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![David Brownell](/assets/img/avatar_default.png)
This turned into a rewrite of Documentation/power/devices.txt: - Provide more of the "big picture" - Fixup some of the horribly ancient/obsolete description of device suspend() semantics; lots of text just got deleted. - Add a decent description of PM_EVENT_* codes, including the new PRETHAW code needed in some swsusp scenarios. - Describe the new PM factorization from Linus: * class suspend, current suspend, then suspend_late * NOT suspend_prepare, it wasn't really usable * resume_early, current resume, class resume. - Updates power/state docs to be correct, and deprecate its usage except for driver testing. Signed-off-by: David Brownell <dbrownell@users.sourceforge.net> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
553 lines
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553 lines
26 KiB
Text
Most of the code in Linux is device drivers, so most of the Linux power
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management code is also driver-specific. Most drivers will do very little;
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others, especially for platforms with small batteries (like cell phones),
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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-to-ram", or (mostly for systems with
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disks) "hibernate" (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 that low-power state. This feature may be disabled using the
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relevant /sys/devices/.../power/wakeup file; enabling it may cost some
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power usage, but let the whole system enter low power states more often.
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Runtime Power Management model:
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Drivers may also enter low power states while the system is running,
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independently of other power management activity. Upstream drivers
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will normally not know (or care) if the device is in some low power
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state when issuing requests; the driver will auto-resume anything
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that's needed when it gets a request.
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This doesn't have, or need much infrastructure; it's just something you
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should do when writing your drivers. For example, clk_disable() unused
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clocks as part of minimizing power drain for currently-unused hardware.
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Of course, sometimes clusters of drivers will collaborate with each
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other, which could involve task-specific power management.
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There's not a lot to be said about those low power states except that they
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are very system-specific, and often device-specific. Also, that if enough
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drivers put themselves into low power states (at "runtime"), the effect may be
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the same as entering some system-wide low-power state (system sleep) ... and
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that synergies exist, so that several drivers using runtime pm might put the
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system 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, no
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more data read or written, and requests from upstream drivers are no longer
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accepted. A given bus or platform may have different 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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Most of the programming interfaces a device driver needs to know about
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relate to that first model: entering a system-wide low power state,
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rather than just minimizing power consumption by one device.
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Bus Driver Methods
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------------------
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The core methods to suspend and resume devices reside in struct bus_type.
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These are mostly of interest to people writing infrastructure for busses
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like PCI or USB, or because they define the primitives that device drivers
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may need to apply in domain-specific ways to their devices:
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struct bus_type {
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...
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int (*suspend)(struct device *dev, pm_message_t state);
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int (*suspend_late)(struct device *dev, pm_message_t state);
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int (*resume_early)(struct device *dev);
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int (*resume)(struct device *dev);
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};
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Bus drivers implement those methods as appropriate for the hardware and
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the drivers using it; PCI works differently from USB, and so on. Not many
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people write bus drivers; most driver code is a "device driver" that
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builds 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. Note that as this is being written,
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only the suspend() and resume() are widely available; not many bus drivers
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leverage all of those phases, or pass them down to lower driver levels.
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/sys/devices/.../power/wakeup files
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-----------------------------------
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All devices in the driver model have two flags to control handling of
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wakeup events, which are hardware signals that can force the device and/or
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system out of a low power state. These are initialized by bus or device
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driver code using device_init_wakeup(dev,can_wakeup).
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The "can_wakeup" flag just records whether the device (and its driver) can
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physically support wakeup events. When that flag is clear, the sysfs
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"wakeup" file is empty, and device_may_wakeup() returns false.
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For devices that can issue wakeup events, a separate flag controls whether
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that device should try to use its wakeup mechanism. The initial value of
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device_may_wakeup() will be true, so that the device's "wakeup" file holds
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the value "enabled". Userspace can change that to "disabled" so that
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device_may_wakeup() returns false; or change it back to "enabled" (so that
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it returns true again).
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EXAMPLE: PCI Device Driver Methods
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-----------------------------------
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PCI framework software calls these methods when the PCI device driver bound
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to a device device has provided them:
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struct pci_driver {
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...
