Good morning everyone.
I recently bought my XPS 13 9370 (2018) w/ Ubuntu 18.04.2 LTS (Dell respin) and a DA300 USB-C adapter.
I had 200$ spare and I got myself a 4k display from samsung (U28E570D), which shipped with a free displayport cable yay.
Long story short, displayport cable only outputs 1440p/60Hz, random HDMI cable outputs native 4k resolution @ 30Hz. Why is that? Linux drivers? Kernel? Cable? Why can't I use 4k/60Hz with displayport?
4K 60Hz requires an ACTUAL GPU not a usb dongle.
4K 60Hz also requires DUAL Display port 1.2 cables.
USB3 is NOT Thunderbolt 3 even though they use the same connector.
An EGPU with dual display port attached to thunderbolt 3 might work but the standard thunderbolt interface only has 1 and only 1 display port output not 2.
You are demanding more bandwidth than HDMI or single Display port supports.
Never going to happen.