the respective subsystem-level callbacks.
+Specifically, if a device's :c:member:`pm_domain` pointer is not NULL, the
+``->suspend()`` callback from the object pointed to by it will be executed
+instead of its subsystem's (e.g. bus type's) ``->suspend()`` callback and
+analogously for all of the remaining callbacks. In other words, power
+management domain callbacks, if defined for the given device, always take
+precedence over the callbacks provided by the device's subsystem (e.g. bus type).
+
+The support for device power management domains is only relevant to platforms
+needing to use the same device driver power management callbacks in many
+different power domain configurations and wanting to avoid incorporating the
+support for power domains into subsystem-level callbacks, for example by
+modifying the platform bus type. Other platforms need not implement it or take
+it into account in any way.
+
+Devices may be defined as IRQ-safe which indicates to the PM core that their
+runtime PM callbacks may be invoked with disabled interrupts (see
+:file:`Documentation/power/runtime_pm.txt` for more information). If an
+IRQ-safe device belongs to a PM domain, the runtime PM of the domain will be
+disallowed, unless the domain itself is defined as IRQ-safe. However, it
+makes sense to define a PM domain as IRQ-safe only if all the devices in it
+are IRQ-safe. Moreover, if an IRQ-safe domain has a parent domain, the runtime
+PM of the parent is only allowed if the parent itself is IRQ-safe too with the
+additional restriction that all child domains of an IRQ-safe parent must also
+be IRQ-safe.
+
+
+Runtime Power Management
+========================
+
+Many devices are able to dynamically power down while the system is still
+running. This feature is useful for devices that are not being used, and
+can offer significant power savings on a running system. These devices
+often support a range of runtime power states, which might use names such
+as "off", "sleep", "idle", "active", and so on. Those states will in some
+cases (like PCI) be partially constrained by the bus the device uses, and will
+usually include hardware states that are also used in system sleep states.
+
+A system-wide power transition can be started while some devices are in low
+power states due to runtime power management. The system sleep PM callbacks
+should recognize such situations and react to them appropriately, but the
+necessary actions are subsystem-specific.
+
+In some cases the decision may be made at the subsystem level while in other
+cases the device driver may be left to decide. In some cases it may be
+desirable to leave a suspended device in that state during a system-wide power
+transition, but in other cases the device must be put back into the full-power
+state temporarily, for example so that its system wakeup capability can be
+disabled. This all depends on the hardware and the design of the subsystem and
+device driver in question.
+
+During system-wide resume from a sleep state it's easiest to put devices into
+the full-power state, as explained in :file:`Documentation/power/runtime_pm.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.
--- /dev/null
+=======================
+Device Power Management
+=======================
+
+.. toctree::
+
+ devices
+ types
+
+.. only:: subproject and html
+
+ Indices
+ =======
+
+ * :ref:`genindex`
--- /dev/null
+==================================
+Device Power Management Data Types
+==================================
+
+.. kernel-doc:: include/linux/pm.h
+++ /dev/null
-Device Power Management
-
-Copyright (c) 2010-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
-Copyright (c) 2010 Alan Stern <stern@rowland.harvard.edu>
-Copyright (c) 2014 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
-
-
-Most of the code in Linux is device drivers, so most of the Linux power
-management (PM) code is also driver-specific. Most drivers will do very
-little; others, especially for platforms with small batteries (like cell
-phones), will do a lot.
-
-This writeup gives an overview of how drivers interact with system-wide
-power management goals, emphasizing the models and interfaces that are
-shared by everything that hooks up to the driver model core. Read it as
-background for the domain-specific work you'd do with any specific driver.
-
-
-Two Models for Device Power Management
-======================================
-Drivers will use one or both of these models to put devices into low-power
-states:
-
- System Sleep model:
- Drivers can enter low-power states as part of entering system-wide
- low-power states like "suspend" (also known as "suspend-to-RAM"), or
- (mostly for systems with disks) "hibernation" (also known as
- "suspend-to-disk").
