DEBUG WITH JTAG

	See the file debug-swd.txt, if you are using a Raspberry Pi 5!

A comfortable way to source-code debug Circle applications is possible using the
JTAG (Joint Test Action Group) interface of the Raspberry Pi. This allows
multi-core debugging too. This has been tested for AArch64 operation on the
Raspberry Pi 3 and 4, which is described herein. General information about JTAG
can be found here:

	https://en.wikipedia.org/wiki/JTAG

You need an USB-to-JTAG interface adapter for JTAG debugging. An inexpensive but
well-working choice are FT232H-based adapters. Search for "JTAG FT232H" in your
favorite online store, to find one. Be sure that it supports the 3.3V level of
your Raspberry Pi!

Once you have it, the adapter has to be connected to the GPIO header of your
Raspberry Pi like that (GPIO numbers are chip numbers, not header positions):

	FT232H	JTAG	GPIO

	ADBUS0	TCK	25
	ADBUS1	TDI	26
	ADBUS2	TDO	24
	ADBUS3	TMS	27
	ACBUS0	/TRST	22
	ACBUS1	/SRST	not connected
		RCLK	not connected
	GND		GND

This website helps you to find the header positions (ALT4 setting):

	https://pinout.xyz/pinout/jtag
	
A configuration file for FT232H-based JTAG interfaces, which we will need later,
can be found here:

	https://sourceforge.net/p/openocd/code/ci/master/tree/tcl/interface/ftdi/um232h.cfg

If you have an other JTAG interface, it is likely, that you can use it too, but
you need your own wiring information and configuration file.

Now you need the software. A free on-chip-debugger software is OpenOCD. Normally
you need to download and build the sources by yourself, because there has been
no release with the latest features for some time. Please open this website for
information on how to build OpenOCD and an required configuration file for the
Raspberry Pi 3 (search for "JTAG Software"):

	https://www.suse.com/c/debugging-raspberry-pi-3-with-jtag/
	
Please copy and save the configuration file (starts with "transport select
jtag") as rpi3.cfg. This file is for the Raspberry Pi 3. For the Raspberry Pi 4
please copy the file rpi3.cfg to rpi4.cfg and apply the patch in
tools/template/rpi4.cfg.diff to the new file:

	$ patch rpi4.cfg < rpi4.cfg.diff

Now you have to add the following setting to your config.txt file on the SD
card, to enable the JTAG GPIO pins:

	enable_jtag_gpio=1

Remember that we are debugging in AArch64 mode here, which is selected by the
provided configuration files. Thus, we have to enable AArch64 mode too, which
is done in the template file boot/config64.txt by default.

You need some (dummy) kernel8.img (kernel8-rpi4.img on Raspberry Pi 4) file on
the SD card for boot. You can use the serial bootloader, which can be built in
boot/ using "make bootloader64" for that purpose.

Now insert your SD card into the Raspberry Pi, connect the JTAG USB adapter to
your host PC, reboot the Raspberry Pi and enter in a shell on the host:

	$ openocd -f um232h.cfg -f rpi3.cfg	# or rpi4.cfg

OpenOCD should start and detect the JTAG interface and connected Raspberry Pi.
Go to your application's source directory and enter:

	$ gdb kernel8.elf

	(gdb) target extended-remote :3333
	(gdb) load
	(gdb) tb main
	(gdb) cont

This will start GDB, connect to the OpenOCD port for core 0, load the program,
set a temporary breakpoint at "main" and runs the program until this breakpoint
is hit. Now you can continue to debug as you want.

For multi-core debugging you have start multiple instances of GBD and to connect
the ports 3334 (core 1), 3335 (core 2) and 3336 (core 3) within GDB.