https://bugzilla.kernel.org/show_bug.cgi?id=208947
--- Comment #13 from Coleman Kane (ckane@colemankane.org) --- Interestingly enough, the description of a problem attempting to be addressed with this fix sounds similar to behavior I experience with this system:
Namely, the Radeon RX 580M connected over DisplayPort advertises 3840x2160@60Hz but I've never gotten the card to successfully link up to the monitor at that resolution. In console mode, this results in the "black screen" where only the built-in display for the laptop works and the external monitor just stays black (falls asleep). In Xorg, if I set it to 3840x2160@29.97Hz, it works however. Additionally, setting the higher resolutions can take a noticeable moment to display.
I have another laptop with another graphics card and was able to test that the display does work at 3840x2160@60Hz.