On my arm-unstable system, the upgrade to Mesa 21 was not well received. My RPi4 uses llvmpipe with lightdm and it fails to successfully switch to graphical mode. I get a black screen with backlight, I can not switch tty, but I can reboot. Rolling back to 20.3.4 restores a working system.
If I switch to kms, I can get to a desktop but compositing is broken. glxinfo shows Zinc (V3D 4.2). vulkaninfo displays information but vkcube fails to run.
Could be… but it was not intentional. I updated and it broke. I then tried kms in a effort to get a functioning system so I could roll back mesa. While using kms, I checked glxinfo and vulkaninfo.
But if I comment out kms in config.txt to revert back to llvmpipe, black screen.
I was afraid that would be the case. I’ll dig in and figure out what I have messed-up.
Edit: I removed vulkan-broadcom and vulkan-mesa-layers, then upgraded with just mesa. I rebooted and it seems to be working fine now with llvmpipe with xrender compositing.