ALSA: usb-audio: add mono main switch to Presonus S1824c

The 1824c does not have the A/B switch that the 1810c has,
but instead it has a mono main switch that sums the two
main output channels to mono.

Signed-off-by: Roy Vegard Ovesen <roy.vegard.ovesen@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Author: royvegard

sound/usb/mixer_s1810c.c

@@ -93,6 +93,7 @@ struct s1810c_ctl_packet {
 
 #define SC1810C_CTL_LINE_SW	0
 #define SC1810C_CTL_MUTE_SW	1
+#define SC1824C_CTL_MONO_SW	2
 #define SC1810C_CTL_AB_SW	3
 #define SC1810C_CTL_48V_SW	4
 
@@ -123,6 +124,7 @@ struct s1810c_state_packet {
 #define SC1810C_STATE_48V_SW	58
 #define SC1810C_STATE_LINE_SW	59
 #define SC1810C_STATE_MUTE_SW	60
+#define SC1824C_STATE_MONO_SW	61
 #define SC1810C_STATE_AB_SW	62
 
 struct s1810_mixer_state {
@@ -502,6 +504,15 @@ static const struct snd_kcontrol_new snd_s1810c_mute_sw = {
 	.private_value = (SC1810C_STATE_MUTE_SW | SC1810C_CTL_MUTE_SW << 8)
 };
 
+static const struct snd_kcontrol_new snd_s1824c_mono_sw = {
+	.iface = SNDRV_CTL_ELEM_IFACE_MIXER,
+	.name = "Mono Main Out Switch",
+	.info = snd_ctl_boolean_mono_info,
+	.get = snd_s1810c_switch_get,
+	.put = snd_s1810c_switch_set,
+	.private_value = (SC1824C_STATE_MONO_SW | SC1824C_CTL_MONO_SW << 8)
+};
+
 static const struct snd_kcontrol_new snd_s1810c_48v_sw = {
 	.iface = SNDRV_CTL_ELEM_IFACE_MIXER,
 	.name = "48V Phantom Power On Mic Inputs Switch",
@@ -588,8 +599,17 @@ int snd_sc1810_init_mixer(struct usb_mixer_interface *mixer)
 	if (ret < 0)
 		return ret;
 
-	ret = snd_s1810c_switch_init(mixer, &snd_s1810c_ab_sw);
-	if (ret < 0)
-		return ret;
+	// The 1824c has a Mono Main switch instead of a
+	// A/B select switch.
+	if (mixer->chip->usb_id == USB_ID(0x194f, 0x010d)) {
+		ret = snd_s1810c_switch_init(mixer, &snd_s1824c_mono_sw);
+		if (ret < 0)
+			return ret;
+	} else if (mixer->chip->usb_id == USB_ID(0x194f, 0x010c)) {
+		ret = snd_s1810c_switch_init(mixer, &snd_s1810c_ab_sw);
+		if (ret < 0)
+			return ret;
+	}
+
 	return ret;
 }