(In reply to Alex Deucher from comment #2)
> When was it last working?  Can you bisect?

It never really did work. As you can see from the first post (and may remember)
there have been a number of issues with this card and the dc stack and they
often precluded even booting up. So far I always had a "real" error message
that stuck out and that I could sort of hang my bug report on. So I thought
that this issue was due to any of them, not something unrelated (which it seems
to be). Also, so far I was always able to at least boot up with dc=0 and have
it working, but that seems to be over with 4.18 (does not get past gdm login
screen without freezing...).

So no, I cannot really bisect as there does not seem to have ever been a
version that actually did work flawlessly, unfortunately. I really willing to
try and help coax some better debug info out of my machine, though, if anyone
can give me any pointers as to how that might be achieved.