[PATCH] ALSA: usb-audio: Add mixer map quirk for Audient iD24

[Peter Drabik <drabik.p@gmail.com>]

The Audient iD24 (2708:000d) exposes feature unit 12 as a 4-channel
"Speaker Playback Volume" control (cmask 0xf, -127..0 dB).  The device
does not actually apply this volume to all of its output channels: the
left main output ignores it and stays at 0 dB, while the right main
output honors it.

When userspace (PulseAudio / PipeWire in a stereo profile) adopts this
control as the master playback volume, any setting below maximum
produces a stereo imbalance on the main outputs.

This was verified against the device's internal meters: with the
control set to 107/127 (-20 dB) on all four channels and a digitally
identical L/R sine played back, both DAW return meters read the same
level while the right main output metered exactly 20 dB below the
left.  Restoring the control to 127 (0 dB) restored the balance.

Rename the control to "Monitor Mix Playback" so that it is not picked
up as the stream's master volume control, in line with similar quirks
for other devices.  The control remains accessible for manual use.

Signed-off-by: Peter Drabik <drabik.p@gmail.com>
---
 sound/usb/mixer_maps.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

--- a/sound/usb/mixer_maps.c
+++ b/sound/usb/mixer_maps.c
@@ -496,6 +496,19 @@
 };

 /*
+ * Audient iD24: feature unit 12 ("Speaker Playback Volume") sits in the
+ * monitor-mixer branch and does not apply volume to all of its channels;
+ * when userspace adopts it as the master playback volume, the left main
+ * output stays at 0 dB while the right one is attenuated, producing a
+ * stereo imbalance.  Rename it so that it is not picked up as the
+ * stream's master volume control.
+ */
+static const struct usbmix_name_map audient_id24_map[] = {
+	{ 12, "Monitor Mix Playback" },	/* FU, partial channel coverage */
+	{}
+};
+
+/*
  * Control map entries
  */

@@ -579,6 +592,11 @@
 		.map = maya44_map,
 	},
 	{
+		/* Audient iD24 */
+		.id = USB_ID(0x2708, 0x000d),
+		.map = audient_id24_map,
+	},
+	{
 		/* KEF X300A */
 		.id = USB_ID(0x27ac, 0x1000),
 		.map = scms_usb3318_map,