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ReStructuredText
292 lines
13 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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.. include:: <isonum.txt>
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===================
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System Sleep States
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===================
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:Copyright: |copy| 2017 Intel Corporation
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:Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
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Sleep states are global low-power states of the entire system in which user
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space code cannot be executed and the overall system activity is significantly
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reduced.
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Sleep States That Can Be Supported
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==================================
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Depending on its configuration and the capabilities of the platform it runs on,
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the Linux kernel can support up to four system sleep states, including
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hibernation and up to three variants of system suspend. The sleep states that
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can be supported by the kernel are listed below.
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.. _s2idle:
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Suspend-to-Idle
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---------------
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This is a generic, pure software, light-weight variant of system suspend (also
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referred to as S2I or S2Idle). It allows more energy to be saved relative to
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runtime idle by freezing user space, suspending the timekeeping and putting all
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I/O devices into low-power states (possibly lower-power than available in the
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working state), such that the processors can spend time in their deepest idle
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states while the system is suspended.
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The system is woken up from this state by in-band interrupts, so theoretically
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any devices that can cause interrupts to be generated in the working state can
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also be set up as wakeup devices for S2Idle.
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This state can be used on platforms without support for :ref:`standby <standby>`
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or :ref:`suspend-to-RAM <s2ram>`, or it can be used in addition to any of the
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deeper system suspend variants to provide reduced resume latency. It is always
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supported if the :c:macro:`CONFIG_SUSPEND` kernel configuration option is set.
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.. _standby:
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Standby
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-------
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This state, if supported, offers moderate, but real, energy savings, while
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providing a relatively straightforward transition back to the working state. No
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operating state is lost (the system core logic retains power), so the system can
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go back to where it left off easily enough.
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In addition to freezing user space, suspending the timekeeping and putting all
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I/O devices into low-power states, which is done for :ref:`suspend-to-idle
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<s2idle>` too, nonboot CPUs are taken offline and all low-level system functions
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are suspended during transitions into this state. For this reason, it should
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allow more energy to be saved relative to :ref:`suspend-to-idle <s2idle>`, but
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the resume latency will generally be greater than for that state.
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The set of devices that can wake up the system from this state usually is
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reduced relative to :ref:`suspend-to-idle <s2idle>` and it may be necessary to
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rely on the platform for setting up the wakeup functionality as appropriate.
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This state is supported if the :c:macro:`CONFIG_SUSPEND` kernel configuration
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option is set and the support for it is registered by the platform with the
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core system suspend subsystem. On ACPI-based systems this state is mapped to
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the S1 system state defined by ACPI.
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.. _s2ram:
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Suspend-to-RAM
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--------------
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This state (also referred to as STR or S2RAM), if supported, offers significant
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energy savings as everything in the system is put into a low-power state, except
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for memory, which should be placed into the self-refresh mode to retain its
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contents. All of the steps carried out when entering :ref:`standby <standby>`
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are also carried out during transitions to S2RAM. Additional operations may
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take place depending on the platform capabilities. In particular, on ACPI-based
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systems the kernel passes control to the platform firmware (BIOS) as the last
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step during S2RAM transitions and that usually results in powering down some
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more low-level components that are not directly controlled by the kernel.
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The state of devices and CPUs is saved and held in memory. All devices are
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suspended and put into low-power states. In many cases, all peripheral buses
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lose power when entering S2RAM, so devices must be able to handle the transition
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back to the "on" state.
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On ACPI-based systems S2RAM requires some minimal boot-strapping code in the
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platform firmware to resume the system from it. This may be the case on other
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platforms too.
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The set of devices that can wake up the system from S2RAM usually is reduced
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relative to :ref:`suspend-to-idle <s2idle>` and :ref:`standby <standby>` and it
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may be necessary to rely on the platform for setting up the wakeup functionality
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as appropriate.
