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

[Daniel Mack <daniel@caiaq.de>]

On Tue, May 11, 2010 at 05:48:11PM +0100, Pedro Ribeiro wrote:
> I'm clueless about the whole underlying process but I don't think it
> is related to playing a file from the disk or another USB device.
> 
> I don't need to play any file for this interference to be heard - I
> just need to "open" the device. By this I mean that if I create a JACK

... which will stream zeros. Sure, that should also trigger the bug
then.

> server or if I open the device using the ALSA backed in a program like
> Mixxx I immediately start hearing the cracks and pops in the speaker.
> But to answer your question, playing a file from the hard drive using
> aplay I can also hear the interference - but only when the sound is
> playing. As soon as the sound stops and the audio device is "closed",
> no interference occurs.

Yes, because the stream is stopped, pending URBs are unlinked etc.
That's completely expected.

> However, when using the JACK server, which probably opens and polls
> the audio card constantly (its a low latency audio server) I don't
> even need to be playing any sound for the interference to be constant.

That shouldn't affect the driver itself but just put the ALSA core under
pressure.

> I can also see it since I have 4 output leds in the USB audio card and
> even if I'm only using 2 channels, the 4 channel leds light up with
> interference as the sound cracks and pops.

No surprise here. The 4 channels are mux'ed in an interleaved fashion,
so if the buffers contain rubbish, you will hear artefacts on all
channels.

Daniel