Re: 2.6.10-mm2: it87 sensor driver stops CPU fan

[Jean Delvare <khali@linux-fr.org>]

Hi Nicolas,

> I confirm that 0x7f is full speed.

So at least the polarity bit is correct, and Gigabyte isn't to blame.

> > Once you know if the polarity is correct, you can try different
> > values of PWM between 0x00 and 0x7F and see how exactly your fan
> > reacts to them.
> 
> That's where things get really really interesting.  As mentioned
> above 0x7f drives the fan full speed (2596 RPM).  Now lowering that
> value slows the CPU fan gradually down to a certain point.  With a
> value of 0x3f the fan turns at 1041 RPM.  But below 0x3f the fan
> starts speeding up again to reach a peak of 2280 RPM with a value
> of 0x31, then it slows  down again toward 0 RPM as the register
> value is decreased down to 0.
> 
> Bit 3 of register 0x14, when set, only modifies the curve so the
> first minimum is instead reached at 0x30 then the peak occurs at 0x1d
> before dropping to 0.
> 
> Changing the PWM base clock select has no effect.

Wow! Unexpected, to say the least. First time I see such a behavior.

Could it be that your CPU fan isn't a simple passive device but one of
these high-tech models with an embedded thermal sensor and automatic
speed adjustment? This would possibly interact with the motherboard PWM
capability and could explain the strange speed curve your obtained.

I would also like you to try a similar test with your case fan. Enable
"smart guardian" mode for this one (by writing 0x73 to register 0x13),
then scan the 0x7f-0x00 range (register 0x16) like you did for your CPU
fan. I wonder if you will obtain the same kind of result or a standard
linear curve.

(Note that PWM2 might not be wired at all on your motherboard, so don't
be surprised if the case fan speed doesn't change at all.)

Thanks,
-- 
Jean Delvare
http://khali.linux-fr.org/