i think setting `display_default_lcd=1` in `config.txt` will force the main framebuffer to be over DPI, then you can just use the old framebuffer mailbox stuff to interface with it as always?
if you want more direct control, you would move on to the HVS stuff:
https://blog.benjdoherty.com/2019/05/21 ... pberry-Pi/
basically, the firmware will configure an output to use a given HVS channel, and then you can freely change the HVS config whenever you want (though that will have some tearing artifacts)
there are 3 HVS channels, and 3 registers pointing to the start of the displaylist within the HVS config memory
https://github.com/librerpi/rpi-open-fi ... #L159-L190
and this is some code i wrote to try and understand the HVS, it will spy on the current state via /dev/mem while under linux, and try to pretty-print whatever is being drawn currently