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    SubjectRe: [Linaro-mm-sig] [RFC] ARM DMA mapping TODO, v1
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    On Thu, 28 Apr 2011 08:40:08 +0200
    Arnd Bergmann <arnd@arndb.de> wrote:

    > On Wednesday 27 April 2011 23:37:51 Benjamin Herrenschmidt wrote:
    > > On Wed, 2011-04-27 at 08:35 +0100, Russell King - ARM Linux wrote:
    > > > On Thu, Apr 21, 2011 at 09:29:16PM +0200, Arnd Bergmann wrote:
    > > > > 2. Implement dma_alloc_noncoherent on ARM. Marek pointed out
    > > > > that this is needed, and it currently is not implemented, with
    > > > > an outdated comment explaining why it used to not be possible
    > > > > to do it.
    > > >
    > > > dma_alloc_noncoherent is an entirely pointless API afaics.
    > >
    > > I was about to ask what the point is ... (what is the expected
    > > semantic ? Memory that is reachable but not necessarily cache
    > > coherent ?)
    >
    > Drivers use this when they explicitly want to manage the caching
    > themselves.

    Not "want to manage". The API is for drivers that "have to" manage the
    cache because of architectures that can't allocate coherent memory.

    > I think this is most interesting on big NUMA systems,
    > where you really want to use fast (local cached) memory and
    > then flush it explicitly to do dma. Very few drivers use this:


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