Unfortunately, there are some limitations:
- memory aperture of his controller is only 16MiB, so it is nearly
unusable for graphic cards
- every attempt to generate type 1 config cycle always causes trap.
These config cycles are disabled now and we don't support cards
with PCIe switch.
- in some cases, attempt to do config cycle to (probably) not-yet ready
card also causes trap. This cannot be detected at runtime, but it seems
like very rare issue.
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D22724
8d617afff8