Serialization Cookbook

• Why do I need to serialize Modules
• How do I serialize Modules with SerializableModuleGenerator

Why do I need to serialize Modules

Chisel provides a very flexible hardware design experience. However, it sometimes becomes too flexible to design a relative big designs, since parameters of module might come from: 1. Global variables; 2. Outer class; 3. Entropies(time, random). It becomes really hard or impossible to describe "how to reproduce this single module?". This forbids doing unit-test for a module generator, and introduces issues in post-synthesis when doing ECO: a change to Module A might lead to change in Module B. Thus SerializableModuleGenerator, SerializableModule[T <: SerializableModuleParameter] and SerializableModuleParameter are provided to solve these issues. For any SerializableModuleGenerator, Chisel can automatically serialize and de-serialize it by adding these constraints:

1. the SerializableModule should not be inner class, since the outer class is a parameter to it;
2. the SerializableModule has and only has one parameter with SerializableModuleParameter as its type.
3. the Module neither depends on global variables nor uses non-reproducible functions(random, time, etc), and this should be guaranteed by user, since Scala cannot detect it.

It can provide these benefits:

1. user can use SerializableModuleGenerator(module: class[SerializableModule], parameter: SerializableModuleParameter) to auto serialize a Module and its parameter.
2. user can nest SerializableModuleGenerator in other serializable parameters to represent a relative large parameter.
3. user can elaborate any SerializableModuleGenerator into a single module for testing.

How do I serialize Modules with SerializableModuleGenerator

It is pretty simple and illustrated by example below, the GCD Module with width as its parameter.

import chisel3._
import chisel3.experimental.{SerializableModule, SerializableModuleGenerator, SerializableModuleParameter}
import upickle.default._

// provide serialization functions to GCDSerializableModuleParameter
object GCDSerializableModuleParameter {
  implicit def rwP: ReadWriter[GCDSerializableModuleParameter] = macroRW
}

// Parameter
case class GCDSerializableModuleParameter(width: Int) extends SerializableModuleParameter

// Module
class GCDSerializableModule(val parameter: GCDSerializableModuleParameter)
    extends Module
    with SerializableModule[GCDSerializableModuleParameter] {
  val io = IO(new Bundle {
    val a = Input(UInt(parameter.width.W))
    val b = Input(UInt(parameter.width.W))
    val e = Input(Bool())
    val z = Output(UInt(parameter.width.W))
  })
  val x = Reg(UInt(parameter.width.W))
  val y = Reg(UInt(parameter.width.W))
  val z = Reg(UInt(parameter.width.W))
  val e = Reg(Bool())
  when(e) {
    x := io.a
    y := io.b
    z := 0.U
  }
  when(x =/= y) {
    when(x > y) {
      x := x - y
    }.otherwise {
      y := y - x
    }
  }.otherwise {
    z := x
  }
  io.z := z
}

using write function in upickle, it should return a json string:

val j = upickle.default.write(
  SerializableModuleGenerator(
    classOf[GCDSerializableModule],
    GCDSerializableModuleParameter(32)
  )
)
// j: String = "{\"parameter\":{\"width\":32},\"generator\":\"repl.MdocSession$MdocApp$GCDSerializableModule\"}"

You can then read from json string and elaborate the Module:

circt.stage.ChiselStage.emitSystemVerilog(
  upickle.default.read[SerializableModuleGenerator[GCDSerializableModule, GCDSerializableModuleParameter]](
    ujson.read(j)
  ).module()
)