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    SubjectRe: [alsa-devel] USB transfer_buffer allocations on 64bit systems
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    On Tue, 11 May 2010 10:24:40 -0400
    Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote:

    > > > > Either the data isn't getting written to the buffer correctly or else
    > > > > the buffer isn't getting sent to the device correctly. Can anybody
    > > > > suggest a means of determining which is the case?
    > > >
    > > > I can't say anything about this log that including only DMA addresses.
    > > > I'm not familiar with how the USB core does DMA stuff. And the USB
    > > > stack design that the USB core does DMA stuff (allocating, mappings,
    > > > etc) makes debugging DMA issues really difficult.
    > >
    > > The DMA stuff is simple enough in this case. The urb->transfer_buffer
    > > address is passed to dma_map_single(), and the DMA address it returns
    > > is stored in urb->transfer_dma. Those are the two values printed out
    > > by the debugging patch.
    >
    > Is that address (urb->transfer_dma) the same as 'virt_to_phys(urb->transfer_buffer)'
    > (if not, then SWIOTLB is being utilized) and is the dma_sync_* done on the
    > urb->transfer_dma (to properly sync the data from the SWIOTLB to the
    > transfer_buffer) before you start using the urb->transfer_buffer?

    Or calling dma_unmap_single.

    Can you tell me all the exact process of DMA that the usb core and the
    driver do?


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