Rockchip released an updated 4.19 kernel here
GitHub - rockchip-linux/kernel at develop-4.19
Perhaps users still building LE with the 4.4 kernel can get it booting?
Because this kernel is also used for Android-Q things like HDR, 4K, 10-bit color, H265, VP9 might work out of the box just like in the 4.4 kernel.
There is also a commit for HDMI Bitstream audio, perhaps it can help to get HD audio passthrough working?
drm: bridge/dw-hdmi: add support for hdmi bitstream audio · rockchip-linux/kernel@924f480 · GitHub
Another benefit of the 4.19 kernel is PS4 bluetooth controllers working now.
It might still take a long time until all features work in mainline so this might be a stopover for some developers and work better than the 4.4 kernel for now?
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And for the mainline kernel, user jernejsk made a patch based on the Raspberry Pi 4 HBR audio passthrough to get it working on Allwinner devices.
Allwinner: WIP audio passthrough · jernejsk/LibreELEC.tv@87cbddb · GitHub
Because Rockchip and Allwinner devices use the same HDMI hardware, someone might get it working on Rockchip devices too or can work with him to test it on Rockchip devices?