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

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Date:	Thu, 20 May 2010 13:42:51 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	dwmw2@...radead.org
Cc:	fujita.tomonori@....ntt.co.jp, 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, 19 May 2010 15:58:37 +0100
David Woodhouse <dwmw2@...radead.org> wrote:

> 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!

Hmm, driver writers should read DMA-API and DMA-API-HOWTO docs.


> 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.

Yeah, I think that that's a valid point (which was also discussed in
the past). However, I tend to agree on:

http://lwn.net/Articles/2270/

I think that forcing kmalloc is not that bad. Surely more code, but
the performance is not notable in most cases. IMHO, exporting cache
line thing everywhere is worse than it.
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