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![Alan Stern](/assets/img/avatar_default.png)
Don't unset the direct_complete flag on devices that have runtime PM disabled, if they are runtime suspended. This is needed because otherwise ancestor devices wouldn't be able to do direct_complete without adding runtime PM support to all its descendants. Also removes pm_runtime_suspended_if_enabled() because it's now unused. Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com> Signed-off-by: Alan Stern <stern@rowland.harvard.edu> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
704 lines
36 KiB
Text
704 lines
36 KiB
Text
Device Power Management
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Copyright (c) 2010-2011 Rafael J. Wysocki <rjw@sisk.pl>, Novell Inc.
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Copyright (c) 2010 Alan Stern <stern@rowland.harvard.edu>
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Copyright (c) 2014 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
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Most of the code in Linux is device drivers, so most of the Linux power
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management (PM) code is also driver-specific. Most drivers will do very
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little; others, especially for platforms with small batteries (like cell
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phones), will do a lot.
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This writeup gives an overview of how drivers interact with system-wide
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power management goals, emphasizing the models and interfaces that are
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shared by everything that hooks up to the driver model core. Read it as
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background for the domain-specific work you'd do with any specific driver.
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Two Models for Device Power Management
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======================================
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Drivers will use one or both of these models to put devices into low-power
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states:
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System Sleep model:
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Drivers can enter low-power states as part of entering system-wide
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low-power states like "suspend" (also known as "suspend-to-RAM"), or
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(mostly for systems with disks) "hibernation" (also known as
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"suspend-to-disk").
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This is something that device, bus, and class drivers collaborate on
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by implementing various role-specific suspend and resume methods to
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cleanly power down hardware and software subsystems, then reactivate
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them without loss of data.
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Some drivers can manage hardware wakeup events, which make the system
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leave the low-power state. This feature may be enabled or disabled
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using the relevant /sys/devices/.../power/wakeup file (for Ethernet
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drivers the ioctl interface used by ethtool may also be used for this
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purpose); enabling it may cost some power usage, but let the whole
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system enter low-power states more often.
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Runtime Power Management model:
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Devices may also be put into low-power states while the system is
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running, independently of other power management activity in principle.
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However, devices are not generally independent of each other (for
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example, a parent device cannot be suspended unless all of its child
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devices have been suspended). Moreover, depending on the bus type the
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device is on, it may be necessary to carry out some bus-specific
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operations on the device for this purpose. Devices put into low power
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states at run time may require special handling during system-wide power
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transitions (suspend or hibernation).
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For these reasons not only the device driver itself, but also the
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appropriate subsystem (bus type, device type or device class) driver and
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the PM core are involved in runtime power management. As in the system
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sleep power management case, they need to collaborate by implementing
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various role-specific suspend and resume methods, so that the hardware
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is cleanly powered down and reactivated without data or service loss.
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There's not a lot to be said about those low-power states except that they are
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very system-specific, and often device-specific. Also, that if enough devices
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have been put into low-power states (at runtime), the effect may be very similar
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to entering some system-wide low-power state (system sleep) ... and that
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synergies exist, so that several drivers using runtime PM might put the system
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into a state where even deeper power saving options are available.
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Most suspended devices will have quiesced all I/O: no more DMA or IRQs (except
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for wakeup events), no more data read or written, and requests from upstream
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drivers are no longer accepted. A given bus or platform may have different
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requirements though.
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Examples of hardware wakeup events include an alarm from a real time clock,
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network wake-on-LAN packets, keyboard or mouse activity, and media insertion
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or removal (for PCMCIA, MMC/SD, USB, and so on).
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Interfaces for Entering System Sleep States
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===========================================
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There are programming interfaces provided for subsystems (bus type, device type,
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device class) and device drivers to allow them to participate in the power
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management of devices they are concerned with. These interfaces cover both
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system sleep and runtime power management.
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Device Power Management Operations
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----------------------------------
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Device power management operations, at the subsystem level as well as at the
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device driver level, are implemented by defining and populating objects of type
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struct dev_pm_ops:
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struct dev_pm_ops {
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int (*prepare)(struct device *dev);
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void (*complete)(struct device *dev);
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int (*suspend)(struct device *dev);
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int (*resume)(struct device *dev);
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int (*freeze)(struct device *dev);
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int (*thaw)(struct device *dev);
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int (*poweroff)(struct device *dev);
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int (*restore)(struct device *dev);
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int (*suspend_late)(struct device *dev);
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int (*resume_early)(struct device *dev);
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int (*freeze_late)(struct device *dev);
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int (*thaw_early)(struct device *dev);
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int (*poweroff_late)(struct device *dev);
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int (*restore_early)(struct device *dev);
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int (*suspend_noirq)(struct device *dev);
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int (*resume_noirq)(struct device *dev);
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int (*freeze_noirq)(struct device *dev);
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int (*thaw_noirq)(struct device *dev);
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int (*poweroff_noirq)(struct device *dev);
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int (*restore_noirq)(struct device *dev);
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int (*runtime_suspend)(struct device *dev);
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int (*runtime_resume)(struct device *dev);
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int (*runtime_idle)(struct device *dev);
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};
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This structure is defined in include/linux/pm.h and the methods included in it
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are also described in that file. Their roles will be explained in what follows.
