I have tested 4.16 and drm-next-4.17 both under wayland and xorg. All only show the monitor as 2 displays with 2560x2880 side by side.
(I previously had an nvidia gpu using their binary blob on this display without issue)
amdgpu doesn't currently have support for tiled displays. It's expected with tiled displays that you see them as you describe.
This is handled at the Desktop level. There are quite a few large format displays that show up as multiple DP MST displays, even a number of early 4k displays. X exposes the tile info via randr 1.5 and the desktop environment should handle it as a single monitor. Mutter handles this in GNOME for example.
Yes it is handled at Desktop level. (https://gitlab.gnome.org/GNOME/mutter/issues/110)
But the tile property has to be exposed from kernel driver as far as i understand. Which was not the case in my setup with amdgpu. With the new patch from here https://gitlab.freedesktop.org/xorg/driver/xf86-video-amdgpu/merge_requests/32 it should work but i can not see any tiled properties.
For this to work the TILE property (blob) of the connector has to be exposed. modetest does not return it.
xrandr --props also does not return the tile property.
I'm having the same issue with a 4K Dell UP2414Q with 3840x2160 as 1920x2160+0+0, 1920x2160 +1920+0.
Is it possible to do a workaround in xorg.conf, similar to nVidia's TwinView and nvidiaXineramaInfoOrder as described in this question?
You need to be using the amdgpu driver if you are running xorg. I'm not sure if the modesetting driver supports the tile property yet.
FYI starting with kernel 5.2 the tile property is propagated properly on Dell UP2715K. The Mate desktop picks this up just fine.
@AlexDeucher I am using xserver-xorg-video-amdgpu. I can use 18.0.1 (No TILE support, need workaround) or 19.0.1 (Cinnamon crashes, need workaround for time being). I'm looking for a xorg config or xrandr command that works like nVidia's TwinView that I can keep in place while I am checking TILE support with this DM and kernel from time to time. (I am new to AMD/Radeon.)
@TomasBzatek I still need a workaround because Cinnamon crashes on 5.2 w/ 19.0.1 TILE support. See https://github.com/linuxmint/cinnamon/issues/8807