4096x2160 monitor has EDID reporting 3840x2160

[Brian Vincent]

I have a LG 31MU97-B monitor.  My understanding is that EDID does not have
a way to report itself as a 4096x2160 monitor, so it reports itself as a
3840x2160 monitor.  The mode that it actually supports for it's native
resolution that I've been using looks like this:

xrandr --newmode "4096x2160_60" 556.730  4096 4104 4136 4176  2160 2208
2216 2222 +hsync +vsync

I can use this mode with X and it works fine, but I've been unable to get
it to work with KMS and Wayland, since it uses the EDID.  EDID has CEA
extensions that can support some 4096x2160 video modes, but I've tried
them, and they don't work since they're not exactly the modeline that it
needs.

I've found these DMT modes in drm_edid.c that seems like they would
probably work:

/* 0x57 - 4096x2160 at 60Hz RB */
{ DRM_MODE("4096x2160", DRM_MODE_TYPE_DRIVER, 556744, 4096, 4104,
   4136, 4176, 0, 2160, 2208, 2216, 2222, 0,
   DRM_MODE_FLAG_PHSYNC | DRM_MODE_FLAG_NVSYNC) },
/* 0x58 - 4096x2160 at 59.94Hz RB */
{ DRM_MODE("4096x2160", DRM_MODE_TYPE_DRIVER, 556188, 4096, 4104,
   4136, 4176, 0, 2160, 2208, 2216, 2222, 0,
   DRM_MODE_FLAG_PHSYNC | DRM_MODE_FLAG_NVSYNC) },

But... even if I manage to get the code to infer modes on my monitor, it'll
reject them since the horizontal resolution is 4096 which is higher than
the 3840 that my monitor's EDID reports.

If I wanted to fix this for everyone, what would the proper fix even look
like?  Should this be added as a quirk for this monitor to force add the
modes?

Thanks,
Brian Vincent
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