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

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 10 May 2010 12:42:34 +0200
"Oskar Schirmer" <os@emlix.com> wrote:

> With dma based spi transmission, data corruption
> is observed occasionally. With dma buffers located
> right next to msg and xfer fields, cache lines
> correctly flushed in preparation for dma usage
> may be polluted again when writing to fields
> in the same cache line.
> 
> Make sure cache fields used with dma do not
> share cache lines with fields changed during
> dma handling. As both fields are part of a
> struct that is allocated via kzalloc, thus
> cache aligned, moving the fields to the 1st
> position and insert padding for alignment
> does the job.

This sounds odd.  Doesn't it imply that some code somewhere is missing
some DMA synchronisation actions?

> 
> v2: add a comment to explain why alignment is needed
> 
> v3: fix the typo in comment and layout (- to end of line)
> 
> diff --git a/drivers/input/touchscreen/ad7877.c b/drivers/input/touchscreen/ad7877.c
> index 885354c..9ebb1b4 100644
> --- a/drivers/input/touchscreen/ad7877.c
> +++ b/drivers/input/touchscreen/ad7877.c
> @@ -153,15 +153,29 @@ enum {
>   */
>  
>  struct ser_req {
> +	u16			sample;
> +	/*
> +	 * DMA (thus cache coherency maintenance) requires the
> +	 * transfer buffers to live in their own cache lines.
> +	 */
> +	char			__padalign[L1_CACHE_BYTES - sizeof(u16)];

It would be better to use __cacheline_aligned, rather than open-coding
things in this manner.