linux-kernel - Re: [PATCH] [POWERPC] Improve (in|out)

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Date:	Wed, 21 May 2008 10:01:22 -0400
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Trent Piepho <tpiepho@...escale.com>
Cc:	Scott Wood <scottwood@...escale.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>, linuxppc-dev@...abs.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [POWERPC] Improve (in|out)_beXX() asm code


On Tue, 2008-05-20 at 15:55 -0700, Trent Piepho wrote:
> here doesn't appear to be any barriers to use for coherent dma other than
> mb() and wmb().
> 
> Correct me if I'm wrong, but I think the sync isn't actually _required_ (by
> memory-barriers.txt's definitions), and it would be enough to use eieio,
> except there is code that doesn't use mmiowb() between I/O access and
> unlocking.
> 
> So, as I understand it, the minimum needed is eieio.  To provide strict
> ordering w.r.t. spin locks without using mmiowb(), you need sync.  To provide
> strict ordering w.r.t. normal memory, you need sync and a compiler barrier.
> 
> Right now no archs provide the last option.  powerpc is currently the middle
> option.  I don't know if anything uses the first option, maybe alpha?  I'm
> almost certain x86 is the middle option (the first isn't possible, the arch
> already has more ordering than that), which is probably why powerpc used that
> option and not the first.

I don't have time for that now. Can you dig into the archives ? The
whole thing has been discussed at lenght already.

Ben.

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