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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. The
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greatest distinction is who writes memory to disk - the firmware or
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the kernel. If the firmware does it, we assume that it also handles
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suspending the system.
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If the kernel does it, then we have three options for putting the system
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to sleep - using the platform driver (e.g. ACPI or other PM
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registers), powering off the system or rebooting the system (for
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testing). The system will support either 'firmware' or 'platform', and
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that is known a priori. But, the user may choose 'shutdown' or
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'reboot' as alternatives.
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Reading from this file will display what the mode is currently set
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to. Writing to this file will accept one of
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'firmware'
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'platform'
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'shutdown'
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'reboot'
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It will only change to 'firmware' or 'platform' if the system supports
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it.
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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 megabytes. 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 500 MB by default.
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