No volume control w RPi4 and M-DAC+ over USB

Volumio Development Bug reports
1

Prerequisites

Volumio Information

Volumio Version: 2.806
Hardware: RPi4
DAC: M-DAC+ (plus) over USB

Debug Log

http://logs.volumio.org/volumio/N569SMJ.html

Steps to Reproduce

  1. Play any source
  2. Operate volume slider on WebUI interface
  3. No change in volume

Additional Information

I have set Hardware Mixer, and tried all 4 "Mixer Control Name" options. Changing to Software Mixer kills audio playback (playback timer advances, but no sound output at all from any source) - PLAYBACK CAN THEN ONLY BE RESTORED WITH A FACTORY RESET OF VOLUMIO.
2

On further examination, selecting either of the first two “Mixer Control Name” options for Hardware Mixer allow playback, but with no volume control. Changing to Software Mixer still behaves the same - playback but no volume control. In either of these modes, the DAC remote transmits play/pause/forward/backward commands to Volumio via USB, but pressing the remote control volume buttons only changes the DAC volume, but not the volume seen on the UI slider in Volumio.

However, if you select the 3rd or 4th “Mixer Control Name” options, all sound is lost. Playback appears to be continuing - the track timer is advancing, the track position slider is moving, and you can play and pause, but there is no sound. And once you have done this, changing back to either the 1st or 2nd “Mixer Control Name” options, or to Software Mixer or even None, will not restore playback. Only a factory reset re-enables playback.

3

Volume and playback controls from devices such as M-DAC, M-DAC+, KEF EGGs and the FLIRC dongle requires USB HID support. I have asked for this previously, and I don’t think it is unreasonable, but no response as yet.

4

USB HID is integrated since 2 releases

5

Just to clarify - playback controls work, only volume doesn’t