This issue is duplicate of the existing Bionic issue, and not a
regression in UC20.

https://wiki.ubuntu.com/BionicBeaver/ReleaseNotes/#Raspberry_Pi

"""
For Ubuntu Core 18 on the Raspberry Pi, the vc4-fkms-v3d overlay is included in 
the default boot configuration. This is known to cause issues on the Raspberry 
Pi 3A+ on which the overlay allocates half the available memory, leaving 
insufficient memory for the system to operate correctly (1848247). The 
recommended workaround is to comment out the dtoverlay line in config.txt on 
the system-boot partition (the first partition on the card), like so:

 #dtoverlay=vc4-fkms-v3d
"""

also see https://bugs.launchpad.net/ubuntu/+source/linux-firmware-
raspi2/+bug/1848247

config.txt allows per-model filters.
https://www.raspberrypi.org/documentation/configuration/config-txt/conditional.md

However there is no filter that will select the 3A+ model alone. because
[pi3+] selects both the small 3A+ and big 3B+.

We can fix one, by breaking the other. And we chose to have 3B+ to be
working out of the box, whilst leaving 3A+ broken => requiring
config.txt tweak.

Ideally, we'd want to have config.txt filter for 3A+, but there isn't
one. Hence the documentation.

Without firmware fix, once we have multipartition boot, we will be able
to boot to an initrd, detect things on runtime, adjust config.txt, soft
reset and have the right dtb overlay loaded despite missing 3A+ filter
selector.

I'm not sure how to mark this bug. As it's a duplicate of a known issue,
and is not currently fixable.

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https://bugs.launchpad.net/bugs/1878432

Title:
  rpi3a+ does not boot with core20

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