I have a mac pro 2010, with a Radeon 7950 Mac version display card.
It is currently driving dual Dell 30" displays 3007WFHPC and working well.
I just purchased a 4k Dell UP3216Q to replace one of displays. when I connect it via the miniDP port, the mac will not boot at all.
If I disconnect the new Dell 4k, and restart the Mac it boots.
Clearly there is an issue with SST/MST on the new display. The 4k display is DP1.2.
Is there a firmware update for the 7950? The following is the Hardware info on the Radeon:
Chipset Model: AMD Radeon HD 7950
PCIe Lane Width: x16
VRAM (Total): 3072 MB
Vendor: ATI (0x1002)
Device ID: 0x679a
Revision ID: 0x0000
ROM Revision: 113-E2080C-101
EFI Driver Version: 01.00.624