Re: [PATCH] USB Audio Class 2 Mixer unit support for GET_CUR, SET_CUR and RANGE

[Takashi Iwai <tiwai@suse.de>]

On Wed, 16 Sep 2026 09:43:14 +0200,
Zipdox wrote:
> 
> On 9/15/26 4:19 PM, Takashi Iwai wrote:
> > But your code also blindly assumes the MU_MIXER_CONTROL that doesn't
> > fit with other units like feature unit?  I guess we have to set the
> > proper Control Selector value depending on the unit type.
> > 
> > After all, this is about the correct setup of wValue.  Let's try to be
> > minimalistic at first -- just tweak wValue in get_ctl_value_v2() and
> > snd_usb_mixer_set_ctl_value() at first.  If this becomes too ugly, we
> > can think of redesigning.
> I assumed that mixer.c was just for the mixer unit, and I seem to have
> completely overlooked the fact that that it also controls feature
> units. I'm not sure how to proceed from here. Do you have any
> suggestions for how we can store the control selector in the
> usb_mixer_elem_info struct? The control field is presently used as the
> input channel number, so maybe we should add a separate field for the
> input channel? I think maybe it's better that someone else fixes this,
> I'm kind of out of my element here.

I took a look back at the current code, and at least, the parse of
UAC2 mixer unit itself is OK -- it handles the bitmap properly.
The problem is the exceptional way to set up wValue for the mixer
unit, and I guess something like below could work.  It just stores the
part of wValue in an additional field, and use it for calculating the
correct wValue with MCN + UAC2_MU_MIXER in high byte.

Just compile tested, so not 100% sure whether it's correct.


thanks,

Takashi

--- a/sound/usb/mixer.c
+++ b/sound/usb/mixer.c
@@ -347,6 +347,10 @@ static int get_ctl_value_v2(struct usb_mixer_elem_info *cval, int request,
 
 	val_size = uac2_ctl_value_size(cval->val_type);
 
+	/* correct wValue for UAC2 mixer control with MCN */
+	if (cval->v2_mixer)
+		validx = cval->v2_mixer | ((validx & 0xff) - 1);
+
 	if (request == UAC_GET_CUR) {
 		bRequest = UAC2_CS_CUR;
 		size = val_size;
@@ -2345,6 +2349,10 @@ static void build_mixer_unit_ctl(struct mixer_build *state,
 
 	snd_usb_mixer_elem_init_std(&cval->head, state->mixer, unitid);
 	cval->control = in_ch + 1; /* based on 1 */
+	/* MCN is calculated later with the input channel based on v2_mixer */
+	if (state->mixer->protocol == UAC_VERSION_2 ||
+	    state->mixer->protocol == UAC_VERSION_3)
+		cval->v2_mixer = (UAC2_MU_MIXER << 8) | (in_ch * num_outs);
 	cval->val_type = USB_MIXER_S16;
 	for (i = 0; i < num_outs; i++) {
 		__u8 *c = uac_mixer_unit_bmControls(desc, state->mixer->protocol);
diff --git a/sound/usb/mixer.h b/sound/usb/mixer.h
index 037b446d8b6f..650f30026177 100644
--- a/sound/usb/mixer.h
+++ b/sound/usb/mixer.h
@@ -97,6 +97,7 @@ struct usb_mixer_elem_info {
 	u8 initialized;
 	u8 min_mute;
 	u8 get_cur_broken;
+	u16 v2_mixer;
 	void *private_data;
 };