1 .. SPDX-License-Identifier: GPL-2.0
2 .. include:: <isonum.txt>
3
4 ===================
5 System Sleep States
6 ===================
7
8 :Copyright: |copy| 2017 Intel Corporation
9
10 :Author: Rafael J. Wysocki <rafael.j.wysocki@in
11
12
13 Sleep states are global low-power states of th
14 space code cannot be executed and the overall
15 reduced.
16
17
18 Sleep States That Can Be Supported
19 ==================================
20
21 Depending on its configuration and the capabil
22 the Linux kernel can support up to four system
23 hibernation and up to three variants of system
24 can be supported by the kernel are listed belo
25
26 .. _s2idle:
27
28 Suspend-to-Idle
29 ---------------
30
31 This is a generic, pure software, light-weight
32 referred to as S2I or S2Idle). It allows more
33 runtime idle by freezing user space, suspendin
34 I/O devices into low-power states (possibly lo
35 working state), such that the processors can s
36 states while the system is suspended.
37
38 The system is woken up from this state by in-b
39 any devices that can cause interrupts to be ge
40 also be set up as wakeup devices for S2Idle.
41
42 This state can be used on platforms without su
43 or :ref:`suspend-to-RAM <s2ram>`, or it can be
44 deeper system suspend variants to provide redu
45 supported if the :c:macro:`CONFIG_SUSPEND` ker
46
47 .. _standby:
48
49 Standby
50 -------
51
52 This state, if supported, offers moderate, but
53 providing a relatively straightforward transit
54 operating state is lost (the system core logic
55 go back to where it left off easily enough.
56
57 In addition to freezing user space, suspending
58 I/O devices into low-power states, which is do
59 <s2idle>` too, nonboot CPUs are taken offline
60 are suspended during transitions into this sta
61 allow more energy to be saved relative to :ref
62 the resume latency will generally be greater t
63
64 The set of devices that can wake up the system
65 reduced relative to :ref:`suspend-to-idle <s2i
66 rely on the platform for setting up the wakeup
67
68 This state is supported if the :c:macro:`CONFI
69 option is set and the support for it is regist
70 core system suspend subsystem. On ACPI-based
71 the S1 system state defined by ACPI.
72
73 .. _s2ram:
74
75 Suspend-to-RAM
76 --------------
77
78 This state (also referred to as STR or S2RAM),
79 energy savings as everything in the system is
80 for memory, which should be placed into the se
81 contents. All of the steps carried out when e
82 are also carried out during transitions to S2R
83 take place depending on the platform capabilit
84 systems the kernel passes control to the platf
85 step during S2RAM transitions and that usually
86 more low-level components that are not directl
87
88 The state of devices and CPUs is saved and hel
89 suspended and put into low-power states. In m
90 lose power when entering S2RAM, so devices mus
91 back to the "on" state.
92
93 On ACPI-based systems S2RAM requires some mini
94 platform firmware to resume the system from it
95 platforms too.
96
97 The set of devices that can wake up the system
98 relative to :ref:`suspend-to-idle <s2idle>` an
99 may be necessary to rely on the platform for s
100 as appropriate.
101
102 S2RAM is supported if the :c:macro:`CONFIG_SUS
103 is set and the support for it is registered by
104 suspend subsystem. On ACPI-based systems it i
105 defined by ACPI.
106
107 .. _hibernation:
108
109 Hibernation
110 -----------
111
112 This state (also referred to as Suspend-to-Dis
113 energy savings and can be used even in the abs
114 for system suspend. However, it requires some
115 system to be present for the underlying CPU ar
116
117 Hibernation is significantly different from an
118 It takes three system state changes to put it
119 state changes to resume it.
120
121 First, when hibernation is triggered, the kern
122 creates a snapshot image of memory to be writt
123 the system goes into a state in which the snap
124 is written out and finally the system goes int
125 which power is cut from almost all of its hard
126 except for a limited set of wakeup devices.
127
128 Once the snapshot image has been written out,
129 special low-power state (like ACPI S4), or it
130 Powering down means minimum power draw and it
131 any system. However, entering a special low-p
132 means of system wakeup to be used (e.g. press
133 opening a laptop lid).
134
135 After wakeup, control goes to the platform fir
136 which boots a fresh instance of the kernel (co
137 the boot loader, depending on the system confi
138 a fresh instance of the kernel to be booted).
