/usr/share/doc/pm-utils-1.4.1/ Current File: //usr/share/doc/pm-utils-1.4.1/HOWTO.modules
How to write a sleep module:

OVERVIEW:
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. Sleep modules can be stacked, which
means that some care must be taken to write a sleep module.  The basic rules 
are as follows:

1: Check to see if someone else has already implemented your sleep method.
   If they have, you should do nothing.

2: Check to see if your sleep method is applicable to this system.  If it
   is not, you should do nothing.

3: If no other module has claimed your sleep method and this system is capable
   of performing your sleep method, then go ahead and define the function that
   implements your sleep method.

As an example of how to do things, take a look at pm/module.d/uswsusp, which
implements all three sleep methods and shows how to use the module helper 
functions add_before_hooks and add_module_help.
 
Currently, three sleep methods are supported:

SUSPEND:
	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.  

Your sleep module must implement do_suspend if you support suspend to ram.

HIBERNATE:
	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 either in a very low-power use mode or completly powered off.

Your sleep module must implement do_hibernate if you support suspend to disk.

SUSPEND_HYBRID:
	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:

Your sleep module must implement do_suspend_hybrid if you support hybrid suspend.