Power Management Interface
  
  
  The power management subsystem provides a unified sysfs interface to 
  userspace, regardless of what architecture or platform one is
  running. The interface exists in /sys/power/ directory (assuming sysfs
  is mounted at /sys). 
  
  /sys/power/state controls system power state. Reading from this file
  returns what states are supported, which is hard-coded to 'standby'
  (Power-On Suspend), 'mem' (Suspend-to-RAM), and 'disk'
  (Suspend-to-Disk). 
  
  Writing to this file one of those strings causes the system to
  transition into that state. Please see the file
  Documentation/power/states.txt for a description of each of those
  states.
  
  
  /sys/power/disk controls the operating mode of the suspend-to-disk

  mechanism. Suspend-to-disk can be handled in several ways. We have a
  few options for putting the system to sleep - using the platform driver

  (e.g. ACPI or other suspend_ops), powering off the system or rebooting the

  system (for testing).

  Additionally, /sys/power/disk can be used to turn on one of the two testing
  modes of the suspend-to-disk mechanism: 'testproc' or 'test'.  If the
  suspend-to-disk mechanism is in the 'testproc' mode, writing 'disk' to
  /sys/power/state will cause the kernel to disable nonboot CPUs and freeze
  tasks, wait for 5 seconds, unfreeze tasks and enable nonboot CPUs.  If it is
  in the 'test' mode, writing 'disk' to /sys/power/state will cause the kernel
  to disable nonboot CPUs and freeze tasks, shrink memory, suspend devices, wait
  for 5 seconds, resume devices, unfreeze tasks and enable nonboot CPUs.  Then,
  we are able to look in the log messages and work out, for example, which code
  is being slow and which device drivers are misbehaving.

  Reading from this file will display all supported modes and the currently
  selected one in brackets, for example
  
  	[shutdown] reboot test testproc
  
  Writing to this file will accept one of

         'platform' (only if the platform supports it)

         'shutdown'
         'reboot'

         'testproc'
         'test'

  /sys/power/image_size controls the size of the image created by
  the suspend-to-disk mechanism.  It can be written a string
  representing a non-negative integer that will be used as an upper

  limit of the image size, in bytes.  The suspend-to-disk mechanism will

  do its best to ensure the image size will not exceed that number.  However,
  if this turns out to be impossible, it will try to suspend anyway using the
  smallest image possible.  In particular, if "0" is written to this file, the
  suspend image will be as small as possible.
  
  Reading from this file will display the current image size limit, which

  is set to 2/5 of available RAM by default.

  
  /sys/power/pm_trace controls the code which saves the last PM event point in
  the RTC across reboots, so that you can debug a machine that just hangs
  during suspend (or more commonly, during resume).  Namely, the RTC is only
  used to save the last PM event point if this file contains '1'.  Initially it
  contains '0' which may be changed to '1' by writing a string representing a
  nonzero integer into it.
  
  To use this debugging feature you should attempt to suspend the machine, then
  reboot it and run
  
  	dmesg -s 1000000 | grep 'hash matches'
  
  CAUTION: Using it will cause your machine's real-time (CMOS) clock to be
  set to a random invalid time after a resume.