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S2RAM is supported if the :c:macro:`CONFIG_SUSPEND` kernel configuration option
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is set and the support for it is registered by the platform with the core system
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suspend subsystem. On ACPI-based systems it is mapped to the S3 system state
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defined by ACPI.
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.. _hibernation:
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Hibernation
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-----------
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This state (also referred to as Suspend-to-Disk or STD) offers the greatest
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energy savings and can be used even in the absence of low-level platform support
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for system suspend. However, it requires some low-level code for resuming the
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system to be present for the underlying CPU architecture.
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Hibernation is significantly different from any of the system suspend variants.
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It takes three system state changes to put it into hibernation and two system
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state changes to resume it.
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First, when hibernation is triggered, the kernel stops all system activity and
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creates a snapshot image of memory to be written into persistent storage. Next,
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the system goes into a state in which the snapshot image can be saved, the image
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is written out and finally the system goes into the target low-power state in
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which power is cut from almost all of its hardware components, including memory,
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except for a limited set of wakeup devices.
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Once the snapshot image has been written out, the system may either enter a
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special low-power state (like ACPI S4), or it may simply power down itself.
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Powering down means minimum power draw and it allows this mechanism to work on
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any system. However, entering a special low-power state may allow additional
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means of system wakeup to be used (e.g. pressing a key on the keyboard or
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opening a laptop lid).
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After wakeup, control goes to the platform firmware that runs a boot loader
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which boots a fresh instance of the kernel (control may also go directly to
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the boot loader, depending on the system configuration, but anyway it causes
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a fresh instance of the kernel to be booted). That new instance of the kernel
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(referred to as the ``restore kernel``) looks for a hibernation image in
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persistent storage and if one is found, it is loaded into memory. Next, all
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activity in the system is stopped and the restore kernel overwrites itself with
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the image contents and jumps into a special trampoline area in the original
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kernel stored in the image (referred to as the ``image kernel``), which is where
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the special architecture-specific low-level code is needed. Finally, the
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image kernel restores the system to the pre-hibernation state and allows user
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space to run again.
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Hibernation is supported if the :c:macro:`CONFIG_HIBERNATION` kernel
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configuration option is set. However, this option can only be set if support
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for the given CPU architecture includes the low-level code for system resume.
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Basic ``sysfs`` Interfaces for System Suspend and Hibernation
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=============================================================
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The power management subsystem provides userspace with a unified ``sysfs``
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interface for system sleep regardless of the underlying system architecture or
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platform. That interface is located in the :file:`/sys/power/` directory
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(assuming that ``sysfs`` is mounted at :file:`/sys`) and it consists of the
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following attributes (files):
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``state``
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This file contains a list of strings representing sleep states supported
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by the kernel. Writing one of these strings into it causes the kernel
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to start a transition of the system into the sleep state represented by
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that string.
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In particular, the "disk", "freeze" and "standby" strings represent the
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:ref:`hibernation <hibernation>`, :ref:`suspend-to-idle <s2idle>` and
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:ref:`standby <standby>` sleep states, respectively. The "mem" string
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is interpreted in accordance with the contents of the ``mem_sleep`` file
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described below.
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If the kernel does not support any system sleep states, this file is
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not present.
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``mem_sleep``
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This file contains a list of strings representing supported system
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suspend variants and allows user space to select the variant to be
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associated with the "mem" string in the ``state`` file described above.
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The strings that may be present in this file are "s2idle", "shallow"
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and "deep". The "s2idle" string always represents :ref:`suspend-to-idle
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<s2idle>` and, by convention, "shallow" and "deep" represent
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:ref:`standby <standby>` and :ref:`suspend-to-RAM <s2ram>`,
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respectively.
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Writing one of the listed strings into this file causes the system
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suspend variant represented by it to be associated with the "mem" string
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in the ``state`` file. The string representing the suspend variant
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currently associated with the "mem" string in the ``state`` file is
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shown in square brackets.
