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