first id like to say sorry for my bad and limited ability of expression with english since its not my first language
well,as we know sierra and high sierra cannot booted without igpu, id like to share some of my "observation" about this problem. in fact i want to write this a while ago, but im busying with other things so i quickly forgot about that
correct me if am wrong,heres is the known ways to boot sierras without igpu
1.flash the card with legacy bios ONLY
if we stripped the UEFI from vbios, polaris would boot fine without igpu or helper card, thats the first hint for me which problem lying in the difference of initialize the EFI between real mac and hackintosh
2.change the device id in UEFI Vbios
if we change the Vbios device id from 67DF to anything not used by any card(like 67DD), the card would boot fine even under UEFI Vbios, i tried to using clover's load vbios function to load the very same bios, i got black screen just as theres no igpu, while flashing it to card and booting with clover results in success booting. thats the second hint for me, solution must lying in the way clover deal with uefi vbios. As we can see, load vbios failed to show same result as flashing the vbios directly into card, thats imply clover would load basic info(like device id) from vbios then passing these info to the system regardless what we changed in vbios file.
Im not intended to nail the exact cause of these problem, but i come up a workaround, if clover could let us pass any info we want to the system(like custom device id), or making a DSDT patch which fake the device id for polaris card, would it solve this problem?