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Date: Thu, 02 Dec 2010 13:30:13 -0700
From: Peter Grehan <grehan@freebsd.org>
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Subject: Re: New powerpc64 snapshot
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> The really bizarre thing, though, is that when the fans begin racing
> and the noise is loud, simply executing "sysctl dev.fcu.0.fans" is
> enough to calm them down to a more manageable 5600--6400 RPM!

  Fan control is handled by a microcontroller. If it doesn't see any 
communication, it assumes that the main CPU has gone away and winds up 
the fans to full-bore (you can see this if you sit at the FreeBSD loader 
prompt for a short time).

  Reading the fan speed is probably enough to fool the micro into 
thinking that something might be happening, so it will turn the fan 
speed back to normal and start it's timeout again.

later,

Peter.