For anyone else struggling to get some Radeon RX6000 GPU's working to mine Eth on Linux, here is what I did to get it working:
Latest kernel 5.12rc7
Install amdgpu-pro 20.40 using this PKGBUILD file. This will download the pro driver version 20.40 and extract the openCL driver to work alongside the open source driver.
Drivers 20.45, 20.50 and 21.10 do not work with the big Navi cards. AMD, this is utterly ridiculous! I was so excited to see 21.10 in the repositories only to find it still doesn't work (clinfo hangs half way through the second card and mining delivers 0.00H/s)
Drivers 20.45 and onward worked fine for me when using only 1 GPU in the first PCIE slot. When I added a second GPU things simply failed.