Hi.
Is mainline support for U-Boot planned for 3C ?
The same goes for 6.x kernel ?
Is DTS from 5.x line suitable for use with 6.x ?
ps. why there’s no subcategory for 3C ? 
Mainline U-Boot / kernel support for ROCK 3C
Is DTS from 5.x line suitable for use with 6.x ?
You have to write a devicetree for 6.x mainline kernel.
As @amazingfate mention, someone first have to upstream a device tree for ROCK 3C to mainline linux.
After it has been accepted and merged into linux-rockchip maintainer tree, that device tree can then be used/imported to create a proper target in mainline U-Boot.
Fortunately the Quartz64-B schematics is very similar to ROCK 3C, I use this device tree for my mainline ROCK 3C needs 
See https://github.com/Kwiboo/u-boot-rockchip/commit/06caaa0f8cc6f10d6818da0d02f43c0841b8ca83
Why Radxa can’t write proper dts file for their product?
Hello? Any answer?
They already sold you hardware where you can boot something. Mission accomplished.
I’ve used the mainline kernel build from minimyth2 on a rock 5a for a long time.
There’s now a dts for the rock 3c as well. Just finished booting 6.5.7 on a rock 3c finally to get some newer device drivers.
What distro did you try? Were you able to get internet on ethernet port or via wifi? Bc I used this device tree on Slarm64 and Manjaro, and I can’t get internet.
I’m running Ubuntu 22, but not that much luck with the onboard networking. They worked to some extent at some point, but lots of hickups, so I haven’t really tried anymore. I needed the kernel to get better support for an USB 2.5G ethernet and an additional M.2 Wifi 6E chip (or actually two with this PCIe splitting setup). They’re working very nicely now.
do you have all 4 usb ports working? I have two bottom ones working, the two on the top, are giving power, but nothing gets detected when something is plugged in?