[PATCH 3.18 10/25] ALSA: usb-audio: Skip broken EU on Dell dock USB-audio

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

3.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Takashi Iwai <tiwai@suse.de>

commit 1d8d6428d1da642ddd75b0be2d1bb1123ff8e017 upstream.

The Dell Dock USB-audio device with 0bda:4014 is behaving notoriously
bad, and we have already applied some workaround to avoid the firmware
hiccup.  Yet we still need to skip one thing, the Extension Unit at ID
4, which doesn't react correctly to the mixer ctl access.

Bugzilla: https://bugzilla.suse.com/show_bug.cgi?id=1090658
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 sound/usb/mixer_maps.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/sound/usb/mixer_maps.c
+++ b/sound/usb/mixer_maps.c
@@ -353,8 +353,11 @@ static struct usbmix_name_map dragonfly_
 /*
  * Dell usb dock with ALC4020 codec had a firmware problem where it got
  * screwed up when zero volume is passed; just skip it as a workaround
+ *
+ * Also the extension unit gives an access error, so skip it as well.
  */
 static const struct usbmix_name_map dell_alc4020_map[] = {
+	{ 4, NULL },	/* extension unit */
 	{ 16, NULL },
 	{ 19, NULL },
 	{ 0 }