rocket-chip/junctions/src/main/scala/nasti.scala

/// See LICENSE for license details.

package junctions
import Chisel._
import scala.math.max
import scala.collection.mutable.ArraySeq
import cde.{Parameters, Field}

case object NastiKey extends Field[NastiParameters]

case class NastiParameters(dataBits: Int, addrBits: Int, idBits: Int)

trait HasNastiParameters {
  implicit val p: Parameters
  val nastiExternal = p(NastiKey)
  val nastiXDataBits = nastiExternal.dataBits
  val nastiWStrobeBits = nastiXDataBits / 8
  val nastiXAddrBits = nastiExternal.addrBits
  val nastiWIdBits = nastiExternal.idBits
  val nastiRIdBits = nastiExternal.idBits
  val nastiXIdBits = max(nastiWIdBits, nastiRIdBits)
  val nastiXUserBits = 1
  val nastiAWUserBits = nastiXUserBits
  val nastiWUserBits = nastiXUserBits
  val nastiBUserBits = nastiXUserBits
  val nastiARUserBits = nastiXUserBits
  val nastiRUserBits = nastiXUserBits
  val nastiXLenBits = 8
  val nastiXSizeBits = 3
  val nastiXBurstBits = 2
  val nastiXCacheBits = 4
  val nastiXProtBits = 3
  val nastiXQosBits = 4
  val nastiXRegionBits = 4
  val nastiXRespBits = 2

  def bytesToXSize(bytes: UInt) = MuxLookup(bytes, UInt("b111"), Array(
    UInt(1) -> UInt(0),
    UInt(2) -> UInt(1),
    UInt(4) -> UInt(2),
    UInt(8) -> UInt(3),
    UInt(16) -> UInt(4),
    UInt(32) -> UInt(5),
    UInt(64) -> UInt(6),
    UInt(128) -> UInt(7)))
}

abstract class NastiModule(implicit val p: Parameters) extends Module
  with HasNastiParameters
abstract class NastiBundle(implicit val p: Parameters) extends ParameterizedBundle()(p)
  with HasNastiParameters

abstract class NastiChannel(implicit p: Parameters) extends NastiBundle()(p)
abstract class NastiMasterToSlaveChannel(implicit p: Parameters) extends NastiChannel()(p)
abstract class NastiSlaveToMasterChannel(implicit p: Parameters) extends NastiChannel()(p)

trait HasNastiMetadata extends HasNastiParameters {
  val addr   = UInt(width = nastiXAddrBits)
  val len    = UInt(width = nastiXLenBits)
  val size   = UInt(width = nastiXSizeBits)
  val burst  = UInt(width = nastiXBurstBits)
  val lock   = Bool()
  val cache  = UInt(width = nastiXCacheBits)
  val prot   = UInt(width = nastiXProtBits)
  val qos    = UInt(width = nastiXQosBits)
  val region = UInt(width = nastiXRegionBits)
}

trait HasNastiData extends HasNastiParameters {
  val data = UInt(width = nastiXDataBits)
  val last = Bool()
}

class NastiIO(implicit val p: Parameters) extends ParameterizedBundle()(p) {
  val aw = Decoupled(new NastiWriteAddressChannel)
  val w  = Decoupled(new NastiWriteDataChannel)
  val b  = Decoupled(new NastiWriteResponseChannel).flip
  val ar = Decoupled(new NastiReadAddressChannel)
  val r  = Decoupled(new NastiReadDataChannel).flip
}

class NastiAddressChannel(implicit p: Parameters) extends NastiMasterToSlaveChannel()(p)
    with HasNastiMetadata

class NastiResponseChannel(implicit p: Parameters) extends NastiSlaveToMasterChannel()(p) {
  val resp = UInt(width = nastiXRespBits)
}

class NastiWriteAddressChannel(implicit p: Parameters) extends NastiAddressChannel()(p) {
  val id   = UInt(width = nastiWIdBits)
  val user = UInt(width = nastiAWUserBits)
}

class NastiWriteDataChannel(implicit p: Parameters) extends NastiMasterToSlaveChannel()(p)
    with HasNastiData {
  val strb = UInt(width = nastiWStrobeBits)
  val user = UInt(width = nastiWUserBits)
}

