246 lines
11 KiB
ReStructuredText
246 lines
11 KiB
ReStructuredText
===================
|
|
System Sleep States
|
|
===================
|
|
|
|
::
|
|
|
|
Copyright (c) 2017 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
|
|
|
|
Sleep states are global low-power states of the entire system in which user
|
|
space code cannot be executed and the overall system activity is significantly
|
|
reduced.
|
|
|
|
|
|
Sleep States That Can Be Supported
|
|
==================================
|
|
|
|
Depending on its configuration and the capabilities of the platform it runs on,
|
|
the Linux kernel can support up to four system sleep states, includig
|
|
hibernation and up to three variants of system suspend. The sleep states that
|
|
can be supported by the kernel are listed below.
|
|
|
|
.. _s2idle:
|
|
|
|
Suspend-to-Idle
|
|
---------------
|
|
|
|
This is a generic, pure software, light-weight variant of system suspend (also
|
|
referred to as S2I or S2Idle). It allows more energy to be saved relative to
|
|
runtime idle by freezing user space, suspending the timekeeping and putting all
|
|
I/O devices into low-power states (possibly lower-power than available in the
|
|
working state), such that the processors can spend time in their deepest idle
|
|
states while the system is suspended.
|
|
|
|
The system is woken up from this state by in-band interrupts, so theoretically
|
|
any devices that can cause interrupts to be generated in the working state can
|
|
also be set up as wakeup devices for S2Idle.
|
|
|
|
This state can be used on platforms without support for :ref:`standby <standby>`
|
|
or :ref:`suspend-to-RAM <s2ram>`, or it can be used in addition to any of the
|
|
deeper system suspend variants to provide reduced resume latency. It is always
|
|
supported if the :c:macro:`CONFIG_SUSPEND` kernel configuration option is set.
|
|
|
|
.. _standby:
|
|
|
|
Standby
|
|
-------
|
|
|
|
This state, if supported, offers moderate, but real, energy savings, while
|
|
providing a relatively straightforward transition back to the working state. No
|
|
operating state is lost (the system core logic retains power), so the system can
|
|
go back to where it left off easily enough.
|
|
|
|
In addition to freezing user space, suspending the timekeeping and putting all
|
|
I/O devices into low-power states, which is done for :ref:`suspend-to-idle
|
|
<s2idle>` 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 :ref:`suspend-to-idle <s2idle>`, but
|
|
the resume latency will generally be greater than for that state.
|
|
|
|
The set of devices that can wake up the system from this state usually is
|
|
reduced relative to :ref:`suspend-to-idle <s2idle>` and it may be necessary to
|
|
rely on the platform for setting up the wakeup functionality as appropriate.
|
|
|
|
This state is supported if the :c:macro:`CONFIG_SUSPEND` kernel configuration
|
|
option is set and the support for it is registered by the platform with the
|
|
core system suspend subsystem. On ACPI-based systems this state is mapped to
|
|
the S1 system state defined by ACPI.
|
|
|
|
.. _s2ram:
|
|
|
|
Suspend-to-RAM
|
|
--------------
|
|
|
|
This state (also referred to as STR or S2RAM), if supported, offers significant
|
|
energy 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 :ref:`standby <standby>`
|
|
are also carried out during transitions to S2RAM. Additional operations may
|
|
take place depending on the platform capabilities. In particular, on ACPI-based
|
|
systems the kernel passes control to the platform firmware (BIOS) as the last
|
|
step during S2RAM transitions and that usually results in powering down some
|
|
more low-level components that are not directly controlled by the kernel.
|
|
|
|
The state of devices and CPUs is saved and held in memory. All devices are
|
|
suspended and put into low-power states. In many cases, all peripheral buses
|
|
lose power when entering S2RAM, so devices must be able to handle the transition
|
|
back to the "on" state.
|
|
|
|
On ACPI-based systems S2RAM requires some minimal boot-strapping code in the
|
|
platform firmware to resume the system from it. This may be the case on other
|
|
platforms too.
|
|
|
|
The set of devices that can wake up the system from S2RAM usually is reduced
|
|
relative to :ref:`suspend-to-idle <s2idle>` and :ref:`standby <standby>` and it
|
|
may be necessary to rely on the platform for setting up the wakeup functionality
|
|
as appropriate.
|
|
|
|
S2RAM is supported if the :c:macro:`CONFIG_SUSPEND` kernel configuration option
|
|
is set and the support for it is registered by the platform with the core system
|
|
suspend subsystem. On ACPI-based systems it is mapped to the S3 system state
|
|
defined by ACPI.
|
|
|
|
.. _hibernation:
|
|
|
|
Hibernation
|
|
-----------
|
|
|
|
This state (also referred to as Suspend-to-Disk or STD) offers the greatest
|
|
energy savings and can be used even in the absence of low-level platform support
|
|
for system suspend. However, it requires some low-level code for resuming the
|
|
system to be present for the underlying CPU architecture.
|
|
|
|
Hibernation is significantly different from any of the system suspend variants.
|
|
It takes three system state changes to put it into hibernation and two system
|
|
state changes to resume it.
|
|
|
|
First, when hibernation is triggered, the kernel stops all system activity and
|
|
creates a snapshot image of memory to be written into persistent storage. Next,
|
|
the system goes into a state in which the snapshot image can be saved, the image
|
|
is written out and finally the system goes into the target low-power state in
|
|
which power is cut from almost all of its hardware components, including memory,
|
|
except for a limited set of wakeup devices.
|
|
|
|
Once the snapshot image has been written out, the system may either enter a
|
|
special low-power state (like ACPI S4), or it may simply power down itself.
