Technically, the Steam Machine supports HDMI 2.1. However, Valve and AMD are not allowed to offer an open-source driver for it.

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The HDMI Forum continues to block open-source HDMI 2.1 driver implementation for Linux, forcing Valve's Steam Machine to be software-limited to HDMI 2.0 despite hardware support. This restriction prevents native 4K at 120Hz without compression and limits VRR functionality. AMD submitted a functional HDMI 2.1 driver in early 2024 but was rejected by the HDMI Forum. Valve confirms they're still working to resolve the issue, while users can work around limitations using chroma subsampling or active DisplayPort-to-HDMI adapters.

Valve: HDMI Forum Continues to Block HDMI 2.1 for Linux

HDMI Forum board of directors
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Christopher Pasqualino / Chairman / Broadcom
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Yeah, Broadcom, famous for friendliness towards end-users and developers. /me says: fuck HDMI, go DisplayPort.

Closed standards should just die off.

This is why I use DisplayPort exclusively on my displays. HDMI can die as a standard.

I did not know HDMI sucked so much. But now I do, and I’m halfway to being a GI Joe.

I wish Tivi model switch to use display port and console switch too. Its should be for consumer beneficial