[alsa-devel] [PATCH] ALSA: hda - hdmi: Fallback to ALSA allocation when selecting CA

Anssi Hannula anssi.hannula at iki.fi
Sun Sep 1 13:36:47 CEST 2013

hdmi_channel_allocation() tries to find a HDMI channel allocation that
matches the number channels in the playback stream and contains only
speakers that the HDMI sink has reported as available via EDID. If no
such allocation is found, 0 (stereo audio) is used.

Using CA 0 causes the audio causes the sink to discard everything except
the first two channels (front left and front right).

However, the sink may be capable of receiving more channels than it has
speakers (and then perform downmix or discard the extra channels), in
which case it is preferable to use a CA that contains extra channels
than to use CA 0 which discards all the non-stereo channels.

Additionally, it seems that HBR (HD) passthrough output does not work on
Intel HDMI codecs when CA is set to 0 (possibly the codec zeroes
channels not present in CA). This happens with all receivers that report
a 5.1 speaker mask since a HBR stream is carried on 8 channels to the

Add a fallback in the CA selection so that the CA channel count at least
matches the stream channel count, even if the stream contains channels
not present in the sink speaker descriptor.

Thanks to GrimGriefer at OpenELEC forums for discovering that changing
the sink speaker mask allowed HBR output.

Reported-by: GrimGriefer
Reported-by: Ashecrow
Reported-by: Frank Zafka <kafkaesque1978 at gmail.com>
Reported-by: Peter Frühberger <fritsch at xbmc.org>
Signed-off-by: Anssi Hannula <anssi.hannula at iki.fi>
Cc: <stable at vger.kernel.org>

Hopefully this fixes HBR (HD passthrough) for the remaining Intel users
who were still experiencing problems.

BTW, the hdmi_channel_allocation() logic seems also otherwise somewhat
suspect to me. Shouldn't we always select an allocation matching ALSA
channel mapping, instead of re-assigning channels received from
userspace "randomly" to sink speakers (in case of unusual sink speaker
Anyway, I let it be for now at least.

 sound/pci/hda/patch_hdmi.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/sound/pci/hda/patch_hdmi.c b/sound/pci/hda/patch_hdmi.c
index 030ca86..354fc55 100644
--- a/sound/pci/hda/patch_hdmi.c
+++ b/sound/pci/hda/patch_hdmi.c
@@ -551,6 +551,17 @@ static int hdmi_channel_allocation(struct hdmi_eld *eld, int channels)
+	if (!ca) {
+		/* if there was no match, select the regular ALSA channel
+		 * allocation with the matching number of channels */
+		for (i = 0; i < ARRAY_SIZE(channel_allocations); i++) {
+			if (channels == channel_allocations[i].channels) {
+				ca = channel_allocations[i].ca_index;
+				break;
+			}
+		}
+	}
 	snd_print_channel_allocation(eld->info.spk_alloc, buf, sizeof(buf));
 	snd_printdd("HDMI: select CA 0x%x for %d-channel allocation: %s\n",
 		    ca, channels, buf);

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