Ok, I have a more formulated question now. TLDR, I got the same result as with 4.12, so there's probably something else going on I'd love some insight on.
Hardware-wise, I'm on a 51nb X210, and have attached a Thunderbolt controller in the NVME slot (i.e root port 9, which is enabled in the device tree. Other PCI cards show up correctly here). Under normal circumstances, the Thunderbolt card shows up as a hidden device, until it is woken by writing certain values to an extended register. Obviously, in order to do this, the device has to be correctly detected (even as a hidden device).
With the stock BIOS, issuing "pci 030000 -i" in an EFI shell results in the first attachment ("working.txt"). Note that here, I can see the entire configuration space including extended registers, which is important & I can then unhide the card.
If I do the same thing with coreboot from either Tianocore's EFI shell or the same one I invoked above on disk, I get the other result ("notworking.txt"). Notice that it has much less output, and the human-readable section actually appears cut off. I'm guessing this means that somehow the rest of the configuration space isn't being mapped properly in the coreboot version, but that might be the wrong word. I'm unfamiliar with the inner workings of this part.
Can anyone shed some light on why I can't seem to access the full config space for my device with coreboot, but can via stock BIOS? This is really the only thing keeping me from straight up switching to coreboot so it'd be dope to get it working...
Cheers,
Rafael