Summary

Fixes mono audio output on RK3576 NanoPi R76S and M5 boards when using HDMI output. The issue was caused by the HDMI codec driver incorrectly advertising capture capability on output-only hardware.

Problem

Users experience mono audio (single channel) when connecting HDMI displays, with kernel errors:
hdmi-audio-codec: Only one simultaneous stream supported!
ASoC: error at snd_soc_dai_startup on i2s-hifi: -22

Root Cause Analysis

Through detailed debug logging, we identified that:

1. The hdmi-codec driver advertises both playback and capture capabilities
2. HDMI-TX hardware is output-only - it should NOT support capture
3. PulseAudio detects capture support and attempts to open capture streams
4. When capture stream opens while playback is active, the codec's busy flag check rejects it
5. This causes audio initialization to partially fail, resulting in mono output

Debug log showing the issue:
[36.916883] startup: stream=0 (PLAYBACK) → SUCCESS
[36.924680] startup: stream=1 (CAPTURE) → FAILS (busy conflict)
Result: Only one stream active = mono audio

Solution

Commit 1: DTS cleanup

• Disable unused spdif_tx3 interface on R76S (no DisplayPort)

Commit 2: Driver fix

• Disable capture capability in hdmi-codec.c by setting channels_min/max = 0
• Prevents PulseAudio from attempting invalid capture operations
• Allows proper stereo playback initialization

Testing

Before: Mono audio + kernel errors
After: Stereo audio, no errors

# Verified on NanoPi R76S with HDMI output
speaker-test -c 2 -t wav -l 1  # Both channels working ✅

dmesg after fix:
[36.916883] startup: stream=0 (PLAYBACK) → SUCCESS
[36.924680] startup: stream=0 (PLAYBACK) → SUCCESS
No capture attempts, no errors ✅