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    SubjectRe: USB mass storage and ARM cache coherency
    Am Dienstag, 16. Februar 2010 10:39:46 schrieb Russell King - ARM Linux:
    > However, because ARM CPUs can now speculatively prefetch, just leaving it
    > at that results in corruption of buffers used for DMA. So we have to
    > invalidate DMA_FROM_DEVICE and DMA_BIDIRECTIONAL buffers on unmap to
    > ensure coherency with DMA operations.
    > 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.


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