class NastiWriteResponseChannel(implicit p: Parameters) extends NastiResponseChannel()(p) {
  val id   = UInt(width = nastiWIdBits)
  val user = UInt(width = nastiBUserBits)
}

class NastiReadAddressChannel(implicit p: Parameters) extends NastiAddressChannel()(p) {
  val id   = UInt(width = nastiRIdBits)
  val user = UInt(width = nastiARUserBits)
}

class NastiReadDataChannel(implicit p: Parameters) extends NastiResponseChannel()(p)
    with HasNastiData {
  val id   = UInt(width = nastiRIdBits)
  val user = UInt(width = nastiRUserBits)
}

object NastiConstants {
  val BURST_FIXED = UInt("b00")
  val BURST_INCR  = UInt("b01")
  val BURST_WRAP  = UInt("b10")

  val RESP_OKAY = UInt("b00")
  val RESP_EXOKAY = UInt("b01")
  val RESP_SLVERR = UInt("b10")
  val RESP_DECERR = UInt("b11")
}

import NastiConstants._

object NastiWriteAddressChannel {
  def apply(id: UInt, addr: UInt, size: UInt,
      len: UInt = UInt(0), burst: UInt = BURST_INCR)
      (implicit p: Parameters) = {
    val aw = Wire(new NastiWriteAddressChannel)
    aw.id := id
    aw.addr := addr
    aw.len := len
    aw.size := size
    aw.burst := burst
    aw.lock := Bool(false)
    aw.cache := UInt("b0000")
    aw.prot := UInt("b000")
    aw.qos := UInt("b0000")
    aw.region := UInt("b0000")
    aw.user := UInt(0)
    aw
  }
}

object NastiReadAddressChannel {
  def apply(id: UInt, addr: UInt, size: UInt,
      len: UInt = UInt(0), burst: UInt = BURST_INCR)
      (implicit p: Parameters) = {
    val ar = Wire(new NastiReadAddressChannel)
    ar.id := id
    ar.addr := addr
    ar.len := len
    ar.size := size
    ar.burst := burst
    ar.lock := Bool(false)
    ar.cache := UInt(0)
    ar.prot := UInt(0)
    ar.qos := UInt(0)
    ar.region := UInt(0)
    ar.user := UInt(0)
    ar
  }
}

object NastiWriteDataChannel {
  def apply(data: UInt, last: Bool = Bool(true))(implicit p: Parameters): NastiWriteDataChannel = {
    val w = Wire(new NastiWriteDataChannel)
    w.strb := Fill(w.nastiWStrobeBits, UInt(1, 1))
    w.data := data
    w.last := last
    w.user := UInt(0)
    w
  }
  def apply(data: UInt, strb: UInt, last: Bool)
           (implicit p: Parameters): NastiWriteDataChannel = {
    val w = apply(data, last)
    w.strb := strb
    w
  }
}

object NastiReadDataChannel {
  def apply(id: UInt, data: UInt, last: Bool = Bool(true), resp: UInt = UInt(0))(
      implicit p: Parameters) = {
    val r = Wire(new NastiReadDataChannel)
    r.id := id
    r.data := data
    r.last := last
    r.resp := resp
    r.user := UInt(0)
    r
  }
}

object NastiWriteResponseChannel {
  def apply(id: UInt, resp: UInt = UInt(0))(implicit p: Parameters) = {
    val b = Wire(new NastiWriteResponseChannel)
    b.id := id
    b.resp := resp
    b.user := UInt(0)
    b
  }
}

class MemIONastiIOConverter(cacheBlockOffsetBits: Int)(implicit p: Parameters) extends MIFModule
    with HasNastiParameters {
  val io = new Bundle {
    val nasti = (new NastiIO).flip
    val mem = new MemIO
  }

  require(mifDataBits == nastiXDataBits, "Data sizes between LLC and MC don't agree")
  val (mif_cnt_out, mif_wrap_out) = Counter(io.mem.resp.fire(), mifDataBeats)

