HOWTO
=====

First, load the sg kernel module.
# modprobe sg

To run the gdb server, do (you do not need sudo if you have set up
permissions correctly):
$ make -C build && sudo ./build/st-util 1234 /dev/sg1

Then, in gdb:
(gdb) target remote :1234

Have fun!

Resetting the chip from GDB
===========================

You may reset the chip using GDB if you want. You'll need to use `target
extended-remote' command like in this session:
(gdb) target extended-remote localhost:1111
Remote debugging using localhost:1111
0x080007a8 in _startup ()
(gdb) kill
Kill the program being debugged? (y or n) y
(gdb) run
Starting program: /home/whitequark/ST/apps/bally/firmware.elf 

Remember that you can shorten the commands. `tar ext :1111' is good enough
for GDB.

Setting up udev rules
=====================

For convenience, you may install udev rules file, 10-stlink.rules, located
in the root of repository. You will need to copy it to /etc/udev/rules.d,
and then either reboot or execute
$ udevadm control --reload-rules

Udev will now create a /dev/stlink file, which will point at appropriate
/dev/sgX device. Good to not accidentally start debugging your flash drive.

Running programs from SRAM
==========================

You can run your firmware directly from SRAM if you want to. Just link
it at 0x20000000 and do
(gdb) load firmware.elf

It will be loaded, and pc will be adjusted to point to start of the
code, if it is linked correctly (i.e. ELF has correct entry point).

Writing to flash
================

The GDB stub ships with a correct memory map, including the flash area.
If you would link your executable to 0x08000000 and then do
(gdb) load firmware.elf
then it would be written to the memory.

FAQ
===

Q: My breakpoints do not work at all or only work once.

A: Optimizations can cause severe instruction reordering. For example,
if you are doing something like `REG = 0x100;' in a loop, the code may
be split into two parts: loading 0x100 into some intermediate register
and moving that value to REG. When you set up a breakpoint, GDB will
hook to the first instruction, which may be called only once if there are
enough unused registers. In my experience, -O3 causes that frequently.

Q: At some point I use GDB command `next', and it hangs.

A: Sometimes when you will try to use GDB `next' command to skip a loop,
it will use a rather inefficient single-stepping way of doing that.
Set up a breakpoint manually in that case and do `continue'.