rocket-chip/src/main/scala/uncore/tilelink2/Arbiter.scala

// See LICENSE.SiFive for license details.

package uncore.tilelink2

import Chisel._
import config._
import diplomacy._

object TLArbiter
{
  // (valids, select) => readys
  type Policy = (Integer, UInt, Bool) => UInt

  val lowestIndexFirst: Policy = (width, valids, select) => ~(leftOR(valids) << 1)(width-1, 0)

  val roundRobin: Policy = (width, valids, select) => {
    val valid = valids(width-1, 0)
    assert (valid === valids)
    val mask = RegInit(~UInt(0, width=width))
    val filter = Cat(valid & ~mask, valid)
    val unready = (rightOR(filter, width*2) >> 1) | (mask << width) // last right shift unneeded
    val readys = ~((unready >> width) & unready(width-1, 0))
    when (select && valid.orR) {
      mask := leftOR(readys & valid, width)
    }
    readys(width-1, 0)
  }

  def lowestFromSeq[T <: TLChannel](edge: TLEdge, sink: DecoupledIO[T], sources: Seq[DecoupledIO[T]]) {
    apply(lowestIndexFirst)(sink, sources.map(s => (edge.numBeats1(s.bits), s)):_*)
  }

  def lowest[T <: TLChannel](edge: TLEdge, sink: DecoupledIO[T], sources: DecoupledIO[T]*) {
    apply(lowestIndexFirst)(sink, sources.toList.map(s => (edge.numBeats1(s.bits), s)):_*)
  }

  def robin[T <: TLChannel](edge: TLEdge, sink: DecoupledIO[T], sources: DecoupledIO[T]*) {
    apply(roundRobin)(sink, sources.toList.map(s => (edge.numBeats1(s.bits), s)):_*)
  }

  def apply[T <: Data](policy: Policy)(sink: DecoupledIO[T], sources: (UInt, DecoupledIO[T])*) {
    if (sources.isEmpty) {
      sink.valid := Bool(false)
    } else {
      val pairs = sources.toList
      val beatsIn = pairs.map(_._1)
      val sourcesIn = pairs.map(_._2)

      // The number of beats which remain to be sent
      val beatsLeft = RegInit(UInt(0))
      val idle = beatsLeft === UInt(0)
      val latch = idle && sink.ready // winner (if any) claims sink

      // Who wants access to the sink?
      val valids = sourcesIn.map(_.valid)
      // Arbitrate amongst the requests
      val readys = Vec(policy(valids.size, Cat(valids.reverse), latch).toBools)
      // Which request wins arbitration?
      val winner = Vec((readys zip valids) map { case (r,v) => r&&v })

      // Confirm the policy works properly
      require (readys.size == valids.size)
      // Never two winners
      val prefixOR = winner.scanLeft(Bool(false))(_||_).init
      assert((prefixOR zip winner) map { case (p,w) => !p || !w } reduce {_ && _})
      // If there was any request, there is a winner
      assert (!valids.reduce(_||_) || winner.reduce(_||_))

      // Track remaining beats
      val maskedBeats = (winner zip beatsIn) map { case (w,b) => Mux(w, b, UInt(0)) }
      val initBeats = maskedBeats.reduce(_ | _) // no winner => 0 beats
      beatsLeft := Mux(latch, initBeats, beatsLeft - sink.fire())

      // The one-hot source granted access in the previous cycle
      val state = RegInit(Vec.fill(sources.size)(Bool(false)))
      val muxState = Mux(idle, winner, state)
      state := muxState

      if (sources.size > 1) {
        val allowed = Mux(idle, readys, state)
        (sourcesIn zip allowed) foreach { case (s, r) =>
          s.ready := sink.ready && r
        }
      } else {
        sourcesIn(0).ready := sink.ready
      }

      sink.valid := Mux(idle, valids.reduce(_||_), Mux1H(state, valids))
      sink.bits := Mux1H(muxState, sourcesIn.map(_.bits))
    }
  }
}

/** Synthesizeable unit tests */
import unittest._

class TestRobin(txns: Int = 128, timeout: Int = 500000)(implicit p: Parameters) extends UnitTest(timeout) {
  val sources = Wire(Vec(6, DecoupledIO(UInt(width=3))))
  val sink = Wire(DecoupledIO(UInt(width=3)))
  val count = RegInit(UInt(0, width=8))

  val lfsr = LFSR16(Bool(true))
  val valid = lfsr(0)
  val ready = lfsr(15)

  sources.zipWithIndex.map { case (z, i) => z.bits := UInt(i) }
  sources(0).valid := valid
  sources(1).valid := Bool(false)
  sources(2).valid := valid
  sources(3).valid := valid
  sources(4).valid := Bool(false)
  sources(5).valid := valid
  sink.ready := ready

  TLArbiter(TLArbiter.roundRobin)(sink, sources.zipWithIndex.map { case (z, i) => (UInt(i), z) }:_*)
  when (sink.fire()) { printf("TestRobin: %d\n", sink.bits) }
  when (!sink.fire()) { printf("TestRobin: idle (%d %d)\n", valid, ready) }

  count := count + UInt(1)
  io.finished := count >= UInt(txns)
}