PM: Update comments describing device power management callbacks
The comments describing device power management callbacks in include/pm.h are outdated and somewhat confusing, so make them reflect the reality more accurately. Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
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Rafael J. Wysocki
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@ -54,118 +54,145 @@ typedef struct pm_message {
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/**
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* struct dev_pm_ops - device PM callbacks
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*
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* Several driver power state transitions are externally visible, affecting
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* Several device power state transitions are externally visible, affecting
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* the state of pending I/O queues and (for drivers that touch hardware)
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* interrupts, wakeups, DMA, and other hardware state. There may also be
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* internal transitions to various low power modes, which are transparent
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* internal transitions to various low-power modes which are transparent
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* to the rest of the driver stack (such as a driver that's ON gating off
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* clocks which are not in active use).
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*
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* The externally visible transitions are handled with the help of the following
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* callbacks included in this structure:
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* The externally visible transitions are handled with the help of callbacks
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* included in this structure in such a way that two levels of callbacks are
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* involved. First, the PM core executes callbacks provided by PM domains,
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* device types, classes and bus types. They are the subsystem-level callbacks
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* supposed to execute callbacks provided by device drivers, although they may
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* choose not to do that. If the driver callbacks are executed, they have to
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* collaborate with the subsystem-level callbacks to achieve the goals
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* appropriate for the given system transition, given transition phase and the
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* subsystem the device belongs to.
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*
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* @prepare: Prepare the device for the upcoming transition, but do NOT change
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* its hardware state. Prevent new children of the device from being
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* registered after @prepare() returns (the driver's subsystem and
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* generally the rest of the kernel is supposed to prevent new calls to the
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* probe method from being made too once @prepare() has succeeded). If
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* @prepare() detects a situation it cannot handle (e.g. registration of a
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* child already in progress), it may return -EAGAIN, so that the PM core
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* can execute it once again (e.g. after the new child has been registered)
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* to recover from the race condition. This method is executed for all
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* kinds of suspend transitions and is followed by one of the suspend
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* callbacks: @suspend(), @freeze(), or @poweroff().
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* The PM core executes @prepare() for all devices before starting to
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* execute suspend callbacks for any of them, so drivers may assume all of
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* the other devices to be present and functional while @prepare() is being
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* executed. In particular, it is safe to make GFP_KERNEL memory
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* allocations from within @prepare(). However, drivers may NOT assume
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* anything about the availability of the user space at that time and it
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* is not correct to request firmware from within @prepare() (it's too
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* late to do that). [To work around this limitation, drivers may
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* register suspend and hibernation notifiers that are executed before the
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* freezing of tasks.]
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* @prepare: The principal role of this callback is to prevent new children of
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* the device from being registered after it has returned (the driver's
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* subsystem and generally the rest of the kernel is supposed to prevent
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* new calls to the probe method from being made too once @prepare() has
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* succeeded). If @prepare() detects a situation it cannot handle (e.g.
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* registration of a child already in progress), it may return -EAGAIN, so
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* that the PM core can execute it once again (e.g. after a new child has
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* been registered) to recover from the race condition.
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* This method is executed for all kinds of suspend transitions and is
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* followed by one of the suspend callbacks: @suspend(), @freeze(), or
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* @poweroff(). The PM core executes subsystem-level @prepare() for all
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* devices before starting to invoke suspend callbacks for any of them, so
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* generally devices may be assumed to be functional or to respond to
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* runtime resume requests while @prepare() is being executed. However,
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* device drivers may NOT assume anything about the availability of user
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* space at that time and it is NOT valid to request firmware from within
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* @prepare() (it's too late to do that). It also is NOT valid to allocate
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* substantial amounts of memory from @prepare() in the GFP_KERNEL mode.
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* [To work around these limitations, drivers may register suspend and
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* hibernation notifiers to be executed before the freezing of tasks.]
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*
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* @complete: Undo the changes made by @prepare(). This method is executed for
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* all kinds of resume transitions, following one of the resume callbacks:
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* @resume(), @thaw(), @restore(). Also called if the state transition
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* fails before the driver's suspend callback (@suspend(), @freeze(),
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* @poweroff()) can be executed (e.g. if the suspend callback fails for one
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* fails before the driver's suspend callback: @suspend(), @freeze() or
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* @poweroff(), can be executed (e.g. if the suspend callback fails for one
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* of the other devices that the PM core has unsuccessfully attempted to
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* suspend earlier).
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* The PM core executes @complete() after it has executed the appropriate
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* resume callback for all devices.
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* The PM core executes subsystem-level @complete() after it has executed
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* the appropriate resume callbacks for all devices.
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*
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* @suspend: Executed before putting the system into a sleep state in which the
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* contents of main memory are preserved. Quiesce the device, put it into
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* a low power state appropriate for the upcoming system state (such as
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* PCI_D3hot), and enable wakeup events as appropriate.
