kernel-ark/Documentation/power/interface.txt
Linus Torvalds 1da177e4c3 Linux-2.6.12-rc2
Initial git repository build. I'm not bothering with the full history,
even though we have it. We can create a separate "historical" git
archive of that later if we want to, and in the meantime it's about
3.2GB when imported into git - space that would just make the early
git days unnecessarily complicated, when we don't have a lot of good
infrastructure for it.

Let it rip!
2005-04-16 15:20:36 -07:00

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