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.
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 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.
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.
