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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.
DisplayPort already exists
Hard to find on non-pc gear, but that’s a fair point
It's usually easy enough to adapt it as needed. It can typically send signals compatible with HDMI and DVI-D just fine.
The passive adapters that connect to DP++ ports probably still rely on this HDMI specific driver/firmware support for these features.