1 .. SPDX-License-Identifier: GPL-2.0
2 .. include:: <isonum.txt>
8 :Copyright: |copy| 2017 Intel Corporation
10 :Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
13 Sleep states are global low-power states of the entire system in which user
14 space code cannot be executed and the overall system activity is significantly
18 Sleep States That Can Be Supported
19 ==================================
21 Depending on its configuration and the capabilities of the platform it runs on,
22 the Linux kernel can support up to four system sleep states, including
23 hibernation and up to three variants of system suspend. The sleep states that
24 can be supported by the kernel are listed below.
31 This is a generic, pure software, light-weight variant of system suspend (also
32 referred to as S2I or S2Idle). It allows more energy to be saved relative to
33 runtime idle by freezing user space, suspending the timekeeping and putting all
34 I/O devices into low-power states (possibly lower-power than available in the
35 working state), such that the processors can spend time in their deepest idle
36 states while the system is suspended.
38 The system is woken up from this state by in-band interrupts, so theoretically
39 any devices that can cause interrupts to be generated in the working state can
40 also be set up as wakeup devices for S2Idle.
42 This state can be used on platforms without support for :ref:`standby <standby>`
43 or :ref:`suspend-to-RAM <s2ram>`, or it can be used in addition to any of the
44 deeper system suspend variants to provide reduced resume latency. It is always
45 supported if the :c:macro:`CONFIG_SUSPEND` kernel configuration option is set.
52 This state, if supported, offers moderate, but real, energy savings, while
53 providing a relatively straightforward transition back to the working state. No
54 operating state is lost (the system core logic retains power), so the system can
55 go back to where it left off easily enough.
57 In addition to freezing user space, suspending the timekeeping and putting all
58 I/O devices into low-power states, which is done for :ref:`suspend-to-idle
59 <s2idle>` too, nonboot CPUs are taken offline and all low-level system functions
60 are suspended during transitions into this state. For this reason, it should
61 allow more energy to be saved relative to :ref:`suspend-to-idle <s2idle>`, but
62 the resume latency will generally be greater than for that state.
64 The set of devices that can wake up the system from this state usually is
65 reduced relative to :ref:`suspend-to-idle <s2idle>` and it may be necessary to
66 rely on the platform for setting up the wakeup functionality as appropriate.
68 This state is supported if the :c:macro:`CONFIG_SUSPEND` kernel configuration
69 option is set and the support for it is registered by the platform with the
70 core system suspend subsystem. On ACPI-based systems this state is mapped to
71 the S1 system state defined by ACPI.
78 This state (also referred to as STR or S2RAM), if supported, offers significant
79 energy savings as everything in the system is put into a low-power state, except
80 for memory, which should be placed into the self-refresh mode to retain its
81 contents. All of the steps carried out when entering :ref:`standby <standby>`
82 are also carried out during transitions to S2RAM. Additional operations may
83 take place depending on the platform capabilities. In particular, on ACPI-based
84 systems the kernel passes control to the platform firmware (BIOS) as the last
85 step during S2RAM transitions and that usually results in powering down some
86 more low-level components that are not directly controlled by the kernel.
88 The state of devices and CPUs is saved and held in memory. All devices are
89 suspended and put into low-power states. In many cases, all peripheral buses
90 lose power when entering S2RAM, so devices must be able to handle the transition
91 back to the "on" state.
93 On ACPI-based systems S2RAM requires some minimal boot-strapping code in the
94 platform firmware to resume the system from it. This may be the case on other
97 The set of devices that can wake up the system from S2RAM usually is reduced
98 relative to :ref:`suspend-to-idle <s2idle>` and :ref:`standby <standby>` and it
99 may be necessary to rely on the platform for setting up the wakeup functionality
102 S2RAM is supported if the :c:macro:`CONFIG_SUSPEND` kernel configuration option
103 is set and the support for it is registered by the platform with the core system
104 suspend subsystem. On ACPI-based systems it is mapped to the S3 system state
112 This state (also referred to as Suspend-to-Disk or STD) offers the greatest
113 energy savings and can be used even in the absence of low-level platform support
114 for system suspend. However, it requires some low-level code for resuming the
115 system to be present for the underlying CPU architecture.
117 Hibernation is significantly different from any of the system suspend variants.
118 It takes three system state changes to put it into hibernation and two system
119 state changes to resume it.
121 First, when hibernation is triggered, the kernel stops all system activity and
122 creates a snapshot image of memory to be written into persistent storage. Next,
123 the system goes into a state in which the snapshot image can be saved, the image
124 is written out and finally the system goes into the target low-power state in
125 which power is cut from almost all of its hardware components, including memory,
126 except for a limited set of wakeup devices.
128 Once the snapshot image has been written out, the system may either enter a
129 special low-power state (like ACPI S4), or it may simply power down itself.
130 Powering down means minimum power draw and it allows this mechanism to work on
131 any system. However, entering a special low-power state may allow additional
132 means of system wakeup to be used (e.g. pressing a key on the keyboard or
133 opening a laptop lid).
135 After wakeup, control goes to the platform firmware that runs a boot loader
136 which boots a fresh instance of the kernel (control may also go directly to
137 the boot loader, depending on the system configuration, but anyway it causes
138 a fresh instance of the kernel to be booted). That new instance of the kernel
139 (referred to as the ``restore kernel``) looks for a hibernation image in
140 persistent storage and if one is found, it is loaded into memory. Next, all
141 activity in the system is stopped and the restore kernel overwrites itself with
142 the image contents and jumps into a special trampoline area in the original
143 kernel stored in the image (referred to as the ``image kernel``), which is where
144 the special architecture-specific low-level code is needed. Finally, the
145 image kernel restores the system to the pre-hibernation state and allows user
148 Hibernation is supported if the :c:macro:`CONFIG_HIBERNATION` kernel
149 configuration option is set. However, this option can only be set if support
150 for the given CPU architecture includes the low-level code for system resume.