  assert(!io.nasti.aw.valid || io.nasti.aw.bits.size === UInt(log2Up(mifDataBits/8)),
    "Nasti data size does not match MemIO data size")
  assert(!io.nasti.ar.valid || io.nasti.ar.bits.size === UInt(log2Up(mifDataBits/8)),
    "Nasti data size does not match MemIO data size")
  assert(!io.nasti.aw.valid || io.nasti.aw.bits.len === UInt(mifDataBeats - 1),
    "Nasti length does not match number of MemIO beats")
  assert(!io.nasti.ar.valid || io.nasti.ar.bits.len === UInt(mifDataBeats - 1),
    "Nasti length does not match number of MemIO beats")

  // according to the spec, we can't send b until the last transfer on w
  val b_ok = Reg(init = Bool(true))
  when (io.nasti.aw.fire()) { b_ok := Bool(false) }
  when (io.nasti.w.fire() && io.nasti.w.bits.last) { b_ok := Bool(true) }

  val id_q = Module(new Queue(UInt(width = nastiWIdBits), 2))
  id_q.io.enq.valid := io.nasti.aw.valid && io.mem.req_cmd.ready
  id_q.io.enq.bits := io.nasti.aw.bits.id
  id_q.io.deq.ready := io.nasti.b.ready && b_ok

  io.mem.req_cmd.bits.addr := Mux(io.nasti.aw.valid, io.nasti.aw.bits.addr, io.nasti.ar.bits.addr) >>
                                UInt(cacheBlockOffsetBits)
  io.mem.req_cmd.bits.tag := Mux(io.nasti.aw.valid, io.nasti.aw.bits.id, io.nasti.ar.bits.id)
  io.mem.req_cmd.bits.rw := io.nasti.aw.valid
  io.mem.req_cmd.valid := (io.nasti.aw.valid && id_q.io.enq.ready) || io.nasti.ar.valid
  io.nasti.ar.ready := io.mem.req_cmd.ready && !io.nasti.aw.valid
  io.nasti.aw.ready := io.mem.req_cmd.ready && id_q.io.enq.ready

  io.nasti.b.valid := id_q.io.deq.valid && b_ok
  io.nasti.b.bits.id := id_q.io.deq.bits
  io.nasti.b.bits.resp := UInt(0)

  io.nasti.w.ready := io.mem.req_data.ready
  io.mem.req_data.valid := io.nasti.w.valid
  io.mem.req_data.bits.data := io.nasti.w.bits.data
  assert(!io.nasti.w.valid || io.nasti.w.bits.strb.andR, "MemIO must write full cache line")

  io.nasti.r.valid := io.mem.resp.valid
  io.nasti.r.bits.data := io.mem.resp.bits.data
  io.nasti.r.bits.last := mif_wrap_out
  io.nasti.r.bits.id := io.mem.resp.bits.tag
  io.nasti.r.bits.resp := UInt(0)
  io.mem.resp.ready := io.nasti.r.ready
}

/** Arbitrate among arbN masters requesting to a single slave */
class NastiArbiter(val arbN: Int)(implicit p: Parameters) extends NastiModule {
  val io = new Bundle {
    val master = Vec(arbN, new NastiIO).flip
    val slave = new NastiIO
  }

  if (arbN > 1) {
    val arbIdBits = log2Up(arbN)

    val ar_arb = Module(new RRArbiter(new NastiReadAddressChannel, arbN))
    val aw_arb = Module(new RRArbiter(new NastiWriteAddressChannel, arbN))

    val slave_r_arb_id = io.slave.r.bits.id(arbIdBits - 1, 0)
    val slave_b_arb_id = io.slave.b.bits.id(arbIdBits - 1, 0)

    val w_chosen = Reg(UInt(width = arbIdBits))
    val w_done = Reg(init = Bool(true))

    when (aw_arb.io.out.fire()) {
      w_chosen := aw_arb.io.chosen
      w_done := Bool(false)
    }

    when (io.slave.w.fire() && io.slave.w.bits.last) {
      w_done := Bool(true)
    }

