ALSA: usb-audio: Assume first control interface is for audio

For devices with more than one control interface, let's assume the first
one contains the audio controls. Unfortunately, there is no field in any
of the descriptors to tell us whether a control interface is for audio
or MIDI controls, so a better check is not easy to implement.

On a composite device with audio and MIDI functions, for example, the
code currently overwrites chip->ctrl_intf, causing operations on the
control interface to fail if they are issued after the device probe.

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

diff --git a/sound/usb/card.c b/sound/usb/card.c
index 9feb00c831a02b791227b75294e6dc8e15975f1f..b443a33d31c98d9ce78b8be8a84f5687b5e6d8fa 100644 (file)
--- a/sound/usb/card.c
+++ b/sound/usb/card.c
@@ -465,7 +465,13 @@ static void *snd_usb_audio_probe(struct usb_device *dev,
                        goto __error;
        }
 
-       chip->ctrl_intf = alts;
+       /*
+        * For devices with more than one control interface, we assume the
+        * first contains the audio controls. We might need a more specific
+        * check here in the future.
+        */
+       if (!chip->ctrl_intf)
+               chip->ctrl_intf = alts;
 
        if (err > 0) {
                /* create normal USB audio interfaces */