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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 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.