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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.

1	System Power Management Sleep States
2	
3	(C) 2014 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
4	
5	The kernel supports up to four system sleep states generically, although three
6	of them depend on the platform support code to implement the low-level details
7	for each state.
8	
9	The states are represented by strings that can be read or written to the
10	/sys/power/state file.  Those strings may be "mem", "standby", "freeze" and
11	"disk", where the last three always represent Power-On Suspend (if supported),
12	Suspend-To-Idle and hibernation (Suspend-To-Disk), respectively.
13	
14	The meaning of the "mem" string is controlled by the /sys/power/mem_sleep file.
15	It contains strings representing the available modes of system suspend that may
16	be triggered by writing "mem" to /sys/power/state.  These modes are "s2idle"
17	(Suspend-To-Idle), "shallow" (Power-On Suspend) and "deep" (Suspend-To-RAM).
18	The "s2idle" mode is always available, while the other ones are only available
19	if supported by the platform (if not supported, the strings representing them
20	are not present in /sys/power/mem_sleep).  The string representing the suspend
21	mode to be used subsequently is enclosed in square brackets.  Writing one of
22	the other strings present in /sys/power/mem_sleep to it causes the suspend mode
23	to be used subsequently to change to the one represented by that string.
24	
25	Consequently, there are two ways to cause the system to go into the
26	Suspend-To-Idle sleep state.  The first one is to write "freeze" directly to
27	/sys/power/state.  The second one is to write "s2idle" to /sys/power/mem_sleep
28	and then to write "mem" to /sys/power/state.  Similarly, there are two ways
29	to cause the system to go into the Power-On Suspend sleep state (the strings to
30	write to the control files in that case are "standby" or "shallow" and "mem",
31	respectively) if that state is supported by the platform.  In turn, there is
32	only one way to cause the system to go into the Suspend-To-RAM state (write
33	"deep" into /sys/power/mem_sleep and "mem" into /sys/power/state).
34	
35	The default suspend mode (ie. the one to be used without writing anything into
36	/sys/power/mem_sleep) is either "deep" (if Suspend-To-RAM is supported) or
37	"s2idle", but it can be overridden by the value of the "mem_sleep_default"
38	parameter in the kernel command line.
39	
40	The properties of all of the sleep states are described below.
41	
42	
43	State:		Suspend-To-Idle
44	ACPI state:	S0
45	Label:		"s2idle" ("freeze")
46	
47	This state is a generic, pure software, light-weight, system sleep state.
48	It allows more energy to be saved relative to runtime idle by freezing user
49	space and putting all I/O devices into low-power states (possibly
50	lower-power than available at run time), such that the processors can
51	spend more time in their idle states.
52	
53	This state can be used for platforms without Power-On Suspend/Suspend-to-RAM
54	support, or it can be used in addition to Suspend-to-RAM to provide reduced
55	resume latency.  It is always supported.
56	
57	
58	State:		Standby / Power-On Suspend
59	ACPI State:	S1
60	Label:		"shallow" ("standby")
61	
62	This state, if supported, offers moderate, though real, power savings, while
63	providing a relatively low-latency transition back to a working system.  No
64	operating state is lost (the CPU retains power), so the system easily starts up
65	again where it left off. 
66	
67	In addition to freezing user space and putting all I/O devices into low-power
68	states, which is done for Suspend-To-Idle too, nonboot CPUs are taken offline
69	and all low-level system functions are suspended during transitions into this
70	state.  For this reason, it should allow more energy to be saved relative to
71	Suspend-To-Idle, but the resume latency will generally be greater than for that
72	state.
73	
74	
75	State:		Suspend-to-RAM
76	ACPI State:	S3
77	Label:		"deep"
78	
79	This state, if supported, offers significant power savings as everything in the
80	system is put into a low-power state, except for memory, which should be placed
81	into the self-refresh mode to retain its contents.  All of the steps carried out
82	when entering Power-On Suspend are also carried out during transitions to STR.
83	Additional operations may take place depending on the platform capabilities.  In
84	particular, on ACPI systems the kernel passes control to the BIOS (platform
85	firmware) as the last step during STR transitions and that usually results in
86	powering down some more low-level components that aren't directly controlled by
87	the kernel.
88	
89	System and device state is saved and kept in memory.  All devices are suspended
90	and put into low-power states.  In many cases, all peripheral buses lose power
91	when entering STR, so devices must be able to handle the transition back to the
92	"on" state.
93	
94	For at least ACPI, STR requires some minimal boot-strapping code to resume the
95	system from it.  This may be the case on other platforms too.
96	
97	
98	State:		Suspend-to-disk
99	ACPI State:	S4
100	Label:		"disk"
101	
102	This state offers the greatest power savings, and can be used even in
103	the absence of low-level platform support for power management. This
104	state operates similarly to Suspend-to-RAM, but includes a final step
105	of writing memory contents to disk. On resume, this is read and memory
106	is restored to its pre-suspend state. 
107	
108	STD can be handled by the firmware or the kernel. If it is handled by
109	the firmware, it usually requires a dedicated partition that must be
110	setup via another operating system for it to use. Despite the
111	inconvenience, this method requires minimal work by the kernel, since
112	the firmware will also handle restoring memory contents on resume. 
113	
114	For suspend-to-disk, a mechanism called 'swsusp' (Swap Suspend) is used
115	to write memory contents to free swap space. swsusp has some restrictive
116	requirements, but should work in most cases. Some, albeit outdated,
117	documentation can be found in Documentation/power/swsusp.txt.
118	Alternatively, userspace can do most of the actual suspend to disk work,
119	see userland-swsusp.txt.
120	
121	Once memory state is written to disk, the system may either enter a
122	low-power state (like ACPI S4), or it may simply power down. Powering
123	down offers greater savings, and allows this mechanism to work on any
124	system. However, entering a real low-power state allows the user to
125	trigger wake up events (e.g. pressing a key or opening a laptop lid).
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