It has recently been worked out why FreeSync support over HDMI is not coming to the open-source drivers, at least not yet … It stems from the HDMI Forum’s decision to prevent public access to the HDMI specification, which in turn does hurt open-source graphics drivers.
Since the HDMI specification is no longer public, but only for “HDMI adopters”, the open-source GPU driver support becomes complicated. In turn, even for those who are part of the HDMI forum, such as AMD, cannot provide an open-source implementation of functions covered by the closed specifications to in turn potentially and indirectly open up aspects of it to the public.
This situation seems to be the main blocker of why FreeSync with the open-source AMDGPU Linux kernel driver only works with DisplayPort and not HDMI connections, which has been one of the main questions over and over in regards to FreeSync Linux support and its current shortcomings .
The current limitations of the HDMI specification compromise many advanced HDMI functionality, such as HDMI 2.1, which is implemented in the open-source graphics drivers.
The X.Org Foundation is asking the HDMI Forum to at least allow certain HDMI 2.1 specification data to be made public so that open-source graphics drivers can in turn (legally) implement the functionality.
Let’s hope this succeeds in 2021 … In the meantime, keep this in mind when determining DisplayPort vs HDMI preferences!