powerd: add support for limiting cpu frequency on adaptive mode

[Kevin Oberman]

> Can someone look on the pr
> (http://www.freebsd.org/cgi/query-pr.cgi?pr=bin/104689) I sent. Maybe
> the patch is dirty but I think that idea is nice and with it battery
> lives really longer.

YES! When I am on battery for an extended time, I am very happy to have
the system max out at a MUCH lower speed if I can keep basic capabilities
for simple stuff like editing and such. Slight slow-downs are not an
issue.

That said, power consumption is NOT intuitive. If I am playing an MP3
(very constant CPU load) at  a speed where the CPU is at about 80%, the
system uses more power than when the CPU is at 30%. In either case, the
MP3 playback is fine. (This was on my T30 with a 1.8G P4M CPU, so it's
not the best for power use.)

I know that Windows wants to reduce maximum CPU speed when running on
battery, so I do believe this is a good thing.
-- 
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at es.net			Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4  EADA 927D EBB3 987B 3751
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