[rv_dm] Debug module does not support sub-word accesses over SBA

[sriyerg]

The debug spec requires the DMI CSR SBCS[SBACCESS] to be implemented as RW, but the design implements it as RO instead.
See: https://github.com/riscv/riscv-debug-spec/raw/4e0bb0fc2d843473db2356623792c6b7603b94d4/riscv-debug-release.pdf
Page 43.

The following logic 'overwrites' any changes made to sbaccess:

opentitan/hw/vendor/pulp_riscv_dbg/src/dm_csrs.sv

Line 527 in 60706c2

sbcs_d.sbaccess = (BusWidth == 32'd64) ? 3'd3 : 3'd2;

On line 431, it captures the data written to this field.

The dm_sba appears to support sub-word accesses, as it sets the byte masks correctly.

The end result is JTAG can only ever initiate 32-bit accesses over SBA.

[tjaychen]

@GregAC
@msfschaffner

[msfschaffner]

There is a PR pending upstream that I think should fix this (pulp-platform/riscv-dbg#106), but since it has not been merged yet we may have to pull that over to our lowRISC fork in the mean time.

We did discuss this on this issue already #4975 and labeled this as a future enhancement. If that is needed for debug to work correctly, though, we may have to reclassify it as P1.

[tjaychen]

is this needed for debug to work correctly? The SBA is just a side band debug bus right?

[msfschaffner]

Yeah I think that is the reason why we had labelled this as P3 in the first place.
Interactive debug should still be possible without it, afaik.

[msfschaffner]

Support for this has been merged upstream: pulp-platform/riscv-dbg#106.

However, there is a fix for a CDC bug outstanding here pulp-platform/riscv-dbg#123 and I would recommend to wait for that one to land before we vendor in anything.

[msfschaffner]

CC @jrrk2

[bluewww]

I reverted the issue that introduced the CDC trst bug here. The feature that introduced this bug will be re-introduced once it's clear whether this fix suffices.

[msfschaffner]

Ah great, thanks for letting us know.
In that case it should be ok to proceed and vendor this in again.

@sriyerg I'll do that later today and will let you know once it is done.

[msfschaffner]

This is now supported - closing.