How to write a sleep module:
Sleep modules are POSIX-compatible shell scripts designed to be sourced by
the pm-utils machinery in order to check that the machine supports a sleep
method and to actually enter that method.
Currently, three sleep methods are supported:
Suspend (also known as suspend-to-ram) is the sleep mode equivalent
to ACPI S3 -- state is saved in system memory, and all other hardware is either
powered off or in power-saving mode. If your sleep module supports suspend,
it MUST define 2 functions:
* check_suspend, which checks to see that the system provides a suspend-to-ram
mechanism, returns 0 if it does, and 1 if it does not.
* do_suspend, which performs just the task of putting the system in suspend.
Hibernate (also know as suspend-to-disk) is the sleep mode equivalent
to ACPI S4 -- the system state is saved to permanent media (like a hard drive),
and the system is iether in a very low-power use mode or completly powered off.
If your sleep module supports hibernate, it MUST define 2 functions:
* check_hibernate, which checks to see if the system provides a suspend-to-disk
mechanism, and returns 0 if it does, and 1 if it does not.
* do_hibernate, which performs just the task of putting the system in hibernate.
Hibernate can optionally provide a third function:
* hibernate_might_work, which should return 0 if the minimum prerequisites
for a sucessful hibernate/thaw cycle are met (ensuring that there is an
active swap device, etc), and 1 otherwise. If your system provides this
method, your do_hibernate function SHOULD check it and:
* emit an error message, and
* return without hibernating the system.
suspend_hybrid is a blend of suspend and hibernate. It performs all
the tasks needed to put the system into hibernate mode (including writing the
memory image to disk), and then puts the system into suspend mode. If
implemented porperly, this should provide the convienence of the resume speed
of suspend with the assurance that your state is saved to the hard drive if
you run out of power. If your sleep module supports suspend_hybrid, it MUST
define 2 functions:
* check_suspend_hybrid, which checks to see that the system provides hybrid
suspend functionality, and returns 0 if it dies and 1 if it does not.
* do_suspend_hybrid, which just performs the task of putting the system into
do_suspend_hybrid SHOULD utilize hibernate_might_work if it is present.
If you define a function named "before_hook", that function will be executed
just before the sleep/resume hooks are executed. If you need to disable a hook,
this is the place to do it.