    for (i <- 0 until arbN) {
      val m_ar = io.master(i).ar
      val m_aw = io.master(i).aw
      val m_r = io.master(i).r
      val m_b = io.master(i).b
      val a_ar = ar_arb.io.in(i)
      val a_aw = aw_arb.io.in(i)
      val m_w = io.master(i).w

      a_ar <> m_ar
      a_ar.bits.id := Cat(m_ar.bits.id, UInt(i, arbIdBits))

      a_aw <> m_aw
      a_aw.bits.id := Cat(m_aw.bits.id, UInt(i, arbIdBits))

      m_r.valid := io.slave.r.valid && slave_r_arb_id === UInt(i)
      m_r.bits := io.slave.r.bits
      m_r.bits.id := io.slave.r.bits.id >> UInt(arbIdBits)

      m_b.valid := io.slave.b.valid && slave_b_arb_id === UInt(i)
      m_b.bits := io.slave.b.bits
      m_b.bits.id := io.slave.b.bits.id >> UInt(arbIdBits)

      m_w.ready := io.slave.w.ready && w_chosen === UInt(i) && !w_done
    }

    io.slave.r.ready := io.master(slave_r_arb_id).r.ready
    io.slave.b.ready := io.master(slave_b_arb_id).b.ready

    io.slave.w.bits := io.master(w_chosen).w.bits
    io.slave.w.valid := io.master(w_chosen).w.valid && !w_done

    io.slave.ar <> ar_arb.io.out

    io.slave.aw.bits <> aw_arb.io.out.bits
    io.slave.aw.valid := aw_arb.io.out.valid && w_done
    aw_arb.io.out.ready := io.slave.aw.ready && w_done

  } else { io.slave <> io.master.head }
}

/** A slave that send decode error for every request it receives */
class NastiErrorSlave(implicit p: Parameters) extends NastiModule {
  val io = (new NastiIO).flip

  when (io.ar.fire()) { printf("Invalid read address %x\n", io.ar.bits.addr) }
  when (io.aw.fire()) { printf("Invalid write address %x\n", io.aw.bits.addr) }

  val r_queue = Module(new Queue(new NastiReadAddressChannel, 2))
  r_queue.io.enq <> io.ar

  val responding = Reg(init = Bool(false))
  val beats_left = Reg(init = UInt(0, nastiXLenBits))

  when (!responding && r_queue.io.deq.valid) {
    responding := Bool(true)
    beats_left := r_queue.io.deq.bits.len
  }

  io.r.valid := r_queue.io.deq.valid && responding
  io.r.bits.id := r_queue.io.deq.bits.id
  io.r.bits.data := UInt(0)
  io.r.bits.resp := RESP_DECERR
  io.r.bits.last := beats_left === UInt(0)

  r_queue.io.deq.ready := io.r.fire() && io.r.bits.last

  when (io.r.fire()) {
    when (beats_left === UInt(0)) {
      responding := Bool(false)
    } .otherwise {
      beats_left := beats_left - UInt(1)
    }
  }

  val draining = Reg(init = Bool(false))
  io.w.ready := draining

  when (io.aw.fire()) { draining := Bool(true) }
  when (io.w.fire() && io.w.bits.last) { draining := Bool(false) }

  val b_queue = Module(new Queue(UInt(width = nastiWIdBits), 2))
  b_queue.io.enq.valid := io.aw.valid && !draining
  b_queue.io.enq.bits := io.aw.bits.id
  io.aw.ready := b_queue.io.enq.ready && !draining
  io.b.valid := b_queue.io.deq.valid && !draining
  io.b.bits.id := b_queue.io.deq.bits
  io.b.bits.resp := Bits("b11")
  b_queue.io.deq.ready := io.b.ready && !draining
}

