https://bugs.freedesktop.org/show_bug.cgi?id=104412

--- Comment #10 from S.H. <anaxi@aikq.org> ---
(In reply to Peter Weber from comment #8)
> 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.
> 
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> 
> [1]
> https://medium.com/@ValdikSS/how-to-use-high-resolutions-with-older-hardware-
> 58577d91b1f8

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

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