6.15 Power and Resource Management

Written by Hiten Pandya and Tom Rhodes.

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 controlled by some sort of BIOS embedded interface, such as Plug and Play BIOS (PNPBIOS), or 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 may want an operating system to monitor system limits (and possibly alert you) 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 available on FreeBSD 5.X and above systems as a default kernel module. For FreeBSD 4.9, ACPI can be enabled by adding the line device acpi to a kernel configuration and rebuilding.

6.15.1 What Is ACPI?

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).

6.15.2 Shortcomings of Advanced Power Management (APM)

The Advanced Power Management (APM) facility control's the power usage of a system based on its activity. The APM BIOS is supplied by the (system) vendor and it is specific to the hardware platform. An APM driver in the OS mediates access to the APM Software Interface, which allows management of power levels.

There are four major problems in APM. Firstly, power management is done by the (vendor-specific) BIOS, and the OS does not have any knowledge of it. One example of this, is when the user sets idle-time values for a hard drive in the APM BIOS, that when exceeded, it (BIOS) would spin down the hard drive, without the consent of the OS. Secondly, the APM logic is embedded in the BIOS, and it operates outside the scope of the OS. This means users can only fix problems in their APM BIOS by flashing a new one into the ROM; which, is a very dangerous procedure, and if it fails, it could leave the system in an unrecoverable state. Thirdly, APM is a vendor-specific technology, which, means that there is a lot or parity (duplication of efforts) and bugs found in one vendor's BIOS, may not be solved in others. Last but not the least, the APM BIOS did not have enough room to implement a sophisticated power policy, or one that can adapt very well to the purpose of the machine.

Plug and Play BIOS (PNPBIOS) was unreliable in many situations. PNPBIOS is 16-bit technology, so the OS has to use 16-bit emulation in order to ``interface'' with PNPBIOS methods.

The FreeBSD APM driver is documented in the apm(4) manual page.

6.15.3 Configuring ACPI

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.

6.15.4 Debugging and Disabling ACPI

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. The acpi(4) driver supports many debugging options, it is even possible to selectively disable some parts of the ACPI system. For more information about debugging facilities, read the acpi(4) manual page.

Sometimes for various reasons, the acpi.ko module must be unloaded. This can only be done at boot time by the loader(8). You can type at loader(8) prompt the command unset acpi_load each time you boot the system, or to stop the autoloading of the acpi(4) driver add the following line to the /boot/loader.conf file:

exec="unset acpi_load"

FreeBSD 5.1-RELEASE and later come with a boot-time menu that controls how FreeBSD is booted. One of the proposed options is to turn off ACPI. So to disable ACPI just select 2. Boot FreeBSD with ACPI disabled in the menu.

This, and other documents, can be downloaded from ftp://ftp.FreeBSD.org/pub/FreeBSD/doc/.

For questions about FreeBSD, read the documentation before contacting <[email protected]>.
For questions about this documentation, e-mail <[email protected]>.