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For now, it should be sufficient to remember that the last three methods are
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specific to runtime power management while the remaining ones are used during
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system-wide power transitions.
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There also is a deprecated "old" or "legacy" interface for power management
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operations available at least for some subsystems. This approach does not use
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struct dev_pm_ops objects and it is suitable only for implementing system sleep
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power management methods. Therefore it is not described in this document, so
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please refer directly to the source code for more information about it.
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Subsystem-Level Methods
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-----------------------
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The core methods to suspend and resume devices reside in struct dev_pm_ops
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pointed to by the ops member of struct dev_pm_domain, or by the pm member of
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struct bus_type, struct device_type and struct class. They are mostly of
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interest to the people writing infrastructure for platforms and buses, like PCI
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or USB, or device type and device class drivers. They also are relevant to the
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writers of device drivers whose subsystems (PM domains, device types, device
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classes and bus types) don't provide all power management methods.
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Bus drivers implement these methods as appropriate for the hardware and the
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drivers using it; PCI works differently from USB, and so on. Not many people
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write subsystem-level drivers; most driver code is a "device driver" that builds
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on top of bus-specific framework code.
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For more information on these driver calls, see the description later;
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they are called in phases for every device, respecting the parent-child
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sequencing in the driver model tree.
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/sys/devices/.../power/wakeup files
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-----------------------------------
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All device objects in the driver model contain fields that control the handling
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of system wakeup events (hardware signals that can force the system out of a
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sleep state). These fields are initialized by bus or device driver code using
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device_set_wakeup_capable() and device_set_wakeup_enable(), defined in
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include/linux/pm_wakeup.h.
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The "power.can_wakeup" flag just records whether the device (and its driver) can
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physically support wakeup events. The device_set_wakeup_capable() routine
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affects this flag. The "power.wakeup" field is a pointer to an object of type
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struct wakeup_source used for controlling whether or not the device should use
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its system wakeup mechanism and for notifying the PM core of system wakeup
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events signaled by the device. This object is only present for wakeup-capable
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devices (i.e. devices whose "can_wakeup" flags are set) and is created (or
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removed) by device_set_wakeup_capable().
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Whether or not a device is capable of issuing wakeup events is a hardware
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matter, and the kernel is responsible for keeping track of it. By contrast,
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whether or not a wakeup-capable device should issue wakeup events is a policy
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decision, and it is managed by user space through a sysfs attribute: the
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"power/wakeup" file. User space can write the strings "enabled" or "disabled"
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to it to indicate whether or not, respectively, the device is supposed to signal
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system wakeup. This file is only present if the "power.wakeup" object exists
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for the given device and is created (or removed) along with that object, by
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device_set_wakeup_capable(). Reads from the file will return the corresponding
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string.
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The "power/wakeup" file is supposed to contain the "disabled" string initially
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for the majority of devices; the major exceptions are power buttons, keyboards,
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and Ethernet adapters whose WoL (wake-on-LAN) feature has been set up with
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ethtool. It should also default to "enabled" for devices that don't generate
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wakeup requests on their own but merely forward wakeup requests from one bus to
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another (like PCI Express ports).
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The device_may_wakeup() routine returns true only if the "power.wakeup" object
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exists and the corresponding "power/wakeup" file contains the string "enabled".
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This information is used by subsystems, like the PCI bus type code, to see
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whether or not to enable the devices' wakeup mechanisms. If device wakeup
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mechanisms are enabled or disabled directly by drivers, they also should use
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device_may_wakeup() to decide what to do during a system sleep transition.
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Device drivers, however, are not supposed to call device_set_wakeup_enable()
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directly in any case.
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It ought to be noted that system wakeup is conceptually different from "remote
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wakeup" used by runtime power management, although it may be supported by the
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same physical mechanism. Remote wakeup is a feature allowing devices in
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low-power states to trigger specific interrupts to signal conditions in which
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they should be put into the full-power state. Those interrupts may or may not
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be used to signal system wakeup events, depending on the hardware design. On
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some systems it is impossible to trigger them from system sleep states. In any
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case, remote wakeup should always be enabled for runtime power management for
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all devices and drivers that support it.
