how to capture kernel panic on headless rpii4?

[tech-lists]

Hi,

What's the best way to capture a drop to ddb before the kernel fully boots?
With some searching I've found a couple of ways, but most pages I've seen are
either for other OSes or they may be stale. So, what's the best way (by "best"
I mean most convenient/easy that gets the job done) with a -current as of
r365355? i.e. it has all the debugging bits you'd expect compiled in as per
-curren generic.

I have a FTDI console cable
https://www.amazon.co.uk/gp/product/B01N2K27P9/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
but it looks like it can't be used with this.

I guess I need an FTDI to TTL 3-wire cable?

Or is there a way to make kernel output write to the usb interface before it
fully boots? Or of making it write to the sd card?

I fitted the rpi4 with an aluminium case that also acts as a passive cooler.
It's in contact with the cpu, however there's no access to the GPIO pins when
the case is on. If i can get output through one of the ports it'd be great.

thanks,
-- 
J.
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