EGL rendering via SDL2 broken under 6.12 on 2712D0

[6by9]

Describe the bug

Reported in https://forums.raspberrypi.com/viewtopic.php?p=2311618

Running /usr/libexec/installed-tests/SDL2/testgl2 from libsdl2-tests with a 6.12 kernel on a 2712D0 gives random noise on the screen. 6.6 kernel or C1 gives correct output.

Steps to reproduce the behaviour

sudo apt install libegl1 libsdl2-tests
/usr/libexec/installed-tests/SDL2/testgl2

Device (s)

Raspberry Pi CM5, Raspberry Pi 5

System

Raspberry Pi OS Bookworm
Tested OK on 6.6.78 kernel.
Fails on 6.12.22

Logs

No response

Additional context

No response

[6by9]

Tested on rpi-6.15.y (6.15.0-rc3) as well and also fails.
However I've just noticed that I get a flicker of the correct image as I quit the application. I think this is the DRM async updates in vc4 again - I'll have a look.

[6by9]

https://github.com/raspberrypi/linux/blob/rpi-6.12.y/drivers/gpu/drm/vc4/vc4_plane.c#L2376
== VC4_GEN_6_C should be >= VC4_GEN_6_C.

#6811