GPD XD+ CleanROM 2.0.9.0 (HDMI Output / 4KTV Fix) (Patreon)
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I've spent some time with our community manager PsyOps over the last two days and managed to do some debugging via proxy through him (as he had one of the affected 4K TVs laying around).
Long story short: the black-screen / no-signal issue with 4K TVs is now fixed!
For those interested in the cause of the issue: HDCP2.0 was to blame...
Our XD+ doesn't support the protocol but claimed it does when interacting with HDCP2.0-capable monitors which caused the aforementioned black-screen / no-signal issue.
I've patched up the XD+ kernel with the required changes to force the system into using HDCP1.1 which resolves the issue.