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1Power Management Interface
2
3
4The power management subsystem provides a unified sysfs interface to
5userspace, regardless of what architecture or platform one is
6running. The interface exists in /sys/power/ directory (assuming sysfs
7is mounted at /sys).
8
9/sys/power/state controls system power state. Reading from this file
10returns what states are supported, which is hard-coded to 'standby'
11(Power-On Suspend), 'mem' (Suspend-to-RAM), and 'disk'
12(Suspend-to-Disk).
13
14Writing to this file one of those strings causes the system to
15transition into that state. Please see the file
16Documentation/power/states.txt for a description of each of those
17states.
18
19
20/sys/power/disk controls the operating mode of the suspend-to-disk
21mechanism. Suspend-to-disk can be handled in several ways. The
22greatest distinction is who writes memory to disk - the firmware or
23the kernel. If the firmware does it, we assume that it also handles
24suspending the system.
25
26If the kernel does it, then we have three options for putting the system
27to sleep - using the platform driver (e.g. ACPI or other PM
28registers), powering off the system or rebooting the system (for
29testing). The system will support either 'firmware' or 'platform', and
30that is known a priori. But, the user may choose 'shutdown' or
31'reboot' as alternatives.
32
33Reading from this file will display what the mode is currently set
34to. Writing to this file will accept one of
35
36 'firmware'
37 'platform'
38 'shutdown'
39 'reboot'
40
41It will only change to 'firmware' or 'platform' if the system supports
42it.
43
44/sys/power/image_size controls the size of the image created by
45the suspend-to-disk mechanism. It can be written a string
46representing a non-negative integer that will be used as an upper
47limit of the image size, in bytes. The suspend-to-disk mechanism will
48do its best to ensure the image size will not exceed that number. However,
49if this turns out to be impossible, it will try to suspend anyway using the
50smallest image possible. In particular, if "0" is written to this file, the
51suspend image will be as small as possible.
52
53Reading from this file will display the current image size limit, which
54is set to 500 MB by default.