139 (referred to as the ``restore kernel``) looks
140 persistent storage and if one is found, it is
141 activity in the system is stopped and the rest
142 the image contents and jumps into a special tr
143 kernel stored in the image (referred to as the
144 the special architecture-specific low-level co
145 image kernel restores the system to the pre-hi
146 space to run again.
147
148 Hibernation is supported if the :c:macro:`CONF
149 configuration option is set. However, this op
150 for the given CPU architecture includes the lo
151
152
153 Basic ``sysfs`` Interfaces for System Suspend
154 ==============================================
155
156 The power management subsystem provides usersp
157 interface for system sleep regardless of the u
158 platform. That interface is located in the :f
159 (assuming that ``sysfs`` is mounted at :file:`
160 following attributes (files):
161
162 ``state``
163 This file contains a list of strings r
164 by the kernel. Writing one of these s
165 to start a transition of the system in
166 that string.
167
168 In particular, the "disk", "freeze" an
169 :ref:`hibernation <hibernation>`, :ref
170 :ref:`standby <standby>` sleep states,
171 is interpreted in accordance with the
172 described below.
173
174 If the kernel does not support any sys
175 not present.
176
177 ``mem_sleep``
178 This file contains a list of strings r
179 suspend variants and allows user space
180 associated with the "mem" string in th
181
182 The strings that may be present in thi
183 and "deep". The "s2idle" string alway
184 <s2idle>` and, by convention, "shallow
185 :ref:`standby <standby>` and :ref:`sus
186 respectively.
187
188 Writing one of the listed strings into
189 suspend variant represented by it to b
190 in the ``state`` file. The string rep
191 currently associated with the "mem" st
192 shown in square brackets.
193
194 If the kernel does not support system
195
196 ``disk``
197 This file controls the operating mode
198 Specifically, it tells the kernel what
199 hibernation image.
200
201 Reading from it returns a list of supp
202
203 ``platform``
204 Put the system into a special
205 make additional wakeup options
206 platform firmware to take a si
207 wakeup.
208
209 It is only available if the pl
210 mechanism to put the system to
211 hibernation image (platforms w
212 example).
213
214 ``shutdown``
215 Power off the system.
216
217 ``reboot``
218 Reboot the system (useful for
219
220 ``suspend``
221 Hybrid system suspend. Put th
222 state selected through the ``m
223 If the system is successfully
224 the hibernation image and cont
225 to restore the previous state
226
227 It is available if system susp
228
229 ``test_resume``
230 Diagnostic operation. Load th
231 just woken up from hibernation
232 instance was a restore kernel
233 resume.
234
235 Writing one of the strings listed abov
236 represented by it to be selected.
237
238 The currently selected option is shown
239 that the operation represented by it w
240 and saving the image when hibernation
241 to :file:`/sys/power/state`.
242
243 If the kernel does not support hiberna
244
245 ``image_size``
246 This file controls the size of hiberna
247
248 It can be written a string representin
249 be used as a best-effort upper limit o
250 hibernation core will do its best to e
251 exceed that number, but if that turns
252 hibernation image will still be create
253 possible. In particular, writing '0'
254 hibernation images to be minimum.
255
256 Reading from it returns the current im
257 around 2/5 of the available RAM size b
258
259 ``pm_trace``
260 This file controls the "PM trace" mech
261 or resume event point in the RTC memor
262 debug hard lockups or reboots due to d
263 during system suspend or resume (which
264
265 If it contains "1", the fingerprint of
266 in turn will be stored in the RTC memo
267 information), so it will survive a sys
268 after storing it and it can be used la
269 caused the crash to happen.
270
271 It contains "0" by default, which may
272 string representing a nonzero integer
273
274 According to the above, there are two ways to
275 :ref:`suspend-to-idle <s2idle>` state. The fi
276 directly to :file:`/sys/power/state`. The sec
277 :file:`/sys/power/mem_sleep` and then to write
278 :file:`/sys/power/state`. Likewise, there are
279 into the :ref:`standby <standby>` state (the s
280 files in that case are "standby" or "shallow"
281 state is supported by the platform. However,
282 system go into the :ref:`suspend-to-RAM <s2ram
283 :file:`/sys/power/mem_sleep` and "mem" into :f
284
285 The default suspend variant (ie. the one to be
286 into :file:`/sys/power/mem_sleep`) is either "
287 supporting :ref:`suspend-to-RAM <s2ram>`) or "
288 by the value of the ``mem_sleep_default`` para
289 On some systems with ACPI, depending on the in
290 default may be "s2idle" even if :ref:`suspend-
291 principle.
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