The USB debug port is of interest - most modern systems seem to provide a debug port capability which allows one to send debug over USB to another machine. To check if your USB controller has this capability, use:
sudo lspci -vvv | grep "Debug port"
and look for a string such as "Capabilities:  Debug port: BAR=1 offset=00a0". You may have more than one of these on your system, so beware you use the correct one.
One selects this mode of earlyprintk debugging using:
for the default first port, or select the Nth debug enabled port using:
One also needs to build a kernel with the following config option enabled:
On my debug set-up I used a NET20DC-USB Hi-Speed USB 2.0 Host-to-Host Debug Device connecting the target machine and a host with which I capture the USB debug using /dev/ttyUSB0 with minicom. So that I won't bore you with the details, this is all explained in the kernel documentation in Documentation/x86/earlyprintk.txt
As it was, I needed to tweak the earlyprintk driver to put in some delays in the EHCI probing and reset code to get it working on my fairly fast target laptop.
My experience with this approach wasn't great - I had to plug/unplug the debug device quite frequently for the earlyprintk EHCI reset and probe to work. Also, the EHCI USB driver initialisation later on in the kernel initialisation hung which wasn't useful.
Overall, I may have had problems with the host/target and/or the NET20DC-USB host-to-host device, but it did allow me to get some debug out, be it rather unreliably.
Probably an easier way to get debug out is just using the boot option:
however this has the problem that the messages are eventually overwritten by the real console.
Finally, for anyone with old legacy serial ports on their machine (which is quite unlikely nowadays with newer hardware), one can use:
where ttySn is the nth tty serial port.
One can also append the ",keep" option to not disable the earlyprintk once the real console is up and running.
So, with earlyprintk, there is some chance of being able to get some form of debug out to a device to allow one to debug kernel problems that occur early in the initialisation phase.