2 - Raspberry PI Community Projects

9.12. Power Management
The topic of power management is often problematic. Indeed, properly suspending the computer
requires that all the computer's device drivers know how to put them to standby, and that
they properly reconfigure the devices upon waking. Unfortunately, there are still many devices
unable to sleep well under Linux, because their manufacturers have not provided the required
specifications.
WORTH FOLLOWING
Soware suspend
The soware suspend banner rallies several recent efforts to integrate reliable
hibernation under Linux, on disk or in memory. Recent kernels are relatively
reliable in that regard, when used in cooperation with tools of the uswsusp
package. Unfortunately the problems related to hibernation are not yet ancient
history, and you should run tests on your hardware before puing too
much faith in its ability to wake from suspend.
For those who want to learn more about how standby works with ACPI,
Mahew Garre has an excellent article about this in his blog.
➨ http://www.advogato.org/article/913.html
9.12.1. Advanced Power Management (APM)
APM (Advanced Power Management) control is present in all Debian kernels, but disabled by
default. To activate it, you add the apm=on option to the kernel parameters passed at boot
time. With LILO, you would add the append="apm=on" directive to the block indicating which
image to boot (in the /etc/lilo.conf file), and relaunch lilo. With GRUB2, you simply add
apm=on to the GRUB_CMDLINE_LINUX= variable in /etc/default/grub, and run updategrub
to regenerate the contents of the boot menu.
The apmd package provides a daemon that looks for events connected to energy management
(switching between AC and battery power on a laptop, etc.), and allows you to run specific commands
in response.
These days, APM is really only justified on older computers that do not support ACPI properly.
In all other cases, ACPI should be used.
9.12.2. Modern power savings: Advanced Configuration and Power Interface (ACPI)
Linux supports ACPI (Advanced Configuration and Power Interface) — the most recent standard
in power management. More powerful and flexible, it is also more complicated to implement.
The acpid package is the counterpart to apmd for the ACPI world.
If you know that your BIOS correctly manages ACPI, then this should be preferred over APM
(removed upon update of the BIOS). When moving from one to the other, you must take care
to remove the apmd package, since keeping it alongside with acpid could cause problems (and
vice-versa).
218 The Debian Administrator's Handbook