How can I activate extra-verbose mode (debugging mode) during Debian boot?

Here is a list of all possible boot parameters: I've never used it, but try adding ignore_loglevel.

I previously mentioned verbose, but actually that only applies to other specific kernel options like acpi, as you can read, above.

With an understanding of your exact hardware, like it seems you have, you can go even deeper with this article on kernel hacking

This file is supposed to be included in /linux/documentationn according to an article on The Linux Documentation Project (TLDP.org)