Enabling above 4g decoding does not seem to work on Asrock z170 Pro4s

[Xelaph]

Hi,
I'm trying to enable above 4g decoding and hopefully resizeable BAR on my Asrock z170 Pro4s. This board does not have a GUI setting for above 4g decoding, so I've tried enabling it with var_setup.efi. I'm using
this BIOS where I patched ReBar on. I enabled PCI_COMMON:0x03 as that seems to be the hidden above 4g decoding setting.

I went through the procedure of flashing ReBarUEFI, which worked fine and I'm able to set a ReBar size through ReBarState. I was unable to do the DSDT patch as AmiBoardInfo did not contain DSDT. However, GPU-Z still reports 4g decoding to not work, and I can also see this through sudo lspci -v | grep "Memory.*64-bit" which should show it on Linux.
It returns:

        Memory at df430000 (64-bit, non-prefetchable) [size=64K]
        Memory at df44e000 (64-bit, non-prefetchable) [size=4K]
        Memory at df44d000 (64-bit, non-prefetchable) [size=4K]
        Memory at df440000 (64-bit, non-prefetchable) [size=16K]
        Memory at df420000 (64-bit, non-prefetchable) [size=64K]
        Memory at df44a000 (64-bit, non-prefetchable) [size=256]
        Memory at c0000000 (64-bit, prefetchable) [size=256M]
        Memory at d0000000 (64-bit, prefetchable) [size=32M]
        Memory at df300000 (64-bit, non-prefetchable) [size=128K]
        Memory at df100000 (64-bit, non-prefetchable) [size=16K]

Does anybody know what I'm doing wrong? I saw in the supported motherboards that someone managed to get it working, so I don't know what I'm doing wrong anymore.

My setup:
Motherboard: Asrock z170 Pro4s
CPU: i5-6500
RAM: Kingston HyperX Fury HX421C14FBK2/8
PSU: Seasonic M12II 620W

[xCuri0]

GPU-Z can be wrong about 4G decoding, just enable rebar for the GPU with 4G on if it doesn't work you can do CMOS reset to be able to boot again

[Koekieezz]

@Koekieezz thanks a lot for the BIOS. I've flashed it and works fine without 4G decoding enabled and pure UEFI. However, when enabling 4G decoding, I get no video output whatsoever, even without any GPU. I have tried the following two settings to enable it. Setting 0x387 to 1 and I've tried setting 0x2E9 to 15.

1. 
OneOf Prompt: "Above 4GB MMIO BIOS assignment", Help: "Enable/Disable above 4GB MemoryMappedIO BIOS assignment
This is disabled automatically when Aperture Size is set to 2048MB.", QuestionFlags: 0x10, QuestionId: 0x2D9, VarStoreId: 0x1, VarOffset: 0x387, Flags: 0x10, Size: 8, Min: 0x0, Max: 0x1, Step: 0x0
				OneOfOption Option: "Enabled" Value: 1
				OneOfOption Option: "Disabled" Value: 0, Default, MfgDefault
2.
OneOf Prompt: "Aperture Size", Help: "Select the Aperture Size

Note : Above 4GB MMIO BIOS assignment is automatically enabled when selecting 2048MB aperture. To use this feature, please disable CSM Support.", QuestionFlags: 0x14, QuestionId: 0x27D7, VarStoreId: 0x1, VarOffset: 0x2E9, Flags: 0x10, Size: 8, Min: 0x0, Max: 0x1F, Step: 0x0
				OneOfOption Option: "128MB" Value: 0
				OneOfOption Option: "256MB" Value: 1, Default, MfgDefault
				OneOfOption Option: "512MB" Value: 3
				OneOfOption Option: "1024MB" Value: 7
				OneOfOption Option: "2048MB" Value: 15
				OneOfOption Option: "4096MB" Value: 31

@Xelaph Wait, the earlier bios i gave have no dsdt patch, you could try this one with QWordField patched: Z170 Pro4S Rebar.zip

Try this one, and see if it works (Try to downgrade again, and update with this bios).

[Koekieezz]

@Xelaph have you tested the bios i gave you above (with dsdt patched)

[xCuri0]

@Koekieezz thanks a lot for the BIOS. I've flashed it and works fine without 4G decoding enabled and pure UEFI. However, when enabling 4G decoding, I get no video output whatsoever, even without any GPU. I have tried the following two settings to enable it. Setting 0x387 to 1 and I've tried setting 0x2E9 to 15.

