linux/Documentation/power/notifiers.txt
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   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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