Steam Machine HDMI 2.1 support held back by open-source driver rules, Valve confirms
Valve has now clarified why the new Steam Machine spec sheet only lists HDMI 2.0, even though the hardware is wired for HDMI 2.1. The company says the port is capable of HDMI 2.1, but SteamOS cannot expose the full feature set because of how the HDMI 2.1 standard is licensed and how Linux display drivers are built.
The root of the problem is the HDMI Forum’s stance on HDMI 2.1. The group does not permit a fully open source implementation of the 2.1 feature set, which blocks the open AMD GPU drivers that SteamOS relies on from implementing everything that is in the spec. AMD engineer Alex Deucher has previously stated that an open source HDMI 2.1 stack would conflict with the Forum’s requirements. Valve told Ars Technica it is “trying to unblock” the situation, but for now it has to validate HDMI 2.1 functions under Windows, using proprietary drivers, rather than directly under SteamOS.
Source: Steam
On the bandwidth side, Steam Machine stretches HDMI 2.0 further than the baseline spec. HDMI 2.0 is nominally capped at 4K 60 Hz, yet Valve lists 4K 120 Hz support on its spec sheet. That mode is reached with chroma subsampling to compress color information and free up bandwidth within the 18 Gbps HDMI 2.0 limit.
Steam Machine supports AMD FreeSync over HDMI, but cannot currently expose HDMI VRR, the standardised variable refresh feature in HDMI 2.1, due to the same driver and licensing issues. Many recent TVs advertise HDMI VRR but do not list FreeSync compatibility, so some living room setups will not get any VRR from the Steam Machine HDMI output.
This reminds of Intel Arc Alchemist HDMI 2.0 situation
This kind of split between what the hardware can do and what the HDMI port actually exposes is already familiar from Intel Arc Alchemist. Intel’s DG2 / Alchemist GPUs only have native HDMI 2.0 support; board partners that ship “HDMI 2.1” ports on Arc A770 and A750 cards do it by routing a DisplayPort 1.4 output through an onboard protocol converter such as Realtek’s RTD2173. That PCON translates DP to HDMI 2.1, which allows 4K 120 Hz and modern TV features, but it also depends on specific driver support, especially under Linux, and still originates from a GPU that speaks HDMI 2.0 on its own.
For Steam Machine buyers, these HDMI details will not stop the box from working as a living room PC, and most users will just pick 4K 60 Hz or 120 Hz and play. For people tracking standards and Linux hardware, it underlines how HDMI 2.1 remains tightly controlled by the HDMI Forum, while PC vendors, whether Valve with SteamOS or Intel with Arc Alchemist, have to work around those limits with driver tricks, protocol converters, or simply by pushing users toward DisplayPort instead.
Source: Ars Technica