/** Take a single Nasti master and route its requests to various slaves
 *  @param nSlaves the number of slaves
 *  @param routeSel a function which takes an address and produces
 *  a one-hot encoded selection of the slave to write to */
class NastiRouter(nSlaves: Int, routeSel: UInt => UInt)(implicit p: Parameters)
    extends NastiModule {

  val io = new Bundle {
    val master = (new NastiIO).flip
    val slave = Vec(nSlaves, new NastiIO)
  }

  val ar_route = routeSel(io.master.ar.bits.addr)
  val aw_route = routeSel(io.master.aw.bits.addr)

  var ar_ready = Bool(false)
  var aw_ready = Bool(false)
  var w_ready = Bool(false)

  io.slave.zipWithIndex.foreach { case (s, i) =>
    s.ar.valid := io.master.ar.valid && ar_route(i)
    s.ar.bits := io.master.ar.bits
    ar_ready = ar_ready || (s.ar.ready && ar_route(i))

    s.aw.valid := io.master.aw.valid && aw_route(i)
    s.aw.bits := io.master.aw.bits
    aw_ready = aw_ready || (s.aw.ready && aw_route(i))

    val chosen = Reg(init = Bool(false))
    when (s.w.fire() && s.w.bits.last) { chosen := Bool(false) }
    when (s.aw.fire()) { chosen := Bool(true) }

    s.w.valid := io.master.w.valid && chosen
    s.w.bits := io.master.w.bits
    w_ready = w_ready || (s.w.ready && chosen)
  }

  val r_invalid = !ar_route.orR
  val w_invalid = !aw_route.orR

  val err_slave = Module(new NastiErrorSlave)
  err_slave.io.ar.valid := r_invalid && io.master.ar.valid
  err_slave.io.ar.bits := io.master.ar.bits
  err_slave.io.aw.valid := w_invalid && io.master.aw.valid
  err_slave.io.aw.bits := io.master.aw.bits
  err_slave.io.w.valid := io.master.w.valid
  err_slave.io.w.bits := io.master.w.bits

  io.master.ar.ready := ar_ready || (r_invalid && err_slave.io.ar.ready)
  io.master.aw.ready := aw_ready || (w_invalid && err_slave.io.aw.ready)
  io.master.w.ready := w_ready || err_slave.io.w.ready

  val b_arb = Module(new RRArbiter(new NastiWriteResponseChannel, nSlaves + 1))
  val r_arb = Module(new JunctionsPeekingArbiter(
    new NastiReadDataChannel, nSlaves + 1,
    // we can unlock if it's the last beat
    (r: NastiReadDataChannel) => r.last))

  for (i <- 0 until nSlaves) {
    b_arb.io.in(i) <> io.slave(i).b
    r_arb.io.in(i) <> io.slave(i).r
  }

  b_arb.io.in(nSlaves) <> err_slave.io.b
  r_arb.io.in(nSlaves) <> err_slave.io.r

  io.master.b <> b_arb.io.out
  io.master.r <> r_arb.io.out
}

/** Crossbar between multiple Nasti masters and slaves
 *  @param nMasters the number of Nasti masters
 *  @param nSlaves the number of Nasti slaves
 *  @param routeSel a function selecting the slave to route an address to */
class NastiCrossbar(nMasters: Int, nSlaves: Int, routeSel: UInt => UInt)
                   (implicit p: Parameters) extends NastiModule {
  val io = new Bundle {
    val masters = Vec(nMasters, new NastiIO).flip
    val slaves = Vec(nSlaves, new NastiIO)
  }

  if (nMasters == 1) {
    val router = Module(new NastiRouter(nSlaves, routeSel))
    router.io.master <> io.masters.head
    io.slaves <> router.io.slave
  } else {
    val routers = Vec.fill(nMasters) { Module(new NastiRouter(nSlaves, routeSel)).io }
    val arbiters = Vec.fill(nSlaves) { Module(new NastiArbiter(nMasters)).io }

    for (i <- 0 until nMasters) {
      routers(i).master <> io.masters(i)
    }

    for (i <- 0 until nSlaves) {
      arbiters(i).master <> Vec(routers.map(r => r.slave(i)))
      io.slaves(i) <> arbiters(i).slave
    }
  }
}

class NastiInterconnectIO(val nMasters: Int, val nSlaves: Int)
                         (implicit p: Parameters) extends Bundle {
  /* This is a bit confusing. The interconnect is a slave to the masters and
   * a master to the slaves. Hence why the declarations seem to be backwards. */
  val masters = Vec(nMasters, new NastiIO).flip
  val slaves = Vec(nSlaves, new NastiIO)
  override def cloneType =
    new NastiInterconnectIO(nMasters, nSlaves).asInstanceOf[this.type]
}

