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Subject: Re: [RFC PATCH 1/4] powerpc 2.6.16-rt17: to build on powerpc w/ RT
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: john stultz <johnstul@us.ibm.com>
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> > Well, I've been wanting to look at your stuff and possibly do the
> > conversion for some time, provided we don't lose performances ... Our
> > current implementation is very optimized to avoid even memory barriers
> > in most cases and I doubt we'll be able to be as fine tuned using your
> > generic code, thus it's a tradeoff decision that we have to do. But
> > then, I need to look into the details before doing any final
> > statement :)
> 
> Ok, although do let me know if you see places where the generic code
> could use any of the optimizations used in powerpc.

Difficult... but maybe. One of the main idea is that the 3 values used
for "calibration" (pre-mult offset, multiplier and post-mult offset) are
in a structure. There is an array of 2 of these and a pointer to the
"current" one. When changing those values, we update the "other" one,
wmb, then flip the pointer. The readers can then mostly be barrier-less
since there is a natural data dependency on the pointer.

Ben.