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int (*suspend)(struct pci_device *pdev, pm_message_t state);
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int (*suspend_late)(struct pci_device *pdev, pm_message_t state);
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int (*resume_early)(struct pci_device *pdev);
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int (*resume)(struct pci_device *pdev);
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};
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Drivers will implement those methods, and call PCI-specific procedures
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like pci_set_power_state(), pci_enable_wake(), pci_save_state(), and
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pci_restore_state() to manage PCI-specific mechanisms. (PCI config space
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could be saved during driver probe, if it weren't for the fact that some
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systems rely on userspace tweaking using setpci.) Devices are suspended
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before their bridges enter low power states, and likewise bridges resume
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before their devices.
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Upper Layers of Driver Stacks
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-----------------------------
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Device drivers generally have at least two interfaces, and the methods
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sketched above are the ones which apply to the lower level (nearer PCI, USB,
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or other bus hardware). The network and block layers are examples of upper
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level interfaces, as is a character device talking to userspace.
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Power management requests normally need to flow through those upper levels,
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which often use domain-oriented requests like "blank that screen". In
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some cases those upper levels will have power management intelligence that
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relates to end-user activity, or other devices that work in cooperation.
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When those interfaces are structured using class interfaces, there is a
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standard way to have the upper layer stop issuing requests to a given
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class device (and restart later):
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struct class {
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...
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int (*suspend)(struct device *dev, pm_message_t state);
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int (*resume)(struct device *dev);
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};
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Those calls are issued in specific phases of the process by which the
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system enters a low power "suspend" state, or resumes from it.
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Calling Drivers to Enter System Sleep States
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============================================
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When the system enters a low power state, each device's driver is asked
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to suspend the device by putting it into 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
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to resume it. The suspend and resume operations always go together, and
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both are multi-phase operations.
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For simple drivers, suspend might quiesce the device using the class code
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and then turn its hardware as "off" as possible with late_suspend. 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 drivers will use more than one device low power
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state, either at runtime or during system sleep states, and might trigger
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system wakeup events.
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Call Sequence Guarantees
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------------------------
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To ensure that bridges and similar links needed 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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Suspending Devices
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------------------
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Suspending a given device is done in several phases. Suspending the
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system always includes every phase, executing calls for every device
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before the next phase begins. Not all busses or classes support all
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these callbacks; and not all drivers use all the callbacks.
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The phases are seen by driver notifications issued in this order:
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1 class.suspend(dev, message) is called after tasks are frozen, for
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devices associated with a class that has such a method. This
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method may sleep.
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Since I/O activity usually comes from such higher layers, this is
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a good place to quiesce all drivers of a given type (and keep such
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code out of those drivers).
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2 bus.suspend(dev, message) is called next. This method may sleep,
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and is often morphed into a device driver call with bus-specific
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parameters and/or rules.
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This call should handle parts of device suspend logic that require
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sleeping. It probably does work to quiesce the device which hasn't
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been abstracted into class.suspend() or bus.suspend_late().
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3 bus.suspend_late(dev, message) is called with IRQs disabled, and
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with only one CPU active. Until the bus.resume_early() phase
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completes (see later), IRQs are not enabled again. This method
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won't be exposed by all busses; for message based busses like USB,
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I2C, or SPI, device interactions normally require IRQs. This bus
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call may be morphed into a driver call with bus-specific parameters.
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This call might save low level hardware state that might otherwise
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be lost in the upcoming low power state, and actually put the
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device into a low power state ... so that in some cases the device
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may stay partly usable until this late. This "late" call may also
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help when coping with hardware that behaves badly.
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The pm_message_t parameter is currently used to refine those semantics
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(described later).
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At the end of those phases, drivers should normally have stopped all I/O
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transactions (DMA, IRQs), saved enough state that they can re-initialize
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or restore previous state (as needed by the hardware), and placed the
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device into a low-power state. On many platforms they will also use
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clk_disable() to gate off one or more clock sources; sometimes they will
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also switch off power supplies, or reduce voltages. Drivers which have
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runtime PM support may already have performed some or all of the steps
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needed to prepare for the upcoming system sleep state.
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When any driver sees that its device_can_wakeup(dev), it should make sure
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to use the relevant hardware signals to trigger a system wakeup event.
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For example, enable_irq_wake() might identify GPIO signals hooked up to
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a switch or other external hardware, and pci_enable_wake() does something
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similar for PCI's PME# signal.
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If a driver (or bus, or class) fails it suspend method, the system won't
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enter the desired low power state; it will resume all the devices it's
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suspended so far.
