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-Power Management Interface
-The power management subsystem provides a unified sysfs interface to
-userspace, regardless of what architecture or platform one is
-running. The interface exists in /sys/power/ directory (assuming sysfs
-is mounted at /sys).
-/sys/power/state controls system power state. Reading from this file
-returns what states are supported, which is hard-coded to 'standby'
-(Power-On Suspend), 'mem' (Suspend-to-RAM), and 'disk'
-Writing to this file one of those strings causes the system to
-transition into that state. Please see the file
-Documentation/power/states.txt for a description of each of those
-/sys/power/disk controls the operating mode of the suspend-to-disk
-mechanism. Suspend-to-disk can be handled in several ways. We have a
-few options for putting the system to sleep - using the platform driver
-(e.g. ACPI or other suspend_ops), powering off the system or rebooting the
-system (for testing).
-Additionally, /sys/power/disk can be used to turn on one of the two testing
-modes of the suspend-to-disk mechanism: 'testproc' or 'test'. If the
-suspend-to-disk mechanism is in the 'testproc' mode, writing 'disk' to
-/sys/power/state will cause the kernel to disable nonboot CPUs and freeze
-tasks, wait for 5 seconds, unfreeze tasks and enable nonboot CPUs. If it is
-in the 'test' mode, writing 'disk' to /sys/power/state will cause the kernel
-to disable nonboot CPUs and freeze tasks, shrink memory, suspend devices, wait
-for 5 seconds, resume devices, unfreeze tasks and enable nonboot CPUs. Then,
-we are able to look in the log messages and work out, for example, which code
-is being slow and which device drivers are misbehaving.
-Reading from this file will display all supported modes and the currently
-selected one in brackets, for example
- [shutdown] reboot test testproc
-Writing to this file will accept one of
- 'platform' (only if the platform supports it)
-/sys/power/image_size controls the size of the image created by
-the suspend-to-disk mechanism. It can be written a string
-representing a non-negative integer that will be used as an upper
-limit of the image size, in bytes. The suspend-to-disk mechanism will
-do its best to ensure the image size will not exceed that number. However,
-if this turns out to be impossible, it will try to suspend anyway using the
-smallest image possible. In particular, if "0" is written to this file, the
-suspend image will be as small as possible.
-Reading from this file will display the current image size limit, which
-is set to 2/5 of available RAM by default.
-/sys/power/pm_trace controls the code which saves the last PM event point in
-the RTC across reboots, so that you can debug a machine that just hangs
-during suspend (or more commonly, during resume). Namely, the RTC is only
-used to save the last PM event point if this file contains '1'. Initially it
-contains '0' which may be changed to '1' by writing a string representing a
-nonzero integer into it.
-To use this debugging feature you should attempt to suspend the machine, then
-reboot it and run
- dmesg -s 1000000 | grep 'hash matches'
-CAUTION: Using it will cause your machine's real-time (CMOS) clock to be
-set to a random invalid time after a resume.