amdgpu does not support 3840x2160 at 30Hz on kaveri apu

[Davide Corrado]

hello, I'd like to report this issue that I am having since I updated my
display (samsung U28E590). The amdgpu does not support the native
resolution of my new monitor, which is 3840x2160*.* Using a HDMI or DVI
connection (I tried both, same results), the maximum supported refresh is
30Hz, so I'm stuck with that (don't have a displayport). The radeon module
works fine, I'm having this issue just when I use amdgpu (which I'd like
to, because performance is better).

Some info of my hardware:

cpu: AMD A10-7870K Radeon R7, 12 Compute Cores 4C+8G
kernel version (I tried different ones and different linux distros, same
results!): 5.9.16-200.fc33.x86_64 #1 SMP Mon Dec 21 14:08:22 UTC 2020
x86_64 x86_64 x86_64 GNU/Linux
Monitor: Samsung U28E590.

description:
If I boot the system using amdgpu and no video mode selection, the system
boots but I don't get a screen during boot and in wayland. I can connect
using ssh, so the system is running fine, just no display; If I force a
full HD resolution with "video:" in the kernel line, I can see the boot
process but the screen disappears when wayland starts (because the default
resolution is 3840x2160 at 30Hz). Using a full HD monitor results in no
issues, so it must be related to this very 4k resolution.

As I have already stated, radeon module works with the same
software/hardware configuration.
thanks you so much for your time :-)

-- 
Davide Corrado
UNIX Team Leader

Via Abramo Lincoln, 25
20129 Milano

Tel +39 3474259950
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