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

[Daniel Mack <daniel@caiaq.de>]

On Tue, May 11, 2010 at 12:06:09PM -0400, Alan Stern wrote:
> On Wed, 12 May 2010, FUJITA Tomonori wrote:
> 
> > > > Can you tell me all the exact process of DMA that the usb core and the
> > > > driver do?
> > > 
> > > 1. The audio driver stores data in urb->transfer_buffer.
> > 
> > How urb->transfer_buffer is allocated?
> 
> By kmalloc().  Right, Daniel?

Yes, and that's precisely the reason for the whole thread ;)

[...]

> > The driver does only DMA_TO_DEVICE? Or you see DMA problems only with
> > DMA_TO_DEVICE?
> 
> The particular test that Pedro is running uses audio output only -- 
> he's sending sound data to a speaker and it comes out noisy.
> 
> But the audio data has to come from somewhere, and I don't remember 
> where.  Pedro, does the noise occur only when you're playing sound that 
> comes from a different USB device?  What happens if you play sounds 
> that are stored on your hard disk, like an MP3 file?

AFAIK that was playback from a file, yes. It would be interesting to
have some results for the other direction as well, that's true.

Thanks,
Daniel