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

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Message-ID: <1274281117.6930.10190.camel@macbook.infradead.org>
Date:	Wed, 19 May 2010 15:58:37 +0100
From:	David Woodhouse <dwmw2@...radead.org>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Cc:	npiggin@...e.de, jw@...ix.com, marc@...silica.com,
	vapier.adi@...il.com, dmitry.torokhov@...il.com, cl@...ux.com,
	penberg@...helsinki.fi, mpm@...enic.com, akpm@...ux-foundation.org,
	os@...ix.com, Michael.Hennerich@...log.com,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
	dg@...ix.com, osw@...ix.com, rientjes@...gle.com,
	dbrownell@...rs.sourceforge.net, grant.likely@...retlab.ca,
	chris@...kel.net, Piet.Delaney@...silica.com
Subject: Re: [LKML] Re: [PATCH v3] ad7877: keep dma rx buffers in seperate
 cache lines

On Wed, 2010-05-19 at 23:38 +0900, FUJITA Tomonori wrote:
> 
> > I don't think we need to hide the fact that some platforms have
> > specific alignment restrictions for DMA. So if any drivers make use
> > of the alignment, I see no problem with __dma_aligned.
> 
> IIRC, such was proposed several times:
> 
> http://www.mail-archive.com/linux-scsi@vger.kernel.org/msg12633.html
> 
> I guess that we agreed that it's better to tell driver writers to just
> use kmalloc. 

Perhaps -- but only a few days ago in this thread, they were being
advised to use ____cacheline_aligned instead!

And for this case it really does seem to make sense to keep the buffer
in the parent structure rather than allocating it separately. The DMA
buffers are tiny and on cache-coherent architectures it's _much_ more
efficient just to have them in the original structure and use
__dma_aligned.

-- 
dwmw2

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