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

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Date:	Fri, 7 May 2010 14:07:53 +0200
From:	Johannes Weiner <hannes@...xchg.org>
To:	Mike Frysinger <vapier.adi@...il.com>
Cc:	Oskar Schirmer <os@...ix.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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] ad7877: keep dma rx buffers in seperate cache lines

On Thu, May 06, 2010 at 02:46:04PM -0400, Mike Frysinger wrote:
> On Thu, May 6, 2010 at 06:37, Oskar Schirmer wrote:
> >  struct ser_req {
> > +       u16                     sample;
> > +       char                    __padalign[L1_CACHE_BYTES - sizeof(u16)];
> > +
> >        u16                     reset;
> >        u16                     ref_on;
> >        u16                     command;
> > -       u16                     sample;
> >        struct spi_message      msg;
> >        struct spi_transfer     xfer[6];
> >  };
> 
> are you sure this is necessary ?  ser_req is only ever used with
> spi_sync() and it's allocated/released on the fly, so how could
> anything be reading that memory between the start of the transmission
> and the return to adi7877 ?

The master driver can.  atmel_spi flushes the cache of the buffers
pretty early when queuing a message and touches the message members
afterwards.

> >  struct ad7877 {
> > +       u16                     conversion_data[AD7877_NR_SENSE];
> > +       char                    __padalign[L1_CACHE_BYTES
> > +                                       - AD7877_NR_SENSE * sizeof(u16)];
> > +
> >        struct input_dev        *input;
> >        char                    phys[32];
> >
> > @@ -182,8 +188,6 @@ struct ad7877 {
> >        u8                      averaging;
> >        u8                      pen_down_acc_interval;
> >
> > -       u16                     conversion_data[AD7877_NR_SENSE];
> > -
> >        struct spi_transfer     xfer[AD7877_NR_SENSE + 2];
> >        struct spi_message      msg;
> 
> i can see the spi_message inside of this struct being a problem
> because the spi transfer is doing asynchronously with spi_async().
> however, i would add a comment right above these two fields with a
> short explanation as to why they're at the start and why the pad
> exists so someone down the line doesnt move it.

Good idea.
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