From 0dca01c37a68017fe7112b46bc4b48c927db18c6 Mon Sep 17 00:00:00 2001
From: David Henningsson <david.henningsson@canonical.com>
Date: Tue, 5 Nov 2013 04:41:06 +0100
Subject: ALSA: usb: supply channel maps even when wChannelConfig is
 unspecified

If wChannelconfig is given for some formats but not others, userspace
might not be able to set the channel map.

This is RFC because I'm not sure what the best behaviour is - to guess
the channel map from the given number of channels (it's quite likely
that one channel is MONO and two channels is FL FR), or just to supply
UNKNOWN for all channels.

But the complete lack of channel map for a format leads userspace to
believe that the format is not available at all. Or am I
misunderstanding how this should be used?

Signed-off-by: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/usb/stream.c b/sound/usb/stream.c
index c4339f9..b43b6ee 100644
--- a/sound/usb/stream.c
+++ b/sound/usb/stream.c
@@ -281,8 +281,6 @@ static struct snd_pcm_chmap_elem *convert_chmap(int channels, unsigned int bits,
 	const unsigned int *maps;
 	int c;
 
-	if (!bits)
-		return NULL;
 	if (channels > ARRAY_SIZE(chmap->map))
 		return NULL;
 
@@ -293,9 +291,19 @@ static struct snd_pcm_chmap_elem *convert_chmap(int channels, unsigned int bits,
 	maps = protocol == UAC_VERSION_2 ? uac2_maps : uac1_maps;
 	chmap->channels = channels;
 	c = 0;
-	for (; bits && *maps; maps++, bits >>= 1) {
-		if (bits & 1)
-			chmap->map[c++] = *maps;
+
+	if (bits) {
+		for (; bits && *maps; maps++, bits >>= 1)
+			if (bits & 1)
+				chmap->map[c++] = *maps;
+	} else {
+		/* If we're missing wChannelConfig, then guess something
+		    to make sure the channel map is not skipped entirely */
+		if (channels == 1)
+			chmap->map[c++] = SNDRV_CHMAP_MONO;
+		else
+			for (; c < channels && *maps; maps++)
+				chmap->map[c++] = *maps;
 	}
 
 	for (; c < channels; c++)
-- 
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