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

 System Power Management Sleep States

 (C) 2014 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>

 The kernel supports up to four system sleep states generically, although three
 of them depend on the platform support code to implement the low-level details
 for each state.

 The states are represented by strings that can be read or written to the
 /sys/power/state file.  Those strings may be "mem", "standby", "freeze" and
 "disk", where the last one always represents hibernation (Suspend-To-Disk) and
 the meaning of the remaining ones depends on the relative_sleep_states command
 line argument.

 For relative_sleep_states=1, the strings "mem", "standby" and "freeze" label the
 available non-hibernation sleep states from the deepest to the shallowest,
 respectively.  In that case, "mem" is always present in /sys/power/state,
 because there is at least one non-hibernation sleep state in every system.  If
 the given system supports two non-hibernation sleep states, "standby" is present
 in /sys/power/state in addition to "mem".  If the system supports three
 non-hibernation sleep states, "freeze" will be present in /sys/power/state in
 addition to "mem" and "standby".

 For relative_sleep_states=0, which is the default, the following descriptions
 apply.

 state:		Suspend-To-Idle
 ACPI state:	S0
 Label:		"freeze"

 This state is a generic, pure software, light-weight, system sleep state.
 It allows more energy to be saved relative to runtime idle by freezing user
 space and putting all I/O devices into low-power states (possibly
 lower-power than available at run time), such that the processors can
 spend more time in their idle states.

 This state can be used for platforms without Power-On Suspend/Suspend-to-RAM
 support, or it can be used in addition to Suspend-to-RAM (memory sleep)
 to provide reduced resume latency.  It is always supported.


 State:		Standby / Power-On Suspend
 ACPI State:	S1
 Label:		"standby"

 This state, if supported, offers moderate, though real, power savings, while
 providing a relatively 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.

 In addition to freezing user space and putting all I/O devices into low-power
 states, which is done for Suspend-To-Idle too, nonboot CPUs are taken offline
 and all low-level system functions are suspended during transitions into this
 state.  For this reason, it should allow more energy to be saved relative to
 Suspend-To-Idle, but the resume latency will generally be greater than for that
 state.


 State:		Suspend-to-RAM
 ACPI State:	S3
 Label:		"mem"

 This state, if supported, offers significant power savings as everything in the
 system is put into a low-power state, except for memory, which should be placed
 into the self-refresh mode to retain its contents.  All of the steps carried out
 when entering Power-On Suspend are also carried out during transitions to STR.
 Additional operations may take place depending on the platform capabilities.  In
 particular, on ACPI systems the kernel passes control to the BIOS (platform
 firmware) as the last step during STR transitions and that usually results in
 powering down some more low-level components that aren't directly controlled by
 the kernel.

 System and device state is saved and kept in memory.  All devices are suspended
 and put into low-power states.  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 it.  This may be the case on other platforms too.


 State:		Suspend-to-disk
 ACPI State:	S4
 Label:		"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' (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).
	System Power Management Sleep States

	(C) 2014 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>

	The kernel supports up to four system sleep states generically, although three
	of them depend on the platform support code to implement the low-level details
	for each state.

	The states are represented by strings that can be read or written to the
	/sys/power/state file. Those strings may be "mem", "standby", "freeze" and
	"disk", where the last one always represents hibernation (Suspend-To-Disk) and
	the meaning of the remaining ones depends on the relative_sleep_states command
	line argument.

	For relative_sleep_states=1, the strings "mem", "standby" and "freeze" label the
	available non-hibernation sleep states from the deepest to the shallowest,
	respectively. In that case, "mem" is always present in /sys/power/state,
	because there is at least one non-hibernation sleep state in every system. If
	the given system supports two non-hibernation sleep states, "standby" is present
	in /sys/power/state in addition to "mem". If the system supports three
	non-hibernation sleep states, "freeze" will be present in /sys/power/state in
	addition to "mem" and "standby".

	For relative_sleep_states=0, which is the default, the following descriptions
	apply.

	state: Suspend-To-Idle
	ACPI state: S0
	Label: "freeze"

	This state is a generic, pure software, light-weight, system sleep state.
	It allows more energy to be saved relative to runtime idle by freezing user
	space and putting all I/O devices into low-power states (possibly
	lower-power than available at run time), such that the processors can
	spend more time in their idle states.

	This state can be used for platforms without Power-On Suspend/Suspend-to-RAM
	support, or it can be used in addition to Suspend-to-RAM (memory sleep)
	to provide reduced resume latency. It is always supported.


	State: Standby / Power-On Suspend
	ACPI State: S1
	Label: "standby"

	This state, if supported, offers moderate, though real, power savings, while
	providing a relatively 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.

	In addition to freezing user space and putting all I/O devices into low-power
	states, which is done for Suspend-To-Idle too, nonboot CPUs are taken offline
	and all low-level system functions are suspended during transitions into this
	state. For this reason, it should allow more energy to be saved relative to
	Suspend-To-Idle, but the resume latency will generally be greater than for that
	state.


	State: Suspend-to-RAM
	ACPI State: S3
	Label: "mem"

	This state, if supported, offers significant power savings as everything in the
	system is put into a low-power state, except for memory, which should be placed
	into the self-refresh mode to retain its contents. All of the steps carried out
	when entering Power-On Suspend are also carried out during transitions to STR.
	Additional operations may take place depending on the platform capabilities. In
	particular, on ACPI systems the kernel passes control to the BIOS (platform
	firmware) as the last step during STR transitions and that usually results in
	powering down some more low-level components that aren't directly controlled by
	the kernel.

	System and device state is saved and kept in memory. All devices are suspended
	and put into low-power states. 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 it. This may be the case on other platforms too.


	State: Suspend-to-disk
	ACPI State: S4
	Label: "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' (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).