Handle backpressure from CoalescerNode

Now do proper sourcegen for the tlCoal edge that's coming out
of the coalescer manager node.  This also prevents inflight table from
being full.

This means we move setting source ID of coalReq to outside the
coalescer, because sourceGen needs looking into response bits as well,
which is easier to do outside coalescer at the toplevel.

FIXME: coalescer unit test is still broken.

src/main/scala/tilelink/Coalescing.scala

@@ -93,7 +93,7 @@ object defaultConfig extends CoalescerConfig(
   wordSizeInBytes = 4,
   // when attaching to SoC, 16 source IDs are not enough due to longer latency
   numOldSrcIds = 16,
-  numNewSrcIds = 4,
+  numNewSrcIds = 8,
   respQueueDepth = 4,
   coalLogSizes = Seq(3),
   sizeEnum = DefaultInFlightTableSizeEnum,
@@ -663,16 +663,12 @@ class MultiCoalescer(
     )
   }
 
-  val sourceGen = Module(
-    new RoundRobinSourceGenerator(log2Ceil(config.numNewSrcIds))
-  )
-  sourceGen.io.gen := io.coalReq.fire // use up a source ID only when request is created
-  sourceGen.io.reclaim.valid := false.B // not used
-  sourceGen.io.reclaim.bits := DontCare // not used
+  val coalesceValid = chosenValid
 
-  val coalesceValid = chosenValid && sourceGen.io.id.valid
-
-  io.coalReq.bits.source := sourceGen.io.id.bits
+  // setting source is deferred, because in order to do proper source ID
+  // generation we also have to look at the responses coming back, which
+  // is easier to do at the toplevel.
+  io.coalReq.bits.source := DontCare
   io.coalReq.bits.mask := mask.asUInt
   io.coalReq.bits.data := data.asUInt
   io.coalReq.bits.size := chosenSize
@@ -732,6 +728,8 @@ class CoalescingUnitImp(outer: CoalescingUnit, config: CoalescerConfig)
   reqQueues.io.coalescable := coalescer.io.coalescable
   reqQueues.io.invalidate := coalescer.io.invalidate
 
+  val uncoalescer = Module(new Uncoalescer(config, nonCoalReqT, coalReqT))
+
   // ===========================================================================
   // Request flow
   // ===========================================================================
@@ -761,8 +759,10 @@ class CoalescingUnitImp(outer: CoalescingUnit, config: CoalescerConfig)
       val deq = reqQueues.io.queue.deq(lane)
       enq.valid := tlIn.a.valid
       enq.bits := req
-      // TODO: deq.ready should respect downstream arbiter
-      deq.ready := true.B
+      // Only allow dequeue when uncoalescer is ready to record the current
+      // queue entries
+      // TODO: deq.ready should also respect downstream arbiter
+      deq.ready := uncoalescer.io.coalReq.ready
       // Stall upstream core or memtrace driver when shiftqueue is not ready
       tlIn.a.ready := enq.ready
       tlOut.a.valid := deq.valid
@@ -831,7 +831,7 @@ class CoalescingUnitImp(outer: CoalescingUnit, config: CoalescerConfig)
   // queues.
 
   // The maximum number of requests from a single lane that can go into a
-  // coalesced request.  Upper bound is min(DEPTH, 2**sourceWidth).
+  // coalesced request.
   val numPerLaneReqs = config.queueDepth
 
   // FIXME: no need to contain maxCoalLogSize data
@@ -911,12 +911,19 @@ class CoalescingUnitImp(outer: CoalescingUnit, config: CoalescerConfig)
       dontTouch(tlOut.d)
   }
 
-  val uncoalescer = Module(
-    new Uncoalescer(config, nonCoalReqT, coalReqT)
-  )
   // connect coalesced request that is newly generated and being recorded in
   // the uncoalescer
   uncoalescer.io.coalReq <> coalescer.io.coalReq
+  // We can't simply use coalescer.io.coalReq.valid here.
+  // coalescer.io.coalReq.valid tells us when there exists a valid coalescing
+  // combination, but not when we can actually fire that to downstream, because
+  // we can still be blocked by source ID clashes due to backpressure.
+  // So, we have to overwrite just the valid bit with the final valid that
+  // indicates when we can send this request out.
+  // NOTE(hansung): this feels slightly awkward.  Maybe doing sourcegen inside
+  // the coalescer so that it gives the final call is better, but that may be
+  // too much IO for the coalescer.
+  uncoalescer.io.coalReq.valid := coalReqValid
   uncoalescer.io.invalidate := coalescer.io.invalidate
   val reqQueueHeads = reqQueues.io.queue.deq.map(_.bits)
   uncoalescer.io.windowElts := reqQueues.io.elts
@@ -984,9 +991,10 @@ class Uncoalescer(
     )
   })
 
-  // Uncoalescer has to be always ready to accept and record new coalesced
-  // requests, so that it doesn't stall the coalescer.
-  io.coalReq.ready := true.B
+  // If inflight table is full, we cannot accept new requests to record them.
+  // This might happen when we sent out many requests and exhausted all source
+  // IDs, but they haven't come back yet.
+  io.coalReq.ready := inflightTable.io.enq.ready
 
   // Construct a new entry for the inflight table using generated coalesced request
   def generateInflightTableEntry: InflightCoalReqTableEntry = {
@@ -1136,7 +1144,6 @@ class InflightCoalReqTable(config: CoalescerConfig) extends Module {
 
   val full = Wire(Bool())
   full := (0 until entries).map(table(_).valid).reduce(_ && _)
-  assert(!full, "inflight table is full and blocking coalescer")
   dontTouch(full)
 
   // Enqueue logic
@@ -1156,9 +1163,8 @@ class InflightCoalReqTable(config: CoalescerConfig) extends Module {
   // Lookup logic
   io.lookup.valid := table(io.lookupSourceId).valid
   io.lookup.bits := table(io.lookupSourceId).bits
-  val lookupFire = io.lookup.ready && io.lookup.valid
   // Dequeue as soon as lookup succeeds
-  when(lookupFire) {
+  when(io.lookup.fire) {
     table(io.lookupSourceId).valid := false.B
   }