Problem: How can I detect when the audio UAC1 gadget is used from a host device?
Background: I created a USB audio gadget using USB Audio Class 1 (UAC1) to send/receive audio over USB from a Linux device to/from a Windows host. The gadget is already working and Windows detects the device as an audio in and output. I am also able to send and receive audio over the gadget using alsa and jack.
Detailed Problem: I'm trying to connect two different audio devices with Jack to route an incoming signal from audio-device-1 to the output of audio-device-2. To do this, I'm working with the virtual wiring of Jack. This works already fine, but I run into the problem that if the gadget device is not actively selected as audio in and output in a DAW or other program on the host computer (e.g. Windows), Jack gets a "poll time out" error and the virtual wiring has to be restarted.
$ jackd -r --name default -d alsa --device hw:2 -r 48000 -p 64 -n 2
jackdmp 1.9.17
....
creating alsa driver ... hw:2|hw:2|64|2|48000|0|0|nomon|swmeter|-|32bit
configuring for 48000Hz, period = 64 frames (1.3 ms), buffer = 2 periods
ALSA: final selected sample format for capture: 24bit little-endian in 3bytes format
ALSA: use 4 periods for capture
ALSA: final selected sample format for playback: 24bit little-endian in 3bytes format
ALSA: use 4 periods for playback
ALSA: poll time out, polled for 2000981 usecs, Retrying with a recovery, retry cnt = 1
ALSA: poll time out, polled for 2000979 usecs, Retrying with a recovery, retry cnt = 2
ALSA: poll time out, polled for 2000982 usecs, Retrying with a recovery, retry cnt = 3
ALSA: poll time out, polled for 2000979 usecs, Retrying with a recovery, retry cnt = 4
ALSA: poll time out, polled for 2000988 usecs, Retrying with a recovery, retry cnt = 5
ALSA: poll time out, polled for 2000989 usecs, Reached max retry cnt = 5, Exiting
JackAudioDriver::ProcessAsync: read error, stopping...
I would like to solve this automatically e.g. via a shell script that detects when the gadget driver is requested by the host and I then start the virtual cabling with Jack.
For this it is necessary to be able to read whether the gadget driver is in use or not.
Is there any directory / file where I can detect, if the audio gadget device is in use?