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From: Segher Boessenkool <segher@kernel.crashing.org>
Subject: Re: [PATCH 2.6.21-rt2] PowerPC: decrementer clockevent driver
Date: Sun, 20 May 2007 18:02:46 +0200
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Daniel Walker <dwalker@mvista.com>, linux-kernel@vger.kernel.org,
	linuxppc-dev@ozlabs.org, Paul Mackerras <paulus@samba.org>,
	mingo@elte.hu, tglx@linutronix.de
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> 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.

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

> 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.


Segher