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>

1 files changed, 6 insertions, 4 deletions

diff --git a/sound/usb/mixer.c b/sound/usb/mixer.c
index 5e4775716607..c8c28cd75534 100644
--- 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;