abstract class NastiInterconnect(implicit p: Parameters) extends NastiModule()(p) {
  val nMasters: Int
  val nSlaves: Int

  lazy val io = new NastiInterconnectIO(nMasters, nSlaves)
}

class NastiRecursiveInterconnect(
    val nMasters: Int, val nSlaves: Int,
    addrmap: AddrMap, base: BigInt)
    (implicit p: Parameters) extends NastiInterconnect()(p) {
  var lastEnd = base
  var slaveInd = 0
  val levelSize = addrmap.size
  val realAddrMap = new ArraySeq[(BigInt, BigInt)](addrmap.size)

  addrmap.zipWithIndex.foreach { case (AddrMapEntry(name, startOpt, region), i) =>
    val start = startOpt.getOrElse(lastEnd)
    val size = region.size

    require(bigIntPow2(size),
      s"Region $name size $size is not a power of 2")
    require(start % size == 0,
      f"Region $name start address 0x$start%x not divisible by 0x$size%x" )
    require(start >= lastEnd,
      f"Region $name start address 0x$start%x before previous region end")

    realAddrMap(i) = (start, size)
    lastEnd = start + size
  }

  val routeSel = (addr: UInt) => {
    Vec(realAddrMap.map { case (start, size) =>
      addr >= UInt(start) && addr < UInt(start + size)
    }).toBits
  }

  val xbar = Module(new NastiCrossbar(nMasters, levelSize, routeSel))
  xbar.io.masters <> io.masters

  addrmap.zip(realAddrMap).zip(xbar.io.slaves).zipWithIndex.foreach {
    case (((entry, (start, size)), xbarSlave), i) => {
      entry.region match {
        case MemSize(_, _) =>
          io.slaves(slaveInd) <> xbarSlave
          slaveInd += 1
        case MemSubmap(_, submap) =>
          if (submap.isEmpty) {
            val err_slave = Module(new NastiErrorSlave)
            err_slave.io <> xbarSlave
          } else {
            val subSlaves = submap.countSlaves
            val outputs = io.slaves.drop(slaveInd).take(subSlaves)
            val ic = Module(new NastiRecursiveInterconnect(1, subSlaves, submap, start))
            ic.io.masters.head <> xbarSlave
            for ((o, s) <- outputs zip ic.io.slaves)
              o <> s
            slaveInd += subSlaves
          }
      }
    }
  }
}

class ChannelHelper(nChannels: Int)
    (implicit val p: Parameters) extends HasNastiParameters {

  val dataBytes = p(MIFDataBits) * p(MIFDataBeats) / 8
  val chanSelBits = log2Ceil(nChannels)
  val selOffset = log2Up(dataBytes)
  val blockOffset = selOffset + chanSelBits

  def getSelect(addr: UInt) =
    if (nChannels > 1) addr(blockOffset - 1, selOffset) else UInt(0)

  def getAddr(addr: UInt) =
    if (nChannels > 1)
      Cat(addr(nastiXAddrBits - 1, blockOffset), addr(selOffset - 1, 0))
    else addr
}

class NastiMemoryInterconnect(
    nBanksPerChannel: Int, nChannels: Int)
    (implicit p: Parameters) extends NastiInterconnect()(p) {

  val nBanks = nBanksPerChannel * nChannels
  val nMasters = nBanks
  val nSlaves = nChannels

  val chanHelper = new ChannelHelper(nChannels)
  def connectChannel(outer: NastiIO, inner: NastiIO) {
    outer <> inner
    outer.ar.bits.addr := chanHelper.getAddr(inner.ar.bits.addr)
    outer.aw.bits.addr := chanHelper.getAddr(inner.aw.bits.addr)
  }

  for (i <- 0 until nChannels) {
    /* Bank assignments to channels are strided so that consecutive banks
     * map to different channels. That way, consecutive cache lines also
     * map to different channels */
    val banks = (i until nBanks by nChannels).map(j => io.masters(j))

    val channelArb = Module(new NastiArbiter(nBanksPerChannel))
    channelArb.io.master <> banks
    connectChannel(io.slaves(i), channelArb.io.slave)
  }
}