|
|
Powering down means minimum power draw and it allows this mechanism to work on
|
|
any system. However, entering a special low-power state may allow additional
|
|
means of system wakeup to be used (e.g. pressing a key on the keyboard or
|
|
opening a laptop lid).
|
|
|
|
After wakeup, control goes to the platform firmware that runs a boot loader
|
|
which boots a fresh instance of the kernel (control may also go directly to
|
|
the boot loader, depending on the system configuration, but anyway it causes
|
|
a fresh instance of the kernel to be booted). That new instance of the kernel
|
|
(referred to as the ``restore kernel``) looks for a hibernation image in
|
|
persistent storage and if one is found, it is loaded into memory. Next, all
|
|
activity in the system is stopped and the restore kernel overwrites itself with
|
|
the image contents and jumps into a special trampoline area in the original
|
|
kernel stored in the image (referred to as the ``image kernel``), which is where
|
|
the special architecture-specific low-level code is needed. Finally, the
|
|
image kernel restores the system to the pre-hibernation state and allows user
|
|
space to run again.
|
|
|
|
Hibernation is supported if the :c:macro:`CONFIG_HIBERNATION` kernel
|
|
configuration option is set. However, this option can only be set if support
|
|
for the given CPU architecture includes the low-level code for system resume.
|
|
|
|
|
|
Basic ``sysfs`` Interfaces for System Suspend and Hibernation
|
|
=============================================================
|
|
|
|
The following files located in the :file:`/sys/power/` directory can be used by
|
|
user space for sleep states control.
|
|
|
|
``state``
|
|
This file contains a list of strings representing sleep states supported
|
|
by the kernel. Writing one of these strings into it causes the kernel
|
|
to start a transition of the system into the sleep state represented by
|
|
that string.
|
|
|
|
In particular, the strings "disk", "freeze" and "standby" represent the
|
|
:ref:`hibernation <hibernation>`, :ref:`suspend-to-idle <s2idle>` and
|
|
:ref:`standby <standby>` sleep states, respectively. The string "mem"
|
|
is interpreted in accordance with the contents of the ``mem_sleep`` file
|
|
described below.
|
|
|
|
If the kernel does not support any system sleep states, this file is
|
|
not present.
|
|
|
|
``mem_sleep``
|
|
This file contains a list of strings representing supported system
|
|
suspend variants and allows user space to select the variant to be
|
|
associated with the "mem" string in the ``state`` file described above.
|
|
|
|
The strings that may be present in this file are "s2idle", "shallow"
|
|
and "deep". The string "s2idle" always represents :ref:`suspend-to-idle
|
|
<s2idle>` and, by convention, "shallow" and "deep" represent
|
|
:ref:`standby <standby>` and :ref:`suspend-to-RAM <s2ram>`,
|
|
respectively.
|
|
|
|
Writing one of the listed strings into this file causes the system
|
|
suspend variant represented by it to be associated with the "mem" string
|
|
in the ``state`` file. The string representing the suspend variant
|
|
currently associated with the "mem" string in the ``state`` file
|
|
is listed in square brackets.
|
|
|
|
If the kernel does not support system suspend, this file is not present.
|
|
|
|
``disk``
|
|
This file contains a list of strings representing different operations
|
|
that can be carried out after the hibernation image has been saved. The
|
|
possible options are as follows:
|
|
|
|
``platform``
|
|
Put the system into a special low-power state (e.g. ACPI S4) to
|
|
make additional wakeup options available and possibly allow the
|
|
platform firmware to take a simplified initialization path after
|
|
wakeup.
|
|
|
|
``shutdown``
|
|
Power off the system.
|
|
|
|
``reboot``
|
|
Reboot the system (useful for diagnostics mostly).
|
|
|
|
``suspend``
|
|
Hybrid system suspend. Put the system into the suspend sleep
|
|
state selected through the ``mem_sleep`` file described above.
|
|
If the system is successfully woken up from that state, discard
|
|
the hibernation image and continue. Otherwise, use the image
|
|
to restore the previous state of the system.
|
|
|
|
``test_resume``
|
|
Diagnostic operation. Load the image as though the system had
|
|
just woken up from hibernation and the currently running kernel
|
|
instance was a restore kernel and follow up with full system
|
|
resume.
|
|
|
|
Writing one of the listed strings into this file causes the option
|
|
represented by it to be selected.
|
|
|
|
The currently selected option is shown in square brackets which means
|
|
that the operation represented by it will be carried out after creating
|
|
and saving the image next time hibernation is triggered by writing
|
|
``disk`` to :file:`/sys/power/state`.
|
|
|
|
If the kernel does not support hibernation, this file is not present.
|
|
|
|
According to the above, there are two ways to make the system go into the
|
|
:ref:`suspend-to-idle <s2idle>` state. The first one is to write "freeze"
|
|
directly to :file:`/sys/power/state`. The second one is to write "s2idle" to
|
|
:file:`/sys/power/mem_sleep` and then to write "mem" to
|
|
:file:`/sys/power/state`. Likewise, there are two ways to make the system go
|
|
into the :ref:`standby <standby>` 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. However, there is only one way to make the
|
|
system go into the :ref:`suspend-to-RAM <s2ram>` state (write "deep" into
|
|
:file:`/sys/power/mem_sleep` and "mem" into :file:`/sys/power/state`).
|
|
|
|
The default suspend variant (ie. the one to be used without writing anything
|
|
into :file:`/sys/power/mem_sleep`) is either "deep" (on the majority of systems
|
|
supporting :ref:`suspend-to-RAM <s2ram>`) 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 ACPI tables, the
|
|
default may be "s2idle" even if :ref:`suspend-to-RAM <s2ram>` is supported.
|