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    SubjectRe: [alsa-devel] USB transfer_buffer allocations on 64bit systems
    On Fri, May 07, 2010 at 11:47:37AM +0200, Clemens Ladisch wrote:
    > Daniel Mack wrote:
    > > The problem is again (summarized):
    > >
    > > On 64bit machines, with 4GB or more, the allocated buffers for USB
    > > transfers might be beyond the 32bit boundary. In this case, the IOMMU
    > > should take care and install DMA bounce buffer to copy over the buffer
    > > before the transfer actually happens. The problem is, however, that this
    > > copy mechanism takes place when the URB with its associated buffer is
    > > submitted, not when the EHCI will actually do the transfer.
    > >
    > > In the particular case of audio drivers, though, the contents of the
    > > buffers are likely to change after the submission. What we do here
    > > is that we map the audio stream buffers which are used by ALSA to
    > > the output URBs, so they're filled asychronously. Once the buffer is
    > > actually sent out on the bus, it is believed to contain proper audio
    > > date. If it doesn't, that's due to too tight audio timing or other
    > > problems. This breaks once buffers are magically bounced in the
    > > background.
    >
    > At least the audio class and ua101 drivers don't do this and fill the
    > buffers before they are submitted.

    Gnaa, you're right. I _thought_ my code does it the way I described, but
    what I wrote is how I _wanted_ to do it, not how it's currently done. I
    have a plan to change this in the future.

    So unfortunately, that doesn't explain it either. Sorry for the noise.

    Daniel


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