ALSA: usb-audio: fix feature unit parser for UAC2

Fix a small off-by-one bug which causes the feature unit to announce a
wrong number of channels. This leads to illegal requests sent to the
firmware eventually.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/usb/mixer.c b/sound/usb/mixer.c
index 97dd17655104a2bfbde0db95b6b6cab1c61d164f..03ce971e002760e84b2d36565ee0cbfa56bebd50 100644 (file)
--- a/sound/usb/mixer.c
+++ b/sound/usb/mixer.c
@@ -1126,7 +1126,7 @@ static int parse_audio_feature_unit(struct mixer_build *state, int unitid, void
        } else {
                struct uac2_feature_unit_descriptor *ftr = _ftr;
                csize = 4;
-               channels = (hdr->bLength - 6) / 4;
+               channels = (hdr->bLength - 6) / 4 - 1;
                bmaControls = ftr->bmaControls;
        }