Raspi 5 DSI Display stuck in splash screen

[heubeck]

Hey there,

started a new project using a Raspberry Pi 5 and the Recalbox 9.2.3 Pulsar.

I've the official Raspberry Touch Display 2 connected via the DSI port and a brand new SanDisk SD Card.
Raspberry PI OS runs perfectly using the DSI Display and also an HDMI Screen in addition.

Recalbox doesn't turn on the DSI Display, but works perfectly on the HDMI Screen (even with DSI Display still connected).

Using display_auto_detect=1 made the DSI Display turn on and show (nearly immediately) the Recalbox splash screen (with the Pacmans, Recalbox logo and version) but keeps stuck there.
SSH and also the WebUI works.
Connecting a HDMI screen doesn't change situation and the HDMI screen keeps off.

Interesting part is, that the animation that usual plays even before the Splash screen isn't shown neither.

Tried dtoverlay=vc4-kms-dsi-generic with various args (pi5, dsi-7inch) as well and some hdmi settings, only change was, that the splash screen may be shown on the HDMI screen and the DSI display is turned on (backlight, without content) - but Recalbox keeps stuck there.

Only resetting display_auto_detect=0 makes Recalbox boot again - but only using HDMI screen, the DSI display then keeps turned off, also no matter what dtoverlay is set.

Would really appreciate some help, thank you!

PS: Goal is, to build a 2-player arcade controller with the DSI Display built in, but also have the option to connect an external screen for gameplay.