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* contents of main memory are preserved. The exact action to perform
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* depends on the device's subsystem (PM domain, device type, class or bus
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* type), but generally the device must be quiescent after subsystem-level
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* @suspend() has returned, so that it doesn't do any I/O or DMA.
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* Subsystem-level @suspend() is executed for all devices after invoking
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* subsystem-level @prepare() for all of them.
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*
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* @resume: Executed after waking the system up from a sleep state in which the
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* contents of main memory were preserved. Put the device into the
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* appropriate state, according to the information saved in memory by the
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* preceding @suspend(). The driver starts working again, responding to
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* hardware events and software requests. The hardware may have gone
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* through a power-off reset, or it may have maintained state from the
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* previous suspend() which the driver may rely on while resuming. On most
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* platforms, there are no restrictions on availability of resources like
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* clocks during @resume().
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* contents of main memory were preserved. The exact action to perform
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* depends on the device's subsystem, but generally the driver is expected
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* to start working again, responding to hardware events and software
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* requests (the device itself may be left in a low-power state, waiting
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* for a runtime resume to occur). The state of the device at the time its
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* driver's @resume() callback is run depends on the platform and subsystem
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* the device belongs to. On most platforms, there are no restrictions on
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* availability of resources like clocks during @resume().
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* Subsystem-level @resume() is executed for all devices after invoking
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* subsystem-level @resume_noirq() for all of them.
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*
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* @freeze: Hibernation-specific, executed before creating a hibernation image.
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* Quiesce operations so that a consistent image can be created, but do NOT
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* otherwise put the device into a low power device state and do NOT emit
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* system wakeup events. Save in main memory the device settings to be
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* used by @restore() during the subsequent resume from hibernation or by
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* the subsequent @thaw(), if the creation of the image or the restoration
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* of main memory contents from it fails.
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* Analogous to @suspend(), but it should not enable the device to signal
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* wakeup events or change its power state. The majority of subsystems
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* (with the notable exception of the PCI bus type) expect the driver-level
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* @freeze() to save the device settings in memory to be used by @restore()
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* during the subsequent resume from hibernation.
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* Subsystem-level @freeze() is executed for all devices after invoking
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* subsystem-level @prepare() for all of them.
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*
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* @thaw: Hibernation-specific, executed after creating a hibernation image OR
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* if the creation of the image fails. Also executed after a failing
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* if the creation of an image has failed. Also executed after a failing
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* attempt to restore the contents of main memory from such an image.
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* Undo the changes made by the preceding @freeze(), so the device can be
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* operated in the same way as immediately before the call to @freeze().
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* Subsystem-level @thaw() is executed for all devices after invoking
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* subsystem-level @thaw_noirq() for all of them. It also may be executed
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* directly after @freeze() in case of a transition error.
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*
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* @poweroff: Hibernation-specific, executed after saving a hibernation image.
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* Quiesce the device, put it into a low power state appropriate for the
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* upcoming system state (such as PCI_D3hot), and enable wakeup events as
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* appropriate.
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* Analogous to @suspend(), but it need not save the device's settings in
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* memory.
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* Subsystem-level @poweroff() is executed for all devices after invoking
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* subsystem-level @prepare() for all of them.
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*
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* @restore: Hibernation-specific, executed after restoring the contents of main
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* memory from a hibernation image. Driver starts working again,
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* responding to hardware events and software requests. Drivers may NOT
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* make ANY assumptions about the hardware state right prior to @restore().
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* On most platforms, there are no restrictions on availability of
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* resources like clocks during @restore().
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* memory from a hibernation image, analogous to @resume().
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*
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* @suspend_noirq: Complete the operations of ->suspend() by carrying out any
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* actions required for suspending the device that need interrupts to be
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* disabled
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* @suspend_noirq: Complete the actions started by @suspend(). Carry out any
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* additional operations required for suspending the device that might be
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* racing with its driver's interrupt handler, which is guaranteed not to
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* run while @suspend_noirq() is being executed.
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* It generally is expected that the device will be in a low-power state
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* (appropriate for the target system sleep state) after subsystem-level
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* @suspend_noirq() has returned successfully. If the device can generate
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* system wakeup signals and is enabled to wake up the system, it should be
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* configured to do so at that time. However, depending on the platform
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* and device's subsystem, @suspend() may be allowed to put the device into
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* the low-power state and configure it to generate wakeup signals, in
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* which case it generally is not necessary to define @suspend_noirq().
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*
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* @resume_noirq: Prepare for the execution of ->resume() by carrying out any
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* actions required for resuming the device that need interrupts to be
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* disabled
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* @resume_noirq: Prepare for the execution of @resume() by carrying out any
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* operations required for resuming the device that might be racing with
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* its driver's interrupt handler, which is guaranteed not to run while
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* @resume_noirq() is being executed.
