In the world of power management ACPI is relatively new to the game. It was first released in 1996 by Compaq/Hewlett-Packard, Intel, Microsoft, Phoenix and Toshiba. These developers aimed to replace the previous industry standard for power management. Their ACPI.info site includes the official specifications, and a list of companies that support ACPI. This is not required reading, but may be of some interest to the curious.
ACPI allows control of power management from within the operating system. The previous industry standard for power management, Advanced Power Management (APM), is controlled at the BIOS level. APM is activated when the system becomes idle--the longer the system idles, the less power it consumes (for example: screen saver, sleep and suspend). In APM, the operating system has no knowledge of when the system will change power states.
ACPI can typically be configured from within the operating system. This is unlike APM where configuration often involves rebooting and entering the BIOS configuration screens to set parameters.
ACPI has several different software components:
a subsystem which controls hardware states and functions that may have previously been in the BIOS configuration
These states include:
thermal control
motherboard configuration
power states (sleep, suspend)
a policy manager, which is software that sits on top of the operating system and allows user input on the system policies
device drivers that control/monitor devices such as a laptop battery, SMBus (communication/transmission path) and EC (embedded controller).
More information on power management in laptops is available from tuxmobil.org. Specifically: Power Management with Linux - APM, ACPI, PMU and the Hardware in Detail section of the Linux on the Road.
Not all systems support both APM and ACPI. I switched because my system only supported ACPI. Pretty easy decision really. If you are switching to get S3 (suspend to RAM) support and you are using a 2.4.x kernel, do not bother. It is not supported.
ACPI4Linux has a list of supported machines/BIOSes and a list of unsupported machines. Please contribute to the list!
For more information about the power management in laptops you may (also) find the Battery Powered Linux Mini-HOWTO useful.
As ACPI support matures in Linux there may be some specific things you should know with regards to your distribution. At this time there is very little news to share, but I hope this section will grow with time. Please email me at <emmajane@xtrinsic.com> if there is anything else that should be reported in this section!
News for your distro
Red Hat Fedora Core 2 now ships with ACPI enabled by default! This is big progress for the ACPI development team. Congratulations to everyone. Fedora Core 2 users please skip right past the installation instructions and go directly to Using ACPI.