One of the limitations of AMD's open-source Linux graphics driver has been the inability to implement HDMI 2.1+ functionality on the basis of legal requirements by the HDMI Forum. AMD engineers had been working to come up with a solution in conjunction with the HDMI Forum for being able to provide HDMI 2.1+ capabilities with their open-source Linux kernel driver, but it looks like those efforts for now have concluded and failed.

For three years there has been a bug report around 4K@120Hz being unavailable via HDMI 2.1 on the AMD Linux driver. Similarly, there have been bug reports like 5K @ 240Hz not possible either with the AMD graphics driver on Linux.

As covered back in 2021, the HDMI Forum closing public specification access is hurting open-source support . AMD as well as the X.Org Foundation have been engaged with the HDMI Forum to try to come up with a solution to be able to provide open-source implementations of the now-private HDMI specs.

AMD Linux engineers have spent months working with their legal team and evaluating all HDMI features to determine if/how they can be exposed in their open-source driver. AMD had code working internally and then the past few months were waiting on approval from the HDMI Forum... Sadly, the HDMI Forum has turned down AMD's request for open-source driver support.

AMD Linux engineer Alex Deucher commented on the ticket:

"The HDMI Forum has rejected our proposal unfortunately. At this time an open source HDMI 2.1 implementation is not possible without running afoul of the HDMI Forum requirements."

Thus a sad day for open-source GPU drivers with no apparent solution to safely implement HDMI 2.1+ functionality within open-source display drivers...

Needless to say, open-source Linux advocates should try to use DisplayPort instead if at all possible.