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*
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* @freeze_noirq: Complete the operations of ->freeze() by carrying out any
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* actions required for freezing the device that need interrupts to be
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* disabled
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* @freeze_noirq: Complete the actions started by @freeze(). Carry out any
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* additional operations required for freezing the device that might be
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* racing with its driver's interrupt handler, which is guaranteed not to
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* run while @freeze_noirq() is being executed.
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* The power state of the device should not be changed by either @freeze()
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* or @freeze_noirq() and it should not be configured to signal system
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* wakeup by any of these callbacks.
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*
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* @thaw_noirq: Prepare for the execution of ->thaw() by carrying out any
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* actions required for thawing the device that need interrupts to be
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* disabled
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* @thaw_noirq: Prepare for the execution of @thaw() by carrying out any
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* operations required for thawing the device that might be racing with its
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* driver's interrupt handler, which is guaranteed not to run while
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* @thaw_noirq() is being executed.
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*
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* @poweroff_noirq: Complete the operations of ->poweroff() by carrying out any
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* actions required for handling the device that need interrupts to be
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* disabled
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* @poweroff_noirq: Complete the actions started by @poweroff(). Analogous to
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* @suspend_noirq(), but it need not save the device's settings in memory.
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*
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* @restore_noirq: Prepare for the execution of ->restore() by carrying out any
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* actions required for restoring the operations of the device that need
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* interrupts to be disabled
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* @restore_noirq: Prepare for the execution of @restore() by carrying out any
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* operations required for thawing the device that might be racing with its
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* driver's interrupt handler, which is guaranteed not to run while
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* @restore_noirq() is being executed. Analogous to @resume_noirq().
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*
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* All of the above callbacks, except for @complete(), return error codes.
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* However, the error codes returned by the resume operations, @resume(),
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* @thaw(), @restore(), @resume_noirq(), @thaw_noirq(), and @restore_noirq() do
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* @thaw(), @restore(), @resume_noirq(), @thaw_noirq(), and @restore_noirq(), do
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* not cause the PM core to abort the resume transition during which they are
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* returned. The error codes returned in that cases are only printed by the PM
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* returned. The error codes returned in those cases are only printed by the PM
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* core to the system logs for debugging purposes. Still, it is recommended
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* that drivers only return error codes from their resume methods in case of an
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* unrecoverable failure (i.e. when the device being handled refuses to resume
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@ -174,31 +201,43 @@ typedef struct pm_message {
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* their children.
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*
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* It is allowed to unregister devices while the above callbacks are being
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* executed. However, it is not allowed to unregister a device from within any
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* of its own callbacks.
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* executed. However, a callback routine must NOT try to unregister the device
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* it was called for, although it may unregister children of that device (for
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* example, if it detects that a child was unplugged while the system was
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* asleep).
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*
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* There also are the following callbacks related to run-time power management
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* of devices:
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* Refer to Documentation/power/devices.txt for more information about the role
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* of the above callbacks in the system suspend process.
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*
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* There also are callbacks related to runtime power management of devices.
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* Again, these callbacks are executed by the PM core only for subsystems
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* (PM domains, device types, classes and bus types) and the subsystem-level
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* callbacks are supposed to invoke the driver callbacks. Moreover, the exact
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* actions to be performed by a device driver's callbacks generally depend on
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* the platform and subsystem the device belongs to.
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*
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* @runtime_suspend: Prepare the device for a condition in which it won't be
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* able to communicate with the CPU(s) and RAM due to power management.
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* This need not mean that the device should be put into a low power state.
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* This need not mean that the device should be put into a low-power state.
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* For example, if the device is behind a link which is about to be turned
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* off, the device may remain at full power. If the device does go to low
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* power and is capable of generating run-time wake-up events, remote
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* wake-up (i.e., a hardware mechanism allowing the device to request a
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* change of its power state via a wake-up event, such as PCI PME) should
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* be enabled for it.
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* power and is capable of generating runtime wakeup events, remote wakeup
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* (i.e., a hardware mechanism allowing the device to request a change of
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* its power state via an interrupt) should be enabled for it.
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*
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* @runtime_resume: Put the device into the fully active state in response to a
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* wake-up event generated by hardware or at the request of software. If
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* necessary, put the device into the full power state and restore its
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* wakeup event generated by hardware or at the request of software. If
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* necessary, put the device into the full-power state and restore its
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* registers, so that it is fully operational.
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*
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* @runtime_idle: Device appears to be inactive and it might be put into a low
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* power state if all of the necessary conditions are satisfied. Check
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* @runtime_idle: Device appears to be inactive and it might be put into a
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* low-power state if all of the necessary conditions are satisfied. Check
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* these conditions and handle the device as appropriate, possibly queueing
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* a suspend request for it. The return value is ignored by the PM core.
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*
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* Refer to Documentation/power/runtime_pm.txt for more information about the
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* role of the above callbacks in device runtime power management.
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*
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*/
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struct dev_pm_ops {
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