FPGA Shells for the Rocket Chip Generator

An FPGA shell is a Chisel module designed to wrap any Rocket Chip subsystem configuration. The goal of the fpga-shell system is to reduce the number of wrappers so as to have only one for each physical device rather than one for every combination of physical device and core configuration.

This repository replaces https://github.com/sifive/fpga-shells

Each shell consists of Overlays which use dependency injection to create and connect peripheral device interfaces in an FPGADesign to the toplevel shell module.

Most devices already have an overlay defined for them in src/main/scala/shell[/xilinx]. If you're using a Xilinx device, you'll probably want to use the xilinx-specific overlay because it defines a few things that you'd otherwise have to specify yourself.

Generally, you'll want to create a device shell that extends Series7Shell or UltraScaleShell. If you need different functionality (or you're not using a Xilinx device), you can extend Shell and implement abstract members. Some Microsemi devices are supported by fgpa-shells as well (and can be found in src/main/scala/shell/microsemi)

For example:

class DeviceShell()(implicit p: Parameters) extends UltraScaleShell {
  // create Overlays
  val myperipheral = Overlay(PeripheralOverlayKey) (new PeripheralOverlay(_,_,_))
  // ...

  // assign abstract members
  val pllReset = InModuleBody { Wire(Bool()) }
  val topDesign = LazyModule(p(DesignKey)(designParameters))

  // ensure clocks are connected
  designParameters(ClockInputOverlayKey).foreach { unused =>
    val source = unused(ClockInputOverlayParams())
    val sink = ClockSinkNode(Seq(ClockSinkParameters()))
    sink := source
  }

  // override module implementation to connect reset
  override lazy val module = new LazyRawModuleImp(this) {
    val reset = IO(Input(Bool()))
    pllReset := reset
  }
}

Each peripheral device to be added to the shell must define an Overlay, which creates the device and connects it to the toplevel shell. In addition, in order to access the overlay, the device needs to have a case class OverlayParams and a case object OverlayKey

case class PeripheralOverlayParams()(implicit val p: Parameters)
case object PeripheralOverlayKey extends Field[Seq[DesignOverlay[PeripheralOverlayParams, PeripheralDesignOutput]]](Nil)

If the device is parameterizable, then each parameter for the device creation can be passed to the PeripheralOverlayParams constructor by adding a field for said parameter. Typically, devices are connected to a TileLink bus for processor control, so PeripheralDesignOutput can usually be substituted with TLInwardNode.

The Overlay extends IOOverlay which is paramerized by the device's IO (in this case PeripheralDeviceIO is a subtype of Data and is a port specification for the peripheral device) and DesignOutput.

abstract class AbstractPeripheralOverlay(val params: PeripheralOverlayParams)
  extends IOOverlay[PeripheralDeviceIO, PeripheralDesignOutput]
{
  // assign abstract member p (used to access overlays with their key)
  // e.g. p(PeripheralOverlayKey) will return a Seq[DesignOverlay[PeripheralOverlayParams, PeripheralDesignOutput]]
  implicit val p = params.p
}

Continuing our example with a DeviceShell shell, the actual overlay is constructed by extending our abstract PeripheralOverlay

class ConcretePeripheralOverlay(val shell: DeviceShell, val name: String, params: PeripheralOverlayParams)
  extends AbstractPeripheralOverlay(params)
{
  val device = LazyModule(new PeripheralDevice(PeripheralDeviceParams(???))) // if your peripheral device isn't parameterizable, then it'll have an empty constructor

  def ioFactory = new PeripheralDeviceIO // ioFactory defines interface of val io
  val designOutput = device.node

  // this is where "code-injection" starts
  val ioSource = BundleBridgeSource(() => device.module.io.cloneType) // create a bridge between device (source) and shell (sink)
  val ioSink = shell { ioSource.makeSink() }

  InModuleBody { ioSource.bundle <> device.module.io }

  shell { InModuleBody {
    val port = ioSink.bundle

    io <> port // io is the bundle of shell-level IO as specified by ioFactory
  } }
}

The actual device implementation (where the device's functionality is defined) will be something like this:

case class PeripheralDeviceParams(param1: Param1Type, ???) // only necessary if your device is parameterizable
class PeripheralDevice(c: PeripheralDeviceParams)(implicit p: Parameters) extends LazyModule {
  
  val node: PeripheralDesignOutput = ???

  // device implementation
  lazy val module = new LazyModuleImp(this) {
    val io = ???
    ???
  }
}

bchetwynd/rocket-chip-fpga-shells

FPGA Shells for the Rocket Chip Generator