|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.