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Subject[PATCH 5] PM: Update comments describing device power management callbacks
Date
From: Rafael J. Wysocki <rjw@sisk.pl>

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include/pm.h are outdated and somewhat confusing, so make them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>
---
include/linux/pm.h | 193 ++++++++++++++++++++++++++++++-----------------------
1 file changed, 110 insertions(+), 83 deletions(-)
Index: linux/include/linux/pm.h
===================================================================
--- linux.orig/include/linux/pm.h
+++ linux/include/linux/pm.h
@@ -54,15 +54,22 @@ typedef struct pm_message {
/**
* struct dev_pm_ops - device PM callbacks
*
- * Several driver power state transitions are externally visible, affecting
+ * Several device power state transitions are externally visible, affecting
* the state of pending I/O queues and (for drivers that touch hardware)
* interrupts, wakeups, DMA, and other hardware state. There may also be
- * internal transitions to various low power modes, which are transparent
+ * internal transitions to various low-power modes which are transparent
* to the rest of the driver stack (such as a driver that's ON gating off
* clocks which are not in active use).
*
- * The externally visible transitions are handled with the help of the following
- * callbacks included in this structure:
+ * The externally visible transitions are handled with the help of callbacks
+ * included in this structure in such a way that two levels of callbacks are
+ * involved. First, the PM core executes callbacks provided by PM domains,
+ * device types, classes and bus types. They are the subsystem-level callbacks
+ * supposed to execute callbacks provided by device drivers, although they may
+ * choose not to do that. If the driver callbacks are executed, they have to
+ * collaborate with the subsystem-level callbacks to achieve the goals
+ * appropriate for the given system transition, given transition phase and the
+ * subsystem the device belongs to.
*
* @prepare: Prepare the device for the upcoming transition, but do NOT change
* its hardware state. Prevent new children of the device from being
@@ -71,101 +78,118 @@ typedef struct pm_message {
* probe method from being made too once @prepare() has succeeded). If
* @prepare() detects a situation it cannot handle (e.g. registration of a
* child already in progress), it may return -EAGAIN, so that the PM core
- * can execute it once again (e.g. after the new child has been registered)
+ * can execute it once again (e.g. after a new child has been registered)
* to recover from the race condition. This method is executed for all
* kinds of suspend transitions and is followed by one of the suspend
* callbacks: @suspend(), @freeze(), or @poweroff().
- * The PM core executes @prepare() for all devices before starting to
- * execute suspend callbacks for any of them, so drivers may assume all of
- * the other devices to be present and functional while @prepare() is being
- * executed. In particular, it is safe to make GFP_KERNEL memory
- * allocations from within @prepare(). However, drivers may NOT assume
- * anything about the availability of the user space at that time and it
- * is not correct to request firmware from within @prepare() (it's too
- * late to do that). [To work around this limitation, drivers may
- * register suspend and hibernation notifiers that are executed before the
+ * The PM core executes subsystem-level @prepare() for all devices before
+ * starting to execute suspend callbacks for any of them, so all devices
+ * may be assumed to be present and functional while @prepare() is being
+ * executed. However, device drivers may NOT assume anything about the
+ * availability of user space at that time and it is NOT valid to request
+ * firmware from within @prepare() (it's too late to do that). It also is
+ * NOT valid to allocate substantial amounts of memory from @prepare() in
+ * the GFP_KERNEL mode. [To work around these limitations, drivers may
+ * register suspend and hibernation notifiers to be executed before the
* freezing of tasks.]
*
* @complete: Undo the changes made by @prepare(). This method is executed for
* all kinds of resume transitions, following one of the resume callbacks:
* @resume(), @thaw(), @restore(). Also called if the state transition
- * fails before the driver's suspend callback (@suspend(), @freeze(),
- * @poweroff()) can be executed (e.g. if the suspend callback fails for one
+ * fails before the driver's suspend callback: @suspend(), @freeze() or
+ * @poweroff(), can be executed (e.g. if the suspend callback fails for one
* of the other devices that the PM core has unsuccessfully attempted to
* suspend earlier).
- * The PM core executes @complete() after it has executed the appropriate
- * resume callback for all devices.
+ * The PM core executes subsystem-level @complete() after it has executed
+ * the appropriate resume callbacks for all devices.
*
* @suspend: Executed before putting the system into a sleep state in which the
- * contents of main memory are preserved. Quiesce the device, put it into
- * a low power state appropriate for the upcoming system state (such as
- * PCI_D3hot), and enable wakeup events as appropriate.
