[clocking] Improve reference clock selection using a multiple-of-fastest strategy

generators/chipyard/src/main/scala/clocking/DividerOnlyClockGenerator.scala

@@ -14,21 +14,65 @@ import scala.collection.immutable.ListMap
   * TODO: figure out how much division is acceptable in our simulators and redefine this.
   */
 object FrequencyUtils {
-  def computeReferenceFrequencyMHz(
+  /**
+    * Adds up the squared error between the generated clocks (refClock / [integer] divider)
+    * and the requested frequencies.
+    *
+    * @param refMHz The candidate reference clock
+    * @param desiredFreqMHz A list of the requested output frequencies
+    */
+  def squaredError(refMHz: Double, desiredFreqMHz: List[Double], sum: Double = 0.0): Double = desiredFreqMHz match {
+    case Nil => sum
+    case desired :: xs =>
+      val divider = Math.round(refMHz / desired)
+      val termError = ((refMHz / divider) - desired) / desired
+      squaredError(refMHz, xs, sum + termError * termError)
+  }
+
+  /**
+    * Picks a candidate reference frequency by doing a brute-force search over
+    * multiples of the fastest requested clock. Choose the smallest multiple that
+    * has an RMS error (across all output frequencies) that is:
+    * 1) zero or failing that,
+    * 2) is within the relativeThreshold of the best or is less than the absoluteThreshold
+    *
+    * @param requestedOutputs The desired output frequencies in MHz
+    * @param maximumAllowableFreqMHz The maximum allowable reference in MHz
+    * @param relativeThreshold See above
+    * @param absoluteThreshold See above
+    */
+  def computeReferenceAsMultipleOfFastestClock(
     requestedOutputs: Seq[ClockParameters],
-    maximumAllowableFreqMHz: Double = 8000.0): ClockParameters = {
+    maximumAllowableFreqMHz: Double,
+    relativeThreshold: Double = 1.10,
+    absoluteThreshold: Double = 0.01): ClockParameters = {
+
     require(requestedOutputs.nonEmpty)
     require(!requestedOutputs.contains(0.0))
-    val freqs = requestedOutputs.map(f => BigInt(Math.round(f.freqMHz * 1000 * 1000)))
-    val refFreq = freqs.reduce((a, b) => a * b / a.gcd(b)).toDouble / (1000 * 1000)
-    assert(refFreq < maximumAllowableFreqMHz,
-      s"Reference frequency ${refFreq} exceeds maximum allowable value of ${maximumAllowableFreqMHz} MHz")
-    ClockParameters(refFreq)
+    val requestedFreqs = requestedOutputs.map(_.freqMHz)
+    val fastestFreq = requestedFreqs.max
+    require(fastestFreq < maximumAllowableFreqMHz)
+
+    val candidateFreqs =
+      Seq.tabulate(Math.ceil(maximumAllowableFreqMHz / fastestFreq).toInt)(i => (i + 1) * fastestFreq)
+    val errorTuples = candidateFreqs.map { f =>
+      f -> Math.sqrt(squaredError(f, requestedFreqs.toList) / requestedFreqs.size)
+    }
+    val minError = errorTuples.map(_._2).min
+    val viableFreqs = errorTuples.collect {
+      case (f, error) if (error <= minError * relativeThreshold) || (minError > 0 && error < absoluteThreshold) => f
+    }
+    ClockParameters(viableFreqs.min)
   }
 }
 
-class SimplePllConfiguration(name: String, val sinks: Seq[ClockSinkParameters]) {
-  val referenceFreqMHz = FrequencyUtils.computeReferenceFrequencyMHz(sinks.flatMap(_.take)).freqMHz
+class SimplePllConfiguration(
+    name: String,
+    val sinks: Seq[ClockSinkParameters],
+    maximumAllowableFreqMHz: Double = 16000.0 ) {
+  val referenceFreqMHz = FrequencyUtils.computeReferenceAsMultipleOfFastestClock(
+    sinks.flatMap(_.take),
+    maximumAllowableFreqMHz).freqMHz
   val sinkDividerMap = ListMap((sinks.map({s => (s, Math.round(referenceFreqMHz / s.take.get.freqMHz).toInt) })):_*)
 
