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