+ * contents of main memory are preserved. The exact action to perform
+ * depends on the device's subsystem (PM domain, device type, class or bus
+ * type), but generally the device must be quiescent after @suspend() has
+ * returned, so that it doesn't do any I/O or DMA.
+ * Subsystem-level @suspend() is executed for all devices after invoking
+ * subsystem-level @prepare() for all of them.
*
* @resume: Executed after waking the system up from a sleep state in which the
- * contents of main memory were preserved. Put the device into the
- * appropriate state, according to the information saved in memory by the
- * preceding @suspend(). The driver starts working again, responding to
- * hardware events and software requests. The hardware may have gone
- * through a power-off reset, or it may have maintained state from the
- * previous suspend() which the driver may rely on while resuming. On most
- * platforms, there are no restrictions on availability of resources like
- * clocks during @resume().
+ * contents of main memory were preserved. Undo the changes made by
+ * the preceding @suspend() and cause the device to become operational
+ * (the exact action to perform depends on the device's subsystem).
+ * The driver starts working again, responding to hardware events and
+ * software requests. The state of the device at the time its driver's
+ * @resume() callback is run depends on the platform and subsystem the
+ * device belongs to. On most platforms, there are no restrictions on
+ * availability of resources like clocks during @resume().
+ * Subsystem-level @resume() is executed for all devices after invoking
+ * subsystem-level @resume_noirq() for all of them.
*
* @freeze: Hibernation-specific, executed before creating a hibernation image.
- * Quiesce operations so that a consistent image can be created, but do NOT
- * otherwise put the device into a low power device state and do NOT emit
- * system wakeup events. Save in main memory the device settings to be
- * used by @restore() during the subsequent resume from hibernation or by
- * the subsequent @thaw(), if the creation of the image or the restoration
- * of main memory contents from it fails.
+ * Analogous to @suspend(), but it should not enable the device to signal
+ * wakeup events. The majority of subsystems (with the notable exception
+ * of the PCI bus type) expect the driver-level @freeze() to save the
+ * device settings in memory to be used by @restore() during the subsequent
+ * resume from hibernation.
+ * Subsystem-level @freeze() is executed for all devices after invoking
+ * subsystem-level @prepare() for all of them.
*
* @thaw: Hibernation-specific, executed after creating a hibernation image OR
- * if the creation of the image fails. Also executed after a failing
+ * if the creation of an image has failed. Also executed after a failing
* attempt to restore the contents of main memory from such an image.
* Undo the changes made by the preceding @freeze(), so the device can be
* operated in the same way as immediately before the call to @freeze().
+ * Subsystem-level @thaw() is executed for all devices after invoking
+ * subsystem-level @thaw_noirq() for all of them. It also may be executed
+ * directly after @freeze() in case of a transition error.
*
* @poweroff: Hibernation-specific, executed after saving a hibernation image.
- * Quiesce the device, put it into a low power state appropriate for the
- * upcoming system state (such as PCI_D3hot), and enable wakeup events as
- * appropriate.
+ * Analogous to @suspend(), but it need not save the the device settings in
+ * memory.
+ * Subsystem-level @poweroff() is executed for all devices after invoking
+ * subsystem-level @prepare() for all of them.
*
* @restore: Hibernation-specific, executed after restoring the contents of main
- * memory from a hibernation image. Driver starts working again,
- * responding to hardware events and software requests. Drivers may NOT
- * make ANY assumptions about the hardware state right prior to @restore().
- * On most platforms, there are no restrictions on availability of
- * resources like clocks during @restore().
- *
- * @suspend_noirq: Complete the operations of ->suspend() by carrying out any
- * actions required for suspending the device that need interrupts to be
- * disabled
- *
- * @resume_noirq: Prepare for the execution of ->resume() by carrying out any
- * actions required for resuming the device that need interrupts to be
- * disabled
- *
- * @freeze_noirq: Complete the operations of ->freeze() by carrying out any
- * actions required for freezing the device that need interrupts to be
- * disabled
- *
- * @thaw_noirq: Prepare for the execution of ->thaw() by carrying out any
- * actions required for thawing the device that need interrupts to be
- * disabled
- *
- * @poweroff_noirq: Complete the operations of ->poweroff() by carrying out any
- * actions required for handling the device that need interrupts to be
- * disabled
- *
- * @restore_noirq: Prepare for the execution of ->restore() by carrying out any
- * actions required for restoring the operations of the device that need
- * interrupts to be disabled
+ * memory from a hibernation image. The state of the device at the time
+ * its driver's @restore() callback is run depends on the platform and
+ * subsystem the device belongs to. On most platforms, there are no
+ * restrictions on availability of resources like clocks during @restore().
