enable_jtag_gpio=1
uart_2ndstage=1
gpio=4=a5
gpio=27=a4
gpio=23-25=a4

root@raspberrypi:~# /home/pi/openocd/src/openocd -s /home/pi/openocd/tcl/ -f rpi4-to-rpi1.tcl

ssh pi@pi4 -L 3333:localhost:3333 -L 4444:localhost:4444

[clever@amd-nixos:~/apps/rpi/rpi-open-firmware]$ nix-shell default.nix -A aarch64-shell
[nix-shell:~/apps/rpi/rpi-open-firmware]$ armv7l-unknown-linux-gnueabihf-gdb
(gdb) set style enabled off
(gdb) target remote :3333
(gdb) x/5i $pc
=> 0xc03dafd8:  bic     r3, r2, #-16777216      ; 0xff000000
   0xc03dafdc:  bic     r3, r3, #14680064       ; 0xe00000
   0xc03dafe0:  cmp     r3, #0
   0xc03dafe4:  beq     0xc03db048
   0xc03dafe8:  tst     r2, #255        ; 0xff