-
- This is something that device, bus, and class drivers collaborate on
- by implementing various role-specific suspend and resume methods to
- cleanly power down hardware and software subsystems, then reactivate
- them without loss of data.
-
- Some drivers can manage hardware wakeup events, which make the system
- leave the low-power state. This feature may be enabled or disabled
- using the relevant /sys/devices/.../power/wakeup file (for Ethernet
- drivers the ioctl interface used by ethtool may also be used for this
- purpose); enabling it may cost some power usage, but let the whole
- system enter low-power states more often.
-
- Runtime Power Management model:
- Devices may also be put into low-power states while the system is
- running, independently of other power management activity in principle.
- However, devices are not generally independent of each other (for
- example, a parent device cannot be suspended unless all of its child
- devices have been suspended). Moreover, depending on the bus type the
- device is on, it may be necessary to carry out some bus-specific
- operations on the device for this purpose. Devices put into low power
- states at run time may require special handling during system-wide power
- transitions (suspend or hibernation).
-
- For these reasons not only the device driver itself, but also the
- appropriate subsystem (bus type, device type or device class) driver and
- the PM core are involved in runtime power management. As in the system
- sleep power management case, they need to collaborate by implementing
- various role-specific suspend and resume methods, so that the hardware
- is cleanly powered down and reactivated without data or service loss.
-
-There's not a lot to be said about those low-power states except that they are
-very system-specific, and often device-specific. Also, that if enough devices
-have been put into low-power states (at runtime), the effect may be very similar
-to entering some system-wide low-power state (system sleep) ... and that
-synergies exist, so that several drivers using runtime PM might put the system
-into a state where even deeper power saving options are available.
-
-Most suspended devices will have quiesced all I/O: no more DMA or IRQs (except
-for wakeup events), no more data read or written, and requests from upstream
-drivers are no longer accepted. A given bus or platform may have different
-requirements though.
-
-Examples of hardware wakeup events include an alarm from a real time clock,
-network wake-on-LAN packets, keyboard or mouse activity, and media insertion
-or removal (for PCMCIA, MMC/SD, USB, and so on).
-
-
-Interfaces for Entering System Sleep States
-===========================================
-There are programming interfaces provided for subsystems (bus type, device type,
-device class) and device drivers to allow them to participate in the power
-management of devices they are concerned with. These interfaces cover both
-system sleep and runtime power management.
-
-
-Device Power Management Operations
-----------------------------------
-Device power management operations, at the subsystem level as well as at the
-device driver level, are implemented by defining and populating objects of type
-struct dev_pm_ops:
-
-struct dev_pm_ops {
- int (*prepare)(struct device *dev);
- void (*complete)(struct device *dev);
- int (*suspend)(struct device *dev);
- int (*resume)(struct device *dev);
- int (*freeze)(struct device *dev);
- int (*thaw)(struct device *dev);
- int (*poweroff)(struct device *dev);
- int (*restore)(struct device *dev);
- int (*suspend_late)(struct device *dev);
- int (*resume_early)(struct device *dev);
- int (*freeze_late)(struct device *dev);
- int (*thaw_early)(struct device *dev);
- int (*poweroff_late)(struct device *dev);
- int (*restore_early)(struct device *dev);
- int (*suspend_noirq)(struct device *dev);
- int (*resume_noirq)(struct device *dev);
- int (*freeze_noirq)(struct device *dev);
- int (*thaw_noirq)(struct device *dev);
- int (*poweroff_noirq)(struct device *dev);
- int (*restore_noirq)(struct device *dev);
- int (*runtime_suspend)(struct device *dev);
- int (*runtime_resume)(struct device *dev);
- int (*runtime_idle)(struct device *dev);
-};
-
-This structure is defined in include/linux/pm.h and the methods included in it
-are also described in that file. Their roles will be explained in what follows.
-For now, it should be sufficient to remember that the last three methods are
-specific to runtime power management while the remaining ones are used during
-system-wide power transitions.
-
-There also is a deprecated "old" or "legacy" interface for power management
-operations available at least for some subsystems. This approach does not use
-struct dev_pm_ops objects and it is suitable only for implementing system sleep
-power management methods. Therefore it is not described in this document, so
-please refer directly to the source code for more information about it.
