I am having a strange bug where I get D6 if I try to boot the system with my monitor connected through DisplayPort, although connecting the same monitor via HDMI boots fine.  I had thought it might be an issue with the DP cable or the miniDP adapter, since I have a 1080 Ti KPE, but if I then plug in the monitor to that port *after* booting then everything works just fine.

I've looked through that thread and have tried all of the fixes and still get the same error.  I am on BIOS ver. 1.29, have not enabled 4G decoding or resizable BAR, and make sure the monitor is powered on before I boot.  Perhaps there is an issue with a corrupted BIOS and I could just reflash it or switch to an older version, although I am not able to use the backup BIOS since those didn't come pre-flashed with a version that supports my CPU.  Is it safe to toggle the BIOS after booting and then flash it from the OS?

Hoggle

PwrdOff
I've looked through that thread and have tried all of the fixes and still get the same error.  I am on BIOS ver. 1.29, have not enabled 4G decoding or resizable BAR, and make sure the monitor is powered on before I boot.  Perhaps there is an issue with a corrupted BIOS and I could just reflash it or switch to an older version, although I am not able to use the backup BIOS since those didn't come pre-flashed with a version that supports my CPU.  Is it safe to toggle the BIOS after booting and then flash it from the OS?

You should be able to flash the bios regardless of the CPU.
 
The other thing I was wondering is did the display port cable come with the monitor or was it one that was bought separately?  Often times display port issues are related to uncertified cables but I haven't seen a failure to boot the system because of it mostly it's just a black screen on the monitor.

The cable is the one that came with the monitor, but I still had to use the DP to miniDP adapter that came with my GPU as it does not have a full size DP port.  I think it must be some kind of motherboard issue because I can get everything working if plug in the monitor after booting the computer, but oddly enough I'm still getting D6 if there's anything plugged into the miniDP port, even if I have another display connected to HDMI.  

Sajin
It’s because of the adapter. Get a dp cable that has a mini dp on one side and a normal dp on the other to fix the issue. I had the same issue with 980 ti kp’s.

Hmm I'll give that a shot, hopefully that's all it is because nothing else I've done works.  I reflashed the BIOS, same version 1.29, and that didn't fix anything, and I also tried connecting the monitor to my other GPU and got the same error, so it's not either of those that's the problem.  FYI, the monitor I'm using is an Asus PA32UCR-K.  I have tried connecting it a few different ways:
- DP to mDP w/ adapter: D6 post code, but eventually gets me to Windows so at least I can use the computer
- HDMI w/o adapter: no error code, but don't see post screen and can't get into bios, also goes into Windows eventually
- HDMI to mDP w/ adapter: everything works this way but limited to 1080p resolution
For now at least it's usable, only that if I need to get into the BIOS I need to swap to HDMI, which is annoying but at least lets me do everything.  Hopefully replacing the adapter resolves it.
 

Mister E...
Another thought:
 
Have you updated your 1080Ti firmware to support Display Port 1.3/1.4?  Details below:
 
NVIDIA Graphics Firmware Update Tool for DisplayPort 1.3 and 1.4 Displays
 

I tried that but it just gives me "the driver could not be updated" error, but I don't think that should be an issue since my monitor only runs on DP 1.2 anyway.  Just to be sure I also installed updated drivers and updated the firmware for my monitor, my display drivers are updated to 531.41.  For now if I need to get into the BIOS I can just connect my TV via HDMI, but annoyingly if I have the TV *and* the monitor connected I get D6 again, which I guess means that it detects something is plugged into the miniDP port, rejects it as a valid display, and then doesn't check if there's anything plugged into the other ports.
 
At the very least, I'm learning a lot through this process haha.  Oh, and I also tried flipping the vBIOS switch to see if that might be it, and it wasn't.
 
EDIT: Got a new DP to miniDP 1.4 cable, and still getting the same error so I'm well and truly out of ideas now.
 
EDIT: Enabling CSM gets rid of the D6 code, though I am still unable to see the post screen or enter the BIOS, and I would rather not leave it enabled anyway.  I also tried seeing if the firmware updater would be able to run with CSM on, but no luck there either, so I think that it simply refuses to run in Win11 in general and I'm not really inclined to make a separate Win10 partition just for the sake of doing the firmware update.
 
UPDATE: Turned out to be the firmware issue after all.  I made a portable Win10 USB and flashed the firmware from there, and I feel like it was just absolutely fitting just how frustratingly slow it is trying to run the computer in that mode because it just added that much more extra tension and drama to the whole situation.  Luckily I have multiple vBIOSes I can screw up on these GPUs so I wouldn't have been completely effed if it hadn't worked, but thankfully it all went smoothly.