Documentation/power/states.txt - kernel/skids - Git at Google


 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.

	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.