-
-
-Subsystem-Level Methods
------------------------
-The core methods to suspend and resume devices reside in struct dev_pm_ops
-pointed to by the ops member of struct dev_pm_domain, or by the pm member of
-struct bus_type, struct device_type and struct class. They are mostly of
-interest to the people writing infrastructure for platforms and buses, like PCI
-or USB, or device type and device class drivers. They also are relevant to the
-writers of device drivers whose subsystems (PM domains, device types, device
-classes and bus types) don't provide all power management methods.
-
-Bus drivers implement these methods as appropriate for the hardware and the
-drivers using it; PCI works differently from USB, and so on. Not many people
-write subsystem-level drivers; most driver code is a "device driver" that builds
-on top of bus-specific framework code.
-
-For more information on these driver calls, see the description later;
-they are called in phases for every device, respecting the parent-child
-sequencing in the driver model tree.
-
-
-/sys/devices/.../power/wakeup files
------------------------------------
-All device objects in the driver model contain fields that control the handling
-of system wakeup events (hardware signals that can force the system out of a
-sleep state). These fields are initialized by bus or device driver code using
-device_set_wakeup_capable() and device_set_wakeup_enable(), defined in
-include/linux/pm_wakeup.h.
-
-The "power.can_wakeup" flag just records whether the device (and its driver) can
-physically support wakeup events. The device_set_wakeup_capable() routine
-affects this flag. The "power.wakeup" field is a pointer to an object of type
-struct wakeup_source used for controlling whether or not the device should use
-its system wakeup mechanism and for notifying the PM core of system wakeup
-events signaled by the device. This object is only present for wakeup-capable
-devices (i.e. devices whose "can_wakeup" flags are set) and is created (or
-removed) by device_set_wakeup_capable().
-
-Whether or not a device is capable of issuing wakeup events is a hardware
-matter, and the kernel is responsible for keeping track of it. By contrast,
-whether or not a wakeup-capable device should issue wakeup events is a policy
-decision, and it is managed by user space through a sysfs attribute: the
-"power/wakeup" file. User space can write the strings "enabled" or "disabled"
-to it to indicate whether or not, respectively, the device is supposed to signal
-system wakeup. This file is only present if the "power.wakeup" object exists
-for the given device and is created (or removed) along with that object, by
-device_set_wakeup_capable(). Reads from the file will return the corresponding
-string.
-
-The "power/wakeup" file is supposed to contain the "disabled" string initially
-for the majority of devices; the major exceptions are power buttons, keyboards,
-and Ethernet adapters whose WoL (wake-on-LAN) feature has been set up with
-ethtool. It should also default to "enabled" for devices that don't generate
-wakeup requests on their own but merely forward wakeup requests from one bus to
-another (like PCI Express ports).
-
-The device_may_wakeup() routine returns true only if the "power.wakeup" object
-exists and the corresponding "power/wakeup" file contains the string "enabled".
-This information is used by subsystems, like the PCI bus type code, to see
-whether or not to enable the devices' wakeup mechanisms. If device wakeup
-mechanisms are enabled or disabled directly by drivers, they also should use
-device_may_wakeup() to decide what to do during a system sleep transition.
-Device drivers, however, are not supposed to call device_set_wakeup_enable()
-directly in any case.
-
-It ought to be noted that system wakeup is conceptually different from "remote
-wakeup" used by runtime power management, although it may be supported by the
-same physical mechanism. Remote wakeup is a feature allowing devices in
-low-power states to trigger specific interrupts to signal conditions in which
-they should be put into the full-power state. Those interrupts may or may not
-be used to signal system wakeup events, depending on the hardware design. On
-some systems it is impossible to trigger them from system sleep states. In any
-case, remote wakeup should always be enabled for runtime power management for
-all devices and drivers that support it.
-
-/sys/devices/.../power/control files
-------------------------------------
-Each device in the driver model has a flag to control whether it is subject to
-runtime power management. This flag, called runtime_auto, is initialized by the
-bus type (or generally subsystem) code using pm_runtime_allow() or
-pm_runtime_forbid(); the default is to allow runtime power management.
