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

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Message-Id: <20100510232015.0a9f653a.akpm@linux-foundation.org>
Date:	Mon, 10 May 2010 23:20:15 -0400
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mike Frysinger <vapier.adi@...il.com>
Cc:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	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>
Subject: Re: [PATCH v3] ad7877: keep dma rx buffers in seperate cache lines

On Tue, 11 May 2010 02:11:41 -0400 Mike Frysinger <vapier.adi@...il.com> wrote:

> > __ __ __ __unsigned __ __ __ __ __ __ __ __pending:1; __ __ __/* P: lock */
> > +
> > + __ __ __ /*
> > + __ __ __ __* DMA (thus cache coherency maintenance) requires the
> > + __ __ __ __* transfer buffers to live in their own cache lines.
> > + __ __ __ __*/
> > + __ __ __ u16 conversion_data[AD7877_NR_SENSE] ____cacheline_aligned;
> > __};

(^^stupid gmail)

> i'm not sure this is correct.  the cached_aligned attribute makes sure
> it starts on a cache boundary, but it doesnt make sure it pads out to
> one.  so it might work more of the time, but i dont think it's
> guaranteed.

yup.  You'd need to put something like

	int pad ____cacheline_aligned;

_after_ the trashable field.

Then look at the .s file and make sure it came out right ;)
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