1. 
OneOf Prompt: "Above 4GB MMIO BIOS assignment", Help: "Enable/Disable above 4GB MemoryMappedIO BIOS assignment
This is disabled automatically when Aperture Size is set to 2048MB.", QuestionFlags: 0x10, QuestionId: 0x2D9, VarStoreId: 0x1, VarOffset: 0x387, Flags: 0x10, Size: 8, Min: 0x0, Max: 0x1, Step: 0x0
				OneOfOption Option: "Enabled" Value: 1
				OneOfOption Option: "Disabled" Value: 0, Default, MfgDefault
2.
OneOf Prompt: "Aperture Size", Help: "Select the Aperture Size

Note : Above 4GB MMIO BIOS assignment is automatically enabled when selecting 2048MB aperture. To use this feature, please disable CSM Support.", QuestionFlags: 0x14, QuestionId: 0x27D7, VarStoreId: 0x1, VarOffset: 0x2E9, Flags: 0x10, Size: 8, Min: 0x0, Max: 0x1F, Step: 0x0
				OneOfOption Option: "128MB" Value: 0
				OneOfOption Option: "256MB" Value: 1, Default, MfgDefault
				OneOfOption Option: "512MB" Value: 3
				OneOfOption Option: "1024MB" Value: 7
				OneOfOption Option: "2048MB" Value: 15
				OneOfOption Option: "4096MB" Value: 31

Disable CSM, if UEFI GOP is working correctly your boot screen should be at your monitors full resolution.

[xCuri0]

@Koekieezz thanks a lot for the BIOS. I've flashed it and works fine without 4G decoding enabled and pure UEFI. However, when enabling 4G decoding, I get no video output whatsoever, even without any GPU. I have tried the following two settings to enable it. Setting 0x387 to 1 and I've tried setting 0x2E9 to 15.

1. 
OneOf Prompt: "Above 4GB MMIO BIOS assignment", Help: "Enable/Disable above 4GB MemoryMappedIO BIOS assignment
This is disabled automatically when Aperture Size is set to 2048MB.", QuestionFlags: 0x10, QuestionId: 0x2D9, VarStoreId: 0x1, VarOffset: 0x387, Flags: 0x10, Size: 8, Min: 0x0, Max: 0x1, Step: 0x0
				OneOfOption Option: "Enabled" Value: 1
				OneOfOption Option: "Disabled" Value: 0, Default, MfgDefault
2.
OneOf Prompt: "Aperture Size", Help: "Select the Aperture Size

Note : Above 4GB MMIO BIOS assignment is automatically enabled when selecting 2048MB aperture. To use this feature, please disable CSM Support.", QuestionFlags: 0x14, QuestionId: 0x27D7, VarStoreId: 0x1, VarOffset: 0x2E9, Flags: 0x10, Size: 8, Min: 0x0, Max: 0x1F, Step: 0x0
				OneOfOption Option: "128MB" Value: 0
				OneOfOption Option: "256MB" Value: 1, Default, MfgDefault
				OneOfOption Option: "512MB" Value: 3
				OneOfOption Option: "1024MB" Value: 7
				OneOfOption Option: "2048MB" Value: 15
				OneOfOption Option: "4096MB" Value: 31

@Xelaph Wait, the earlier bios i gave have no dsdt patch, you could try this one with QWordField patched: Z170 Pro4S Rebar.zip

Try this one, and see if it works (Try to downgrade again, and update with this bios).

This BIOS doesn't need DSDT patch, it clearly gets them from external reference.

                If ((M64L == Zero))
                {
                    CreateDWordField (BUF0, \_SB.PCI0._Y0F._LEN, MSLN)  // _LEN: Length
                    MSLN = Zero
                }
                Else
                {
                    CreateQWordField (BUF0, \_SB.PCI0._Y0F._LEN, M2LN)  // _LEN: Length
                    CreateQWordField (BUF0, \_SB.PCI0._Y0F._MIN, M2MN)  // _MIN: Minimum Base Address
                    CreateQWordField (BUF0, \_SB.PCI0._Y0F._MAX, M2MX)  // _MAX: Maximum Base Address
                    M2LN = M64L /* External reference */
                    M2MN = M64B /* External reference */
                    M2MX = ((M2MN + M2LN) - One)
                }

[Xelaph]

@Xelaph have you tested the bios i gave you above (with dsdt patched)
@Koekieezz, tried this one and got the same result, no video out.

@xCuri0 CSM is disabled, the splash is at full resolution. I also tried enabling Secure Boot, but got no different result.

I had one try where I got a blinking cursor instead of no video, but I've not been able to reproduce that. I'm forced to clear the CMOS after each attempt as the whole pc just locks up.

Personally I'm completely out of ideas, I'm willing to try stuff if you have any suggestions. Otherwise I'll go look for a slightly newer motherboard that does have native support for 4G decoding.