Update: Syncretic has done it again, you can download the patched ROM from MacRumors for AMD RX 6600 / RX 6800 /RX 6900 XT cards.
Original article below.
It has been nearly a year since I wrote the end of the classic Mac Pro after selling my classic Mac Pros for a 2019 and yet here we are, OpenCore 0.7.9 can run macOS 12.3.1 with very good results.
The big news comes from MacVidCards.Eu and the 6000 AMD GPUs. MacVidCards.Eu is an affiliate of MacVidCards.com, but I'm not sure of the business relationship. MacVidCards.com is a service that flashes GPUs with custom firmware that is Mac EFI compatible.
In my excitement and haste to post a video, I incorrectly stated that it's a RX 6800 XT and not a 6800.
The Mac Pros EFI implementation predates UEFI, the Unified Extensible Firmware Interface, which replaced BIOS computers. Apple's implementation uses Universal Graphics Adapter Protocol (UGA). The more modern UEFI replaced UGA with Graphics Output Protocol (GOP) Thus, any UEFI GPU will not output video before drivers are loaded. This meant for years, Mac users who bought any sort of non-OEM GPUs did not display a boot screen until OpenCore. OpenCore is a boot loader, meaning it launches before the operating system and has the ability to perform functions before the OS is loaded. It can inject the low-level driver support, thus giving classic Mac Pro users a boot screen, among many other features.
MacVidcards.com offered an alternative for aftermarket GPU upgrades. It would flash GPUs with its custom hacked ROMs for a fee. MacVidCards.eu had some business arrangement with MacVidCards.com, as MacVidCards.com didn't ship to Europe. MacVidCards.Eu is now selling flashed 6600 XTs and 6800 XTs for classic Mac Pros with screenshots to back up the claim. I'm inclined to believe these are real, and here's why.
Syncretic of the SurPlus fame had a look at the ROMs found on the 6000 series AMD GPUs and postulated it was due to bad code on the ROM. During the init, the ROM checked for UEFI HII (Human Interface Infrastructure) protocols but didn't have any error handling. Apple's EFI implementation does not have UEFI HII support. Thus the ROM on the card would look for these settings, and it'd fail to return a value. When the GPU hit the unexpected null state, it'd hang, thus interrupting the boot process.
Synchretic theorized patching this error in the ROM on the GPU would allow the boot sequence to continue, and thus you could use the GPU.
My guess is that MacVidCards.eu figured out how to do this and is now selling these GPUs. MacVidCards.com, interestingly is not. I have to stress that I haven't had any firm confirmations that these are real, but I'd most likely wager they are.
First, it's coming from a reliable source. The MacVidCards group(s) have shipped working EFI hacked GPUs for years. Even if the MacVidCards.com in the US has a negative reputation for customer service, plenty of people can attest their products work.
Second, thanks to community research, we know (at least part of) the scope of the problem for these GPUs. It's not an insurmountable fix
Third and final, the benchmarks pass the "sniff test" . They are about 6% slower in the Metal scores for a 6800 vs a 2019 Mac Pro with the same GPU. This checks out as PCIe 2.0 vs 3.0 generally only incurs about 5% hit for PCIe 3.0 GPUs as GPUs aren't as bandwidth-intensive as most people assume they are. Side Note: This will likely change with technologies like DirectStorage in Windows, where the GPU can bypass the CPU for accessing NVMe but for now, there's not a huge advantage for larger PCIe buses when concerning GPUs.
While currently this is only avaliable in Europe, this news should make all Mac Pro owners excited as it means there's just a few more drops left in the tank for the classic Mac Pros. Perhaps we will see a community solution for the ROM now that we know it's possible.