[PATCH] ALSA: usb-audio: ignore broken processing/extension unit

[tom.ty89 at gmail.com]

From: Tom Yan <tom.ty89 at gmail.com>

Some devices have broken extension unit where getting current value doesn't work. Attempt that once when creating mixer control for it. If it fails, just ignore it, so that it won't cripple the device entirely (and/or make the error floods).
---
 sound/usb/mixer.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/sound/usb/mixer.c b/sound/usb/mixer.c
index eab0fd4fd7c3..e0b7174c1043 100644
--- a/sound/usb/mixer.c
+++ b/sound/usb/mixer.c
@@ -2367,7 +2367,7 @@ static int build_audio_procunit(struct mixer_build *state, int unitid,
 	int num_ins;
 	struct usb_mixer_elem_info *cval;
 	struct snd_kcontrol *kctl;
-	int i, err, nameid, type, len;
+	int i, err, nameid, type, len, val;
 	const struct procunit_info *info;
 	const struct procunit_value_info *valinfo;
 	const struct usbmix_name_map *map;
@@ -2470,6 +2470,12 @@ static int build_audio_procunit(struct mixer_build *state, int unitid,
 			break;
 		}
 
+		err = get_cur_ctl_value(cval, cval->control << 8, &val);
+		if (err < 0) {
+			usb_mixer_elem_info_free(cval);
+			return -EINVAL;
+		}
+
 		kctl = snd_ctl_new1(&mixer_procunit_ctl, cval);
 		if (!kctl) {
 			usb_mixer_elem_info_free(cval);
-- 
2.28.0