[pulseaudio-discuss] review+pull-request: Passthrough support

[pl bossart]

> Yes. The C bits are usually ignored by the receiver since a lot of
> sources don't configure them well. But it turns out that in this case
> setting the non-audio bit is a mandatory setting.

I tried to implement this audio/non-audio switch by playing with ALSA
controls, however there are a couple of problems:
- the non-audio mode can only be set when the device is closed. There
are some protections in alsa-lib, meaning we would need to set this
mode on resume and go back to audio mode on suspend.
- it took me a while to realize that everything I programmed using
alsa controls (snd_ctl*) or configured with iecset on a command line
was just overwritten with default values when playback starts. the
alsa-lib hdmi/iec958 plugin resets all the values that you carefully
programmed with snd_ctl* calls, you really have to pass an argument to
this device when it is opened. Boils down to adding ",AES0=0x06" to
the device name in passthrough mode.

>> Is the ALSA device correctly being configured to 192 kHz?
>
> I will have to recheck for these two cases. I guess it is since I can
> see the D+ logo, but let me confirm for these two cases.

The frequency is correctly set:

more /proc/asound/card0/pcm3p/sub0/hw_params
access: MMAP_INTERLEAVED
format: S16_LE
subformat: STD
channels: 2
rate: 192000 (192000/1)
period_size: 8192
buffer_size: 16384