/** Allows users to switch between various memory configurations.  Note that
  * this is a dangerous operation: not only does switching the select input to
  * this module violate Nasti, it also causes the memory of the machine to
  * become garbled.  It's expected that select only changes at boot time, as
  * part of the memory controller configuration. */
class NastiMemorySelectorIO(val nBanks: Int, val maxMemChannels: Int, nConfigs: Int)
                           (implicit p: Parameters)
                           extends NastiInterconnectIO(nBanks, maxMemChannels) {
  val select  = UInt(INPUT, width = log2Up(nConfigs))
  override def cloneType =
    new NastiMemorySelectorIO(nMasters, nSlaves, nConfigs).asInstanceOf[this.type]
}

class NastiMemorySelector(nBanks: Int, maxMemChannels: Int, configs: Seq[Int])
                         (implicit p: Parameters)
                         extends NastiInterconnect()(p) {
  val nMasters = nBanks
  val nSlaves  = maxMemChannels
  val nConfigs = configs.size

  override lazy val io = new NastiMemorySelectorIO(nBanks, maxMemChannels, nConfigs)

  def muxOnSelect(up: DecoupledIO[Bundle], dn: DecoupledIO[Bundle], active: Bool): Unit = {
    when (active) { dn.bits  := up.bits  }
    when (active) { up.ready := dn.ready }
    when (active) { dn.valid := up.valid }
  }

  def muxOnSelect(up: NastiIO, dn: NastiIO, active: Bool): Unit = {
    muxOnSelect(up.aw, dn.aw, active)
    muxOnSelect(up.w,  dn.w,  active)
    muxOnSelect(dn.b,  up.b,  active)
    muxOnSelect(up.ar, dn.ar, active)
    muxOnSelect(dn.r,  up.r,  active)
  }

  def muxOnSelect(up: Vec[NastiIO], dn: Vec[NastiIO], active: Bool) : Unit = {
    for (i <- 0 until up.size)
      muxOnSelect(up(i), dn(i), active)
  }

  /* Disconnects a vector of Nasti ports, which involves setting them to
   * invalid.  Due to Chisel reasons, we need to also set the bits to 0 (since
   * there can't be any unconnected inputs). */
  def disconnectSlave(slave: Vec[NastiIO]) = {
    slave.foreach{ m =>
      m.aw.valid := Bool(false)
      m.aw.bits  := m.aw.bits.fromBits( UInt(0) )
      m.w.valid  := Bool(false)
      m.w.bits   := m.w.bits.fromBits( UInt(0) )
      m.b.ready  := Bool(false)
      m.ar.valid := Bool(false)
      m.ar.bits  := m.ar.bits.fromBits( UInt(0) )
      m.r.ready  := Bool(false)
    }
  }

  def disconnectMaster(master: Vec[NastiIO]) = {
    master.foreach{ m =>
      m.aw.ready := Bool(false)
      m.w.ready  := Bool(false)
      m.b.valid  := Bool(false)
      m.b.bits   := m.b.bits.fromBits( UInt(0) )
      m.ar.ready := Bool(false)
      m.r.valid  := Bool(false)
      m.r.bits   := m.r.bits.fromBits( UInt(0) )
    }
  }

  /* Provides default wires on all our outputs. */
  disconnectMaster(io.masters)
  disconnectSlave(io.slaves)

  /* Constructs interconnects for each of the layouts suggested by the
   * configuration and switches between them based on the select input. */
  configs.zipWithIndex.foreach{ case (nChannels, select) =>
    val nBanksPerChannel = nBanks / nChannels
    val ic = Module(new NastiMemoryInterconnect(nBanksPerChannel, nChannels))
    disconnectMaster(ic.io.slaves)
    disconnectSlave(ic.io.masters)
    muxOnSelect(   io.masters, ic.io.masters, io.select === UInt(select))
    muxOnSelect(ic.io.slaves,     io.slaves,  io.select === UInt(select))
  }
}