linux-kernel - Re: [RFC] ARM DMA mapping TODO, v1

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Date:	Thu, 28 Apr 2011 07:45:37 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Arnd Bergmann <arnd@...db.de>, linaro-mm-sig@...ts.linaro.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] ARM DMA mapping TODO, v1

On Wed, 2011-04-27 at 10:52 +0100, Catalin Marinas wrote:
> 
> It's not broken since we moved to using Normal non-cacheable memory
> for the coherent DMA buffers (as long as you flush the cacheable alias
> before using the buffer, as we already do). The ARM ARM currently says
> unpredictable for such situations but this is being clarified in
> future updates and the Normal non-cacheable vs cacheable aliases can
> be used (given correct cache maintenance before using the buffer).

Don't you have a risk where speculative loads or prefetches might bring
back some stuff into the cache via the cachable mapping ? Is that an
issue ? As long as it's non-dirty and the cachable mapping isn't
otherwise used, I suppose it might be a non-issue, tho I've seen in
powerpc land cases of processors that can checkstop if a subsequent non
cachable access "hits" the stuff that was loaded in the cache.

Cheers,
Ben.


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