I too have the Mac Pro 2019 and a new Sapphire Nitro RX 6900 XT. I realize it's a waiting game for Big Sur and/or Catalina, but I expected it to work in bootcamp. I had left the RX 580 in there. I have a promise pegasus RAID too, so I had
the Promise in slot 1 ( MPX card, covering slot 2)
the Rx 580 in slot 3 (a half width MPX)
The RX 6900 XT in Slot 4 with 2 of the baby 8 to 6X2 power cables from the belkin kit wired to the board for power
It booted no problem with no cables attached to the RX6900
In Catalina and Big Sur, Slot 4 showed up as occupied by an AMD GPU with no kext (as expected)
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In Bootcamp Windows 10 (latest), I did see a MS Generic display driver in addition to the AMD RX580. However plugging the HDMI cable into the RX 6900 did nothing.
I downloaded the specific adrenaline drivers for the RX 6900 XT from AMD's web site (dated Jan 21 2021) and attempted to install. No dice. It said no recognized AMD card available.
Thinking there may be a conflict with the RX580, I removed it and booted again. Just darkness, lol. I can tell it booted. I guess I'll make sure to set up RDP this time.
What is the trick? This seems totally chicken/egg. The AMD software won't install as it doesn't see its own card somehow. If there is a card conflict, you have to run blind while one driverless card is inserted? Someone had a month of late nights in a row when they were designing this driver system.
My system is a little unconventional. I have the memory full and the 28 core Xeon. It will be amazing when the card is going.