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    SubjectRe: USB mass storage and ARM cache coherency
    On Wed, 2010-02-17 at 10:15 +0100, Oliver Neukum wrote:
    > We should have changed the subject line.
    > There's a second problem. It turns out that on ARM
    > mapping for DMA must not be done if PIO will be used. Some HCDs
    > use PIO for some transfers but DMA for others. The generic layer
    > must learn about this.

    Ah, that makes a lot of sense and the same problem would happen on
    any non-DMA coherent architecture, including some embedded ppc's.

    I can see why the dma unmap would invalidate the dcache and blow
    away the PIO.

    What bugs me here is that the dma_map_* operation should always
    be done at the lowest level, ie, the actual HCD driver, and thus
    it should be up to the HCD to decide whether to dma_map or not
    depending on whether it's going to do DMA or not. I haven't
    scrutinized USB lately but if that isn't the case and the dma_map_*
    operations are done behind your back by the USB core then that needs to
    be changed in a way or another, or hooked at least.


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