linux-kernel - Re: [PATCH 2.6.21-rt2] PowerPC: decrementer clockevent driver

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Date:	Mon, 21 May 2007 07:26:54 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Segher Boessenkool <segher@...nel.crashing.org>
Cc:	Paul Mackerras <paulus@...ba.org>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, Daniel Walker <dwalker@...sta.com>,
	linuxppc-dev@...abs.org, tglx@...utronix.de
Subject: Re: [PATCH 2.6.21-rt2] PowerPC: decrementer clockevent driver

On Sun, 2007-05-20 at 18:02 +0200, Segher Boessenkool wrote:
> > In fact, while it's never worded explicitely in the spec, it's always
> > been strongly in the "spirit" of the architecture that the timebase and
> > decrementer have a constant frequency.
> 
> The architecture mentions varying time base frequencies,
> and how to deal with this, actually.  It makes no
> recommendations one way or the other.

I might be mixing up with PAPR... anyway, it's very stupid to vary it
imho. And as I just said, I have about 0 plan to imlement support for
such a contraption.

> Fixed frequencies are easier for almost everything of
> course :-)

Yup.

> > This is why processors like the
> > 970 allow for an external sourcing for when they are used in setups
> > where the various clocks are slewed for power management.
> 
> Clock spreading on the core clock is the bigger problem,
> lack of accuracy on the order of 1% is unacceptable for
> certain applications.

That too.

Ben.


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