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+Suspend notifiers
+        (C) 2007 Rafael J. Wysocki <rjw@sisk.pl>, GPL
+
+There are some operations that device drivers may want to carry out in their
+.suspend() routines, but shouldn't, because they can cause the hibernation or
+suspend to fail. For example, a driver may want to allocate a substantial amount
+of memory (like 50 MB) in .suspend(), but that shouldn't be done after the
+swsusp's memory shrinker has run.
+
+Also, there may be some operations, that subsystems want to carry out before a
+hibernation/suspend or after a restore/resume, requiring the system to be fully
+functional, so the drivers' .suspend() and .resume() routines are not suitable
+for this purpose.  For example, device drivers may want to upload firmware to
+their devices after a restore from a hibernation image, but they cannot do it by
+calling request_firmware() from their .resume() routines (user land processes
+are frozen at this point).  The solution may be to load the firmware into
+memory before processes are frozen and upload it from there in the .resume()
+routine.  Of course, a hibernation notifier may be used for this purpose.
+
+The subsystems that have such needs can register suspend notifiers that will be
+called upon the following events by the suspend core:
+
+PM_HIBERNATION_PREPARE  The system is going to hibernate or suspend, tasks will
+                        be frozen immediately.
+
+PM_POST_HIBERNATION     The system memory state has been restored from a
+                        hibernation image or an error occured during the
+                        hibernation.  Device drivers' .resume() callbacks have
+                        been executed and tasks have been thawed.
+
+PM_SUSPEND_PREPARE      The system is preparing for a suspend.
+
+PM_POST_SUSPEND         The system has just resumed or an error occured during
+                        the 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 .