System Power Management States
  
  
  The kernel supports three power management states generically, though
  each is dependent on platform support code to implement the low-level
  details for each state. This file describes each state, what they are
  commonly called, what ACPI state they map to, and what string to write
  to /sys/power/state to enter that state
  
  
  State:		Standby / Power-On Suspend
  ACPI State:	S1
  String:		"standby"
  
  This state offers minimal, though real, power savings, while providing
  a very low-latency transition back to a working system. No operating
  state is lost (the CPU retains power), so the system easily starts up
  again where it left off. 
  
  We try to put devices in a low-power state equivalent to D1, which
  also offers low power savings, but low resume latency. Not all devices
  support D1, and those that don't are left on. 
  
  A transition from Standby to the On state should take about 1-2
  seconds. 
  
  
  State:		Suspend-to-RAM
  ACPI State:	S3
  String:		"mem"
  
  This state offers significant power savings as everything in the
  system is put into a low-power state, except for memory, which is
  placed in self-refresh mode to retain its contents. 
  
  System and device state is saved and kept in memory. All devices are
  suspended and put into D3. In many cases, all peripheral buses lose
  power when entering STR, so devices must be able to handle the
  transition back to the On state. 
  
  For at least ACPI, STR requires some minimal boot-strapping code to
  resume the system from STR. This may be true on other platforms. 
  
  A transition from Suspend-to-RAM to the On state should take about
  3-5 seconds. 
  
  
  State:		Suspend-to-disk
  ACPI State:	S4
  String:		"disk"
  
  This state offers the greatest power savings, and can be used even in
  the absence of low-level platform support for power management. This
  state operates similarly to Suspend-to-RAM, but includes a final step
  of writing memory contents to disk. On resume, this is read and memory
  is restored to its pre-suspend state. 
  
  STD can be handled by the firmware or the kernel. If it is handled by
  the firmware, it usually requires a dedicated partition that must be
  setup via another operating system for it to use. Despite the
  inconvenience, this method requires minimal work by the kernel, since
  the firmware will also handle restoring memory contents on resume. 

  For suspend-to-disk, a mechanism called 'swsusp' (Swap Suspend) is used
  to write memory contents to free swap space. swsusp has some restrictive
  requirements, but should work in most cases. Some, albeit outdated,
  documentation can be found in Documentation/power/swsusp.txt.
  Alternatively, userspace can do most of the actual suspend to disk work,
  see userland-swsusp.txt.

  
  Once memory state is written to disk, the system may either enter a
  low-power state (like ACPI S4), or it may simply power down. Powering
  down offers greater savings, and allows this mechanism to work on any
  system. However, entering a real low-power state allows the user to

  trigger wake up events (e.g. pressing a key or opening a laptop lid).

  
  A transition from Suspend-to-Disk to the On state should take about 30
  seconds, though it's typically a bit more with the current
  implementation.