fbdev frame buffer format not supported

I'm trying to get weston with fbdev backend working. The simple framebuffer setup by bootloader accepts just the two formats "r5g6b5" or "a8b8g8r8". Our format setting is "a8b8g8r8" and the output of fbset as expected is:

mode "1024x768-0"
 #D: 0.000 MHz, H: 0.000 kHz, V: 0.000 Hz
        geometry 1024 768 1024 768 32
        timings 0 0 0 0 0 0 0
        accel false
        rgba 8/0,8/8,8/16,8/24

endmode

After starting weston with fbev backend it fails with Frame buffer uses an unsupported format.

The complete output:

[23:19:39.873] weston 5.0.0
               https://wayland.freedesktop.org
               Bug reports to: https://gitlab.freedesktop.org/wayland/weston/issues/
               Build: unknown (not built from git or tarball)
[23:19:39.873] Command line: weston --tty=1
[23:19:39.873] OS: Linux, 4.19.0-xilinx-v2019.1, #1 SMP PREEMPT Thu Mar 18 20:46:07 UTC 2021, armv7l
[23:19:39.874] Using config file '/home/root/.config/weston.ini'
[23:19:39.874] Output repaint window is 7 ms maximum.
[23:19:39.874] Loading module '/usr/lib/libweston-5/fbdev-backend.so'
[23:19:39.876] initializing fbdev backend
[23:19:39.878] Opening fbdev frame buffer.
[23:19:39.878] Calculating pixman format from:
                - type: 0 (aux: 0)
                - visual: 2
                - bpp: 32 (grayscale: 0)
                - red: offset: 0, length: 8, MSB: 0
                - green: offset: 8, length: 8, MSB: 0
                - blue: offset: 16, length: 8, MSB: 0
                - transp: offset: 24, length: 8, MSB: 0
[23:19:39.878] Frame buffer uses an unsupported format.
[23:19:39.878] Failed to get frame buffer info: No such file or directory
[23:19:39.878] Creating frame buffer head failed.
[23:19:39.898] fatal: failed to create compositor backend

Which formats supports the fbdev frame buffer? What is the background of not supporting "a8b8g8r8?