linux-kernel - Re: [PATCH] dmatest: flush and invalidate destination buffer before DMA

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Date:	Mon, 05 Jan 2009 18:29:56 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Atsushi Nemoto <anemo@....ocn.ne.jp>
CC:	"haavard.skinnemoen@...el.com" <haavard.skinnemoen@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hskinnemoen@...el.com" <hskinnemoen@...el.com>,
	"Sosnowski, Maciej" <maciej.sosnowski@...el.com>,
	"ralf@...ux-mips.org" <ralf@...ux-mips.org>
Subject: Re: [PATCH] dmatest: flush and invalidate destination buffer before
 DMA

Atsushi Nemoto wrote:
> On Mon, 5 Jan 2009 11:31:57 -0700, "Dan Williams" <dan.j.williams@...el.com> wrote:
> Yes, MIPS and ARM do different thing on partial cache line.  But I
> suppose this belongs to "implementation dependent" area of DMA API so
> users of the API should not depend on it.  (Well, maybe I'm biased to
> MIPS ;-))
> 
> In general, drivers must not put normal data and DMA buffer on same
> cacheline anyway to avoid unexpected writeback and data loss.  So this
> ambiguity is not a problem.  IMHO writeback of the partial line for
> DMA_FROM_DEVICE just _hides_ abusing of the DMA API and potential data
> loss.

Hmm... one implementation does the right thing in all cases.  The other 
silently allows data corruption unless each callsite that accidentally 
or purposely passes cacheline-unaligned buffers adds extra maintenance 
code.  Which implementation are you biased towards now? :-).

--
Dan

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