[alsa-devel] USB transfer_buffer allocations on 64bit systems

Clemens Ladisch clemens at ladisch.de
Fri May 7 11:47:37 CEST 2010

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.

> So - long story short: these audio buffers need to be DMA coherent.

Does the USB API actually guarantee that all controllers use DMA, i.e.,
that the buffers can be filled after submission?


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