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 17:40:53 +0200,
Takashi Iwai wrote:
> 
> 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.

... and this version was obviously half-baked, forgotten the
counterpart.  A revised patch is below.


Takashi

-- 8< --
diff --git a/sound/usb/mixer.c b/sound/usb/mixer.c
index 33a6a1281410..0977fc58d0b5 100644
--- 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;
@@ -478,6 +482,10 @@ int snd_usb_mixer_set_ctl_value(struct usb_mixer_elem_info *cval,
 		}
 
 		request = UAC2_CS_CUR;
+
+		/* correct wValue for UAC2 mixer control with MCN */
+		if (cval->v2_mixer)
+			validx = cval->v2_mixer | ((validx & 0xff) - 1);
 	}
 
 	value_set = convert_bytes_value(cval, value_set);
@@ -2345,6 +2353,10 @@ 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;
 };