PM: docs: Delete the obsolete states.txt document

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 three always represent Power-On Suspend (if supported),

Suspend-To-Idle and hibernation (Suspend-To-Disk), respectively.

The meaning of the "mem" string is controlled by the /sys/power/mem_sleep file.

It contains strings representing the available modes of system suspend that may

be triggered by writing "mem" to /sys/power/state.  These modes are "s2idle"

(Suspend-To-Idle), "shallow" (Power-On Suspend) and "deep" (Suspend-To-RAM).

The "s2idle" mode is always available, while the other ones are only available

if supported by the platform (if not supported, the strings representing them

are not present in /sys/power/mem_sleep).  The string representing the suspend

mode to be used subsequently is enclosed in square brackets.  Writing one of

the other strings present in /sys/power/mem_sleep to it causes the suspend mode

to be used subsequently to change to the one represented by that string.

Consequently, there are two ways to cause the system to go into the

Suspend-To-Idle sleep state.  The first one is to write "freeze" directly to

/sys/power/state.  The second one is to write "s2idle" to /sys/power/mem_sleep

and then to write "mem" to /sys/power/state.  Similarly, there are two ways

to cause the system to go into the Power-On Suspend sleep state (the strings to

write to the control files in that case are "standby" or "shallow" and "mem",

respectively) if that state is supported by the platform.  In turn, there is

only one way to cause the system to go into the Suspend-To-RAM state (write

"deep" into /sys/power/mem_sleep and "mem" into /sys/power/state).

The default suspend mode (ie. the one to be used without writing anything into

/sys/power/mem_sleep) is either "deep" (if Suspend-To-RAM is supported) or

"s2idle", but it can be overridden by the value of the "mem_sleep_default"

parameter in the kernel command line.  On some ACPI-based systems, depending on

the information in the FADT, the default may be "s2idle" even if Suspend-To-RAM

is supported.

The properties of all of the sleep states are described below.

State:		Suspend-To-Idle

ACPI state:	S0

Label:		"s2idle" ("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 to provide reduced

resume latency.  It is always supported.

State:		Standby / Power-On Suspend

ACPI State:	S1

Label:		"shallow" ("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:		"deep"

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).