From: Takashi Iwai <tiwai@suse.de>
Subject: Revive usb-audio Keep Interface mixer
Patch-mainline: Never, only for compatibility
References: bsc#1181014

It seems that the lack of the old mixer element "Keep Interface"
confuses alsactl and others.  Revive the dummy mixer element again
just for compatibility.

Signed-off-by: Takashi Iwai <tiwai@suse.de>

---
 sound/usb/mixer.c |   38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

--- a/sound/usb/mixer.c
+++ b/sound/usb/mixer.c
@@ -3508,6 +3508,40 @@ static int snd_usb_mixer_status_create(s
 	return 0;
 }
 
+/* Dummy mixer ctl just for compatibility */
+static int keep_iface_ctl_get(struct snd_kcontrol *kcontrol,
+			      struct snd_ctl_elem_value *ucontrol)
+{
+	return 0;
+}
+
+static int keep_iface_ctl_put(struct snd_kcontrol *kcontrol,
+			      struct snd_ctl_elem_value *ucontrol)
+{
+	return 0;
+}
+
+static const struct snd_kcontrol_new keep_iface_ctl = {
+	.iface = SNDRV_CTL_ELEM_IFACE_CARD,
+	.name = "Keep Interface",
+	.info = snd_ctl_boolean_mono_info,
+	.get = keep_iface_ctl_get,
+	.put = keep_iface_ctl_put,
+};
+
+static int create_keep_iface_ctl(struct usb_mixer_interface *mixer)
+{
+	struct snd_kcontrol *kctl = snd_ctl_new1(&keep_iface_ctl, mixer);
+
+	/* need only one control per card */
+	if (snd_ctl_find_id(mixer->chip->card, &kctl->id)) {
+		snd_ctl_free_one(kctl);
+		return 0;
+	}
+
+	return snd_ctl_add(mixer->chip->card, kctl);
+}
+
 int snd_usb_create_mixer(struct snd_usb_audio *chip, int ctrlif)
 {
 	static const struct snd_device_ops dev_ops = {
@@ -3559,6 +3593,10 @@ int snd_usb_create_mixer(struct snd_usb_
 	if (err < 0)
 		goto _error;
 
+	err = create_keep_iface_ctl(mixer);
+	if (err < 0)
+		goto _error;
+
 	err = snd_usb_mixer_apply_create_quirk(mixer);
 	if (err < 0)
 		goto _error;