[alsa-devel] USB composite audio device problem with newer kernel versions

[Pecsi, Gergely]

Hi all,

We are having issues with a special multiple interface audio device. The card has 3 devices with the same VID/PID, 2 of them is a mono output device and the third one has an input and an output channel also. This third device does not function properly with newer kernel versions, the audio capture is not working.

Tests:
-our Debian Jessie based distro with kernel 3.16      -> OK
-our Debian Jessie based distro with kernel 4.1.25  -> OK
-our Debian Jessie based distro with kernel 4.4.1    -> NOK
-our Debian Jessie based distro with kernel 4.6        -> NOK
-our Debian Jessie based distro with kernel 4.7        -> NOK
-Debian testing daily snapshot with kernel 4.6.0      -> NOK

In the NOK cases, when we try to capture the stream with arecord, we get: "audio open error: No such file or directory".
We can also see that in the NOK cases the arecord labels the capture device with device number 0, but in the OK cases it labels it with device number 2. Aplay labels these devices with device number 2 in any case.

Arecord list output in the good case:
card 5: Module [Spc Module], device 2: USB Audio [USB Audio #2]
  Subdevices: 1/1
  Subdevice #0: subdevice #0

and in the bad case:
card 5: Module [Spc Module], device 0: USB Audio [USB Audio]
  Subdevices: 1/1
  Subdevice #0: subdevice #0

Relevant parts of alsa-info.sh output attached.

What could be the root of the problem?
What areas in the alsa codebase could be involved?
Should we report an ALSA bug?

Best regards,
Gergely Pecsi
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