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Q. How can I determine if my kernel correctly detects my PCI Bridge?

A. As discussed above, the NVIDIA driver must have access to the PCI Bridge
   that each NVIDIA GPU is connected to in order to configure SLI or MultiGPU
   correctly. The following steps will identify whether the kernel correctly
   recognizes the PCI Bridge:

      o Identify both NVIDIA GPUs:

            % /sbin/lspci | grep -i vga

            0a:00.0 VGA compatible controller: nVidia Corporation [...]
            81:00.0 VGA compatible controller: nVidia Corporation [...]


      o Verify that each GPU is connected to a bus connected to the Root
        Bridge (note that the GPUs in the above example are on buses 0a and
        81):

            % /sbin/lspci -t

        good:

            -+-[0000:80]-+-00.0
             |           +-01.0
             |           \-0e.0-[0000:81]----00.0
            ...
             \-[0000:00]-+-00.0
                         +-01.0
                         +-01.1
                         +-0e.0-[0000:0a]----00.0

        bad:

            -+-[0000:81]---00.0
            ...
             \-[0000:00]-+-00.0
                         +-01.0
                         +-01.1
                         +-0e.0-[0000:0a]----00.0

        Note that in the first example, bus 81 is connected to Root Bridge
        80, but that in the second example there is no Root Bridge 80 and bus
        81 is incorrectly connected at the base of the device tree. In the
        bad case, the only solution is to upgrade your kernel to one that
        properly detects your PCI bus layout.