ALSA: usb-audio - Don't expose broken dB ranges

Some crappy USB-audio devices give broken dB ranges, e.g. both min and max
are 0dB.  This confuses the volume control that prefers dB expression such
as alsactl or PulseAudio.  In such a case, it's much better not to expose
the broken dB information.

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

diff --git a/sound/usb/mixer.c b/sound/usb/mixer.c
index 5e47757166074833159007d725d6dd2ede025bfd..c8c28cd755348c63746e54ee654639299fc45dc2 100644 (file)
--- a/sound/usb/mixer.c
+++ b/sound/usb/mixer.c
@@ -1097,11 +1097,13 @@ static void build_feature_ctl(struct mixer_build *state, void *raw_desc,
                append_ctl_name(kctl, control == UAC_FU_MUTE ?
                                " Switch" : " Volume");
                if (control == UAC_FU_VOLUME) {
-                       kctl->tlv.c = mixer_vol_tlv;
-                       kctl->vd[0].access |= 
-                               SNDRV_CTL_ELEM_ACCESS_TLV_READ |
-                               SNDRV_CTL_ELEM_ACCESS_TLV_CALLBACK;
                        check_mapped_dB(map, cval);
+                       if (cval->dBmin < cval->dBmax) {
+                               kctl->tlv.c = mixer_vol_tlv;
+                               kctl->vd[0].access |= 
+                                       SNDRV_CTL_ELEM_ACCESS_TLV_READ |
+                                       SNDRV_CTL_ELEM_ACCESS_TLV_CALLBACK;
+                       }
                }
                break;