21:9 monitor resolution incorrect since 4.14 kernel

[Neil Mayhew]

I have a 21:9 ultrawide monitor connected to an RX 570 with the amdgpu
driver. I'm still using the 4.14 kernel since with later kernels I can't
get the driver to allow use of the full resolution of 2560x1080 and I'm
limited to regular HD, ie 1920x1080. The latest kernel I've tried is
5.4-rc2. My distro is NixOS unstable-small. I have two other monitors
(16:9 and 16:10) connected to the same graphics card.

Is there anything I can do to work around this or try to fix it? I had a
look at the driver sources but couldn't find where the mode is set. I'm
happy to try things if someone can point me in the right direction, even
if it's just to gather better info for a bug report. Using such an old
kernel is becoming really inconvenient.

Some additional info about the situation on 4.14 that may or may not help:

1. The resolution is limited with Wayland but not with X11

2. When the system boots up the resolution is square and the image is
squished horizontally with many columns dropped, both during the boot
process and at the gdm login screen. It's not until after I log with X11
that the resolution is set correctly.

3. The output of xrandr --props is below.

TIA for any help.

--Neil

Screen 0: minimum 320 x 200, current 6160 x 1080, maximum 16384 x 16384
DP-1 connected primary 2560x1080+1920+0 (normal left inverted right x
axis y axis) 798mm x 334mm
    _MUTTER_PRESENTATION_OUTPUT: 0
    EDID:
        00ffffffffffff001e6df9765de80500
        091c010380502278eaca95a6554ea126
        0f5054256b807140818081c0a9c0b300
        d1c08100d1cfcd4600a0a0381f403020
        3a001e4e3100001a003a801871382d40
        582c4500132a2100001e000000fd0038
        4b1e5a18000a202020202020000000fc
        004c4720554c545241574944450a01b5
        02031af12309070747100403011f1312
        8301000065030c0010008c0ad08a20e0
        2d10103e96001e4e31000018295900a0
        a038274030203a001e4e3100001a0000
        00000000000000000000000000000000
        00000000000000000000000000000000
        0000000000ff003830394e544b464244
        3136350a0000000000000000000000a4
    dither: off
        supported: off, on
    audio: auto
        supported: off, on, auto
    scaling mode: None
        supported: None, Full, Center, Full aspect
    underscan vborder: 0
        range: (0, 128)
    underscan hborder: 0
        range: (0, 128)
    underscan: off
        supported: off, on, auto
    coherent: 1
        range: (0, 1)
    link-status: Good
        supported: Good, Bad
    CONNECTOR_ID: 48
        supported: 48
    non-desktop: 0
        supported: 0, 1
   2560x1080     59.98 +  74.99*
   1920x1080     74.99    59.96    50.00    59.99    59.94    59.93 
   1680x1050     59.95    59.88 

[etc.]
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