NVIDIA Tegra X2 & Xavier Get HDMI Audio With Linux 4.21
Thierry Reding of NVIDIA sent in the Tegra DRM driver updates this week for the upcoming Linux 4.21 cycle. He commented, "These changes contain a couple of minor fixes for host1x and the Falcon library in Tegra DRM. There are also a couple of missing pieces that finally enable support for host1x, VIC and display on Tegra194. I've also added a patch that enables audio over HDMI using the SOR which has been tested, and works, on both Tegra186 and Tegra194."
The mainline support for the recent NVIDIA SoCs are improving bit by bit. The Tegra186 is the X2 and the Tegra194 is the latest-generation Xavier SoC. So HDMI should be working and even the open-source display support for Xavier come Linux 4.21 when released towards the end of Q1'2019.
NVIDIA does contribute a bit to Nouveau in the realm of Tegra, but for the best experience you really need to be using their proprietary graphics driver, especially with most Tegra users leveraging CUDA and NVIDIA's proprietary compute interfaces to take full advantage of the SoC's capabilities.