6.4. Adding a RoCC Accelerator

RoCC accelerators are lazy modules that extend the LazyRoCC class. Their implementation should extends the LazyRoCCModule class.

class CustomAccelerator(opcodes: OpcodeSet)
    (implicit p: Parameters) extends LazyRoCC(opcodes) {
  override lazy val module = new CustomAcceleratorModule(this)

class CustomAcceleratorModule(outer: CustomAccelerator)
    extends LazyRoCCModuleImp(outer) {
  val cmd = Queue(io.cmd)
  // The parts of the command are as follows
  // inst - the parts of the instruction itself
  //   opcode
  //   rd - destination register number
  //   rs1 - first source register number
  //   rs2 - second source register number
  //   funct
  //   xd - is the destination register being used?
  //   xs1 - is the first source register being used?
  //   xs2 - is the second source register being used?
  // rs1 - the value of source register 1
  // rs2 - the value of source register 2

The opcodes parameter for LazyRoCC is the set of custom opcodes that will map to this accelerator. More on this in the next subsection.

The LazyRoCC class contains two TLOutputNode instances, atlNode and tlNode. The former connects into a tile-local arbiter along with the backside of the L1 instruction cache. The latter connects directly to the L1-L2 crossbar. The corresponding Tilelink ports in the module implementation’s IO bundle are atl and tl, respectively.

The other interfaces available to the accelerator are mem, which provides access to the L1 cache; ptw which provides access to the page-table walker; the busy signal, which indicates when the accelerator is still handling an instruction; and the interrupt signal, which can be used to interrupt the CPU.

Look at the examples in generators/rocket-chip/src/main/scala/tile/LazyRoCC.scala for detailed information on the different IOs.

6.4.1. Adding RoCC accelerator to Config

RoCC accelerators can be added to a core by overriding the BuildRoCC parameter in the configuration. This takes a sequence of functions producing LazyRoCC objects, one for each accelerator you wish to add.

For instance, if we wanted to add the previously defined accelerator and route custom0 and custom1 instructions to it, we could do the following.

class WithCustomAccelerator extends Config((site, here, up) => {
  case BuildRoCC => Seq((p: Parameters) => LazyModule(
    new CustomAccelerator(OpcodeSet.custom0 | OpcodeSet.custom1)(p)))

class CustomAcceleratorConfig extends Config(
  new WithCustomAccelerator ++
  new RocketConfig)

To add RoCC instructions in your program, use the RoCC C macros provided in tests/rocc.h. You can find examples in the files tests/accum.c and charcount.c.