Add a possibility to specify expected IR lengths for JTAG capabilities of debug probes

[Yatekii]

Add a possibility to specify expected IR lengths for JTAG capabilities of debug probes

[Yatekii]

Looking great! I am not sure we should use terms such as Cortex-M0+ rather than thumbv6 because other stuff uses it that way, but other than that its nice!

[seanmlyons22]

@Yatekii, how do you think this should be done? One idea I have is that something can be added to the target.yaml file that describes the scan chain. This could also be a solution for #1683 where the yaml file could contain the index of the wanted DAP.

Both could be optional, where if not provided we fall back to the default behavior as it is today.

[Yatekii]

Yeah, I think the target specifications are the right place :) Is this a fixed value? We already have special values for denoting the DAP on some multidrop chips (rp2040) or unconventionaly enumerated DAPs :)
So this should fit right in :)

[seanmlyons22]

Is this a fixed value?

Yep! Its fixed in the HW, but not by the JTAG spec. Different vendors can implement different ir lengths.

I found a scanchain description as part of the System Description File which looks like it would do exactly the trick.

Converting this to yaml would look like:

scanchain:
    - !Dap
        name: core0
        type: Cortex-M0+
        ir_len: 4
        protocol: 
        - jtag
        - swd
    - !Device
        name: ICEPICK
        type: Other
        ir_len: 6

Following that format means I think we could try to automatically parse and populate this from target-gen if it exists in the pack. Then, if the target file has a scanchain specified, we could use that to specify the ir_lengths before connect.

[seanmlyons22]

I have the scanchain added to the target now.

Do you have any recommendations on how to connect that to configure_jtag?

Is it through session? I basically need something that knows about the interface and target.

[Yatekii]

I would give it the interface and target as separate params directly :)