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



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