rp-selfdebug

This wraps the CMSIS DAP code in a rust wrapper, and then uses that with the pi pico's dbgforce ability to debug core0 from core1 or vice versa.

Make sure you have cloned this repo using submodules.

I created this because I wanted to be able to debug my keyboard firmware running USB code without having a debugger attached. The firmware doesn't need two cores but it does need to use the USB code, so just using pico-debug wasn't an option.

Prior art

The most obvious inspiration for this is pico-debug which would have been a good option but the repository was archived and if I was going to use USB, I thought I might as well use it in Rust since that's what the rest of the keyboard firmware was written in.

I have also been looking at picoprobe which was easier to compile as it just used cmake and didn't need CrossWorks.

And of course rust-dap should also get a mention, which I looked into using, but I had problems using it, I suspect the reason was due to it not handling error cases correctly and putting 0xFF into the USB bulk response packet, just propagating Result out using ? in the process method.

Configuring

You can configure the following parameters of CMSIS DAP:

• CPU_CLK (default 120000000 [120MHz])
• DAP_DEFAULT_SWJ_CLOCK (default 24000000 [24MHz])
• DAP_PACKET_COUNT (default 8) by editing your .cargo/config.toml, to include:

rustflags = [
  // ...
  "--cfg", "CMSIS_DAP_CPU_CLOCK=\"120000000\"",
  "--cfg", "CMSIS_DAP_DEFAULT_SWJ_CLOCK=\"24000000\"",
  "--cfg", "CMSIS_DAP_PACKET_COUNT=\"8\"",
  // ...
]

If you want to configure something else (see the file CMSIS_Config/DAP_config.h for more configuration options), feel free to make a PR, or just use this repository as an example.

Known problems

rust-lld: error: undefined symbol: __gnu_thumb1_case_uhi

The issue here is that with the -Os optimisation GCC is using a "compact switch" which requires helper functions/tables (https://chromium.googlesource.com/chromiumos/platform/ec/+/refs/heads/master/core/cortex-m0/thumb_case.S), but libgcc.a isn't getting linked. To combat this, the libgcc.a directory is added to the linker search paths (see build.rs) but you have to add -lgcc to your linker, e.g. put the following into .cargo/config.toml

@@ -1,11 +1,12 @@
 [target.'cfg(all(target_arch = "arm", target_os = "none"))']
 # runner = "elf2uf2-rs -d"
 runner = "probe-rs run --chip RP2040 --probe 2E8A:000C"

 rustflags = [
   "-C", "link-arg=-Tlink.x",
   "-C", "link-arg=-Tdefmt.x",
+  "-C", "link-args=-lgcc"
 ]

 [build]
 target = "thumbv6m-none-eabi"

TODOs

• Test flashing core-1 apps. This might require compiling two separate binaries, the CMSIS-DAP one first, and then the core 1 application, linked using the -R/--just-symbols flag pointing to the CMSIS-DAP application's ELF.