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