[Bug 196980] [pci] PCI Resource Allocation Causing 2 em devices to fail during initialization

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=196980

--- Comment #5 from John Baldwin <jhb at FreeBSD.org> ---
This is not caused by MSI.  The root issue is that the PCI bus only
pre-reserves BARs for leaf devices, not the I/O windows of PCI-PCI bridges. 
The agp0 device probes before pcibX and it allocates a random page of physical
address space that overlaps with the PCI-PCI bridge window.  That then causes a
conflict with reserving the PCI-PCI window.

I have another PR I can't find at the moment where I've discussed the possible
solutions.  The most expedient is probably to have the PCI bus pre-reserve the
resources for memory windows (I/O windows are much more complicated, but memory
windows would be good enough for all the instances of this bug that I've seen
so far).

Also, this is not a regression in 10.2.  I believe it has been an issue since
NEW_PCIB was added, so 10.0 (and in older branches with the option enabled).

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