I have a cheap old 1GB Dell OEM card with DisplayPort, its specs from Dell say it should do 4k. It does 3840x1080@120hz fine but any attempts to get 5120x1440 (slightly less than 4k) result in a recognizable but messed up screen - see photo from curved monitor. 5120x1440@60 and @120 are in the automatically determined display options list so Linux thinks they are in range.
If I use cvt to create a 5120x1440@30 modeline that has the same effect.
It does the same when using radeon or amdgpu drivers (as seen in lspci -v). It auto detects to radeon, I can force to use amdgpu with kernel boot flags. Looks like amdgpu came as part of the default Fedora kernel.
Monitor is a new Samsung G95 that does 5120x1440@120 fine on another PC (with nvidia card) with the same cable.
Oddly the official Samsung DP cable that came with the monitor does not work at all even for BIOS/POST screens.
01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Oland [Radeon HD 8570 / R7 240/340 / Radeon 520 OEM] (prog-if 00 [VGA controller])
Subsystem: Dell Radeon R5 240 OEM
Flags: bus master, fast devsel, latency 0, IRQ 138
Memory at e0000000 (64-bit, prefetchable) [size=256M]
Memory at f7200000 (64-bit, non-prefetchable) [size=256K] I/O ports at e000 [size=256]
Expansion ROM at 000c0000 [disabled] [size=128K]
Capabilities:  Vendor Specific Information: Len=08 <?>
Capabilities:  Power Management version 3
Capabilities:  Express Legacy Endpoint, MSI 00
Capabilities: [a0] MSI: Enable+ Count=1/1 Maskable- 64bit+
Capabilities:  Vendor Specific Information: ID=0001 Rev=1 Len=010 <?>
Capabilities:  Advanced Error Reporting
Capabilities:  Resizable BAR <?>
Capabilities:  Secondary PCI Express
Kernel driver in use: amdgpu
Kernel modules: radeon, amdgpu
Kernel 5.8.13, Fedora 31, amdgpu 3.38.0 20150101, xorg-x11-drv-amdgpu-19.1.0-1.fc31.x86_64
TechPowerUp says it is a rebrand of the HD 8570:
Dell specs: https://www.dell.com/support/article/en-uk/sln285319/technical-specifications-and-other-details-for-video-graphics-adapters?lang=en