Negative parameter indexes

[MikeOpenHWGroup]

A recent addition to the CV32E40X (sha 4206fff on 2021-04-27) introduced "negative indexes" for the PMA_CFG parameter. An example of this is bhv/cv32e40x_wrapper.sv:

module cv32e40x_wrapper
  import cv32e40x_pkg::*;
#(
  parameter NUM_MHPMCOUNTERS             =  1,
  parameter int unsigned PMA_NUM_REGIONS =  0,
  parameter pma_region_t PMA_CFG [PMA_NUM_REGIONS-1:0] = '{default:PMA_R_DEFAULT}
)

The default value of PMA_NUM_REGIONS is zero which means the range of PMA_CFG is [-1:0]. Most SV simulators will accept this, but Metrics DSIM interprets SV parameters as unsigned values (which, I believe is correct) and therefore the size of PMA_CFG is (2**31)-1 which exceeds a built-in DSIM limit.

[Silabs-ArjanB]

Hi @silabs-oivind I believe my original proposal would prevent this.

[silabs-oivind]

Hi @MikeOpenHWGroup,
I've updated the CV32E40X RTL to work around this.
However, if I read the section about range specification in the SystemVerilog LRM correctly, range boundaries are allowed to be any integer value, "positive, negative, or zero".

[MikeOpenHWGroup]

Thanks @silabs-oivind, that did the trick. Just for safety's sake it would also be good to do the same for the 40x_wrapper.

FWIW, I've looked into this and agree with you that the LRM supports negative range boundaries. The Metrics DSIM simulator supports these as expected, so I was in error thinking this was the issue. It turns out the problem is that DSIM interprets parameters as unsigned values, so if PUM_NUM_REGIONS is zero, then PMA_NUM_REGIONS-1 is resolved as a very large positive number, not -1.