-
-The setting can be adjusted by user space by writing either "on" or "auto" to
-the device's power/control sysfs file. Writing "auto" calls pm_runtime_allow(),
-setting the flag and allowing the device to be runtime power-managed by its
-driver. Writing "on" calls pm_runtime_forbid(), clearing the flag, returning
-the device to full power if it was in a low-power state, and preventing the
-device from being runtime power-managed. User space can check the current value
-of the runtime_auto flag by reading the file.
-
-The device's runtime_auto flag has no effect on the handling of system-wide
-power transitions. In particular, the device can (and in the majority of cases
-should and will) be put into a low-power state during a system-wide transition
-to a sleep state even though its runtime_auto flag is clear.
-
-For more information about the runtime power management framework, refer to
-Documentation/power/runtime_pm.txt.
-
-
-Calling Drivers to Enter and Leave System Sleep States
-======================================================
-When the system goes into a sleep state, each device's driver is asked to
-suspend the device by putting it into a state compatible with the target
-system state. That's usually some version of "off", but the details are
-system-specific. Also, wakeup-enabled devices will usually stay partly
-functional in order to wake the system.
-
-When the system leaves that low-power state, the device's driver is asked to
-resume it by returning it to full power. The suspend and resume operations
-always go together, and both are multi-phase operations.
-
-For simple drivers, suspend might quiesce the device using class code
-and then turn its hardware as "off" as possible during suspend_noirq. The
-matching resume calls would then completely reinitialize the hardware
-before reactivating its class I/O queues.
-
-More power-aware drivers might prepare the devices for triggering system wakeup
-events.
-
-
-Call Sequence Guarantees
-------------------------
-To ensure that bridges and similar links needing to talk to a device are
-available when the device is suspended or resumed, the device tree is
-walked in a bottom-up order to suspend devices. A top-down order is
-used to resume those devices.
-
-The ordering of the device tree is defined by the order in which devices
-get registered: a child can never be registered, probed or resumed before
-its parent; and can't be removed or suspended after that parent.
-
-The policy is that the device tree should match hardware bus topology.
-(Or at least the control bus, for devices which use multiple busses.)
-In particular, this means that a device registration may fail if the parent of
-the device is suspending (i.e. has been chosen by the PM core as the next
-device to suspend) or has already suspended, as well as after all of the other
-devices have been suspended. Device drivers must be prepared to cope with such
-situations.
-
-
-System Power Management Phases
-------------------------------
-Suspending or resuming the system is done in several phases. Different phases
-are used for freeze, standby, and 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 freeze, standby or memory sleep state,
-the phases are:
-
- prepare, suspend, suspend_late, suspend_noirq.
-
- 1. The prepare phase is meant to prevent races by preventing new devices
- from being registered; the PM core would never know that all the
- children of a device had been suspended if new children could be
- registered at will. (By contrast, 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.
-
- For devices supporting runtime power management, the return value of the
- prepare callback can be used to indicate to the PM core that it may
- safely leave the device in runtime suspend (if runtime-suspended
- already), provided that all of the device's descendants are also left in
- runtime suspend. Namely, if the prepare callback returns a positive
- number and that happens for all of the descendants of the device too,
- and all of them (including the device itself) are runtime-suspended, the
- PM core will skip the suspend, suspend_late and suspend_noirq suspend
- phases as well as the resume_noirq, resume_early and resume phases of
- the following system resume for all of these devices. In that case,
- the complete callback will be called directly after the prepare callback
- and is entirely responsible for bringing the device back to the
- functional state as appropriate.
-
- Note that this direct-complete procedure applies even if the device is
- disabled for runtime PM; only the runtime-PM status matters. It follows
- that if a device has system-sleep callbacks but does not support runtime
- PM, then its prepare callback must never return a positive value. This
- is because all devices are initially set to runtime-suspended with
- runtime PM disabled.
-
- 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 freeze, standby or memory sleep, the phases are:
-
- resume_noirq, resume_early, resume, complete.
