[alsa-devel] Attempting to understand odd snd-usb-audio code and behavior

[Keith A. Milner]

On Wednesday 23 Aug 2017 14:52:43 Clemens Ladisch wrote:
> Keith A. Milner wrote:
> >>> kernel: [ 1987.452947] usb 1-1: Unable to change format on ep #8e:
> >>> already in use> 
> > So the error messages are due to a userspace app trying to open playback
> > and capture at the same time?
> 
> For this device, playback requires a running capture stream to determine
> the rate of packets.

Interestingly, the device works for playback (using aplay) until I try to 
capture, when it stops working until I reset it.

> This error happens when the application tries to
> open the capture stream after the playback stream.  (If the format does
> not actually need to change, the error message is misleading.)
> 
> Your actual problem is that the device does not send any capture packets.
> 

Presumably this should (in principle) be taken care of by the driver. The 
application should not care. So the driver should detect the dependency and 
open the corresponding capture stream if a playback stream is opened?

There's definitely something hooky with this device and the capture stream 
though, which I need to explore further. So far I cannot get capture working 
at all.

Cheers,

Keith