   private val preamble = s"""
@@ -41,8 +85,10 @@ class SimplePllConfiguration(name: String, val sinks: Seq[ClockSinkParameters])
     }
 
    val summaryString =  preamble + outputSummaries.mkString("\n")
-   ElaborationArtefacts.add(s"${name}.freq-summary", summaryString)
-   println(summaryString)
+   def emitSummaries(): Unit = {
+     ElaborationArtefacts.add(s"${name}.freq-summary", summaryString)
+     println(summaryString)
+   }
 }
 
 case class DividerOnlyClockGeneratorNode(pllName: String)(implicit valName: ValName)
@@ -54,7 +100,7 @@ case class DividerOnlyClockGeneratorNode(pllName: String)(implicit valName: ValN
       "All output clocks in group must set their take parameters. Use a ClockGroupDealiaser")
     ClockSinkParameters(
       name = Some(s"${pllName}_reference_input"),
-      take = Some(FrequencyUtils.computeReferenceFrequencyMHz(u.head.members.flatMap(_.take)))) }
+      take = Some(ClockParameters(new SimplePllConfiguration(pllName, u.head.members).referenceFreqMHz))) }
   )
 
 /**
@@ -79,6 +125,7 @@ class DividerOnlyClockGenerator(pllName: String)(implicit p: Parameters, valName
 
     val referenceFreq = refSinkParam.take.get.freqMHz
     val pllConfig = new SimplePllConfiguration(pllName, outSinkParams.members)
+    pllConfig.emitSummaries()
 
     val dividedClocks = mutable.HashMap[Int, Clock]()
     def instantiateDivider(div: Int): Clock = {

generators/chipyard/src/test/scala/clocking/SimplePllConfigurationSpec.scala

@@ -5,17 +5,25 @@ import freechips.rocketchip.prci._
 
 class SimplePllConfigurationSpec extends org.scalatest.FlatSpec {
 
-    def conf(freqMHz: Iterable[Double]): SimplePllConfiguration = new SimplePllConfiguration("test",
+    def genConf(freqMHz: Iterable[Double]): SimplePllConfiguration = new SimplePllConfiguration(
+      "testPLL",
       freqMHz.map({ f => ClockSinkParameters(
         name = Some(s"desiredFreq_$f"),
-        take = Some(ClockParameters(f))) }).toSeq)
+        take = Some(ClockParameters(f))) }).toSeq,
+      maximumAllowableFreqMHz = 16000.0)
 
-    def tryConf(freqMHz: Double*): Unit = {
-      val freqStr = freqMHz.mkString(", ")
-      it should s"configure for ${freqStr} MHz" in { conf(freqMHz) }
+    def trySuccessfulConf(requestedFreqs: Seq[Double], expected: Double): Unit = {
+      val freqStr = requestedFreqs.mkString(", ")
+      it should s"select a reference of ${expected} MHz for ${freqStr} MHz" in { 
+        val conf = genConf(requestedFreqs)
+        conf.emitSummaries
+        assert(expected == conf.referenceFreqMHz)
+      }
     }
 
-    tryConf(3200.0, 1600.0, 1000.0, 100.0)
-    tryConf(3200.0, 1600.0)
-    tryConf(3200.0, 1066.7)
+    trySuccessfulConf(Seq(3200.0, 1600.0, 1000.0, 100.0), 16000.0)
+    trySuccessfulConf(Seq(3200.0, 1600.0), 3200.0)
+    trySuccessfulConf(Seq(3200.0, 1066.7), 3200.0)
+    trySuccessfulConf(Seq(100, 50, 6.67), 100)
+    trySuccessfulConf(Seq(1, 2, 3, 5, 7, 11, 13).map(_ * 10.0), 1560.0)
 }

generators/firechip/src/main/scala/FireSim.scala

@@ -118,6 +118,7 @@ class WithFireSimSimpleClocks extends Config((site, here, up) => {
       }
 
       val pllConfig = new SimplePllConfiguration("FireSim RationalClockBridge", clockGroupEdge.sink.members)
+      pllConfig.emitSummaries
       val rationalClockSpecs = for ((sinkP, division) <- pllConfig.sinkDividerMap) yield {
         RationalClock(sinkP.name.get, 1, division)
       }