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Note that drivers may need to perform different actions based on the target
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system lowpower/sleep state. At this writing, there are only platform
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specific APIs through which drivers could determine those target states.
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Device Low Power (suspend) States
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---------------------------------
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Device low-power states aren't very standard. One device might only handle
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"on" and "off, while another might support a dozen different versions of
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"on" (how many engines are active?), plus a state that gets back to "on"
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faster than from a full "off".
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Some busses define rules about what different suspend states mean. PCI
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gives one example: after the suspend sequence completes, a non-legacy
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PCI device may not perform DMA or issue IRQs, and any wakeup events it
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issues would be issued through the PME# bus signal. Plus, there are
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several PCI-standard device states, some of which are optional.
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In contrast, integrated system-on-chip processors often use irqs as the
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wakeup event sources (so drivers would call enable_irq_wake) and might
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be able to treat DMA completion as a wakeup event (sometimes DMA can stay
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active too, it'd only be the CPU and some peripherals that sleep).
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Some details here may be platform-specific. Systems may have devices that
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can be fully active in certain sleep states, such as an LCD display that's
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refreshed using DMA while most of the system is sleeping lightly ... and
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its frame buffer might even be updated by a DSP or other non-Linux CPU while
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the Linux control processor stays idle.
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Moreover, the specific actions taken may depend on the target system state.
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One target system state might allow a given device to be very operational;
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another might require a hard shut down with re-initialization on resume.
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And two different target systems might use the same device in different
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ways; the aforementioned LCD might be active in one product's "standby",
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but a different product using the same SOC might work differently.
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Meaning of pm_message_t.event
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-----------------------------
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Parameters to suspend calls include the device affected and a message of
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type pm_message_t, which has one field: the event. If driver does not
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recognize the event code, suspend calls may abort the request and return
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a negative errno. However, most drivers will be fine if they implement
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PM_EVENT_SUSPEND semantics for all messages.
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The event codes are used to refine the goal of suspending the device, and
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mostly matter when creating or resuming system memory image snapshots, as
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used with suspend-to-disk:
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PM_EVENT_SUSPEND -- quiesce the driver and put hardware into a low-power
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state. When used with system sleep states like "suspend-to-RAM" or
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"standby", the upcoming resume() call will often be able to rely on
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state kept in hardware, or issue system wakeup events. When used
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instead with suspend-to-disk, few devices support this capability;
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most are completely powered off.
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PM_EVENT_FREEZE -- quiesce the driver, but don't necessarily change into
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any low power mode. A system snapshot is about to be taken, often
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followed by a call to the driver's resume() method. Neither wakeup
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events nor DMA are allowed.
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PM_EVENT_PRETHAW -- quiesce the driver, knowing that the upcoming resume()
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will restore a suspend-to-disk snapshot from a different kernel image.
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Drivers that are smart enough to look at their hardware state during
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resume() processing need that state to be correct ... a PRETHAW could
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be used to invalidate that state (by resetting the device), like a
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shutdown() invocation would before a kexec() or system halt. Other
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drivers might handle this the same way as PM_EVENT_FREEZE. Neither
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wakeup events nor DMA are allowed.
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To enter "standby" (ACPI S1) or "Suspend to RAM" (STR, ACPI S3) states, or
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the similarly named APM states, only PM_EVENT_SUSPEND is used; for "Suspend
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to Disk" (STD, hibernate, ACPI S4), all of those event codes are used.
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There's also PM_EVENT_ON, a value which never appears as a suspend event
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but is sometimes used to record the "not suspended" device state.
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Resuming Devices
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----------------
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Resuming is done in multiple phases, much like suspending, with all
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devices processing each phase's calls before the next phase begins.
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The phases are seen by driver notifications issued in this order:
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1 bus.resume_early(dev) is called with IRQs disabled, and with
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only one CPU active. As with bus.suspend_late(), this method
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won't be supported on busses that require IRQs in order to
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interact with devices.
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This reverses the effects of bus.suspend_late().
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2 bus.resume(dev) is called next. This may be morphed into a device
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driver call with bus-specific parameters; implementations may sleep.
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This reverses the effects of bus.suspend().
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3 class.resume(dev) is called for devices associated with a class
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that has such a method. Implementations may sleep.
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This reverses the effects of class.suspend(), and would usually
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reactivate the device's I/O queue.
