states.txt 3.1 KB

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