Linux.Desktop.Hacks [Electronic resources] نسخه متنی

Hack 96. Control Your Power with ACPI

Extend the life of your laptop battery by
limiting your computer's appetite for
power.

The Advanced Configuration
and Power
Interface (ACPI) is an open industry specification that allows for
power management on laptops, desktops, and servers.
Compaq/Hewlett-Packard, Intel, Microsoft, Phoenix, and Toshiba first
released it in 1996. These developers aimed to replace Advanced Power
Management (APM), the previous industry standard for power
management.

You can configure ACPI and control it from within the operating
system. This is a step forward from APM, which was affected only by
system idle time and could be configured only from within the BIOS
screens during startup. ACPI can be used by most new laptops,
although some brands have specific configuration issues. Be sure to
do a web search for your specific machine to see what limitations
your system has.

ACPI has several different software components:

A subsystem, which monitors and affects hardware, including thermal
control, motherboard configuration, and power states

A policy manager, which allows the user to modify system states

Drivers to control and monitor devices, such as laptop batteries,
SMBus (communications/transmission path), and EC (embedded
controller)

11.6.1. Installing ACPI

A number of things must be in place to
use
ACPI successfully. Your kernel must have the correct configuration,
you must have the relevant modules loaded, and you should be running
an application to monitor the status of your system.

To confirm that the kernel is properly configured, launch a kernel
configuration tool and go to the General Setup section. Inside this
section, confirm that Power Management Support is enabled. Also, make
sure APM is disabled. Select all options to do with ACPI that are
relevant to your hardware. You can choose either M
for modules or * to compile directly into the
kernel. Save the new configuration and exit. Then compile your kernel
and modules, as discussed in [Hack #88] .

ACPI is always under revision. It is available in later versions of
the 2.4.x series kernel (2.4.22 and higher) and all 2.6.x series
kernels. You should always use the latest stable version of the
kernel that your system can support. Even the most recent kernel can
have minor bug fixes available as a patch, so be sure to check for
one at http://acpi.sourceforge.net. If your
distribution has already patched your kernel, you might have
difficulty applying a second patch for ACPI. Read
/usr/src/kernel-source-<version>/README to
see if your kernel has already been patched.

If you are already running a kernel with ACPI support, you can check
the ACPI revision date with the following command:

foo@bar:~$ cat /proc/acpi/info

This might give you detailed list or only a version number. You are
looking for the line that starts with version:.

11.6.2. Load Related Modules

The next step is to check to see that
each ACPI module is loaded after your machine boots. You can do this
with the lsmod command. You are looking for the
following options: button,
battery, fan,
ac, thermal, and
processor. If you chose Y
instead of M (modules) when you compiled your
kernel, you will not see this list, because the components were
compiled into the kernel itself. Otherwise, the output should look
similar to this:

    Module                  Size  Used by
button                  2420   0 (unused)
battery                 5960   0 (unused)
ac                      1832   0 (unused)
fan                     1608   0 (unused)
thermal                 6664   0 (unused)
processor               8664   0 [thermal]

If you compiled ACPI support as modules and you do not see the ACPI
modules listed, you need to load the modules by hand. The modules
should be in
/lib/modules/<kernel-version>/kernel/drivers/acpi/.

To prevent having to load the modules each time you reboot, you can
do one of two things: compile them directly into the kernel (a bit
late for that, though, eh?) or add them to your
/etc/modules file. If you do not already have a
copy of the file, just create a new one and add each module name
(remember, no .o) on a separate line. You can
also try running update-modules which should
automatically update your /etc/modules.conf
configuration file.

11.6.3. Use ACPI

You can install a few different applications/daemons on your system:
acpid (the daemon that controls your hardware
states), and a monitoring program. Be sure to remove the APM daemon
(apmd) if you are running ACPI. In addition to
using an application to monitor your system, you can also check the
ACPI files individually. Look in the /proc/acpi
directory for various things of importance. For example, if you want
to look at your battery status you need to read the battery state
file:

foo@bar:~$ cat /proc/acpi/battery/BAT0/state
present:                 yes
capacity state:          ok
charging state:          charging
present rate:            37 mW
remaining capacity:      44400 mWh
present voltage:         12456 mV

More information on ACPI in Linux, including information on power
management (sleep and suspend), is available at http://acpi.sourceforge.net.

Emma Jane Hogbin