#438 Software breakpoints on m4k broken.
I've been setting up to debug an Edgerouter-X, which uses the mt7621 SoC, which in turn is a dual-core, 4 thread m4k.
I found that software breakpoints were badly broken when attempting to debug the Linux kernel. While the mt7621 has hardware breakpoints, it only has two, and the brokenness in the software breakpoints makes it awkward to even continue from a hardware breakpoint.
There are two issues:
1. The writes that insert the SDBBP instruction or restore the original instruction don't flush the cache. Apparently the m4k has some caching modes that weren't foreseen by the code.
2. The debug entry cause on SDBBP instruction was not derived correctly. It seems in order to identify the breakpoint cause on SDBBP exception, it's necessary to look at the Bp bit of the Debug register (CP0, 23,0). I did try and get this from the Cause register (CP0,13,0), which according to the m4k docs should have a distinct code in the Exc Code field, but this didn't work in testing, so I guess under EJTAG debugging this cause is only indicated in the Debug register.
I have a small patch prepared that fixes both issues, which I will upload it to gerrit and see about getting it landed.
The patch is uploaded now: https://review.openocd.org/c/openocd/+/8519. I would appreciate some feedback.