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At the end of those phases, drivers should normally be as functional as
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they were before suspending: I/O can be performed using DMA and IRQs, and
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the relevant clocks are gated on. The device need not be "fully on"; it
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might be in a runtime lowpower/suspend state that acts as if it were.
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However, the details here may again be platform-specific. For example,
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some systems support multiple "run" states, and the mode in effect at
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the end of resume() might not be the one which preceded suspension.
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That means availability of certain clocks or power supplies changed,
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which could easily affect how a driver works.
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Drivers need to be able to handle hardware which has been reset since the
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suspend methods were called, for example by complete reinitialization.
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This may be the hardest part, and the one most protected by NDA'd documents
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and chip errata. It's simplest if the hardware state hasn't changed since
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the suspend() was called, but that can't always be guaranteed.
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Drivers must also be prepared to notice that the device has been removed
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while the system was powered off, whenever that's physically possible.
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PCMCIA, MMC, USB, Firewire, SCSI, and even IDE are common examples of busses
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where common Linux platforms will see such removal. Details of how drivers
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will notice and handle such removals are currently bus-specific, and often
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involve a separate thread.
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Note that the bus-specific runtime PM wakeup mechanism can exist, and might
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be defined to share some of the same driver code as for system wakeup. For
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example, a bus-specific device driver's resume() method might be used there,
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so it wouldn't only be called from bus.resume() during system-wide wakeup.
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See bus-specific information about how runtime wakeup events are handled.
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System Devices
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--------------
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System devices follow a slightly different API, which can be found in
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include/linux/sysdev.h
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drivers/base/sys.c
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System devices will only be suspended with interrupts disabled, and after
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all other devices have been suspended. On resume, they will be resumed
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before any other devices, and also with interrupts disabled.
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That is, IRQs are disabled, the suspend_late() phase begins, then the
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sysdev_driver.suspend() phase, and the system enters a sleep state. Then
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the sysdev_driver.resume() phase begins, followed by the resume_early()
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phase, after which IRQs are enabled.
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Code to actually enter and exit the system-wide low power state sometimes
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involves hardware details that are only known to the boot firmware, and
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may leave a CPU running software (from SRAM or flash memory) that monitors
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the system and manages its wakeup sequence.
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Runtime Power Management
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========================
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Many devices are able to dynamically power down while the system is still
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running. This feature is useful for devices that are not being used, and
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can offer significant power savings on a running system. These devices
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often support a range of runtime power states, which might use names such
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as "off", "sleep", "idle", "active", and so on. Those states will in some
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cases (like PCI) be partially constrained by a bus the device uses, and will
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usually include hardware states that are also used in system sleep states.
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However, note that if a driver puts a device into a runtime low power state
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and the system then goes into a system-wide sleep state, it normally ought
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to resume into that runtime low power state rather than "full on". Such
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distinctions would be part of the driver-internal state machine for that
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hardware; the whole point of runtime power management is to be sure that
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drivers are decoupled in that way from the state machine governing phases
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of the system-wide power/sleep state transitions.
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Power Saving Techniques
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-----------------------
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Normally runtime power management is handled by the drivers without specific
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userspace or kernel intervention, by device-aware use of techniques like:
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Using information provided by other system layers
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- stay deeply "off" except between open() and close()
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- if transceiver/PHY indicates "nobody connected", stay "off"
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- application protocols may include power commands or hints
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Using fewer CPU cycles
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- using DMA instead of PIO
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- removing timers, or making them lower frequency
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- shortening "hot" code paths
|
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- eliminating cache misses
|
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- (sometimes) offloading work to device firmware
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|
|
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Reducing other resource costs
|
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- gating off unused clocks in software (or hardware)
|
|
- switching off unused power supplies
|
|
- eliminating (or delaying/merging) IRQs
|
|
- tuning DMA to use word and/or burst modes
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|
|
|
Using device-specific low power states
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- using lower voltages
|
|
- avoiding needless DMA transfers
|
|
|
|
Read your hardware documentation carefully to see the opportunities that
|
|
may be available. If you can, measure the actual power usage and check
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|
it against the budget established for your project.
