The Q is about a bare console, without any graphical environment.
1.FullHD (1920x1080) - via HDMI-A-2
2.UHD (3840x2160) - via DP-1.
When the computer starts, DRM/KMS chooses the lowest resolution of two available monitors. This is completely logical and normal from the point of view of the default DRM settings. However, because of this, only 1/4 of the screen area is occupied on UHD.
If you flip the HDMI (FullHD monitor), then everything is ok on UHD. Passing parameters to the kernel:
drm_kms_helper.edid_firmware=DP-1:edid/dp/dp.bin video=DP-1:3840x2160 drm_kms_helper.edid_firmware=HDMI-A-2:edid/hdmi/hdmi.bin video=HDMI-A-2:1920x1080
does not help. (edid here is just in case, because it automatically comes normal automatically). Used various variants of these options, does not work if both monitors are connected.
The conclusion is that with both monitors turned on, the DP-1 cannot be set to a resolution higher than 1920x1080. However, it can be underestimated for both DP and HDMI.
Yes, the problem is solved by creating a systemd unit to start "fbset -xres 3840 -yres 2160" when starting the computer.
But I left this in case a solution native to KMS cannot be found. In general, how to specify either the main monitor via KMS, or increase the resolution to UHD? Is this possible in principle?