MRFIXIT Rockchip Kernel:
This is a fork of Rockchip's BSP kernel, located here: https://github.com/rockchip-linux/kernel
This fork has lots of patches, mainline backports, and custom fixes.
All work has specifically been to support these boards in MrFixIt's releases (found in his other GitHub repositories): RockPi4, Rock64, Rockbox, Pinebook Pro, and Rockpro64