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

[Zipdox]

On 9/28/26 12:59 PM, Takashi Iwai wrote:
> I thought of it, too.  OTOH, it'll be a bit tricky, because the range
> isn't usually covering -32767 but only a part.  So -32768 won't be
> reached continuously.
>
> I'm inclined to have a simple mute switch in addition.
>
>
> thanks,
>
> Takashi

Actually I think this seems to work. We might have to do something with
dBmin and dBmax as well though, I don't fully understand their function.

---
 sound/usb/mixer.c | 26 +++++++++++++++++++++-----
 sound/usb/mixer.h |  2 +-
 2 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/sound/usb/mixer.c b/sound/usb/mixer.c
index 91d4c7373871..1653bb47ed6c 100644
--- a/sound/usb/mixer.c
+++ b/sound/usb/mixer.c
@@ -358,7 +358,7 @@ static int request_ctl_v2(struct usb_mixer_elem_info *cval, u8 request,
 		return -EIO;
 
 	/* correct wValue for UAC2 mixer control with MCN */
-	if (cval->v2_mixer)
+	if (cval->mixer_unit == 2)
 		validx = (UAC2_MU_MIXER << 8) | to_mcn(cval, validx);
 
 	memset(buf, 0, size);
@@ -400,9 +400,14 @@ static int get_cur_ctl_value(struct usb_mixer_elem_info *cval,
 {
 	validx += cval->idx_off;
 
-	return (cval->head.mixer->protocol == UAC_VERSION_1) ?
+	int value = (cval->head.mixer->protocol == UAC_VERSION_1) ?
 		request_ctl_v1(cval, UAC_GET_CUR, validx, value_ret) :
 		get_ctl_value_v2(cval, validx, value_ret);
+
+	/* Map -32768 (mute in mixer unit spec) to minimum */
+	if(cval->mixer_unit && value == -32768) value = cval->min;
+
+	return value;
 }
 
 /* channel = 0: master, 1 = first channel */
@@ -539,11 +544,16 @@ 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)
+		/* correct wValue for UAC2 mixer unit control with MCN */
+		if (cval->mixer_unit == 2)
 			validx = (UAC2_MU_MIXER << 8) | to_mcn(cval, validx);
 	}
 
+	/* Remap the lowest value to -32768 (mute per the spec for mixer units) */
+	if(cval->mixer_unit && value_set < (cval->min + cval->res)){
+		value_set = -32768;
+	}
+
 	value_set = convert_bytes_value(cval, value_set);
 	buf[0] = value_set & 0xff;
 	buf[1] = (value_set >> 8) & 0xff;
@@ -2390,7 +2400,11 @@ static void build_mixer_unit_ctl(struct mixer_build *state,
 	cval->control = in_ch + 1; /* based on 1 */
 	if (state->mixer->protocol == UAC_VERSION_2 ||
 	    state->mixer->protocol == UAC_VERSION_3)
-		cval->v2_mixer = true;
+	{
+		cval->mixer_unit = 2;
+	}else{
+		cval->mixer_unit = 1;
+	}
 	cval->num_outputs = num_outs;
 	cval->val_type = USB_MIXER_S16;
 	for (i = 0; i < num_outs; i++) {
@@ -2408,6 +2422,8 @@ static void build_mixer_unit_ctl(struct mixer_build *state,
 		usb_mixer_elem_info_free(cval);
 		return;
 	}
+	/* We will map the lowest setting to mute (-32768 per the spec) */
+	cval->min -= cval->res;
 
 	kctl = snd_ctl_new1(&usb_feature_unit_ctl, cval);
 	if (!kctl) {
diff --git a/sound/usb/mixer.h b/sound/usb/mixer.h
index cf45c39cbccc..2ace9b95da0a 100644
--- a/sound/usb/mixer.h
+++ b/sound/usb/mixer.h
@@ -98,7 +98,7 @@ struct usb_mixer_elem_info {
 	u8 min_mute;
 	u8 get_cur_broken;
 	u8 num_outputs;
-	bool v2_mixer;
+	int mixer_unit;
 	void *private_data;
 };
 
--
2.47.3