Radeon Pro WX 4100 specs quoted from an AMD press release upon announcement seem to indicate that it is capable of driving 4 4k monitors, however, xrandr/Xorg will only allow it to drive 2 monitors at any one time (irrespective of configured resolution). xrandr --verbose lists each of the 4 MiniDP ports as "CRTCs: 0 1" and xrandr --listproviders lists "Provider 0: id: 0x58 cap: 0xf, Source Output, Sink Output, Source Offload, Sink Offload crtcs: 5 outputs: 4 associated providers: 0 name:POLARIS11 @ pci:0000:81:000". Kernel 4.9.8 (haven't tested on Windows or anything else as those are generally not available to test on this machine).
Please attach the Xorg log file and dmesg output corresponding to the problem. This is up to the kernel driver. You could try the amd-staging-4.9 branch of https://cgit.freedesktop.org/~agd5f/linux/ with CONFIG_DRM_AMD_DC enabled to see if that works better.
Created attachment 129461 [details] Xorg.log
Created attachment 129462 [details] dmesg grep amd, drm, dmar (in case its relevant)
Attached (possibly) relevant logs. Will test on the staging branch when I get a chance (probably in a week or two). I figured the issue would be with the kernel driver but didnt see an obviously better place to file a bug, should I have filed on kernel bugzilla?
This bugzilla is preferred for kernel driver issues as well. I reassigned this report to the correct product and component. Can you also attach the output of xrandr --verbose?
Created attachment 129463 [details] xrandr --verbose
Created attachment 129464 [details] [review] possible fix This patch should fix the issue.
Confirmed the attached patch fixes the issue.
Fixed in stable/LTS/mainline.
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