Debugging revealed that this is related to the vc4-kms-v3d overlay and
specifically to the cma dtparam.

1. The kernel boots fine if vc4-kms-v3d is *not* loaded.

2. The kernel does *not* boot with any of the following config settings:

dtoverlay=vc4-kms-v3d,cma-96
dtoverlay=vc4-kms-v3d,cma-128
dtoverlay=vc4-kms-v3d,cma-192

3. The kernel boots fine with any other setting like:

dtoverlay=vc4-kms-v3d
dtoverlay=vc4-kms-v3d,cma-64
dtoverlay=vc4-kms-v3d,cma-256

4. The kernel also boots fine if the CMA size is specified on the
commandline. I.e. this boots fine:

cma=128M (in commandline.txt)
dtoverlay=vc4-kms-v3d

The above allocates 128M of CMA, just like dtoverlay=vc4-kms-v3d,cma-128
does but for some reason the allocation via the DT is problematic and
breaks boot.

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https://bugs.launchpad.net/bugs/2146871

Title:
  Boot failure on Pi 3B+

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