Feature: Alternate Manchester SWO implementation using DMA

[ssimek]

Fast DMA-based SWO Manchester decoding

Detailed description

The current interrupt-based SWO decoding suffers (IMO) from several issues

• it is limited to ~100 kHz (~50 kbit/s raw, i.e. ~25 kbit/s decoded SWO console) signal
  • while this is usually sufficient for text-based debug output, it gets worse when more data needs to be sent out/graphed/etc
  • it also unnecessarily impacts performance of the tested code, because the MCU has to wait for the ITM to be available
• high frequency hangs the probe due to an overload of interrupt requests
  • this happens easily when starting with new hardware before the init code is debugged

This PR contains a full reimplementation of the SWO Manchester decoder using DMA that makes capture a lot more efficient:

1. all edge times of the signal are captured using a timer
2. DMA is used to record the timings into a circular buffer
3. the buffer is periodically processed in batches, transformig the edge stream into a byte stream for sending in another circular buffer, resulting in effective processing time per sample on the order of several clock cycles
4. the output buffer is processed in a lower-priority ISR as time permits

The result is the ability to process SWO signal up to ~3 MHz with the probe being more resilient against higher frequency signals (it just fails to decode it properly). Continuous streaming at these speeds obviously makes USB a bottleneck (1.5 Mbit = ~180 kB/s, with the USB controller seemingly being able to handle up to about 70 kB/s without double buffering). But for reasonably intermittent flows, one big benefit is the lightened load on the target.

Your checklist for this pull request

• [x] I've read the Code of Conduct
• [x] I've read the guidelines for contributing to this repository
• [ ] It builds for hardware native (see Building the firmware)
  • it does, but it doesn't fit, neither did the original one, at least not with GCC 13.3 I'm using - it fits with LTO, but then the GDB server doesn't work for some reason. I was developing it with a few targets removed so it fit
• [x] It builds as BMDA (see Building the BMDA)
• [x] I've tested it to the best of my ability
• [x] My commit messages provide a useful short description of what the commits do

Closing issues

None that I'm aware of

[dragonmux]

Note: We are aware of the breakage on the lint pass from GitHub's deployment of Ubuntu 24.04 LTS, we will get this fixed in the mean time.

Edit: This has now been fixed, when you rebase this PR on main next, that fix will automatically get pulled in and used.