I set up a system with two PCI-E devices, one at PCI:1:0:0 and one (the primary) at PCI:2:0:0. Without specifying the BusID, the X server decides that they're both primary,
(!!) More than one possible primary device found
(--) PCI: (0@1:0:0) unknown vendor (0x10de) unknown chipset (0x05fd) rev 161, Mem @ 0xc6000000/16777216, 0xa0000000/268435456, 0xc4000000/33554432, I/O @ 0x00003000/128
(--) PCI: (0@2:0:0) unknown vendor (0x10de) unknown chipset (0x06ea) rev 161, Mem @ 0xc2000000/16777216, 0xb0000000/268435456, 0xc0000000/33554432, I/O @ 0x00002000/128
and then that neither are primary,
(II) Primary Device is:
(EE) No devices detected.
and then fails to start:
Fatal server error:
no screens found
This is a problem at least as far back as server-1.5-branch. Specifying BusIDs works around the problem except that the server decides not to POST the secondary GPU.
Created attachment 20720 [details]
Created attachment 20721 [details]
*** This bug has been marked as a duplicate of bug 18321 ***