1=============================
2Suspend/Hibernation Notifiers
3=============================
4
5::
6
7 Copyright (c) 2016 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
8
9There are some operations that subsystems or drivers may want to carry out
10before hibernation/suspend or after restore/resume, but they require the system
11to be fully functional, so the drivers' and subsystems' ``->suspend()`` and
12``->resume()`` or even ``->prepare()`` and ``->complete()`` callbacks are not
13suitable for this purpose.
14
15For example, device drivers may want to upload firmware to their devices after
16resume/restore, but they cannot do it by calling :c:func:`request_firmware()`
17from their ``->resume()`` or ``->complete()`` callback routines (user land
18processes are frozen at these points).  The solution may be to load the firmware
19into memory before processes are frozen and upload it from there in the
20``->resume()`` routine.  A suspend/hibernation notifier may be used for that.
21
22Subsystems or drivers having such needs can register suspend notifiers that
23will be called upon the following events by the PM core:
24
25``PM_HIBERNATION_PREPARE``
26	The system is going to hibernate, tasks will be frozen immediately. This
27	is different from ``PM_SUSPEND_PREPARE`` below,	because in this case
28	additional work is done between the notifiers and the invocation of PM
29	callbacks for the "freeze" transition.
30
31``PM_POST_HIBERNATION``
32	The system memory state has been restored from a hibernation image or an
33	error occurred during hibernation.  Device restore callbacks have been
34	executed and tasks have been thawed.
35
36``PM_RESTORE_PREPARE``
37	The system is going to restore a hibernation image.  If all goes well,
38	the restored image kernel will issue a ``PM_POST_HIBERNATION``
39	notification.
40
41``PM_POST_RESTORE``
42	An error occurred during restore from hibernation.  Device restore
43	callbacks have been executed and tasks have been thawed.
44
45``PM_SUSPEND_PREPARE``
46	The system is preparing for suspend.
47
48``PM_POST_SUSPEND``
49	The system has just resumed or an error occurred during suspend.  Device
50	resume callbacks have been executed and tasks have been thawed.
51
52It is generally assumed that whatever the notifiers do for
53``PM_HIBERNATION_PREPARE``, should be undone for ``PM_POST_HIBERNATION``.
54Analogously, operations carried out for ``PM_SUSPEND_PREPARE`` should be
55reversed for ``PM_POST_SUSPEND``.
56
57Moreover, if one of the notifiers fails for the ``PM_HIBERNATION_PREPARE`` or
58``PM_SUSPEND_PREPARE`` event, the notifiers that have already succeeded for that
59event will be called for ``PM_POST_HIBERNATION`` or ``PM_POST_SUSPEND``,
60respectively.
61
62The hibernation and suspend notifiers are called with :c:data:`pm_mutex` held.
63They are defined in the usual way, but their last argument is meaningless (it is
64always NULL).
65
66To register and/or unregister a suspend notifier use
67:c:func:`register_pm_notifier()` and :c:func:`unregister_pm_notifier()`,
68respectively (both defined in :file:`include/linux/suspend.h`).  If you don't
69need to unregister the notifier, you can also use the :c:func:`pm_notifier()`
70macro defined in :file:`include/linux/suspend.h`.
71