Fan control for PowerMac7_3 (#3)

[Paul Mackerras]

H. Peter Anvin writes:

> It's the backside fan that oscillates; backside_fan_pwm varies between 
> 30 and 100 in what is pretty much a squarewave.  See attached graph (and 
> note how the other fans vary with workload.)

The sharp rises look like the code thinks it gets into an
over-temperature situation and turns the fans on full blast.  It could
be worth putting some printks in the overtemp code.

> I probably need to write a "power virus" program for the G5 to really 
> test out the high end (a power virus is a program which keeps the chip 
> running as hard as it can; generally keep all pipelines stuffed.)

Hmmm, I should see if I can dig such a thing out of somewhere in IBM.

Paul.