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 23:52:26 +1000
From:	Nick Piggin <npiggin@...e.de>
To:	Johannes Weiner <jw@...ix.com>
Cc:	David Woodhouse <dwmw2@...radead.org>,
	Marc Gauthier <marc@...silica.com>,
	Mike Frysinger <vapier.adi@...il.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Matt Mackall <mpm@...enic.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oskar Schirmer <os@...ix.com>,
	Michael Hennerich <Michael.Hennerich@...log.com>,
	"linux-input@...r.kernel.org" <linux-input@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Daniel Glöckner <dg@...ix.com>,
	Oliver Schneidewind <osw@...ix.com>,
	David Rientjes <rientjes@...gle.com>,
	David Brownell <dbrownell@...rs.sourceforge.net>,
	Grant Likely <grant.likely@...retlab.ca>,
	Chris Zankel <chris@...kel.net>,
	Piet Delaney <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 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.

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