[PATCH] ALSA: usb-audio: Drop bogus dB range in too low level

[Takashi Iwai]

Some USB audio firmware seem to report broken dB values for the volume
controls, and this screws up applications like PulseAudio who blindly
trusts the given data.  For example, Edifier G2000 reports a PCM
volume from -128dB to -127dB, and this results in barely inaudible
sound.

This patch adds a sort of sanity check at parsing the dB values in
USB-audio driver and disables the dB reporting if the range looks
bogus.  Here, we assume -96dB as the bottom line of the max dB.

Note that, if one can figure out that proper dB range later, it can be
patched in the mixer maps.

BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=211929
Cc: <stable at vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai at suse.de>
---
 sound/usb/mixer.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/sound/usb/mixer.c b/sound/usb/mixer.c
index b1c78db0d470..b004b2e63a5d 100644
--- a/sound/usb/mixer.c
+++ b/sound/usb/mixer.c
@@ -1307,6 +1307,17 @@ static int get_min_max_with_quirks(struct usb_mixer_elem_info *cval,
 			/* totally crap, return an error */
 			return -EINVAL;
 		}
+	} else {
+		/* if the max volume is too low, it's likely a bogus range;
+		 * here we use -96dB as the threshold
+		 */
+		if (cval->dBmax <= -9600) {
+			usb_audio_info(cval->head.mixer->chip,
+				       "%d:%d: bogus dB values (%d/%d), disabling dB reporting\n",
+				       cval->head.id, mixer_ctrl_intf(cval->head.mixer),
+				       cval->dBmin, cval->dBmax);
+			cval->dBmin = cval->dBmax = 0;
+		}
 	}
 
 	return 0;
-- 
2.26.2