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.
I have one of these, but I don’t believe they work with HDMI 2.1 VRR (or at least I’ve never been able to get it to work).
With a LG B9, I have both G-Sync and HDMI 2.1 VRR support, but no FreeSync. I can get most of the HDMI 2.1 feature set working with a DP->HDMI 2.1 adapter, except for any form of VRR. That’s with a RX 9070, but similar situation on a 6800 XT previously.
Yeah. If you just want HDR, Cable Matters is the way to go.
If you buy one and flash it with a sketchy firmware, you can get VRR. But my understanding is the HDR is a smaller range. How much that matters when the vast majority of games aren’t taking advantage of HDR is up to you.
Personally? I love how much HDR pops. I more or less need VRR to not have to care about frame tearing and the like. And when I am running games on an underpowered glorified AMD NUC (or, in my case, ACTUALLY an AMD NUC)… my framerates are neither high nor stable.
I care about HDR (on Linux) way less than I do VRR in general.
Is that the “sketchy” firmware you’re referring to?
I’m explicitly not going to link to it as I can’t personally vet how safe it is or its origins but:
No. Check the various issues related to the subject matter. And hobbyist threads on sites like resetera.