80 lines
3.6 KiB
ReStructuredText
80 lines
3.6 KiB
ReStructuredText
===========================================
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Power Management Interface for System Sleep
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===========================================
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Copyright (c) 2016 Intel Corp., Rafael J. Wysocki <rafael.j.wysocki@intel.com>
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The power management subsystem provides userspace with a unified sysfs interface
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for system sleep regardless of the underlying system architecture or platform.
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The interface is located in the /sys/power/ directory (assuming that sysfs is
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mounted at /sys).
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/sys/power/state is the system sleep state control file.
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Reading from it returns a list of supported sleep states, encoded as:
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- 'freeze' (Suspend-to-Idle)
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- 'standby' (Power-On Suspend)
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- 'mem' (Suspend-to-RAM)
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- 'disk' (Suspend-to-Disk)
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Suspend-to-Idle is always supported. Suspend-to-Disk is always supported
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too as long the kernel has been configured to support hibernation at all
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(ie. CONFIG_HIBERNATION is set in the kernel configuration file). Support
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for Suspend-to-RAM and Power-On Suspend depends on the capabilities of the
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platform.
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If one of the strings listed in /sys/power/state is written to it, the system
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will attempt to transition into the corresponding sleep state. Refer to
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Documentation/admin-guide/pm/sleep-states.rst for a description of each of
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those states.
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/sys/power/disk controls the operating mode of hibernation (Suspend-to-Disk).
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Specifically, it tells the kernel what to do after creating a hibernation image.
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Reading from it returns a list of supported options encoded as:
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- 'platform' (put the system into sleep using a platform-provided method)
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- 'shutdown' (shut the system down)
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- 'reboot' (reboot the system)
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- 'suspend' (trigger a Suspend-to-RAM transition)
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- 'test_resume' (resume-after-hibernation test mode)
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The currently selected option is printed in square brackets.
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The 'platform' option is only available if the platform provides a special
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mechanism to put the system to sleep after creating a hibernation image (ACPI
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does that, for example). The 'suspend' option is available if Suspend-to-RAM
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is supported. Refer to Documentation/power/basic-pm-debugging.rst for the
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description of the 'test_resume' option.
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To select an option, write the string representing it to /sys/power/disk.
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/sys/power/image_size controls the size of hibernation images.
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It can be written a string representing a non-negative integer that will be
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used as a best-effort upper limit of the image size, in bytes. The hibernation
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core will do its best to ensure that the image size will not exceed that number.
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However, if that turns out to be impossible to achieve, a hibernation image will
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still be created and its size will be as small as possible. In particular,
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writing '0' to this file will enforce hibernation images to be as small as
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possible.
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Reading from this file returns the current image size limit, which is set to
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around 2/5 of available RAM by default.
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/sys/power/pm_trace controls the PM trace mechanism saving the last suspend
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or resume event point in the RTC across reboots.
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It helps to debug hard lockups or reboots due to device driver failures that
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occur during system suspend or resume (which is more common) more effectively.
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If /sys/power/pm_trace contains '1', the fingerprint of each suspend/resume
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event point in turn will be stored in the RTC memory (overwriting the actual
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RTC information), so it will survive a system crash if one occurs right after
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storing it and it can be used later to identify the driver that caused the crash
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to happen (see Documentation/power/s2ram.rst for more information).
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Initially it contains '0' which may be changed to '1' by writing a string
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representing a nonzero integer into it.
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