SPI display as a secondary display?

[amiotk]

Hello

I would like to run a simple GTK3 GUI on a 3.5" SPI ILI9341 display. I can get an image displayed on the LCD by using FBTFT and writing to /dev/fb1 - this works great. Is there any way of setting the X server to treat /dev/fb1 as a secondary display, or am I living in a dream?
The use case is to have HDMI display as primary and SPI as secondary display running standard Linux app.
I know there are different solutions like GTKFB for GTK2 or QT for embedded, but sticking in GTK3 would save me from rewriting the app.

Thanks

[notro]

I don't know if the X server can mix and match DRM and FB devices and I haven't looked much at the userspace side of graphics myself.

A search turned up this recent post:
Dual display support with legacy driver (HDMI + SPI-LCD on RPi4)
https://forums.raspberrypi.com/viewtopic.php?t=306624

There's a a new DRM driver coming up that should work with your display, but it's not in mainline yet. It will be backported to rpi-linux-5.15 at some point.
https://github.com/notro/panel-mipi-dbi/wiki

[amiotk]

@notro thanks for the reply. Based on the Raspberry Pi forum topic you found, I think I should be able to pull this off.
The new DRM drivers look really great, but I'm stuck with 4.4 kernel, so most probably can't use them.

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