153 Basic ``sysfs`` Interfaces for System Suspend and Hibernation
154 =============================================================
156 The power management subsystem provides userspace with a unified ``sysfs``
157 interface for system sleep regardless of the underlying system architecture or
158 platform. That interface is located in the :file:`/sys/power/` directory
159 (assuming that ``sysfs`` is mounted at :file:`/sys`) and it consists of the
160 following attributes (files):
163 This file contains a list of strings representing sleep states supported
164 by the kernel. Writing one of these strings into it causes the kernel
165 to start a transition of the system into the sleep state represented by
168 In particular, the "disk", "freeze" and "standby" strings represent the
169 :ref:`hibernation <hibernation>`, :ref:`suspend-to-idle <s2idle>` and
170 :ref:`standby <standby>` sleep states, respectively. The "mem" string
171 is interpreted in accordance with the contents of the ``mem_sleep`` file
174 If the kernel does not support any system sleep states, this file is
178 This file contains a list of strings representing supported system
179 suspend variants and allows user space to select the variant to be
180 associated with the "mem" string in the ``state`` file described above.
182 The strings that may be present in this file are "s2idle", "shallow"
183 and "deep". The "s2idle" string always represents :ref:`suspend-to-idle
184 <s2idle>` and, by convention, "shallow" and "deep" represent
185 :ref:`standby <standby>` and :ref:`suspend-to-RAM <s2ram>`,
188 Writing one of the listed strings into this file causes the system
189 suspend variant represented by it to be associated with the "mem" string
190 in the ``state`` file. The string representing the suspend variant
191 currently associated with the "mem" string in the ``state`` file is
192 shown in square brackets.
194 If the kernel does not support system suspend, this file is not present.
197 This file controls the operating mode of hibernation (Suspend-to-Disk).
198 Specifically, it tells the kernel what to do after creating a
201 Reading from it returns a list of supported options encoded as:
204 Put the system into a special low-power state (e.g. ACPI S4) to
205 make additional wakeup options available and possibly allow the
206 platform firmware to take a simplified initialization path after
209 It is only available if the platform provides a special
210 mechanism to put the system to sleep after creating a
211 hibernation image (platforms with ACPI do that as a rule, for
215 Power off the system.
218 Reboot the system (useful for diagnostics mostly).
221 Hybrid system suspend. Put the system into the suspend sleep
222 state selected through the ``mem_sleep`` file described above.
223 If the system is successfully woken up from that state, discard
224 the hibernation image and continue. Otherwise, use the image
225 to restore the previous state of the system.
227 It is available if system suspend is supported.
230 Diagnostic operation. Load the image as though the system had
231 just woken up from hibernation and the currently running kernel
232 instance was a restore kernel and follow up with full system
235 Writing one of the strings listed above into this file causes the option
236 represented by it to be selected.
238 The currently selected option is shown in square brackets, which means
239 that the operation represented by it will be carried out after creating
240 and saving the image when hibernation is triggered by writing ``disk``
241 to :file:`/sys/power/state`.
243 If the kernel does not support hibernation, this file is not present.
246 This file controls the size of hibernation images.
248 It can be written a string representing a non-negative integer that will
249 be used as a best-effort upper limit of the image size, in bytes. The
250 hibernation core will do its best to ensure that the image size will not
251 exceed that number, but if that turns out to be impossible to achieve, a
252 hibernation image will still be created and its size will be as small as
253 possible. In particular, writing '0' to this file causes the size of
254 hibernation images to be minimum.
256 Reading from it returns the current image size limit, which is set to
257 around 2/5 of the available RAM size by default.
260 This file controls the "PM trace" mechanism saving the last suspend
261 or resume event point in the RTC memory across reboots. It helps to
262 debug hard lockups or reboots due to device driver failures that occur
263 during system suspend or resume (which is more common) more effectively.
265 If it contains "1", the fingerprint of each suspend/resume event point
266 in turn will be stored in the RTC memory (overwriting the actual RTC
267 information), so it will survive a system crash if one occurs right
268 after storing it and it can be used later to identify the driver that
269 caused the crash to happen.
271 It contains "0" by default, which may be changed to "1" by writing a
272 string representing a nonzero integer into it.
274 According to the above, there are two ways to make the system go into the
275 :ref:`suspend-to-idle <s2idle>` state. The first one is to write "freeze"
276 directly to :file:`/sys/power/state`. The second one is to write "s2idle" to
277 :file:`/sys/power/mem_sleep` and then to write "mem" to
278 :file:`/sys/power/state`. Likewise, there are two ways to make the system go
279 into the :ref:`standby <standby>` state (the strings to write to the control
280 files in that case are "standby" or "shallow" and "mem", respectively) if that
281 state is supported by the platform. However, there is only one way to make the
282 system go into the :ref:`suspend-to-RAM <s2ram>` state (write "deep" into
283 :file:`/sys/power/mem_sleep` and "mem" into :file:`/sys/power/state`).
285 The default suspend variant (ie. the one to be used without writing anything
286 into :file:`/sys/power/mem_sleep`) is either "deep" (on the majority of systems
287 supporting :ref:`suspend-to-RAM <s2ram>`) or "s2idle", but it can be overridden
288 by the value of the ``mem_sleep_default`` parameter in the kernel command line.
289 On some systems with ACPI, depending on the information in the ACPI tables, the
290 default may be "s2idle" even if :ref:`suspend-to-RAM <s2ram>` is supported in