6.8. Keys, Traits, and Configs

You have probably seen snippets of Chisel referencing keys, traits, and configs by this point. This section aims to elucidate the interactions between these Chisel/Scala components, and provide best practices for how these should be used to create a parameterized design and configure it.

We will continue to use the GCD example.

6.8.1. Keys

Keys specify some parameter which controls some custom widget. Keys should typically be implemented as Option types, with a default value of None that means no change in the system. In other words, the default behavior when the user does not explicitly set the key should be a no-op.

Keys should be defined and documented in sub-projects, since they generally deal with some specific block, and not system-level integration. (We make an exception for the example GCD widget).

case object GCDKey extends Field[Option[GCDParams]](None)

The object within a key is typically a case class XXXParams, which defines a set of parameters which some block accepts. For example, the GCD widget’s GCDParams parameterizes its address, operand widths, whether the widget should be connected by Tilelink or AXI4, and whether the widget should use the blackbox-Verilog implementation, or the Chisel implementation.

case class GCDParams(
  address: BigInt = 0x2000,
  width: Int = 32,
  useAXI4: Boolean = false,
  useBlackBox: Boolean = true)

Accessing the value stored in the key is easy in Chisel, as long as the implicit p: Parameters object is being passed through to the relevant module. For example, p(GCDKey).get.address returns the address field of GCDParams. Note this only works if GCDKey was not set to None, so your Chisel should check for that case!

6.8.2. Traits

Typically, most custom blocks will need to modify the behavior of some pre-existing block. For example, the GCD widget needs the DigitalTop module to instantiate and connect the widget via Tilelink, generate a top-level gcd_busy port, and connect that to the module as well. Traits let us do this without modifying the existing code for the DigitalTop, and enables compartmentalization of code for different custom blocks.

Top-level traits specify that the DigitalTop has been parameterized to read some custom key and optionally instantiate and connect a widget defined by that key. Traits should not mandate the instantiation of custom logic. In other words, traits should be written with CanHave semantics, where the default behavior when the key is unset is a no-op.

Top-level traits should be defined and documented in subprojects, alongside their corresponding keys. The traits should then be added to the DigitalTop being used by Chipyard.

Below we see the traits for the GCD example. The Lazy trait connects the GCD module to the Diplomacy graph, while the Implementation trait causes the DigitalTop to instantiate an additional port and concretely connect it to the GCD module.

trait CanHavePeripheryGCD { this: BaseSubsystem =>
  private val portName = "gcd"

  // Only build if we are using the TL (nonAXI4) version
  val gcd = p(GCDKey) match {
    case Some(params) => {
      if (params.useAXI4) {
        val gcd = LazyModule(new GCDAXI4(params, pbus.beatBytes)(p))
        pbus.toSlave(Some(portName)) {
          gcd.node :=
          AXI4Buffer () :=
          TLToAXI4 () :=
          // toVariableWidthSlave doesn't use holdFirstDeny, which TLToAXI4() needsx
          TLFragmenter(pbus.beatBytes, pbus.blockBytes, holdFirstDeny = true)
      } else {
        val gcd = LazyModule(new GCDTL(params, pbus.beatBytes)(p))
        pbus.toVariableWidthSlave(Some(portName)) { gcd.node }
    case None => None
// DOC include end: GCD lazy trait

// DOC include start: GCD imp trait
trait CanHavePeripheryGCDModuleImp extends LazyModuleImp {
  val outer: CanHavePeripheryGCD
  val gcd_busy = outer.gcd match {
    case Some(gcd) => {
      val busy = IO(Output(Bool()))
      busy := gcd.module.io.gcd_busy
    case None => None

These traits are added to the default DigitalTop in Chipyard.

class DigitalTop(implicit p: Parameters) extends ChipyardSystem
  with testchipip.CanHaveTraceIO // Enables optionally adding trace IO
  with testchipip.CanHaveBackingScratchpad // Enables optionally adding a backing scratchpad
  with testchipip.CanHavePeripheryBlockDevice // Enables optionally adding the block device
  with testchipip.CanHavePeripheryTLSerial // Enables optionally adding the backing memory and serial adapter
  with sifive.blocks.devices.uart.HasPeripheryUART // Enables optionally adding the sifive UART
  with sifive.blocks.devices.gpio.HasPeripheryGPIO // Enables optionally adding the sifive GPIOs
  with sifive.blocks.devices.spi.HasPeripherySPIFlash // Enables optionally adding the sifive SPI flash controller
  with sifive.blocks.devices.spi.HasPeripherySPI // Enables optionally adding the sifive SPI port
  with icenet.CanHavePeripheryIceNIC // Enables optionally adding the IceNIC for FireSim
  with chipyard.example.CanHavePeripheryInitZero // Enables optionally adding the initzero example widget
  with chipyard.example.CanHavePeripheryGCD // Enables optionally adding the GCD example widget
  with chipyard.example.CanHavePeripheryStreamingFIR // Enables optionally adding the DSPTools FIR example widget
  with chipyard.example.CanHavePeripheryStreamingPassthrough // Enables optionally adding the DSPTools streaming-passthrough example widget
  with nvidia.blocks.dla.CanHavePeripheryNVDLA // Enables optionally having an NVDLA
  override lazy val module = new DigitalTopModule(this)

class DigitalTopModule[+L <: DigitalTop](l: L) extends ChipyardSystemModule(l)
  with testchipip.CanHaveTraceIOModuleImp
  with sifive.blocks.devices.uart.HasPeripheryUARTModuleImp
  with sifive.blocks.devices.gpio.HasPeripheryGPIOModuleImp
  with sifive.blocks.devices.spi.HasPeripherySPIFlashModuleImp
  with sifive.blocks.devices.spi.HasPeripherySPIModuleImp
  with chipyard.example.CanHavePeripheryGCDModuleImp
  with freechips.rocketchip.util.DontTouch

6.8.3. Config Fragments

Config fragments set the keys to a non-default value. Together, the collection of config fragments which define a configuration generate the values for all the keys used by the generator.

For example, the WithGCD config fragment is parameterized by the type of GCD widget you want to instantiate. When this config fragment is added to a config, the GCDKey is set to a instance of GCDParams, informing the previously mentioned traits to instantiate and connect the GCD widget appropriately.

class WithGCD(useAXI4: Boolean, useBlackBox: Boolean) extends Config((site, here, up) => {
  case GCDKey => Some(GCDParams(useAXI4 = useAXI4, useBlackBox = useBlackBox))

We can use this config fragment when composing our configs.

class GCDTLRocketConfig extends Config(
  new chipyard.example.WithGCD(useAXI4=false, useBlackBox=false) ++          // Use GCD Chisel, connect Tilelink
  new freechips.rocketchip.subsystem.WithNBigCores(1) ++
  new chipyard.config.AbstractConfig)


Readers who want more information on the configuration system may be interested in reading Context-Dependent-Environments.