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|
|
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Examples: USB hosts, system timer, system CPU
|
|
----------------------------------------------
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USB host controllers make interesting, if complex, examples. In many cases
|
|
these have no work to do: no USB devices are connected, or all of them are
|
|
in the USB "suspend" state. Linux host controller drivers can then disable
|
|
periodic DMA transfers that would otherwise be a constant power drain on the
|
|
memory subsystem, and enter a suspend state. In power-aware controllers,
|
|
entering that suspend state may disable the clock used with USB signaling,
|
|
saving a certain amount of power.
|
|
|
|
The controller will be woken from that state (with an IRQ) by changes to the
|
|
signal state on the data lines of a given port, for example by an existing
|
|
peripheral requesting "remote wakeup" or by plugging a new peripheral. The
|
|
same wakeup mechanism usually works from "standby" sleep states, and on some
|
|
systems also from "suspend to RAM" (or even "suspend to disk") states.
|
|
(Except that ACPI may be involved instead of normal IRQs, on some hardware.)
|
|
|
|
System devices like timers and CPUs may have special roles in the platform
|
|
power management scheme. For example, system timers using a "dynamic tick"
|
|
approach don't just save CPU cycles (by eliminating needless timer IRQs),
|
|
but they may also open the door to using lower power CPU "idle" states that
|
|
cost more than a jiffie to enter and exit. On x86 systems these are states
|
|
like "C3"; note that periodic DMA transfers from a USB host controller will
|
|
also prevent entry to a C3 state, much like a periodic timer IRQ.
|
|
|
|
That kind of runtime mechanism interaction is common. "System On Chip" (SOC)
|
|
processors often have low power idle modes that can't be entered unless
|
|
certain medium-speed clocks (often 12 or 48 MHz) are gated off. When the
|
|
drivers gate those clocks effectively, then the system idle task may be able
|
|
to use the lower power idle modes and thereby increase battery life.
|
|
|
|
If the CPU can have a "cpufreq" driver, there also may be opportunities
|
|
to shift to lower voltage settings and reduce the power cost of executing
|
|
a given number of instructions. (Without voltage adjustment, it's rare
|
|
for cpufreq to save much power; the cost-per-instruction must go down.)
|
|
|
|
|
|
/sys/devices/.../power/state files
|
|
==================================
|
|
For now you can also test some of this functionality using sysfs.
|
|
|
|
DEPRECATED: USE "power/state" ONLY FOR DRIVER TESTING, AND
|
|
AVOID USING dev->power.power_state IN DRIVERS.
|
|
|
|
THESE WILL BE REMOVED. IF THE "power/state" FILE GETS REPLACED,
|
|
IT WILL BECOME SOMETHING COUPLED TO THE BUS OR DRIVER.
|
|
|
|
In each device's directory, there is a 'power' directory, which contains
|
|
at least a 'state' file. The value of this field is effectively boolean,
|
|
PM_EVENT_ON or PM_EVENT_SUSPEND.
|
|
|
|
* Reading from this file displays a value corresponding to
|
|
the power.power_state.event field. All nonzero values are
|
|
displayed as "2", corresponding to a low power state; zero
|
|
is displayed as "0", corresponding to normal operation.
|
|
|
|
* Writing to this file initiates a transition using the
|
|
specified event code number; only '0', '2', and '3' are
|
|
accepted (without a newline); '2' and '3' are both
|
|
mapped to PM_EVENT_SUSPEND.
|
|
|
|
On writes, the PM core relies on that recorded event code and the device/bus
|
|
capabilities to determine whether it uses a partial suspend() or resume()
|
|
sequence to change things so that the recorded event corresponds to the
|
|
numeric parameter.
|
|
|
|
- If the bus requires the irqs-disabled suspend_late()/resume_early()
|
|
phases, writes fail because those operations are not supported here.
|
|
|
|
- If the recorded value is the expected value, nothing is done.
|
|
|
|
- If the recorded value is nonzero, the device is partially resumed,
|
|
using the bus.resume() and/or class.resume() methods.
|
|
|
|
- If the target value is nonzero, the device is partially suspended,
|
|
using the class.suspend() and/or bus.suspend() methods and the
|
|
PM_EVENT_SUSPEND message.
|
|
|
|
Drivers have no way to tell whether their suspend() and resume() calls
|
|
have come through the sysfs power/state file or as part of entering a
|
|
system sleep state, except that when accessed through sysfs the normal
|
|
parent/child sequencing rules are ignored. Drivers (such as bus, bridge,
|
|
or hub drivers) which expose child devices may need to enforce those rules
|
|
on their own.
|