The TL;DR is that the organization that controls the HDMI standard won’t allow any open source implementation of HDMI 2.1.
So the hardware is fully capable of it, but they’ll get in trouble if them officially implement it.
Instead it’s officially HDMI 2 (which maxes out at 4k @ 60Hz), but through a technique called chroma sub-sampling they’ve been able to raise that up to 4k @ 120Hz.
However there are some minor reductions in picture quality because of this, and the whole thing would be much easier if the HDMI forum would be more consumer friendly.
In the meantime, the Steam Machine also has display port as a completely issue free display option.
Countdown until some community member patches 2.1 support in
Not that easy.
To get HDMI 2.1 support for the Gabe Cube itself essentially requires kernel level patches. Which on a “normal” Linux device is possible (but ill advised) but on these atomic distros where even something like syncthing involves shenanigans to keep active week to week? Ain’t happening. Because HDMI is not just mapping data to pins and using the right codecs. There are a LOT of handshakes involved along the way (which is also the basis for HDCP which essentially all commercial streaming services utilize to some degree).
There ARE methods (that I have personally used) to take a DP->HDMI dongle and flash a super sketchy Chinese (the best source for sketchy tech) firmware to effectively cheat the handshakes. It isn’t true HDMI 2.1 but it provides VRR and “good enough for 2025” HDR at 4k/120Hz. But… I would wager money that is violating at least one law or another.
So expect a lot of those “This ini change fixes all of Windows 11. Just give money to my patreon for it” level fixes. And… idiots will believe it since you can use a dongle to already get like HDMI 2.05 or whatever with no extra effort. And there will likely be a LOT of super sketchy dongles on AliExpress that come pre-flashed that get people up to 2.09 (which is genuinely good enough for most people). But it is gonna be a cluster.
And that is why all of us with AMD NUCs already knew what a clusterfuck this was going to be.
There are also ways to fake the handshake in software. I personally did not try that but from what I have seen on message boards? It is VERY temporary (potentially having to redo every single time you change inputs on your TV/receiver) and it is unclear if the folk who think it works actually tested anything or just said “My script printed out ‘Handshake Successful’, it works with this game that doesn’t even output HDR!”
Cable Matters sells plenty of different DP->HDMI 2.1 adapters that work with VRR. The main issue here is that you won’t get CEC if you use those.
Russian is also good
If would need to be patched in on Linux kernel level, which is annoying to say the least.