Suspend notifiers

  	(C) 2007-2011 Rafael J. Wysocki <rjw@sisk.pl>, GPL
  
  There are some operations that subsystems or drivers may want to carry out
  before hibernation/suspend or after restore/resume, but they require the system
  to be fully functional, so the drivers' and subsystems' .suspend() and .resume()
  or even .prepare() and .complete() callbacks are not suitable for this purpose.
  For example, device drivers may want to upload firmware to their devices after
  resume/restore, but they cannot do it by calling request_firmware() from their
  .resume() or .complete() routines (user land processes are frozen at these
  points).  The solution may be to load the firmware into memory before processes
  are frozen and upload it from there in the .resume() routine.
  A suspend/hibernation notifier may be used for this purpose.
  
  The subsystems or drivers having such needs can register suspend notifiers that
  will be called upon the following events by the PM 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 occurred during
  			hibernation.  Device drivers' restore callbacks have

  			been executed and tasks have been thawed.

  PM_RESTORE_PREPARE	The system is going to restore a hibernation image.

  			If all goes well, the restored kernel will issue a

  			PM_POST_HIBERNATION notification.

  PM_POST_RESTORE		An error occurred during restore from hibernation.
  			Device drivers' restore callbacks have been executed

  			and tasks have been thawed.

  PM_SUSPEND_PREPARE	The system is preparing for suspend.

  PM_POST_SUSPEND		The system has just resumed or an error occurred during

  			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 .