linux-kernel - Re: [PATCH v3] ad7877: keep dma rx buffers in seperate cache lines

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Message-ID: <AANLkTinp6wpCTgvq2Vqj4GZRcsM8-vdOm4oI6MMKaedj@mail.gmail.com>
Date:	Mon, 10 May 2010 17:27:30 -0400
From:	Mike Frysinger <vapier.adi@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Oskar Schirmer <os@...ix.com>,
	Michael Hennerich <Michael.Hennerich@...log.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Mike Frysinger <vapier@...too.org>,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
	Daniel Glöckner <dg@...ix.com>,
	Oliver Schneidewind <osw@...ix.com>,
	Johannes Weiner <jw@...ix.com>
Subject: Re: [PATCH v3] ad7877: keep dma rx buffers in seperate cache lines

On Mon, May 10, 2010 at 17:22, Andrew Morton wrote:
> On Mon, 10 May 2010 12:42:34 +0200 "Oskar Schirmer" wrote:
>> With dma based spi transmission, data corruption
>> is observed occasionally. With dma buffers located
>> right next to msg and xfer fields, cache lines
>> correctly flushed in preparation for dma usage
>> may be polluted again when writing to fields
>> in the same cache line.
>>
>> Make sure cache fields used with dma do not
>> share cache lines with fields changed during
>> dma handling. As both fields are part of a
>> struct that is allocated via kzalloc, thus
>> cache aligned, moving the fields to the 1st
>> position and insert padding for alignment
>> does the job.
>
> This sounds odd.  Doesn't it imply that some code somewhere is missing
> some DMA synchronisation actions?

i think it's kind of dumb and induces this sort of bug
semi-frequently, but it is what the current DMA API requires (see like
Documentation/spi/spi-summary)
-mike
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