While I am so happy to have my J-Link back, the couple of weeks without it have been very productive in terms of open source contributions. After finding out that OpenOCD didn't support raw JTAG mode on CMSIS-DAP adapters, I bit that off as a potential project, and eventually got it working, then did some performance tuning, and I'm pretty pleased with it now. With my LPC-Link2, it can program the flash in an ATMEGA32U4 over JTAG at about 1/4-1/3 of the speed of the J-Link (which is kind of a speed demon). I'm going to let it soak on GitHub for a while, then clean it up and submit it to OpenOCD once it's had a bit of scrutiny.
Implementing this required getting pretty familiar with the CMSIS-DAP source code and protocol, and at some point I realized that it wouldn't be super hard to port the SWD/JTAG debugger part of the CMSIS-DAP firmware over to any USB capable microcontroller. Full CMSIS-DAP support requires the debugger part, plus a serial port and a mass storage device emulator that's also capable of flashing a .bin file to a chip, but I'm not doing this to provide a USB interface to a custom board (like the ones on the mbed site), so I've skipped those two. I'll probably add in the serial port sometime, because I wrote serial bridge code for teensy-openocd-remote-bitbang already.
This was mainly a matter of getting rid of Keil-specific code, plus a small amount of debugging:
The _OUT functions (e.g. PIN_SWDIO_OUT) take a uint32_t with a boolean value in the LSB, but often junk in the higher bits.
A 32-bit processor is expected, so there was one point where I needed to add (uint32_t) casts to avoid losing the high bytes of a word.
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