RP2040 coredump on crash

This 🚧 WORK IN PROGRESS 🚧 project is an add-on you can add to your normal flash image, and it provides an alternative entry-point which presents itself as an USB mass storage device with a core-dump file on it.

The expected use-case is that you use the watchdog alternate entry function of the RP2040 (see RP2040 datasheet §2.8.1.1 Watchdog Boot) to enter this mode (save registers in TODO first).

I do far too much RP2040 development on the train with my old laptop which only has one working USB port, so I cannot plug in a debugger (which would be more faff anyway to carry around). As they say, necessity is the mother of invention, and I can't think of a better platform for this than the RP2040.

Progress

Currently it is only a port of the RP2040 bootrom that can be written into the flash and boot to presenting itself as a mass-storage device.

TODO:

• Generate actual coredump file:
  • Dump memory;
  • Space for registers;
  • Check RAM usage so it can be reserved -- maybe possible to keep this under 256 bytes to only use the RAM which the bootrom thrashes by writing the boot2 loader to it.
    • Turns out the USB MSD code is using memory from the USB peripheral for .bss and stack, though there is still some stack used by the preceeding code, from the CRASHDMP.XXD:

      41ea0 040e 1050 0000 0000 0100 0000 1b0a 0010  ...P............
      41eb0 0000 0000 ec09 1050 0000 0000 990f 0010  .......P........
      41ec0 0000 1150 0500 0000 0100 0000 ad0f 0010  ...P............
      41ed0 0000 1150 ad11 0010 0000 0000 00f0 0140  ...P...........@
      41ee0 4000 0000 0000 fc1f 011f 0420 0000 0018  @.......... ....
      41ef0 0000 0000 1724 0010 0000 fc1f ad02 0010  .....$..........
      

      but 96 bytes isn't too bad.
• Link to the end of the flash, link as an ELF library and provide an example that uses it as a library, rather than just linking an ELF executable.
• Optimize space (can probably reuse some functions from the bootrom).
  • Probably worth providing two variants, a portable one and a B2 optimized one.
• Do hand-over for flashing (writing firmware to the mass storage device).
  • Currently it just resets to the actual bootrom via the reset_usb_boot function. This means you need to flash twice, once to get it into actual

Compiling

Since this isn't burned in to the ROM, there is no need for it to be compiled with the exact compiler that I used, nevertheless you can use the Nix flake to do so.

You can use the usual compile steps:

cmake -Bbuild -GNinja -DCMAKE_EXPORT_COMPILE_COMMANDS=On
cmake --build build

And you can run it using elf2uf2/elf2uf2-rs, e.g.

elf2uf2-rs -d build/bootrom.elf