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Re: Cpu frequency scaling

To: "Ralf Baechle" <ralf@linux-mips.org>, "Krishnakumar. R" <krishnakumar@naturesoft.net>
Subject: Re: Cpu frequency scaling
From: "Kevin D. Kissell" <kevink@mips.com>
Date: Sun, 12 Jan 2003 20:01:02 +0100
Cc: <linux-mips@linux-mips.org>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <200301101600.26246.krishnakumar@naturesoft.net> <20030112180917.A18654@linux-mips.org>
Sender: linux-mips-bounce@linux-mips.org
> > Can frequency scaling (through software) 
> > be done on mips using linux ?
> > 
> > Is  such a feature feasible in mips ??
> > 
> > I could not find any documentation nor patches 
> > for  frequency
> > scaling on mips at 
> > http://www.brodo.de/cpufreq/
> > :-(
> 
> None of the currently supported MIPS CPUs support such a feature in
> hardware as such our support is already complete and by definition
> bug free ;-)

Actually, that's not *quite* true.  A number of MIPS CPUs and
cores that are otherwise supported by Linux (e.g. 4K, 5K) have
a "reduced power" mode which is modulated by the CP0 "RP"
bit. In general, this bit does nothing in the CPU itself, however.
It was intended that it be connected to system-level logic for
frequency scaling (1/n normal clock, or CPU=Bus).  So on
most systems it does nothing, and on the ones where it does
do something, it's entirely system dependent.  I don't have an
Alchemy AU1000 spec handy, but since they've integrated
a lot of other logic with their CPU, and since they designed
their component to go into low-power devices, it wouldn't
surprise me in the least if they do something well-defined
with the RP bit.

So, to get back to the original question, something highly
platform dependent *could* be done using MIPS/Linux, 
via /proc/cpu or some kind of system call, but I don't believe 
anyone has made such a hook generally available as yet.

            Kevin K.

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