Hello!

I'm using a LG 27UD88-W, which featurese 1 x DP, 1 x USB-C (DP) and 2 x HDMI.
In my desktop is a Radeon RX 560 which offers DP "1.4" and HDMI "2.0b" and I
also use a ThinkPad X220 which offers DP "1.1".

USB-C -> No hardware available
DP    -> Reserved for ThinkPad, so I can use 3480x2180@30 Hz (probably even 44
Hz [1])
HDMI  -> RX 560

The DP of the RX 560 works fine at 3480x2180@60Hz, but via HDMI it fails with
Fedora 27 and kernel "3.14". Only 1920x1***@30Hz is possible any higher
resolution results in visual garbage on the screen or "NO SIGNAL". Finally I
remembered, that kernel 3.15 will over the new graphics stack "AMDGPU DC". I
booted a live ISO with kernel 3.15 and passed the requried option "amdgpu.dc=1"
and got 3480x2180@30Hz. GNOME also offers 60Hz as option, but that fails and
cannot be used, screen goes blank, until GNOME reverts the settings. I'm afraid
the previous people are right, HDMI2.0b and 4K@60Hz are broken.

HINT: Don't used "DEEP COLOR" (LG OPTION for 8/10BIT COLOR?) with HDMI or
everything is busted, but that is a minor issue, as for now.

[1]
https://medium.com/@ValdikSS/how-to-use-high-resolutions-with-older-hardware-58577d91b1f8