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