One of the limitations of the AMDGPU Linux kernel graphics driver has been the lack of its support for HDMI 2.1 and later. AMD has wanted to support HDMI 2.1+ functionality under Linux but
it's been legally blocked by the HDMI Forum. But anxious independent users have been working on open-source patches for wiring up HDMI 2.1 into the AMDGPU driver outside of the realm of AMD and the HDMI Forum's blessings.
Besides
AMDGPU patches working on HDMI Gaming Features worked on independently and
developed using public code/knowledge and trial and error, today an independent developer announced his "working" HDMI 2.1 Fixed Rate Link (FRL) support for the AMDGPU driver.
The independent user has a kernel tree on GitHub with HDMI FRL support working on AMDGPU to enable HDMI 2.1 bandwidth for AMD GPUs. This HDMI 2.1 bandwidth work is successfully using FRL training, HDR works, and other basic functionality appears to be working but only a Radeon RX 9070 XT graphics card has been tested. Additionally, features like Display Stream Compression and YCbCr 4:2:0 also have yet to be implemented.
The independent developer worked on these patches by analyzing differences in the Radeon GPU register states between Windows and Linux, closing looking at the Windows driver behavior, and also learning some insight from the AMD-Xilinx HDMI FRL training code.
Those interested can find these experimental, out-of-tree patches on
this GitHub repository. Further details on the effort can be found via
this Reddit thread that was started today.
While it's interesting and the passionate work of open-source developers applauded, it's likely not to yield any short-term improvement for AMD Radeon Linux users at large. Besides those wanting to roll their own kernel build with these patches, these patches certainly wouldn't be accepted for upstreaming to the AMDGPU driver given HDMI Forum's rejection already and needing to clear AMD's legal review. It may help apply pressure on the HDMI Forum to change course especially if Valve and other stakeholders get involved, but this has been an unfortunate issue going back years already. AMD Linux users are best off just using DisplayPort where possible. Or if the HDMI Forum doesn't revert their policy, perhaps in some future AMD GPUs we'll find more of the HDMI display functionality moved into their binary firmware blobs to make it easier on the open-source driver.
