(In reply to Peter Weber from comment #8)
> Hello!
>=20
> 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".
>=20
> USB-C -> No hardware available
> DP    -> Reserved for ThinkPad, so I can use 3480x2180@30 Hz (p=
robably even
> 44 Hz [1])
> HDMI  -> RX 560
>=20
> 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&=
quot;. Finally I
> remembered, that kernel 3.15 will over the new graphics stack "AM=
DGPU DC". I
> booted a live ISO with kernel 3.15 and passed the requried option
> "amdgpu.dc=3D1" and got 3480x2180@30Hz. GNOME also offer=
s 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=
4;60Hz are
> broken.
>=20
>=20
>=20
>=20
> 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.
>=20
>=20
>=20
>=20
>=20
>=20
>=20
>=20
>=20
>=20
>=20
>=20
>=20
>=20
> [1]
> https://medium.com/@ValdikSS/how-to-use-high-res=
olutions-with-older-hardware-
> 58577d91b1f8

You need at least Kernel-4.15 which is in rc-state at the moment. Afaik, Ke=
rnel
3.14 is way too old for amdgpu and amdgpu-dc.