rocket-chip/src/main/scala/util/Arbiters.scala

package util
import Chisel._
import config._

/** A generalized locking RR arbiter that addresses the limitations of the
 *  version in the Chisel standard library */
abstract class HellaLockingArbiter[T <: Data](typ: T, arbN: Int, rr: Boolean = false)
    extends Module {

  val io = new Bundle {
    val in = Vec(arbN, Decoupled(typ.cloneType)).flip
    val out = Decoupled(typ.cloneType)
  }

  def rotateLeft[T <: Data](norm: Vec[T], rot: UInt): Vec[T] = {
    val n = norm.size
    Vec.tabulate(n) { i =>
      Mux(rot < UInt(n - i), norm(UInt(i) + rot), norm(rot - UInt(n - i)))
    }
  }

  val lockIdx = Reg(init = UInt(0, log2Up(arbN)))
  val locked = Reg(init = Bool(false))

  val choice = if (rr) {
    PriorityMux(
      rotateLeft(Vec(io.in.map(_.valid)), lockIdx + UInt(1)),
      rotateLeft(Vec((0 until arbN).map(UInt(_))), lockIdx + UInt(1)))
  } else {
    PriorityEncoder(io.in.map(_.valid))
  }

  val chosen = Mux(locked, lockIdx, choice)

  for (i <- 0 until arbN) {
    io.in(i).ready := io.out.ready && chosen === UInt(i)
  }

  io.out.valid := io.in(chosen).valid
  io.out.bits := io.in(chosen).bits
}

/** This locking arbiter determines when it is safe to unlock
 *  by peeking at the data */
class HellaPeekingArbiter[T <: Data](
      typ: T, arbN: Int,
      canUnlock: T => Bool,
      needsLock: Option[T => Bool] = None,
      rr: Boolean = false)
    extends HellaLockingArbiter(typ, arbN, rr) {

  def realNeedsLock(data: T): Bool =
    needsLock.map(_(data)).getOrElse(Bool(true))

  when (io.out.fire()) {
    when (!locked && realNeedsLock(io.out.bits)) {
      lockIdx := choice
      locked := Bool(true)
    }
    // the unlock statement takes precedent
    when (canUnlock(io.out.bits)) {
      locked := Bool(false)
    }
  }
}

/** This arbiter determines when it is safe to unlock by counting transactions */
class HellaCountingArbiter[T <: Data](
      typ: T, arbN: Int, count: Int,
      val needsLock: Option[T => Bool] = None,
      rr: Boolean = false)
    extends HellaLockingArbiter(typ, arbN, rr) {

  def realNeedsLock(data: T): Bool =
    needsLock.map(_(data)).getOrElse(Bool(true))

  // if count is 1, you should use a non-locking arbiter
  require(count > 1, "CountingArbiter cannot have count <= 1")

  val lock_ctr = Counter(count)

  when (io.out.fire()) {
    when (!locked && realNeedsLock(io.out.bits)) {
      lockIdx := choice
      locked := Bool(true)
      lock_ctr.inc()
    }

    when (locked) {
      when (lock_ctr.inc()) { locked := Bool(false) }
    }
  }
}