When the CPU is idle, it runs at a lower speed. There is a bug in coreboot
that the motherboard makes a high pitched whine under this scenario.
A workaround is to disable CPU throttling (the CPU then runs always at full
speed).
This has the effect of reducing battery life. Also, the backlight is fixed at
100% (another limitation in coreboot when native graphics is used instead of
the nonfree VBIOS).
Options 5-8 in the GRUB menu do the same as options 1-4, but without
idle=halt.
This will mean that the CPU throttles again (lower speed) when idle, and will
reduce power usage during most use cases (therefore increasing battery life).
