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 CEA-861-B/C/D.
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...
WDYT?