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

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Message-ID: <4BE9C258.10706@cs.helsinki.fi>
Date:	Tue, 11 May 2010 23:47:20 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Mike Frysinger <vapier.adi@...il.com>
CC:	Christoph Lameter <cl@...ux.com>, Matt Mackall <mpm@...enic.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oskar Schirmer <os@...ix.com>,
	Michael Hennerich <Michael.Hennerich@...log.com>,
	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>, Nick Piggin <npiggin@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	David Brownell <dbrownell@...rs.sourceforge.net>,
	Grant Likely <grant.likely@...retlab.ca>
Subject: Re: [PATCH v3] ad7877: keep dma rx buffers in seperate cache lines

Mike Frysinger wrote:
> On Tue, May 11, 2010 at 16:38, Pekka Enberg wrote:
>> Mike Frysinger wrote:
>>> that is a question for David/Grant.  i'm not the SPI core maintainer,
>>> i'm merely watching over some SPI drivers.  however, this answer also
>>> doesnt sound like it's thinking big enough because what you're
>>> proposing isnt specific to the SPI bus -- any time a DMA safe buffer
>>> is needed dynamically, this function could be used.
>> Well, we have dma_alloc_coherent(), shouldn't you be using that instead?
> 
> my understanding is that dma_alloc_coherent() gives you a buffer that
> is always coherent.  the SPI layers take care of flushing and such on
> the fly which means allocating coherent memory is overkill and bad for
> performance.

OK, I'm out of my expert area here but if dma_alloc_coherent() doesn't 
work for you, you should probably extend the DMA API, not kmalloc().

			Pekka
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