[ltp] General question on BIOS update

[Michael Karcher]

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Am Mittwoch, den 10.12.2008, 11:18 -0500 schrieb Dan Maranville:
> For myself I am trying to keep the bios completely up to date in order
> to see if/when the cpu whine on battery is fixed.
Probably its unfixable. Many laptop BIOSes use deeper power save modes
for the processor on battery. This makes the differences in power
consumption between idle and non-idle bigger. As there are very sharp
power peaks on every interrupt, with a timer interrupt at a steady
frequency you will get this whine. The remedy is to use a tickless
kernel that saves more power and lowers the timer interrupt frequency as
much as possible (so no 100, 300 or 1000 Hz anymore). The other remedy
would be to disable the C4 power saving mode by direct chipset register
patch, which would probably have the effect of wasting power but keep
noise production to the level you are used to from AC operation. If you
already use a tickless kernel, try powertop to find out what is
generating more than 100 interrupts per second.

Another possible source of whine-on-battery is the backlight. Backlight
dimming is done using PWM (that means: supply voltage to the inverter is
pulsed), with PWM frequencies around 300-400Hz. You won't here it at
100% PWM duty. Thinkpads usually dim the backlight on battery (if the
setup entry for brightness is auto. Put it to high to disable), so the
PWM duty cycle will change and might affect whining noise from the
inverter.

Regards,
  Michael Karcher

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