Documentation/power/states.txt

   1 System Power Management Sleep States
   2
   3 (C) 2014 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
   4
   5 The kernel supports up to four system sleep states generically, although three
   6 of them depend on the platform support code to implement the low-level details
   7 for each state.
   8
   9 The states are represented by strings that can be read or written to the
  10 /sys/power/state file.  Those strings may be "mem", "standby", "freeze" and
  11 "disk", where the last one always represents hibernation (Suspend-To-Disk) and
  12 the meaning of the remaining ones depends on the relative_sleep_states command
  13 line argument.
  14
  15 For relative_sleep_states=1, the strings "mem", "standby" and "freeze" label the
  16 available non-hibernation sleep states from the deepest to the shallowest,
  17 respectively.  In that case, "mem" is always present in /sys/power/state,
  18 because there is at least one non-hibernation sleep state in every system.  If
  19 the given system supports two non-hibernation sleep states, "standby" is present
  20 in /sys/power/state in addition to "mem".  If the system supports three
  21 non-hibernation sleep states, "freeze" will be present in /sys/power/state in
  22 addition to "mem" and "standby".
  23
  24 For relative_sleep_states=0, which is the default, the following descriptions
  25 apply.
  26
  27 state:          Suspend-To-Idle
  28 ACPI state:     S0
  29 Label:          "freeze"
  30
  31 This state is a generic, pure software, light-weight, system sleep state.
  32 It allows more energy to be saved relative to runtime idle by freezing user
  33 space and putting all I/O devices into low-power states (possibly
  34 lower-power than available at run time), such that the processors can
  35 spend more time in their idle states.
  36
  37 This state can be used for platforms without Power-On Suspend/Suspend-to-RAM
  38 support, or it can be used in addition to Suspend-to-RAM (memory sleep)
  39 to provide reduced resume latency.  It is always supported.
  40
  41
  42 State:          Standby / Power-On Suspend
  43 ACPI State:     S1
  44 Label:          "standby"
  45
  46 This state, if supported, offers moderate, though real, power savings, while
  47 providing a relatively low-latency transition back to a working system.  No
  48 operating state is lost (the CPU retains power), so the system easily starts up
  49 again where it left off.
  50
  51 In addition to freezing user space and putting all I/O devices into low-power
  52 states, which is done for Suspend-To-Idle too, nonboot CPUs are taken offline
  53 and all low-level system functions are suspended during transitions into this
  54 state.  For this reason, it should allow more energy to be saved relative to
  55 Suspend-To-Idle, but the resume latency will generally be greater than for that
  56 state.
  57
  58
  59 State:          Suspend-to-RAM
  60 ACPI State:     S3
  61 Label:          "mem"
  62
  63 This state, if supported, offers significant power savings as everything in the
  64 system is put into a low-power state, except for memory, which should be placed
  65 into the self-refresh mode to retain its contents.  All of the steps carried out
  66 when entering Power-On Suspend are also carried out during transitions to STR.
  67 Additional operations may take place depending on the platform capabilities.  In
  68 particular, on ACPI systems the kernel passes control to the BIOS (platform
  69 firmware) as the last step during STR transitions and that usually results in
  70 powering down some more low-level components that aren't directly controlled by
  71 the kernel.
  72
  73 System and device state is saved and kept in memory.  All devices are suspended
  74 and put into low-power states.  In many cases, all peripheral buses lose power
  75 when entering STR, so devices must be able to handle the transition back to the
  76 "on" state.
  77
  78 For at least ACPI, STR requires some minimal boot-strapping code to resume the
  79 system from it.  This may be the case on other platforms too.
  80
  81
  82 State:          Suspend-to-disk
  83 ACPI State:     S4
  84 Label:          "disk"
  85
  86 This state offers the greatest power savings, and can be used even in
  87 the absence of low-level platform support for power management. This
  88 state operates similarly to Suspend-to-RAM, but includes a final step
  89 of writing memory contents to disk. On resume, this is read and memory
  90 is restored to its pre-suspend state.
  91
  92 STD can be handled by the firmware or the kernel. If it is handled by
  93 the firmware, it usually requires a dedicated partition that must be
  94 setup via another operating system for it to use. Despite the
  95 inconvenience, this method requires minimal work by the kernel, since
  96 the firmware will also handle restoring memory contents on resume.
  97
  98 For suspend-to-disk, a mechanism called 'swsusp' (Swap Suspend) is used
  99 to write memory contents to free swap space. swsusp has some restrictive
 100 requirements, but should work in most cases. Some, albeit outdated,
 101 documentation can be found in Documentation/power/swsusp.txt.
 102 Alternatively, userspace can do most of the actual suspend to disk work,
 103 see userland-swsusp.txt.
 104
 105 Once memory state is written to disk, the system may either enter a
 106 low-power state (like ACPI S4), or it may simply power down. Powering
 107 down offers greater savings, and allows this mechanism to work on any
 108 system. However, entering a real low-power state allows the user to
 109 trigger wake up events (e.g. pressing a key or opening a laptop lid).