[alsa-devel] snd_mixer_load error with composite USB device

[Alex]

Hi Daniel and Clemens,

I have been testing the sdr-widget and when alsamixer is run, there is a 
snd_mixer_load error - Invalid Argument.

The sdr-widget firmware with UAC1 descriptors works under Windows for audio 
control (muting and volume).  The UAC2 firmware works under Linux and OSX for 
audio control as well.

However, firmware with UAC1 does not work under Linux for audio control.  I 
traced the error to the fact that the sdr-widget enumerates as a composite USB 
device and the audio control interface is not interface #0.

The current Linux driver for UAC1 appears to assume that the audio control 
interface is #0.  It should not make that assumption.

Alex