[PATCH 4.9 10/27] ALSA: usb-audio: Add mute TLV for playback volumes on C-Media devices

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

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

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

From: Takashi Iwai <tiwai@suse.de>

commit 0f174b3525a43bd51f9397394763925e0ebe7bc7 upstream.

C-Media devices (at least some models) mute the playback stream when
volumes are set to the minimum value.  But this isn't informed via TLV
and the user-space, typically PulseAudio, gets confused as if it's
still played in a low volume.

This patch adds the new flag, min_mute, to struct usb_mixer_elem_info
for indicating that the mixer element is with the minimum-mute volume.
This flag is set for known C-Media devices in
snd_usb_mixer_fu_apply_quirk() in turn.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=196669
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 sound/usb/mixer.c        |    2 ++
 sound/usb/mixer.h        |    1 +
 sound/usb/mixer_quirks.c |    6 ++++++
 3 files changed, 9 insertions(+)

--- a/sound/usb/mixer.c
+++ b/sound/usb/mixer.c
@@ -542,6 +542,8 @@ int snd_usb_mixer_vol_tlv(struct snd_kco
 
 	if (size < sizeof(scale))
 		return -ENOMEM;
+	if (cval->min_mute)
+		scale[0] = SNDRV_CTL_TLVT_DB_MINMAX_MUTE;
 	scale[2] = cval->dBmin;
 	scale[3] = cval->dBmax;
 	if (copy_to_user(_tlv, scale, sizeof(scale)))
--- a/sound/usb/mixer.h
+++ b/sound/usb/mixer.h
@@ -64,6 +64,7 @@ struct usb_mixer_elem_info {
 	int cached;
 	int cache_val[MAX_CHANNELS];
 	u8 initialized;
+	u8 min_mute;
 	void *private_data;
 };
 
--- a/sound/usb/mixer_quirks.c
+++ b/sound/usb/mixer_quirks.c
@@ -1873,6 +1873,12 @@ void snd_usb_mixer_fu_apply_quirk(struct
 		if (unitid == 7 && cval->control == UAC_FU_VOLUME)
 			snd_dragonfly_quirk_db_scale(mixer, cval, kctl);
 		break;
+	/* lowest playback value is muted on C-Media devices */
+	case USB_ID(0x0d8c, 0x000c):
+	case USB_ID(0x0d8c, 0x0014):
+		if (strstr(kctl->id.name, "Playback"))
+			cval->min_mute = 1;
+		break;
 	}
 }