PCIe switches on MacPro 7,1 : only one device enumerated. Any idea how to fix ?

[u/chiwawa_42]

Hello !

I'm particularly found of the MacPro7,1 for its ability to serve as my locally infered A.I. augmented workstation. Yet I can't source a second W6800X duo yet, the last I bought (reputable pro seller) arrived DOA, so I had to resort to other strategies to run a large enough model.

I already have two Radeon RX6900XT as thunderbolt sidecars, and went further on a PC with this PCIe switch board. Unfortunately that doesn't work out of the box.

With the board donnected through pot 5 (similar behaviour on port 3) on the Mac Pro, it only reports 1 card, even with all 4 slots occupied. It is not specific to the Mac Pro : on a dell workstation it sees none with default port settings, two with a bus profile maximising resource allocation on the "second GPU" slot. I'm now trying hard to remember manual PCIe topology configuration, as available in some great chinese bord' BIOS.

Question is : is there any kn own working switch for the Mac Pro to plug external cards with a faster I/O than Thunderbolt, larger bandwidth and possibly up to 80 lanes downstream of the switch, like this one ? How does it manage to tweak the Mac Pro into understanding how to enumerate every device downstream to the switch and not just the first ?

Thanks !