[alsa-devel] [RFC/RFT] HDMI/DP CEA-861-E+ channel allocations 0x20+

Anssi Hannula anssi.hannula at iki.fi
Fri Nov 1 22:43:12 CET 2013

Hi all!

Currently we allow the use of CA values up to 0x31 as defined in
CEA-861-E/F. However, only CA values up to 0x19 are defined in

HDMI specification 1.4b specifies that the CA field is to be filled
according to CEA-861-D, and DisplayPort 1.1a according to CEA-861-C.

The ELD (EDID-Like Data) format as specified by Intel HDA specification
1.0a has a speaker allocation bitmask that only accommodates speakers
present in CEA-861-D; all of the 0x20+ CAs contain speakers that do not
have a corresponding bit in ELD.

Using a CA value unsupported by sink will cause either a completely
silent output or stereo output, so I think we should try to prevent
selecting such channel maps, if feasible.

However, before doing anything, I wonder if they are actually supported
by some newer receivers (mine is 4 years old). It'd be good if someone
with a newish receiver could try the below :)
To test this, one can run (replacing XX and YY with appropriate values
from "aplay -L") on *sound git master*:
speaker-test -c6 -Dhdmi:CARD=XX,DEV=YY -m FL,FR,RL,RR,FLH,FRH
If you get some output for the RL/RR speakers, that should mean that
0x20+ CAs are supported. If there is no output except on FL/FR and there
is proper output without the "-m FL,FR,RL,RR,FLH,FRH", this means that
0x20+ CAs are not supported.

I think there are about these options for us to take:
a) drop 0x20+ CAs from channel_allocations altogether
b) put the 0x20+ CAs under a module parameter
c) only allow 0x20+ CAs if any CEA-861-E+ only speakers are specified
   in EDID. However, as ELD doesn't have bits for these, we'd have to
   employ some non-standard bits in ELD or communicate directly with
   video driver.
d) do nothing, allow the 0x20+ CAs.

IMHO we should do something, since players using ALSA channel mapping
could just automatically select a manual channel map that uses an
unsupported 0x20+ CA if the source audio stream contains such channels...


Anssi Hannula

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