Documentation/power/states.txt - kernel/msm-4.19 - Gitiles

 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 wrtie "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).
	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 wrtie "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).