Thanks to everybody, for all the hints. I think the way to search in, is to create an edid file and load it at boot. I searched in this direction before, I just abandoned that way because it did not work.
Now the question I have is: How to create that edid file. The tool I found creates a file in a format, that is not accepted by the kernel. https://github.com/akatrevorjay/edid-generator/issues/11#issuecomment-531120166 I am using latest Debian (Buster 10.1) and put the generated .bin file in /lib/firmware/edid/. I added at boot: drm_kms_helper.edid_firmware=edid/3840x2160_24.00_rb.bin $ ls -trl 3840x2160* -rw-r--r-- 1 xxx xxx 492 Aug 24 20:57 3840x2160_24.00_rb.S -rw-r--r-- 1 xxx xxx 134 Aug 24 20:58 3840x2160_24.00_rb.bin -rw-r--r-- 1 xxx xxx 388 Aug 24 20:59 3840x2160_24.00_rb.bin.ihex -rw-r--r-- 1 xxx xxx 838 Aug 24 20:59 3840x2160_24.00_rb.c Output of dmesg contains - Missing trailing ) on purpose [drm:drm_load_edid_firmware [drm]] *ERROR* Size of EDID firmware "edid/3840x2160_24.00_rb.bin" is invalid (expected 6272, got 134
