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

[Paul Mundt]

On Tue, May 11, 2010 at 11:47:20PM +0300, Pekka Enberg wrote:
> Mike Frysinger wrote:
> >On Tue, May 11, 2010 at 16:38, Pekka Enberg wrote:
> >>Mike Frysinger wrote:
> >>>that is a question for David/Grant.  i'm not the SPI core maintainer,
> >>>i'm merely watching over some SPI drivers.  however, this answer also
> >>>doesnt sound like it's thinking big enough because what you're
> >>>proposing isnt specific to the SPI bus -- any time a DMA safe buffer
> >>>is needed dynamically, this function could be used.
> >>Well, we have dma_alloc_coherent(), shouldn't you be using that instead?
> >
> >my understanding is that dma_alloc_coherent() gives you a buffer that
> >is always coherent.  the SPI layers take care of flushing and such on
> >the fly which means allocating coherent memory is overkill and bad for
> >performance.
> 
> OK, I'm out of my expert area here but if dma_alloc_coherent() doesn't 
> work for you, you should probably extend the DMA API, not kmalloc().
> 
Note that the DMA API already has dma_alloc_noncoherent() for these
sorts of cases. If the driver is taking care of cache maintenance then
dma_alloc_noncoherent() is certainly a reasonable way to go.

Most architectures today simply wrap dma_alloc_noncoherent() to
dma_alloc_coherent(), but if there were more users of the API then
that would quickly change.
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