I have a shiny new Pi4B with a 4" touchscreen case (one of those XPT2046 ones). I also bought a portable 13" screen. The idea is to have a super-compact mini-computer on the go, and to be able to use a wireless keayboard/mouse and the larger screen to be more comfortable.

Out of the box, running an up-to-date Raspbian Buster, the dual screens worked - BUT the touch functionality on the small one (the large one doesn't have touch) did not work. It was a bit roundabout since the manufacturer didn't provide drivers, only a pre-patched Raspbian image that I didn't really feel like using, but I ended up finding drivers that worked from here. After that the small screen worked, in landscape mode, with touch. But now the larger display didn't show anything anymore. It's getting juice and detecting HDMI, but it's just black.

Another weird thing is that the Screen Layout Editor utility is gone from the menu (but can be launched from terminal or file explorer) and when launched, does not let me change anything anymore: resolution, rotation are all locked to 'default'. That was not the case before I fiddled with the touch functionality. This apparently happens when the 3D display drivers are disabled (source). Not sure how I did that - to be honest I'm not even sure what that means - and not sure how I enable them again.

If I just boot with the 13" screen on HDMI0, everything works peachy. I tried multiple things:

  • uninstalling fbturbo drivers: no effect
  • commenting out each line in config.txt one by one: no improvement
  • adding dtoverlay=vc4-fkms-v3c and max_framebuffer=2: slight improvement: the second screen now shows debug lines during boot and shutdown, and after boot shows a lonely prompt (just the underscore) at top left.

tvservice -s -v 2 and 7 report the correct displays and resolution, although it shows the second screen as CEA (not sure if that's normal).

I'm new to Linux and the Pi 4 (have a Pi 3 media center running OSMC though) and I assume I messed up somewhere along the line, but does anyone have any idea how I can get both screens to work? Or should I just start from scratch again?

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Update for future generations of newbie tinkerers: I ended up going back to stock Raspbian and starting over. Everything worked great out of the box. I managed to get touch working merely by enabling SPI and adding dtoverlay=ads7846 (and its usual arguments) at the bottom of config.txt. This lets me keep using the screen layout editor too. However I have other issues now, will make another question.

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