There is something weird about your cable links - they don't appear to show a specific cable. I think I can fix them:Vega II Duo (PCIe slots 1 & 2), and WX 9100 (PCIe slot 5)
Cable with the above AGDCiagnose is a USB-C to DisplayPort cable between the Vega II Duo's port-two and the Dell UP3218K.
(Also, dude, thank you so much for your help. I see you all over the web fiddling with macOS graphics and such; it must be a sizable time-investment. You don't have to do this, heh!)
Edit 1: Also tried this exact configuration with another DisplayPort 1.4 cable I have, with both A. a USB-C-to-female-DisplayPort adapter connected to the Vega II Duo, and B. a MiniDisplayPort-to-DisplayPort adapter connected to the WX 9100. Unfortunately, doesn't seem to be the cable, or the graphics-card.
The only part of the setup I can't really swap out is the 8k display itself; however, it renders at 8k just fine under Windows, so I suspect that's not it?
USB-C to DisplayPort cable - this says 4K60 which is good enough for HBR2 but not HBR3 (required for 8K30.
USB-C-to-female-DisplayPort - I have this. It says DisplayPort 1.4 and 8K so it should be good enough.
MiniDisplayPort-to-DisplayPort - says 4K so it might not be good enough.
Consider cables that say they support DisplayPort 1.4 (also check for VESA certified USB-C adapters and VESA certified 8K cables):
Club 3D CAC-1115 Mini DisplayPort to DisplayPort cable (supports DisplayPort 1.4).
Moshi bidirectional USB-C to DisplayPort cable (supports HBR3 - also sold in the Apple store). There is a less expensive cable from Monoprice that is also HBR3 and bidirectional (but not VESA certified).
I suppose the cables might be ok if it's working in Windows but I'm not sure.
A couple things from your AGDCDiagnose:
- There's a note that says "Warning: output to stdout (1) bypassed collector". This means you copied output from Terminal.app? It's probably better to direct output to a file like this:
Code:/System/Library/Extensions/AppleGraphicsControl.kext/Contents/MacOS/AGDCDiagnose -a > AGDCDiagnose_a.txt 2>&1 - I see the Dell is connected correctly using HBR3 and the EDID looks good (it's from the left tile which I guess is from the first DisplayPort input of the Dell?)
* 1: [DP 1.2 4 x HBR2] Status: [4 x HBR3 7777]
I wonder if the behavior would change if you used the second DisplayPort input of the Dell instead? - AGDCDiagnose only shows the EDID from the hardware. It doesn't show the EDID after patches or overlay is applied. Patches and overlays come from the /System/Library/Displays/Contents/Resources/Overrides/ folder. Dell UP3218K has patches but not an overlay (overlays come from mtdd files - they are EDIDs with some blank info that comes from the real EDID I guess?). To see the patched EDID, you need the EDID from SwitchResX and ioreg and the contents of the related override files.
- To get the override files: For Dell displays, look in the DisplayVendorID-10ac and DisplayVendorID-90ac (if it exists) folders in the Overrides folder. The UP3218K is DisplayProductID-4147. For your display, there may also be a SwitchResX override named DisplayYearManufacture-2019-DisplayWeekManufacture-9 (the manufacture date is specific to your display).
- To get the EDID from SwitchResX - There is an "Export EDID" button.
- To get the EDIDs from ioreg:
Code:ioreg -lw0 | perl -e ' $thepath=""; while (<>) { if ( /^([ |]*)\+\-o (.+) </ ) { $indent = (length $1) / 2; $name = $2; $thepath =~ s|^((/[^/]*){$indent}).*|$1/$name| } if ( /^[ |]*"([^"]+)" = <(00ffffffffffff00.*)>/i ) { print $thepath . "/" . $1 . " = <" . $2 . ">\n" } } ' - The Dell appears to have a high "Link training count" of 17 or 20. I don't know if that's normal or not since this is the first output I've seen that reports this value. I have an older output for the same display that does not report the count.
- The output shows a strange border value
Border {left,right} {0, 3840}. I would expect this to be zero. Maybe the tile info is confusing macOS? But some people don't have a problem. Is this a change due to Catalina 10.15.4? - If the display supported YCbCr 4:2:0 then you could do 8K 60Hz with a single HBR3 connection. But this is probably not possible since the panels are limited to 1080 MHz (otherwise Dell would have added 4:2:0 to the EDID).
- I made a custom EDID (attached). You can try installing it (remove the existing override files for the UP3218K). It just the EDID for the first tile with the Tile information removed.
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