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

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Date:	Wed, 19 May 2010 09:37:45 -0700
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Cc:	npiggin@...e.de, jw@...ix.com, dwmw2@...radead.org,
	marc@...silica.com, vapier.adi@...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, May 19, 2010 at 11:38:34PM +0900, FUJITA Tomonori wrote:
> On Wed, 19 May 2010 23:52:26 +1000
> Nick Piggin <npiggin@...e.de> wrote:
> 
> > On Wed, May 19, 2010 at 03:44:30PM +0200, Johannes Weiner wrote:
> > > On Wed, May 19, 2010 at 11:36:56PM +1000, Nick Piggin wrote:
> > > > On Wed, May 19, 2010 at 02:17:47PM +0100, David Woodhouse wrote:
> > > > >
> > > > > The 'cacheline aligned' misconception did manage to get into the ad7877
> > > > > driver in commit 3843384a though -- it now uses ____cacheline_aligned
> > > > > instead of __attribute__((__aligned__(ARCH_KMALLOC_MINALIGN))) as it
> > > > > should.
> > > > 
> > > > OK so long as there is not a "must be cacheline aligned" requirement.
> > > > Your proposal for a __dma_aligned attribute in an arch header looks
> > > > like a good idea there.
> > > 
> > > Would you happen to know of other potential users?  At this point I'd
> > > much rather just allocate the buffers dynamically and hide the issue
> > > nicely behind kmalloc().
> > 
> > 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.

It really dpeends on the size of the buffer. When I need a single byte I
really do not want to mess with separate kmalloced buffer.

If somebody coudl pick David's patch that would be great.

-- 
Dmitry
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