-
- 1. The resume_noirq callback methods should perform any actions needed
- before the driver's interrupt handlers are invoked. This generally
- means undoing the actions of the suspend_noirq phase. If the bus type
- permits devices to share interrupt vectors, like PCI, the method should
- bring the device and its driver into a state in which the driver can
- recognize if the device is the source of incoming interrupts, if any,
- and handle them correctly.
-
- For example, the PCI bus type's ->pm.resume_noirq() puts the device into
- the full-power state (D0 in the PCI terminology) and restores the
- standard configuration registers of the device. Then it calls the
- device driver's ->pm.resume_noirq() method to perform device-specific
- actions.
-
- 2. The resume_early methods should prepare devices for the execution of
- the resume methods. This generally involves undoing the actions of the
- preceding suspend_late phase.
-
- 3 The resume methods should bring the device back to its operating
- state, so that it can perform normal I/O. This generally involves
- undoing the actions of the suspend phase.
-
- 4. The complete phase should undo the actions of the prepare phase. 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.
-
- Moreover, if the preceding prepare callback returned a positive number,
- the device may have been left in runtime suspend throughout the whole
- system suspend and resume (the suspend, suspend_late, suspend_noirq
- phases of system suspend and the resume_noirq, resume_early, resume
- phases of system resume may have been skipped for it). In that case,
- the complete callback is entirely responsible for bringing the device
- back to the functional state after system suspend if necessary. [For
- example, it may need to queue up a runtime resume request for the device
- for this purpose.] To check if that is the case, the complete callback
- can consult the device's power.direct_complete flag. Namely, if that
- flag is set when the complete callback is being run, it has been called
- directly after the preceding prepare and special action may be required
- to make the device work correctly afterward.
-
-At the end of these phases, drivers should be as functional as they were before
-suspending: I/O can be performed using DMA and IRQs, and the relevant clocks are
-gated on.
-
-However, the details here may again be platform-specific. For example,
-some systems support multiple "run" states, and the mode in effect at
-the end of resume might not be the one which preceded suspension.
-That means availability of certain clocks or power supplies changed,
-which could easily affect how a driver works.
-
-Drivers need to be able to handle hardware which has been reset since 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 other
-sleep states, because it involves creating and saving a system image.
-Therefore there are more phases for hibernation, with a different set of
-callbacks. These phases always run after tasks have been frozen and 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 freeze, 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. A power domain may also be
-nested inside another power domain. The nested domain is referred to as the
-sub-domain of the parent domain.
-
-Support for power domains is provided through the 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
-type).
-
-The support for device power management domains is only relevant to platforms
-needing to use the same device driver power management callbacks in many
-different power domain configurations and wanting to avoid incorporating the
-support for power domains into subsystem-level callbacks, for example by
-modifying the platform bus type. Other platforms need not implement it or take
-it into account in any way.
-
-Devices may be defined as IRQ-safe which indicates to the PM core that their
-runtime PM callbacks may be invoked with disabled interrupts (see
-Documentation/power/runtime_pm.txt for more information). If an IRQ-safe
-device belongs to a PM domain, the runtime PM of the domain will be
-disallowed, unless the domain itself is defined as IRQ-safe. However, it
-makes sense to define a PM domain as IRQ-safe only if all the devices in it
-are IRQ-safe. Moreover, if an IRQ-safe domain has a parent domain, the runtime
-PM of the parent is only allowed if the parent itself is IRQ-safe too with the
-additional restriction that all child domains of an IRQ-safe parent must also
-be IRQ-safe.
-
-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.
* example, if it detects that a child was unplugged while the system was
* asleep).
*
- * Refer to Documentation/power/devices.txt for more information about the role
- * of the above callbacks in the system suspend process.
- *
* There also are callbacks related to runtime power management of devices.
* Again, as a rule these callbacks are executed by the PM core for subsystems
* (PM domains, device types, classes and bus types) and the subsystem-level
projects / GitHub / LineageOS / android_kernel_motorola_exynos9610.git / commitdiff