[coreboot] Cannot access PCI Extended Capabilities registers with Coreboot?

[Rafael Send]

Hi,
I've been trying to get a PCIe add-in card working in my X210 M.2 slot for
a couple weeks.

There is a hack to get it recognized which involves writing to one of the
Extended Capability registers from an EFI shell, but apparently something
about the way Coreboot initializes PCI (?) makes these inaccessible.

It appears to work fine on stock BIOS, so I'm wondering (1) what about
Coreboot might cause this issue and (2) how would one go about debugging /
fixing it?

Cheers,
Rafael