I have looked into the CMA setting issue a bit. This is what I have found so far.

The rpi4 needs CMA to be in ZONE_DMA (lower 1GB of memory) as this is the only area that the peripherals on the rpi4 can address.

The DT sets the allowed range to allocate the CMA from (arch/arm/boot/dts/bcm2711.dtsi#L869), but it seems to not work here. What does work is instead to set the offset manually. I replaced "cma=256MB" with "cma=256M@704M" and then it boots. Note that it has to be 256M instead of 256MB.

Removing the cma option on the command line was known as a workaround. Without that we would fall back to the build config of 64MB cma which was located at offset 0x38000000. This left 64MB at the end of ZONE_DMA, and I chose offset 704M so that those 64MB would still be free. Not sure if that is needed or not. The crashkernel needs to be in ZONE_DMA as well but it seems to be set to 0 size.

I have tested on 5.7 rc2 from rawhide.

This probably belongs in a bug report. What would be the correct place to file that? From what I can tell upstream has been tested with cma settings without problems (as long as the requested CMA size can fit in ZONE_DMA). From that it seems like fedora-specific issue. Not sure though.