tilelink: fix Fragmenter source re-use bug (#888)

Consider the following waveform for two 4-beat bursts:
---A----A------------
-------D-----DDD-DDDD
Under TL rules, the second A can use the same source as the first A,
because the source is released for reuse on the first response beat.

However, if we fragment the requests, it looks like this:
---3210-3210---------
-------3-----210-3210
... now we've broken the rules because 210 are twice inflight.

To solve this, we alternate an a.source bit every time D completes a txn.

src/main/scala/tilelink/Fragmenter.scala

@@ -21,10 +21,8 @@ class TLFragmenter(val minSize: Int, val maxSize: Int, val alwaysMin: Boolean =
   require (isPow2 (minSize))
   require (minSize < maxSize)
 
-  // EarlyAck means that 1.999 transactions can be inflight at a time
-  // Thus, we need an extra toggle bit to prevent source collisions
   val fragmentBits = log2Ceil(maxSize / minSize)
-  val toggleBits = if (earlyAck) 1 else 0
+  val toggleBits = 1
   val addedBits = fragmentBits + toggleBits
 
   def expandTransfer(x: TransferSizes) = if (!x) x else {
@@ -141,9 +139,28 @@ class TLFragmenter(val minSize: Int, val maxSize: Int, val alwaysMin: Boolean =
       val counterBits = log2Up(maxSize/beatBytes)
       val maxDownSize = if (alwaysMin) minSize else min(manager.maxTransfer, maxSize)
 
+      // Consider the following waveform for two 4-beat bursts:
+      // ---A----A------------
+      // -------D-----DDD-DDDD
+      // Under TL rules, the second A can use the same source as the first A,
+      // because the source is released for reuse on the first response beat.
+      //
+      // However, if we fragment the requests, it looks like this:
+      // ---3210-3210---------
+      // -------3-----210-3210
+      // ... now we've broken the rules because 210 are twice inflight.
+      //
+      // This phenomenon means we can have essentially 2*maxSize/minSize-1
+      // fragmented transactions in flight per original transaction source.
+      //
+      // To keep the source unique, we encode the beat counter in the low
+      // bits of the source. To solve the overlap, we use a toggle bit.
+      // Whatever toggle bit the D is reassembling, A will use the opposite.
+
       // First, handle the return path
       val acknum = RegInit(UInt(0, width = counterBits))
       val dOrig = Reg(UInt())
+      val dToggle = Reg(Bool())
       val dFragnum = out.d.bits.source(fragmentBits-1, 0)
       val dFirst = acknum === UInt(0)
       val dLast = dFragnum === UInt(0)
@@ -162,7 +179,10 @@ class TLFragmenter(val minSize: Int, val maxSize: Int, val alwaysMin: Boolean =
 
       when (out.d.fire()) {
         acknum := Mux(dFirst, dFirst_acknum, acknum - ack_decrement)
-        when (dFirst) { dOrig := dFirst_size }
+        when (dFirst) {
+          dOrig := dFirst_size
+          dToggle := out.d.bits.source(fragmentBits)
+        }
       }
 
       // Swallow up non-data ack fragments
@@ -240,22 +260,14 @@ class TLFragmenter(val minSize: Int, val maxSize: Int, val alwaysMin: Boolean =
       val new_gennum = ~(~old_gennum1 | (aMask >> log2Ceil(beatBytes))) // ~(~x|y) is width safe
       val aFragnum = ~(~(old_gennum1 >> log2Ceil(minSize/beatBytes)) | (aFragOH1 >> log2Ceil(minSize)))
       val aLast = aFragnum === UInt(0)
+      val aToggle = !Mux(aFirst, dToggle, RegEnable(dToggle, aFirst))
 
       when (out.a.fire()) { gennum := new_gennum }
 
-      // We need to alternate bits by source to handle the 1.999 txns inflight per Id
-      val toggleBitOpt = if (!earlyAck) None else {
-        val state = Reg(UInt(width = edgeIn.client.endSourceId))
-        val toggle = Wire(init = UInt(0, width = edgeIn.client.endSourceId))
-        when (in_a.fire() && aLast) { toggle := UIntToOH(in_a.bits.source) }
-        state := state ^ toggle
-        Some(state(in_a.bits.source))
-      }
-
       repeater.io.repeat := !aHasData && aFragnum =/= UInt(0)
       out.a <> in_a
       out.a.bits.address := in_a.bits.address | ~(old_gennum1 << log2Ceil(beatBytes) | ~aOrigOH1 | aFragOH1 | UInt(minSize-1))
-      out.a.bits.source := Cat(Seq(in_a.bits.source) ++ toggleBitOpt.toList ++ Seq(aFragnum))
+      out.a.bits.source := Cat(Seq(in_a.bits.source) ++ Seq(aToggle.asUInt, aFragnum))
       out.a.bits.size := aFrag
 
       // Optimize away some of the Repeater's registers