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/sys/devices/.../power/control files
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------------------------------------
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Each device in the driver model has a flag to control whether it is subject to
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runtime power management. This flag, called runtime_auto, is initialized by the
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bus type (or generally subsystem) code using pm_runtime_allow() or
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pm_runtime_forbid(); the default is to allow runtime power management.
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The setting can be adjusted by user space by writing either "on" or "auto" to
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the device's power/control sysfs file. Writing "auto" calls pm_runtime_allow(),
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setting the flag and allowing the device to be runtime power-managed by its
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driver. Writing "on" calls pm_runtime_forbid(), clearing the flag, returning
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the device to full power if it was in a low-power state, and preventing the
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device from being runtime power-managed. User space can check the current value
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of the runtime_auto flag by reading the file.
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The device's runtime_auto flag has no effect on the handling of system-wide
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power transitions. In particular, the device can (and in the majority of cases
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should and will) be put into a low-power state during a system-wide transition
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to a sleep state even though its runtime_auto flag is clear.
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For more information about the runtime power management framework, refer to
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Documentation/power/runtime_pm.txt.
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Calling Drivers to Enter and Leave System Sleep States
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======================================================
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When the system goes into a sleep state, each device's driver is asked to
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suspend the device by putting it into a state compatible with the target
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system state. That's usually some version of "off", but the details are
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system-specific. Also, wakeup-enabled devices will usually stay partly
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functional in order to wake the system.
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When the system leaves that low-power state, the device's driver is asked to
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resume it by returning it to full power. The suspend and resume operations
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always go together, and both are multi-phase operations.
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For simple drivers, suspend might quiesce the device using class code
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and then turn its hardware as "off" as possible during suspend_noirq. The
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matching resume calls would then completely reinitialize the hardware
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before reactivating its class I/O queues.
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More power-aware drivers might prepare the devices for triggering system wakeup
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events.
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Call Sequence Guarantees
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------------------------
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To ensure that bridges and similar links needing to talk to a device are
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available when the device is suspended or resumed, the device tree is
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walked in a bottom-up order to suspend devices. A top-down order is
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used to resume those devices.
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The ordering of the device tree is defined by the order in which devices
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get registered: a child can never be registered, probed or resumed before
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its parent; and can't be removed or suspended after that parent.
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The policy is that the device tree should match hardware bus topology.
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(Or at least the control bus, for devices which use multiple busses.)
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In particular, this means that a device registration may fail if the parent of
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the device is suspending (i.e. has been chosen by the PM core as the next
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device to suspend) or has already suspended, as well as after all of the other
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devices have been suspended. Device drivers must be prepared to cope with such
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situations.
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System Power Management Phases
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------------------------------
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Suspending or resuming the system is done in several phases. Different phases
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are used for freeze, standby, and memory sleep states ("suspend-to-RAM") and the
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hibernation state ("suspend-to-disk"). Each phase involves executing callbacks
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for every device before the next phase begins. Not all busses or classes
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support all these callbacks and not all drivers use all the callbacks. The
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various phases always run after tasks have been frozen and before they are
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unfrozen. Furthermore, the *_noirq phases run at a time when IRQ handlers have
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been disabled (except for those marked with the IRQF_NO_SUSPEND flag).
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All phases use PM domain, bus, type, class or driver callbacks (that is, methods
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defined in dev->pm_domain->ops, dev->bus->pm, dev->type->pm, dev->class->pm or
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dev->driver->pm). These callbacks are regarded by the PM core as mutually
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exclusive. Moreover, PM domain callbacks always take precedence over all of the
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other callbacks and, for example, type callbacks take precedence over bus, class
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and driver callbacks. To be precise, the following rules are used to determine
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which callback to execute in the given phase:
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1. If dev->pm_domain is present, the PM core will choose the callback
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included in dev->pm_domain->ops for execution
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2. Otherwise, if both dev->type and dev->type->pm are present, the callback
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included in dev->type->pm will be chosen for execution.
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3. Otherwise, if both dev->class and dev->class->pm are present, the
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callback included in dev->class->pm will be chosen for execution.
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4. Otherwise, if both dev->bus and dev->bus->pm are present, the callback
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included in dev->bus->pm will be chosen for execution.
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This allows PM domains and device types to override callbacks provided by bus
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types or device classes if necessary.
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The PM domain, type, class and bus callbacks may in turn invoke device- or
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driver-specific methods stored in dev->driver->pm, but they don't have to do
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that.
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If the subsystem callback chosen for execution is not present, the PM core will
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execute the corresponding method from dev->driver->pm instead if there is one.
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Entering System Suspend
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-----------------------
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When the system goes into the freeze, standby or memory sleep state,
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the phases are:
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prepare, suspend, suspend_late, suspend_noirq.
