ALSA: usb: supply channel maps even when wChannelConfig is unspecified (0dca01c3) · Commits · 戴 / test

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>

sound/usb/stream.c

+13 −5

Original line number	Diff line number	Diff line
		@@ -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) {
		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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