(In reply to Maik Freudenberg from comment #65)
> Now with the acpi bug fixed, I applied both of your v2 patchsets with some
> nip/tuck to a 4.16rc4+ kernel and ran into the next oddity: while the pci
> quirk in general works, re-reading the header type will always return 0x00
> so the hda device is not added.
> lspci confirms that the quirk is otherwise working:
> 00: de 10 92 12 07 00 10 00 a1 00 02 03 00 00 80 00
> 480: 00 00 00 00 17 00 00 00 00 00 00 02 00 00 00 00

Hm, the multifunction bit is set. Try adding an msleep(100) after setting the
bit at offset 0x488 and before re-reading the header type register. 100 ms is
likely way too much, this is just to ensure it's not too short.

Also, try changing DECLARE_PCI_FIXUP_CLASS_HEADER to
DECLARE_PCI_FIXUP_CLASS_EARLY. Shouldn't make a difference but maybe I missed
something.

Does it work if you just set gpu->multifunction = 1?