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The HDMI Forum rejected AMD's open source HDMI 2.1 implementation
(www.gamingonlinux.com)
This is a most excellent place for technology news and articles.
USB C is just a connector, you might be referring to Displayport over USB C which is basically just the same standard with a different connector at the end. That or Thunderbolt I guess
I thought thunderbolt was DP passthrough as well