WSL2-Linux-Kernel/Documentation/power/notifiers.txt

Suspend notifiers
	(C) 2007 Rafael J. Wysocki <rjw@sisk.pl>, GPL

There are some operations that device drivers may want to carry out in their
.suspend() routines, but shouldn't, because they can cause the hibernation or
suspend to fail. For example, a driver may want to allocate a substantial amount
of memory (like 50 MB) in .suspend(), but that shouldn't be done after the
swsusp's memory shrinker has run.

Also, there may be some operations, that subsystems want to carry out before a
hibernation/suspend or after a restore/resume, requiring the system to be fully
functional, so the drivers' .suspend() and .resume() routines are not suitable
for this purpose.  For example, device drivers may want to upload firmware to
their devices after a restore from a hibernation image, but they cannot do it by
calling request_firmware() from their .resume() routines (user land processes
are frozen at this point).  The solution may be to load the firmware into
memory before processes are frozen and upload it from there in the .resume()
routine.  Of course, a hibernation notifier may be used for this purpose.

The subsystems that have such needs can register suspend notifiers that will be
called upon the following events by the suspend core:

PM_HIBERNATION_PREPARE	The system is going to hibernate or suspend, tasks will
			be frozen immediately.

PM_POST_HIBERNATION	The system memory state has been restored from a
			hibernation image or an error occured during the
			hibernation.  Device drivers' .resume() callbacks have
			been executed and tasks have been thawed.

PM_RESTORE_PREPARE	The system is going to restore a hibernation image.
			If all goes well the restored kernel will issue a
			PM_POST_HIBERNATION notification.

PM_POST_RESTORE		An error occurred during the hibernation restore.
			Device drivers' .resume() callbacks have been executed
			and tasks have been thawed.

PM_SUSPEND_PREPARE	The system is preparing for a suspend.

PM_POST_SUSPEND		The system has just resumed or an error occured during
			the suspend.	Device drivers' .resume() callbacks have
			been executed and tasks have been thawed.

It is generally assumed that whatever the notifiers do for
PM_HIBERNATION_PREPARE, should be undone for PM_POST_HIBERNATION.  Analogously,
operations performed for PM_SUSPEND_PREPARE should be reversed for
PM_POST_SUSPEND.  Additionally, all of the notifiers are called for
PM_POST_HIBERNATION if one of them fails for PM_HIBERNATION_PREPARE, and
all of the notifiers are called for PM_POST_SUSPEND if one of them fails for
PM_SUSPEND_PREPARE.

The hibernation and suspend notifiers are called with pm_mutex held.  They are
defined in the usual way, but their last argument is meaningless (it is always
NULL).  To register and/or unregister a suspend notifier use the functions
register_pm_notifier() and unregister_pm_notifier(), respectively, defined in
include/linux/suspend.h .  If you don't need to unregister the notifier, you can
also use the pm_notifier() macro defined in include/linux/suspend.h .