linux/Documentation/power/states.txt
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   2System Power Management States
   3
   4
   5The kernel supports three power management states generically, though
   6each is dependent on platform support code to implement the low-level
   7details for each state. This file describes each state, what they are
   8commonly called, what ACPI state they map to, and what string to write
   9to /sys/power/state to enter that state
  10
  11
  12State:          Standby / Power-On Suspend
  13ACPI State:     S1
  14String:         "standby"
  15
  16This state offers minimal, though real, power savings, while providing
  17a very low-latency transition back to a working system. No operating
  18state is lost (the CPU retains power), so the system easily starts up
  19again where it left off. 
  20
  21We try to put devices in a low-power state equivalent to D1, which
  22also offers low power savings, but low resume latency. Not all devices
  23support D1, and those that don't are left on. 
  24
  25A transition from Standby to the On state should take about 1-2
  26seconds. 
  27
  28
  29State:          Suspend-to-RAM
  30ACPI State:     S3
  31String:         "mem"
  32
  33This state offers significant power savings as everything in the
  34system is put into a low-power state, except for memory, which is
  35placed in self-refresh mode to retain its contents. 
  36
  37System and device state is saved and kept in memory. All devices are
  38suspended and put into D3. In many cases, all peripheral buses lose
  39power when entering STR, so devices must be able to handle the
  40transition back to the On state. 
  41
  42For at least ACPI, STR requires some minimal boot-strapping code to
  43resume the system from STR. This may be true on other platforms. 
  44
  45A transition from Suspend-to-RAM to the On state should take about
  463-5 seconds. 
  47
  48
  49State:          Suspend-to-disk
  50ACPI State:     S4
  51String:         "disk"
  52
  53This state offers the greatest power savings, and can be used even in
  54the absence of low-level platform support for power management. This
  55state operates similarly to Suspend-to-RAM, but includes a final step
  56of writing memory contents to disk. On resume, this is read and memory
  57is restored to its pre-suspend state. 
  58
  59STD can be handled by the firmware or the kernel. If it is handled by
  60the firmware, it usually requires a dedicated partition that must be
  61setup via another operating system for it to use. Despite the
  62inconvenience, this method requires minimal work by the kernel, since
  63the firmware will also handle restoring memory contents on resume. 
  64
  65For suspend-to-disk, a mechanism called 'swsusp' (Swap Suspend) is used
  66to write memory contents to free swap space. swsusp has some restrictive
  67requirements, but should work in most cases. Some, albeit outdated,
  68documentation can be found in Documentation/power/swsusp.txt.
  69Alternatively, userspace can do most of the actual suspend to disk work,
  70see userland-swsusp.txt.
  71
  72Once memory state is written to disk, the system may either enter a
  73low-power state (like ACPI S4), or it may simply power down. Powering
  74down offers greater savings, and allows this mechanism to work on any
  75system. However, entering a real low-power state allows the user to
  76trigger wake up events (e.g. pressing a key or opening a laptop lid).
  77
  78A transition from Suspend-to-Disk to the On state should take about 30
  79seconds, though it's typically a bit more with the current
  80implementation. 
  81
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