Is there generally interest in coreboot porting more of these? The following are already supported: https://github.com/coreboot/coreboot/tree/main/src/mainboard/apple. This seems to be the current state of the world: https://ch1p.io/coreboot-macbook-support/
I started to write a long guide on how to port coreboot to new boards, but realized it was probably better to start the discussion first.
Pros would be: We can probably configure a bunch of stuff Apple hid or disabled at a low level, run me_cleaner.py (Intel ME delete/disable), update FSP/refcode, mess with GOP settings, whatever (within the limits of Intel docs, and our understanding of the Apple EC, etc). It should be possible to run standard BIOS or EFI, which is probably not that interesting, or grub directly in fw (can be nice for linux, since disk can be decrypted and booted from firmware). Usually the FSP has a nice long list of things that can be changed, and in the future it's possible we escape the FSP (Intel blob for RAM and CPU bringup).
Cons would be: I have no idea if TianoCore can actually boot OS X directly. It can run in 32 bit or 64 bit mode, and it can definitely be modified to do whatever to make OS X happy. Things like me_cleaner can break weird stuff, and it can take a lot of iteration to make a port good enough for you, and even more to make it good enough for everyone. There are boards that cannot yet be ported, e.g. X99 (some iMacs, I think), and really only depends on having similar north/southbridge & CPU (and probably FSP) in coreboot already.
Disclaimer, I don't have a working intel mac, but I have ported quite a few x86 boards over the years, and these older intel macs might be easier than more recent boards. I don't think they have Intel BootGuard enabled, which means custom firmware should be possible if desired. I wouldn't flash anything you find on the internet, and I would definitely take an external backup of the flash chip first!
I started to write a long guide on how to port coreboot to new boards, but realized it was probably better to start the discussion first.
Pros would be: We can probably configure a bunch of stuff Apple hid or disabled at a low level, run me_cleaner.py (Intel ME delete/disable), update FSP/refcode, mess with GOP settings, whatever (within the limits of Intel docs, and our understanding of the Apple EC, etc). It should be possible to run standard BIOS or EFI, which is probably not that interesting, or grub directly in fw (can be nice for linux, since disk can be decrypted and booted from firmware). Usually the FSP has a nice long list of things that can be changed, and in the future it's possible we escape the FSP (Intel blob for RAM and CPU bringup).
Cons would be: I have no idea if TianoCore can actually boot OS X directly. It can run in 32 bit or 64 bit mode, and it can definitely be modified to do whatever to make OS X happy. Things like me_cleaner can break weird stuff, and it can take a lot of iteration to make a port good enough for you, and even more to make it good enough for everyone. There are boards that cannot yet be ported, e.g. X99 (some iMacs, I think), and really only depends on having similar north/southbridge & CPU (and probably FSP) in coreboot already.
Disclaimer, I don't have a working intel mac, but I have ported quite a few x86 boards over the years, and these older intel macs might be easier than more recent boards. I don't think they have Intel BootGuard enabled, which means custom firmware should be possible if desired. I wouldn't flash anything you find on the internet, and I would definitely take an external backup of the flash chip first!
