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    SubjectRe: USB mass storage and ARM cache coherency
    Am Dienstag, 16. Februar 2010 14:40:45 schrieb Shilimkar, Santosh:
    > > > If the CPU writes to a DMA_FROM_DEVICE buffer between map and unmap, the
    > > > writes can sit in the cache, and on unmap, they will be discarded.
    > > >
    > > > Cleaning the cache on unmap is not an option; that too can lead to DMA
    > > > buffer corruption in the DMA case.
    > >
    > > I am afraid for these controllers the controller driver must be responsible
    > > for all DMA and cache issues. Indicating the exact requirements to the
    > > upper layer would be a battle already lost.
    > > so the safe choice is not to set has_dma and the generic layer will leave
    > > the issue to the lower level.
    > This means don't use dma at all which will almost kill the performance.

    Why would you be unable to map a buffer in the hcd driver when you know
    that you'll use DMA?


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