Root/Documentation/power/notifiers.txt

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 or suspend, tasks will
19            be frozen immediately.
20
21PM_POST_HIBERNATION The system memory state has been restored from a
22            hibernation image or an error occurred during
23            hibernation. Device drivers' restore callbacks have
24            been executed and tasks have been thawed.
25
26PM_RESTORE_PREPARE The system is going to restore a hibernation image.
27            If all goes well, the restored kernel will issue a
28            PM_POST_HIBERNATION notification.
29
30PM_POST_RESTORE An error occurred during restore from hibernation.
31            Device drivers' restore callbacks have been executed
32            and tasks have been thawed.
33
34PM_SUSPEND_PREPARE The system is preparing for suspend.
35
36PM_POST_SUSPEND The system has just resumed or an error occurred during
37            suspend. Device drivers' resume callbacks have been
38            executed and tasks have been thawed.
39
40It is generally assumed that whatever the notifiers do for
41PM_HIBERNATION_PREPARE, should be undone for PM_POST_HIBERNATION. Analogously,
42operations performed for PM_SUSPEND_PREPARE should be reversed for
43PM_POST_SUSPEND. Additionally, all of the notifiers are called for
44PM_POST_HIBERNATION if one of them fails for PM_HIBERNATION_PREPARE, and
45all of the notifiers are called for PM_POST_SUSPEND if one of them fails for
46PM_SUSPEND_PREPARE.
47
48The hibernation and suspend notifiers are called with pm_mutex held. They are
49defined in the usual way, but their last argument is meaningless (it is always
50NULL). To register and/or unregister a suspend notifier use the functions
51register_pm_notifier() and unregister_pm_notifier(), respectively, defined in
52include/linux/suspend.h . If you don't need to unregister the notifier, you can
53also use the pm_notifier() macro defined in include/linux/suspend.h .
54

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