Open Source Limitations on HDMI 2.1 Support in Linux Environments

Broken HDMI 2.1 functionality in AMD’s open-source Linux driver will remain broken. The HDMI Forum has rejected a proposed open-source driver for the HDMI 2.1 spec, meaning that DisplayPort is still the preferred interface for high resolutions and refresh rates on open-source platforms.

The modern HDMI 2.1 spec uses Fixed Rate Link (FRL) signaling technology to deliver uncompressed video beyond 4K 60Hz. This technology is also responsible for variable refresh rate (VRR) support. But the AMD open-source Linux driver cannot utilize HDMI 2.1’s FRL system. As a result, Linux users who try to use the HDMI port on their AMD GPU are stuck with sub-optimal video quality.

This problem is the subject of an ongoing, three-year-old, extremely long support ticket . AMD Linux engineer Alex Deucher occasionally comments on the support ticket and recently gave the open-source community a glimmer of hope—AMD built a functional open-source HDMI 2.1 driver! The manufacturer hoped to launch its driver in late 2023 but required approval from the HDMI Forum.

Somewhat unsurprisingly, the HDMI Forum said “no” to the open-source driver. Alex Deucher relayed the bad news in the aforementioned support ticket.

“The HDMI Forum has rejected our proposal unfortunately. At this time an open source HDMI 2.1 implementation is not possible without running afoul of the HDMI Forum requirements.”

Members of the HDMI Forum, including AMD, may participate in the development of HDMI technology. But as Phoronix explained in 2021, the HDMI 2.1 specification shut the door on HDMI public specification access. An open-source implementation, such as the one created by AMD, would expose HDMI 2.1 spec info to the public. The HDMI Forum wants to keep things locked down and has the legal precedent to do so.

Interestingly, AMD’s open-source driver does support DisplayPort to HDMI 2.1 adapters. Most high-quality monitors support DisplayPort, though this functionality may be useful when connecting your system to a TV.

Note that AMD’s product listings do not mention HDMI 2.1 incompatibility with Linux. Regardless of who’s responsible for these incompatibilities, AMD needs to be more forthright with its customers. A GPU’s HDMI output isn’t some super-complicated thing—it’s basic functionality. And even if the average Linux user prefers DisplayPort, they may want to use HDMI for a secondary display or TV. This is particularly true in the current era of Linux gaming.

Going forward, open-source supporters should place their full weight behind DisplayPort. Some Linux users believe that AMD can still implement full HDMI 2.1 functionality through a closed-source binary blob, though this seems like an unlikely solution. It certainly isn’t the preferable solution, at least for open-source enthusiasts.

Source: AMD via Phoronix

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