Re: [OpenOCD-devel] Improving cmsis-dap performance

[Angus G. <gu...@pr...>]

Hi Andreas,

Thanks for clarifying a great many things.

I did a bit more reading of the source. I hadn't realised that both
JTAG and SWD DAP ops go via dap_ops -> cmsis_dap_transport ->
jtag_interface.swd -> cmsis_dap_swd_driver to generate the USBHID
commands. I actually thought JTAG/CMSIS-DAP hadn't been implemented in
openocd yet, my mistake.

Now I see cmsis_dap_swd_driver is used here not as a way of
implementing SWD operations, but just a way of implementating DAP
operations in a way that the DAP transport can access them.

Does that sound right?

Exploring possibilities, I think we could remove the
cmsis_dap_swd_driver instance and move the read_red/write_reg stuff up
into the transport. However there's got to be a convenient interface
between them:

- If we do the actual USB operations as part of the jtag_interface, we
  need a way for the transport to queue generic CMSIS-DAP commands on
  the jtag_interface. Maybe a custom command that's just raw CMSIS-DAP
  bytes? Or a dap_driver to the jtag_interface, similar to swd_driver?

- Maybe it would be possible to do the USB operations on the transport
  side? This doesn't gel with the description of a transport as
  relating only to the on-wire signalling not the messaging
  protcol. To work the jtag_interface would need a way to queue
  CMSIS-DAP commands on the transport for custom commands,
  adapter_khz, etc.

- Or alternatively we leave it as it is, but maybe label the
  cmsis_dap_swd_driver instance more clearly to make it clearer that
  it's not an SWD-only interface, but just a convenient interface for
  any DAP register operations?


I'd be happy to put time towards exploring any of these though (or
some fourth choice) if it's helpful?


Cheers,


Angus