+ * Subsystem-level @restore() is executed for all devices after invoking
+ * subsystem-level @restore_noirq() for all of them.
+ *
+ * @suspend_noirq: Complete the actions started by @suspend(). Carry out any
+ * additional operations required for suspending the device that might be
+ * racing with its driver's interrupt handler, which is guaranteed not to
+ * run while @suspend_noirq() is being executed.
+ *
+ * @resume_noirq: Prepare for the execution of @resume() by carrying out any
+ * operations required for resuming the device that might be racing with
+ * its driver's interrupt handler, which is guaranteed not to run while
+ * @resume_noirq() is being executed.
+ *
+ * @freeze_noirq: Complete the actions started by @freeze(). Carry out any
+ * additional operations required for freezing the device that might be
+ * racing with its driver's interrupt handler, which is guaranteed not to
+ * run while @freeze_noirq() is being executed.
+ *
+ * @thaw_noirq: Prepare for the execution of @thaw() by carrying out any
+ * operations required for thawing the device that might be racing with its
+ * driver's interrupt handler, which is guaranteed not to run while
+ * @thaw_noirq() is being executed.
+ *
+ * @poweroff_noirq: Complete the actions started by @poweroff(). Carry out any
+ * additional operations required for powering off the device that might be
+ * racing with its driver's interrupt handler, which is guaranteed not to
+ * run while @poweroff_noirq() is being executed.
+ *
+ * @restore_noirq: Prepare for the execution of @restore() by carrying out any
+ * operations required for thawing the device that might be racing with its
+ * driver's interrupt handler, which is guaranteed not to run while
+ * @restore_noirq() is being executed.
*
* All of the above callbacks, except for @complete(), return error codes.
* However, the error codes returned by the resume operations, @resume(),
- * @thaw(), @restore(), @resume_noirq(), @thaw_noirq(), and @restore_noirq() do
+ * @thaw(), @restore(), @resume_noirq(), @thaw_noirq(), and @restore_noirq(), do
* not cause the PM core to abort the resume transition during which they are
- * returned. The error codes returned in that cases are only printed by the PM
+ * returned. The error codes returned in those cases are only printed by the PM
* core to the system logs for debugging purposes. Still, it is recommended
* that drivers only return error codes from their resume methods in case of an
* unrecoverable failure (i.e. when the device being handled refuses to resume
@@ -174,29 +198,32 @@ typedef struct pm_message {
* their children.
*
* It is allowed to unregister devices while the above callbacks are being
- * executed. However, it is not allowed to unregister a device from within any
- * of its own callbacks.
+ * executed. However, it is NOT allowed to unregister a device from within any
+ * of its driver's callbacks.
*
- * There also are the following callbacks related to run-time power management
- * of devices:
+ * There also are callbacks related to runtime power management of devices.
+ * Again, these callbacks are executed by the PM core only for subsystems
+ * (PM domains, device types, classes and bus types) and the subsystem-level
+ * callbacks are supposed to invoke the driver callbacks. Moreover, the exact
+ * actions to be performed by a device driver's callbacks generally depend on
+ * the platform and subsystem the device belongs to.
*
* @runtime_suspend: Prepare the device for a condition in which it won't be
* able to communicate with the CPU(s) and RAM due to power management.
- * This need not mean that the device should be put into a low power state.
+ * This need not mean that the device should be put into a low-power state.
* For example, if the device is behind a link which is about to be turned
* off, the device may remain at full power. If the device does go to low
- * power and is capable of generating run-time wake-up events, remote
- * wake-up (i.e., a hardware mechanism allowing the device to request a
- * change of its power state via a wake-up event, such as PCI PME) should
- * be enabled for it.
+ * power and is capable of generating runtime wakeup events, remote wakeup
+ * (i.e., a hardware mechanism allowing the device to request a change of
+ * its power state via an interrupt) should be enabled for it.
*
* @runtime_resume: Put the device into the fully active state in response to a
- * wake-up event generated by hardware or at the request of software. If
- * necessary, put the device into the full power state and restore its
+ * wakeup event generated by hardware or at the request of software. If
+ * necessary, put the device into the full-power state and restore its
* registers, so that it is fully operational.
*
- * @runtime_idle: Device appears to be inactive and it might be put into a low
- * power state if all of the necessary conditions are satisfied. Check
+ * @runtime_idle: Device appears to be inactive and it might be put into a
+ * low-power state if all of the necessary conditions are satisfied. Check
* these conditions and handle the device as appropriate, possibly queueing
* a suspend request for it. The return value is ignored by the PM core.
*/

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