test: Prefix top-level TB parameters

[andreaskurth]

We use the -G simulator option to define the value of the top-level TB
parameters, and this option apparently overwrites the value of all
parameters in the hierarchy of the TB. To prevent collisions with
parameters below the top-level TB, we now prefix all top-level TB
parameters with Tb or TB_.

[zarubaf]

Just in case you didn’t know -g (lower case) should not override all parameters but just set the top-level ones. So technically that should solve that issue.

[andreaskurth]

Just in case you didn’t know -g (lower case) should not override all parameters but just set the top-level ones. So technically that should solve that issue.

Are you sure? The help message says "Specify generic/parameter default value".

[WRoenninger]

Have been using the -g and Tb prefixing for my last few testbenches. When I recall correctly had some issues with recursive module instantiations with the -g option.

[zarubaf]

Pretty sure, yes. I think that is just an unfortunate naming of the help option. But basically that is what you want to achieve, (re-)setting the default parameter. I have no experience with the option and recursive instantiations :-)

[WRoenninger]

Was using recursiveness with the tc_sram from tech_cells_generic, to break down the sram unto a supported macro size. Had there the same issue with the provided sim script as it is also using -G there.

[andreaskurth]

Alright, let's find out and solve this once and for all. Here's a simple experiment to determine the proper way of setting a parameter in the top-level module without affecting parameters with the same name in other modules:

my_module.sv

module my_module #(
  parameter int unsigned MyParam = 32'hdefa0172,
  parameter int unsigned MyOtherParam = 32'hdefa0173
);
  initial begin
    $display("%m.MyParam = 32'h%0x", MyParam);
    $display("%m.MyOtherParam = 32'h%0x", MyOtherParam);
  end
endmodule

my_toplevel.sv

module my_toplevel #(
  parameter int unsigned MyParam = 32'hdefa0170,
  parameter int unsigned MyOtherParam = 32'hdefa0171
);
  initial begin
    $display("%m.MyParam = 32'h%0x", MyParam);
    $display("%m.MyOtherParam = 32'h%0x", MyOtherParam);
  end

  my_module #(
    .MyParam (32'h17)
  ) some_module ();
endmodule

Invocation

Without arguments

$ vsim -c -do 'run -a; exit' my_toplevel
# my_toplevel.MyParam = 32'hdefa0170
# my_toplevel.MyOtherParam = 32'hdefa0171
# my_toplevel.some_module.MyParam = 32'h17
# my_toplevel.some_module.MyOtherParam = 32'hdefa0173

No surprises here. The parameters that are not assigned by an instantiation retain their default value, and the value of MyParam is properly passed to the some_module instance.

Using -g to set value of parameter not passed in instantiation

$ vsim -c -g MyParam="32'h42" -do 'run -a; exit' my_toplevel
# my_toplevel.MyParam = 32'h42
# my_toplevel.MyOtherParam = 32'hdefa0171
# my_toplevel.some_module.MyParam = 32'h17
# my_toplevel.some_module.MyOtherParam = 32'hdefa0173

No surprises here. The -g option sets MyParam of the top-level module to 32'h42, but for the some_module instance MyParam is not affected.

Using -g to set value of parameter also passed in instantiation

$ vsim -c -g MyOtherParam="32'h42" -do 'run -a; exit' my_toplevel
# my_toplevel.MyParam = 32'hdefa0170
# my_toplevel.MyOtherParam = 32'h42
# my_toplevel.some_module.MyParam = 32'h17
# my_toplevel.some_module.MyOtherParam = 32'h42

This might come unexpected. -g MyOtherParam not only sets the (default) value of MyOtherParam for the top-level module, but also for the some_module instance, where the parameter is not assigned at instantiation.

With -G

$ vsim -c -G MyParam="32'h42" -do 'run -a; exit' my_toplevel
# my_toplevel.MyParam = 32'h42
# my_toplevel.MyOtherParam = 32'hdefa0171
# my_toplevel.some_module.MyParam = 32'h42
# my_toplevel.some_module.MyOtherParam = 32'hdefa0173

$ vsim -c -G MyOtherParam="32'h42" -do 'run -a; exit' my_toplevel
# my_toplevel.MyParam = 32'hdefa0170
# my_toplevel.MyOtherParam = 32'h42
# my_toplevel.some_module.MyParam = 32'h17
# my_toplevel.some_module.MyOtherParam = 32'h42

No surprise in either case, I would say. -G overrides the value of all parameters with a given name, regardless of whether that parameter is assigned at instantiation or not.

Verdict

It seems there is no argument to vsim that sets the value of a parameter only for the top-level module. vsim -h | grep -i param only lists -g and -G (and a logging option). 👎

The next best thing we can do is to use -g, prefix all TB parameters with Tb, and prohibit any non-testbench module to have a parameter starting with Tb. (These rules do not prevent problems with nested TBs, but I think that is currently not a use case for us.) What a complicated way to meet a need that I consider pretty basic for simulating parametrized RTL designs.