Hi there,

I have a ASUS P9X79 PRO motherboard with an i7-4930K and 32GB of ram, gpu is a Sapphire RX 580 Nitro+ 8GB.

The issue is that, after having successfully added your .ffs module to the bios, and also enabled the bios submenu for Above 4G Decoding, and having enabled the 4G Decoding itself, the board POSTs just fine but it doesn't boot anything, it just stucks to a black screen.

If i disable the Above 4G option, i can boot to windows just fine. Also another thing is that adding your module to the bios, breaks the sata contoller somehow (it gives code 10 in device manager, sata controller is a Marvell 91xx, but this is not a problem as i am booting from a NVMe drive).

I didn't do the UEFIPatch part, and the program said that it cannot find anything to patch.

Also i didn't do the DSDT patching step as you indicated in the guide that HEDT boards do not need it.

(But i still tried to look in the extracted DSDT file from the AmiBoardInfo module if i could find any of the lines you pointed to in the guide, and i couldn't find anything that matched)

I tried also updating the GOP in the vga bios to the last version, but to no avail.

UPDATE by xCuri0:
The issue above has been fixed by a new UEFIPatch (make sure to uncomment it) and DSDT patch, however there have been some reports of NVIDIA GPUs still not working on Windows.

UPDATE from @nefh:
dmesg_2GPU_KWin.txt

Since you've renamed the topic, I thought about some summary that could be pasted into first post.
I've recorded also some boot comparison videos for illustration, hope it's easier to get it this way.

All of the following issues happen only with ReBAR>1GB, only when RTX is receiving 64bit address space for BAR1.
In all cases ended in successful boot, the ReBAR works correct.

1. Boot to Windows - bad case
a) If RTX is connected to any display, even as a secondary input, then booting stops after POST, when Windows bootloader starts. The PC responds with instant turn off for power button push.
b) With Radeon connected alone or with no cables attached at all, the boot proceeds without any problems:

2. Boot to Linux - better case
Connecting display to RTX results in visibility problems during booting.
a) When display shows output from Radeon with secondary input connected to RTX, there is black screen instead of bootloaders UI. Any graphics seems to be a problem, even AMI logo is missing in POST screen. The visibility comes back during text mode.
b) When display shows output from RTX, the screen stays entirely black for whole boot sequence.
However, in all cases we have successful boot and proper display in desktop mode.

3. Misc info
The issue is independent to bootloaders. No matter if boot goes through rEFInd, GRUB or straight to Windows, the behavior is the same. Removing Radeon entirely and keeping only RTX also gives no effect - issues stays the same. RTX3090 VBIOS is the newest one.

Thanks for all your support and great work!