I'm not aware that RK hardware supports 10-bit output at the moment.
In any case, I noticed it only on 4K 10-bit HDR HEVC content. I could probably narrow down the exact cause but I'll have to dig through my library to find appropriate examples.
Posts by danijelt
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After additional testing, I can't reproduce the HDMI bootup issue again, so I'm writing it off as a bad connection at that moment.
Regarding signal loss when starting playback, it actually happens only when playing 4K 10-bit HEVC HDR content, so I guess it's just due to setting up video mode.
I have created the pull request: https://github.com/LibreELEC/LibreELEC.tv/pull/7570
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I was able to port LibreELEC to Orange Pi 4 LTS by adding patches for YT8531 and OPi 4 LTS device tree (they can be found here, in sources for manjaro-arm).
It generally works, 10-bit 4K HEVC plays fine, Atmos 7.1 sound is sent via HDMI, CEC controls work, but there are some issues with HDMI video output, mainly when booting up it can take some time for the TV/monitor to detect signal, and a brief loss of signal when starting and stopping playback. Are such issues known on other RK3399 boards?
Also, is there some documentation about the build system (except for the basics of make/clean) and guidelines for adding support for a new device? Would such contribution be accepted? I see that other RK3399 devices are the ones supported in the mainline kernel.