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It is very important to utilize hardware resources in an efficient manner. Before ACPI was introduced, it was very difficult and inflexible for operating systems to manage the power usage and thermal properties of a system. The hardware was either controlled by some sort of BIOS embedded interface, i.e.: Plug and Play BIOS (PNPBIOS), Advanced Power Management (APM) and so on. Power and Resource Management is one of the key components of a modern operating system. For example, you would want an operating system to monitor system limits (and possibly take an action), in case your system temperature increased unexpectedly.
In this section of the FreeBSD Handbook, we will provide comprehensive information about ACPI. References will be provided for further reading, at the end. Please be aware that ACPI is only available on FreeBSD 5.X and above.
Advanced Configuration and Power Interface (ACPI) is a standard written by an alliance of vendors to provide a standard interface for hardware resources and power management (hence the name). It is a key element in Operating System-directed configuration and Power Management, i.e.: it provides more control and flexibility to the operating system (OS). Modern systems ``stretched'' the limits of the current Plug and Play interfaces (such as APM, which is used in FreeBSD 4.X), prior to the introduction of ACPI. ACPI is the direct successor to APM (Advanced Power Management).
The acpi.ko driver is loaded by default at start up by the loader(8) and should not be compiled into the kernel. The reasoning behind this is that modules are easier to work with, say if switching to another acpi.ko without doing a kernel rebuild. This has the advantage of making testing easier. Another reason is that starting ACPI after a system has been brought up is not too useful, and in some cases can be fatal. In doubt, just disable ACPI all together. This driver should not and can not be unloaded because the system bus uses it for various hardware interactions. ACPI can be disabled with the acpiconf(8) utility. In fact most of the interaction with ACPI can be done via acpiconf(8). Basically this means, if anything about ACPI is in the dmesg(8) output, then most likely it is already running.
Note: ACPI and APM cannot coexist and should be used separately. The last one to load will terminate if the driver notices the other running.
In the simplest form, ACPI can be used to put the system into a sleep mode with acpiconf(8), the -s flag, and a 1-5 option. Most users will only need 1. Option 5 will do a soft-off which is the same action as:
# halt -p
The other options are available. Check out the acpiconf(8) manual page for more information.
Almost everything in ACPI is transparent, until it does not work. That is usually when you as a user will know there is something not working properly.
This, and other documents, can be downloaded from ftp://ftp.FreeBSD.org/pub/FreeBSD/doc/.
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For questions about this documentation, e-mail <doc@FreeBSD.org>.