alsaloop: link existing mixer controls

[GitHub issues - opened <github@alsa-project.org>]

alsa-project/alsa-utils issue #303 was opened from bravl:

Hi,

I'm trying to link the volume controls of 2 soundcards together. We've got 2 souncards, an UAC2Gadget card and a custom soundcard. We use alsaloop to create the following setup:

```
UAC2Gadget record -> alsaloop -> Custom soundcard speaker out
Custom soundcard mic in -> alsaloop -> UAC2Gadget soundcard playback
```

For this the alsaloop.conf looks like
```
-P hw:CARD=customsoundcard,DEV=0 -C hw:CARD=UAC2Gadget,DEV=0 -S 5 -T 1"                                                          
-C hw:CARD=customsoundcard,DEV=1 -P hw:CARD=UAC2Gadget,DEV=0 -S 5 -T 2                                                                                                               
```

This setup works perfectly. But we would like to control the volume of the custom soundcard's speaker out via the UAC2Gadgets' PCM Record volume.

The cards expose the following controls:
```sh
$ amixer -c1 controls #custom soundcard
numid=1,iface=MIXER,name='Master Volume'
numid=2,iface=MIXER,name='Speaker Volume'

$ amixer -c2 controls #UAC2Gadget
numid=3,iface=MIXER,name='PCM Playback Switch'
numid=4,iface=MIXER,name='PCM Playback Volume'
numid=6,iface=MIXER,name='PCM Capture Switch'
numid=7,iface=MIXER,name='PCM Capture Volume'
numid=1,iface=PCM,name='Capture Pitch 1000000'
numid=8,iface=PCM,name='Capture Rate'
numid=2,iface=PCM,name='Playback Pitch 1000000'
numid=5,iface=PCM,name='Playback Rate'
```

Adding `-m "name=Speaker Volume"` creates a new linked control to the UAC2Gadget card called "Speaker Volume" as expected.
Adding `-m "name=Speaker Volume"@name="PCM Capture Volume"` does not work. Resulting in:

```
Unable to create control '0,MIXER,0,0,PCM Capture Volume,0': Device or resource busy
playback hw:CARD=customsoundcard,DEV=0/capture hw:CARD=UAC2Gadget,DEV=0: Unable to remove control '0,MIXER,0,0,PCM Capture Volume,0': Invalid argument
```

Looking at the alsaloop code, it seems that this is expected behaviour because it's trying to remove a control which was created by the UAC2Gadget driver.

Is the above conclusion correct or is there a different way to allow alsaloop to sync 2 existing driver controls?

Thanks

Issue URL     : https://github.com/alsa-project/alsa-utils/issues/303
Repository URL: https://github.com/alsa-project/alsa-utils