A20 frequency: allow overclocking/overvolage or not

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

Hello,
so I have code to allow changing the A20 core frequency (to e.g. lower
the power input when idle). I used CPU freq/voltage tables from linux.
It is possible to run the A20 at 1008Mhz (instead of 960) by increasing
the CPU volatge to 1425mv, which is out of spec (the datasheet doens't
mention frequency, but the max voltage for CPU core is 1400mv).
My question is: should I commit the code with 1008Mhz, without it,
or conditionally allow it (either with sysctl or a compile-time option) ?

The problem with allowing it unconditionally is that, even through
the system boots at 960Mhz/1400mv, starting estd will cause the system to
use the out of spec voltage under load.

My prefered way of handling this would be to add two sysctls:
machdep.frequency.min and machdep.frequency.max, user-settable.
machdep.frequency.available will then only report the entries from
the CPU freq/voltage table that are between the bounds, and estd will only
use thoses.

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--