1Suspend notifiers
2	(C) 2007-2011 Rafael J. Wysocki <rjw@sisk.pl>, GPL
3
4There are some operations that subsystems or drivers may want to carry out
5before hibernation/suspend or after restore/resume, but they require the system
6to be fully functional, so the drivers' and subsystems' .suspend() and .resume()
7or even .prepare() and .complete() callbacks are not suitable for this purpose.
8For example, device drivers may want to upload firmware to their devices after
9resume/restore, but they cannot do it by calling request_firmware() from their
10.resume() or .complete() routines (user land processes are frozen at these
11points).  The solution may be to load the firmware into memory before processes
12are frozen and upload it from there in the .resume() routine.
13A suspend/hibernation notifier may be used for this purpose.
14
15The subsystems or drivers having such needs can register suspend notifiers that
16will be called upon the following events by the PM core:
17
18PM_HIBERNATION_PREPARE	The system is going to hibernate, tasks will be frozen
19			immediately. This is different from PM_SUSPEND_PREPARE
20			below because here we do additional work between notifiers
21			and drivers freezing.
22
23PM_POST_HIBERNATION	The system memory state has been restored from a
24			hibernation image or an error occurred during
25			hibernation.  Device drivers' restore callbacks have
26			been executed and tasks have been thawed.
27
28PM_RESTORE_PREPARE	The system is going to restore a hibernation image.
29			If all goes well, the restored kernel will issue a
30			PM_POST_HIBERNATION notification.
31
32PM_POST_RESTORE		An error occurred during restore from hibernation.
33			Device drivers' restore callbacks have been executed
34			and tasks have been thawed.
35
36PM_SUSPEND_PREPARE	The system is preparing for suspend.
37
38PM_POST_SUSPEND		The system has just resumed or an error occurred during
39			suspend.  Device drivers' resume callbacks have been
40			executed and tasks have been thawed.
41
42It is generally assumed that whatever the notifiers do for
43PM_HIBERNATION_PREPARE, should be undone for PM_POST_HIBERNATION.  Analogously,
44operations performed for PM_SUSPEND_PREPARE should be reversed for
45PM_POST_SUSPEND.  Additionally, all of the notifiers are called for
46PM_POST_HIBERNATION if one of them fails for PM_HIBERNATION_PREPARE, and
47all of the notifiers are called for PM_POST_SUSPEND if one of them fails for
48PM_SUSPEND_PREPARE.
49
50The hibernation and suspend notifiers are called with pm_mutex held.  They are
51defined in the usual way, but their last argument is meaningless (it is always
52NULL).  To register and/or unregister a suspend notifier use the functions
53register_pm_notifier() and unregister_pm_notifier(), respectively, defined in
54include/linux/suspend.h .  If you don't need to unregister the notifier, you can
55also use the pm_notifier() macro defined in include/linux/suspend.h .
56