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

Suspend notifiers
	(C) 2007-2011 Rafael J. Wysocki <rjw@sisk.pl>, GPL

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

The subsystems or drivers having such needs can register suspend notifiers that
will be called upon the following events by the PM core:

PM_HIBERNATION_PREPARE	The system is going to hibernate, tasks will be frozen
			immediately. This is different from PM_SUSPEND_PREPARE
			below because here we do additional work between notifiers
			and drivers freezing.

PM_POST_HIBERNATION	The system memory state has been restored from a
			hibernation image or an error occurred during
			hibernation.  Device drivers' restore 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 restore from hibernation.
			Device drivers' restore callbacks have been executed
			and tasks have been thawed.

PM_SUSPEND_PREPARE	The system is preparing for suspend.

PM_POST_SUSPEND		The system has just resumed or an error occurred during
			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 .