Experimental code ready for testing to enable HDMI 2.1 FRL with AMDGPU on Linux

Unfortunately, the HDMI Forum refused to allow HDMI 2.1+ features in the open source driver, but that hasn't stopped one developer.

Writing in a Reddit post, the developer noted they've "successfully managed to implement HDMI FRL in AMDGPU, enabling full HDMI 2.1 bandwidth on your AMD GPUs" and they're now looking for testers. The code is up on GitHub, so anyone can take a look and see how it works.

From the Reddit post:

The current state:

- FRL training works

- Video and audio work

- HDR works, VRR does not also works

- Hotplug, DPMS work

- Dynamic selection of the required FRL rate for a given mode is implemented

Caveats:

- Only DCN 4.0.1 (9070 XT) has been tested. Other GPUs should work similarly, but these paths are completely untested. There may be some clock dependencies there that I've not implemented or figured out yet. Hence me looking for testers today :) But do prepare for the possibility that the kernel might not even boot.

- Untested DCN generations include DCN 3.1 - DCN 3.6. DCN 3.0 (RX 6000 series) has not been implemented yet, but I believe it should be pretty similar to the newer DCNs.

- There is still some weirdness with my TV (Samsung S95B), where it will sometimes reject training higher FRL rates unless i restart the TV. So if you get no picture, try a lower resolution / refresh rate, or try restarting your TV.

- Support DSC is not implemented.

A murky legal situation though - as it's not entirely clear if the HDMI Forum can (or will be able to) block people going with a trial and error approach to getting more modern HDMI features working in the open source drivers. We know Valve have also been looking into certain features for the upcoming Steam Machine too.