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Date: Mon, 22 Mar 2010 15:22:12 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Johannes Weiner <hannes@cmpxchg.org>
cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
       john stultz <johnstul@us.ibm.com>, Richard Henderson <rth@twiddle.net>,
       lkml <linux-kernel@vger.kernel.org>, Matt Turner <mattst88@gmail.com>
Subject: Re: [RFC][PATCH] Convert alpha to use clocksource
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On Mon, 22 Mar 2010, Johannes Weiner wrote:
> > Though, even after a glance over the clocksource code, I've not
> > gotten yet to how one could estimate the "shift" value...
> > Any hints?
> 
> I had the same problem with xtensa and added a comment about what I
> did in there, maybe it helps:
> 
> 	arch/xtensa/kernel/time.c
> 
> I took the upper bound of the multiplicator (nsecs per counter unit)
> and subtracted its logarithm from my available 32 bits.  The result
> is the highest possible shift value that works for the clocksource.

clocks_calc_mult_shift() is what you are looking for.

Thanks,

	tglx