Raspberry Pi 4 boot hangs in sched_idletd.

[Robert Crowston]

> Is there some sort of guide or instructions for using a JTAG debugger
> on the FreeBSD kernel available somewhere?

I haven't seen anything. Once you have the JTAG up and running, it typically presents itself to you as an GDB server, so you connect to it from gdb, add the symbols for your kernel so you can make sense of what's going on, and treat it much like a regular executable.

I found this page invaluable for the configuration needed to work on the Pi: https://metebalci.com/blog/bare-metal-raspberry-pi-3b-jtag/#enabling-jtag-on-raspberry-pi
I use this USB-attached JTAG: https://uk.farnell.com/ftdi/c232hm-ddhsl-0/cable-usb-mpsse-0-25a-3-3v-o-p/dp/2352015?CMP=i-ddd7-00001003 (on a MacBook)

You'll need to build openOCD and a version of gdb targeting your intended architecture.

There are a few hoops to jump through but I don't think it's too difficult at all.

Let me know if you get stuck.