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If the kernel does not support system suspend, this file is not present.
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``disk``
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This file controls the operating mode of hibernation (Suspend-to-Disk).
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Specifically, it tells the kernel what to do after creating a
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hibernation image.
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Reading from it returns a list of supported options encoded as:
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``platform``
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Put the system into a special low-power state (e.g. ACPI S4) to
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make additional wakeup options available and possibly allow the
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platform firmware to take a simplified initialization path after
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wakeup.
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It is only available if the platform provides a special
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mechanism to put the system to sleep after creating a
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hibernation image (platforms with ACPI do that as a rule, for
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example).
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``shutdown``
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Power off the system.
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``reboot``
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Reboot the system (useful for diagnostics mostly).
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``suspend``
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Hybrid system suspend. Put the system into the suspend sleep
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state selected through the ``mem_sleep`` file described above.
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If the system is successfully woken up from that state, discard
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the hibernation image and continue. Otherwise, use the image
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to restore the previous state of the system.
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It is available if system suspend is supported.
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``test_resume``
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Diagnostic operation. Load the image as though the system had
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just woken up from hibernation and the currently running kernel
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instance was a restore kernel and follow up with full system
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resume.
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Writing one of the strings listed above into this file causes the option
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represented by it to be selected.
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The currently selected option is shown in square brackets, which means
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that the operation represented by it will be carried out after creating
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and saving the image when hibernation is triggered by writing ``disk``
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to :file:`/sys/power/state`.
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If the kernel does not support hibernation, this file is not present.
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``image_size``
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This file controls the size of hibernation images.
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It can be written a string representing a non-negative integer that will
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be used as a best-effort upper limit of the image size, in bytes. The
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hibernation core will do its best to ensure that the image size will not
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exceed that number, but if that turns out to be impossible to achieve, a
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hibernation image will still be created and its size will be as small as
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possible. In particular, writing '0' to this file causes the size of
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hibernation images to be minimum.
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Reading from it returns the current image size limit, which is set to
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around 2/5 of the available RAM size by default.
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``pm_trace``
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This file controls the "PM trace" mechanism saving the last suspend
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or resume event point in the RTC memory across reboots. It helps to
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debug hard lockups or reboots due to device driver failures that occur
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during system suspend or resume (which is more common) more effectively.
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If it contains "1", the fingerprint of each suspend/resume event point
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in turn will be stored in the RTC memory (overwriting the actual RTC
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information), so it will survive a system crash if one occurs right
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after storing it and it can be used later to identify the driver that
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caused the crash to happen.
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It contains "0" by default, which may be changed to "1" by writing a
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string representing a nonzero integer into it.
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According to the above, there are two ways to make the system go into the
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:ref:`suspend-to-idle <s2idle>` state. The first one is to write "freeze"
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directly to :file:`/sys/power/state`. The second one is to write "s2idle" to
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:file:`/sys/power/mem_sleep` and then to write "mem" to
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:file:`/sys/power/state`. Likewise, there are two ways to make the system go
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into the :ref:`standby <standby>` state (the strings to write to the control
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files in that case are "standby" or "shallow" and "mem", respectively) if that
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state is supported by the platform. However, there is only one way to make the
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system go into the :ref:`suspend-to-RAM <s2ram>` state (write "deep" into
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:file:`/sys/power/mem_sleep` and "mem" into :file:`/sys/power/state`).
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The default suspend variant (ie. the one to be used without writing anything
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into :file:`/sys/power/mem_sleep`) is either "deep" (on the majority of systems
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supporting :ref:`suspend-to-RAM <s2ram>`) or "s2idle", but it can be overridden
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by the value of the ``mem_sleep_default`` parameter in the kernel command line.
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On some systems with ACPI, depending on the information in the ACPI tables, the
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default may be "s2idle" even if :ref:`suspend-to-RAM <s2ram>` is supported in
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principle.
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