Thunderbolt is a switch for the sets of signals feed into the controller. For the video it is more like a/b switch though. Pragmatically can't chop the DP stream into subpieces. The TB controller takes two DP inputs. If you leave one of those inputs empty then something isn't going to get fed in a two port set up.
It is "active" if don't use the other one is bit of dancing on a pin. The HDMI port on the Vega II cards is "active" then up until you consume the DP stream it is sharing/switched on with the TB controller.
Looking again at the Mac Pro specs the 580X can drive two XDRs (via the two different pairs on the host ) and the Vega II Solo can drive two XDRs . So Apple probably did split the two across two TB controllers (because now can't drive a 3rd XDR ). Which back to the "any port". For the XDR it won't. In the context, of the Vega II 'solo' the TBv3 ports on the host system won't work. Those TBv3 ports won't work for most 5K monitors also from the last several years (which need two stream inputs ).
So if someone had a base config Mac Pro with a 580X and an XDR hooked up and then swapped out the 580X for a Vega II 'Solo' then they'd have to move the TB cable connection to one of the GPU card's edge connectors to make it work. (that's a bit quirky).
But yeah, for a single sub 4K monitor, it is all any TBv3 port would do.
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The tangent started with not generic displays but the XDR.
back at
post 14,741
"...
The XDR can be attached to any TB3 port. Including the top mounted if that’s how you want to roll.
As long as you have a MPX GPU video will be routed to all ports. ..."
I thought there might be but there isn't. In retrospect, it probably makes it easier to pass the Thunderbolt certification that there is at least one video stream for the Vega II 'solo' case. But does come at a trade-off is that two stream displays won't work.
Pragmatically, those one display stream feed controllers probably will end up being used as "data only" ports in more than a few contexts. 2 out of 3 monitors Apple is pitching don't really work well (XDR and Ultrafine 5k ).
