Parameters¶

A configuration file can declare parameters in a top-level params block and refer to them anywhere else in the file. This keeps a value that appears in several places — a cluster count, a base address, a region size — in one spot, and lets it be overridden from the command line without editing the configuration.

name: my_noc

params:
  num_clusters: 4
  cluster_base: 0x1000_0000
  cluster_size: 0x0004_0000

endpoints:
  - name: "cluster"
    array: "${num_clusters}"
    addr_range:
      base: "${cluster_base}"
      size: "${cluster_size}"
    # ...

Parameters are a property of FlooGen's input, not of its output. They are resolved before the configuration is validated, so the generated SystemVerilog and SystemRDL contain the resolved values inlined — there is nothing left to override downstream. The declared values are additionally emitted as localparams in the generated package for reference (see Generated localparams).

References and expressions¶

A reference is written ${...}, and the text inside the braces is an expression over the declared parameters, not just a name:

params:
  num_x: 4
  num_y: 8
  num_tiles: "${num_x * num_y}"        # 32
  addr_bits: "${1 << 10}"              # 1024

Parameters may reference each other in any order — a declaration can use one written below it. A reference cycle is reported as an error.

Types are inferred¶

There is no type annotation: the type of a parameter is the type of its value. That matters because of how a reference is substituted:

So "${a}-${b}" with a: 1 and b: 2 is the string "1-2" — it is not evaluated as arithmetic, because only the inside of the braces is an expression. If you want the subtraction, write "${a - b}".

Supported expression syntax¶

Arithmetic (+ - * / // % **), bit operations (<< >> | & ^ ~), comparisons (== != < <= > >=) and boolean operators (and or not) over declared parameters and literals.

There are deliberately no function calls, attribute accesses, subscripts, comprehensions or conditionals. A configuration file that can generate structure rather than values stops being checkable against the JSON schema and stops being readable; if you need eight endpoints, declare eight endpoints or use the array support.

Overriding from the command line¶

Any command that reads a configuration accepts -P / --param:

floogen rtl -c my_noc.yml -o output -P num_clusters=8 -P cluster_base=0x2000_0000

The value is parsed as the type inferred from the declaration, so 0x, 0b and _ separators work for integers, and true/false for booleans. Overriding a parameter that is not declared is an error rather than a silent no-op, so that a typo in -P is reported instead of being ignored.

Note that an override makes the generated output depend on more than the configuration file alone. When archiving or reporting a problem, record the -P flags alongside the configuration.

Generated localparams¶

Every declared parameter is emitted as a localparam in the generated package, with its name converted to camel case and its SystemVerilog type inferred from its value:

package floo_my_noc_noc_pkg;

  ////////////////////
  //   Parameters   //
  ////////////////////

  localparam int unsigned NumClusters = 4;
  localparam int unsigned ClusterBase = 268435456;
  localparam bit EnableDebug = 1'b1;

The width is chosen by magnitude rather than declared, so a base address above 4 GiB does not silently truncate.

All declared parameters are emitted, including ones the configuration never references, so that the generated package documents the full set a configuration exposes rather than only the subset that happened to be used.

Relation to the JSON schema¶

The JSON schema describes the configuration after parameters are resolved. A file that writes size: "${cluster_size}" into an integer-typed field will therefore not validate against the schema directly, even though FlooGen accepts it.

YAML anchors¶

YAML anchors and aliases (&name / *name) still work and are the right tool for reusing a whole subtree via merge keys (<<: *base). They cannot be overridden from the command line and do not support arithmetic, which is what params adds.