ALSA: usb-audio - Add volume range check and warn if it too big

Signed-off-by: Alexey Fisher <bug-track@fisher-privat.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/mixer.c

@@ -987,6 +987,7 @@ static void build_feature_ctl(struct mixer_build *state, void *raw_desc,
 	struct snd_kcontrol *kctl;
 	struct usb_mixer_elem_info *cval;
 	const struct usbmix_name_map *map;
+	unsigned int range;
 
 	control++; /* change from zero-based to 1-based value */
 
@@ -1136,6 +1137,21 @@ static void build_feature_ctl(struct mixer_build *state, void *raw_desc,
 
 	}
 
+	range = (cval->max - cval->min) / cval->res;
+	/* Are there devices with volume range more than 255? I use a bit more
+	 * to be sure. 384 is a resolution magic number found on Logitech
+	 * devices. It will definitively catch all buggy Logitech devices.
+	 */
+	if (range > 384) {
+		snd_printk(KERN_WARNING "usb_audio: Warning! Unlikely big "
+			   "volume range (=%u), cval->res is probably wrong.",
+			   range);
+		snd_printk(KERN_WARNING "usb_audio: [%d] FU [%s] ch = %d, "
+			   "val = %d/%d/%d", cval->id,
+			   kctl->id.name, cval->channels,
+			   cval->min, cval->max, cval->res);
+	}
+
 	snd_printdd(KERN_INFO "[%d] FU [%s] ch = %d, val = %d/%d/%d\n",
 		    cval->id, kctl->id.name, cval->channels, cval->min, cval->max, cval->res);
 	add_control_to_empty(state, kctl);