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

[Alan Stern]

On Fri, 7 May 2010, Daniel Mack wrote:

> > Or to be precise it could be done with coherent memory, but you'd risk
> > transfering partially updated buffers, as you cannot know when DMA will
> > be done and we don't guarantee that DMA will be done right as we transfer.
> 
> If the memory is coherent, it should be possible. And if buffers are
> only partially updated, the audio buffer scheduling is too tight. IOW,
> the configured buffer size of audio clients in userspace is too small.

Even with coherent memory, this dynamic updating of buffer contents 
isn't a good idea.  There's no guarantee that coherent memory won't be 
implemented using a bounce buffer during URB submission.

Alan Stern

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