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1. The prepare phase is meant to prevent races by preventing new devices
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from being registered; the PM core would never know that all the
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children of a device had been suspended if new children could be
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registered at will. (By contrast, devices may be unregistered at any
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time.) Unlike the other suspend-related phases, during the prepare
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phase the device tree is traversed top-down.
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After the prepare callback method returns, no new children may be
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registered below the device. The method may also prepare the device or
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driver in some way for the upcoming system power transition, but it
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should not put the device into a low-power state.
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For devices supporting runtime power management, the return value of the
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prepare callback can be used to indicate to the PM core that it may
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safely leave the device in runtime suspend (if runtime-suspended
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already), provided that all of the device's descendants are also left in
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runtime suspend. Namely, if the prepare callback returns a positive
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number and that happens for all of the descendants of the device too,
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and all of them (including the device itself) are runtime-suspended, the
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PM core will skip the suspend, suspend_late and suspend_noirq suspend
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phases as well as the resume_noirq, resume_early and resume phases of
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the following system resume for all of these devices. In that case,
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the complete callback will be called directly after the prepare callback
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and is entirely responsible for bringing the device back to the
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functional state as appropriate.
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Note that this direct-complete procedure applies even if the device is
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disabled for runtime PM; only the runtime-PM status matters. It follows
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that if a device has system-sleep callbacks but does not support runtime
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PM, then its prepare callback must never return a positive value. This
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is because all devices are initially set to runtime-suspended with
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runtime PM disabled.
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2. The suspend methods should quiesce the device to stop it from performing
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I/O. They also may save the device registers and put it into the
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appropriate low-power state, depending on the bus type the device is on,
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and they may enable wakeup events.
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3 For a number of devices it is convenient to split suspend into the
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"quiesce device" and "save device state" phases, in which cases
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suspend_late is meant to do the latter. It is always executed after
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runtime power management has been disabled for all devices.
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4. The suspend_noirq phase occurs after IRQ handlers have been disabled,
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which means that the driver's interrupt handler will not be called while
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the callback method is running. The methods should save the values of
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the device's registers that weren't saved previously and finally put the
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device into the appropriate low-power state.
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The majority of subsystems and device drivers need not implement this
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callback. However, bus types allowing devices to share interrupt
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vectors, like PCI, generally need it; otherwise a driver might encounter
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an error during the suspend phase by fielding a shared interrupt
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generated by some other device after its own device had been set to low
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power.
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At the end of these phases, drivers should have stopped all I/O transactions
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(DMA, IRQs), saved enough state that they can re-initialize or restore previous
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state (as needed by the hardware), and placed the device into a low-power state.
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On many platforms they will gate off one or more clock sources; sometimes they
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will also switch off power supplies or reduce voltages. (Drivers supporting
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runtime PM may already have performed some or all of these steps.)
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If device_may_wakeup(dev) returns true, the device should be prepared for
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generating hardware wakeup signals to trigger a system wakeup event when the
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system is in the sleep state. For example, enable_irq_wake() might identify
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GPIO signals hooked up to a switch or other external hardware, and
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pci_enable_wake() does something similar for the PCI PME signal.
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If any of these callbacks returns an error, the system won't enter the desired
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low-power state. Instead the PM core will unwind its actions by resuming all
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the devices that were suspended.
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Leaving System Suspend
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----------------------
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When resuming from freeze, standby or memory sleep, the phases are:
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resume_noirq, resume_early, resume, complete.
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1. The resume_noirq callback methods should perform any actions needed
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before the driver's interrupt handlers are invoked. This generally
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means undoing the actions of the suspend_noirq phase. If the bus type
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permits devices to share interrupt vectors, like PCI, the method should
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bring the device and its driver into a state in which the driver can
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recognize if the device is the source of incoming interrupts, if any,
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and handle them correctly.
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For example, the PCI bus type's ->pm.resume_noirq() puts the device into
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the full-power state (D0 in the PCI terminology) and restores the
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standard configuration registers of the device. Then it calls the
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device driver's ->pm.resume_noirq() method to perform device-specific
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actions.
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2. The resume_early methods should prepare devices for the execution of
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the resume methods. This generally involves undoing the actions of the
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preceding suspend_late phase.
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3 The resume methods should bring the device back to its operating
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state, so that it can perform normal I/O. This generally involves
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undoing the actions of the suspend phase.
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4. The complete phase should undo the actions of the prepare phase. Note,
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however, that new children may be registered below the device as soon as
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the resume callbacks occur; it's not necessary to wait until the
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complete phase.
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Moreover, if the preceding prepare callback returned a positive number,
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the device may have been left in runtime suspend throughout the whole
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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.
|
|
|
|
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.
|
|
|
|
|
|
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.
|