Merge pull request #975 from freechipsproject/async_reg

Cleanup some register primitives

src/main/scala/tilelink/IntNodes.scala

@@ -6,6 +6,7 @@ import Chisel._
 import chisel3.internal.sourceinfo.SourceInfo
 import freechips.rocketchip.config.Parameters
 import freechips.rocketchip.diplomacy._
+import freechips.rocketchip.util.SynchronizerShiftReg
 import scala.collection.mutable.ListBuffer
 import scala.math.max
 
@@ -139,7 +140,7 @@ class IntXing(sync: Int = 3)(implicit p: Parameters) extends LazyModule
     }
 
     (io.in zip io.out) foreach { case (in, out) =>
-      out := (0 to sync).foldLeft(in) { case (a, _) => RegNext(a) }
+      out := SynchronizerShiftReg(in, sync)
     }
   }
 }

src/main/scala/util/AsyncQueue.scala

@@ -13,27 +13,12 @@ object GrayCounter {
   }
 }
 
-object UIntSyncChain {
-  def apply(in: UInt, sync: Int, name: String = "gray"): UInt = {
-    val syncv = List.tabulate(sync) { i =>
-      Module (new AsyncResetRegVec(w = in.getWidth, 0)).suggestName(s"${name}_sync_${i}")
-    }
-    syncv.last.io.d := in
-    syncv.last.io.en := Bool(true)
-      (syncv.init zip syncv.tail).foreach { case (sink, source) =>
-        sink.io.d := source.io.q
-        sink.io.en := Bool(true)
-      }
-    syncv.head.io.q
-  }
-}
-
 class AsyncValidSync(sync: Int, desc: String) extends Module {
   val io = new Bundle {
     val in = Bool(INPUT)
     val out = Bool(OUTPUT)
   }
-  io.out := UIntSyncChain(io.in.asUInt, sync, desc)(0)
+  io.out := AsyncResetSynchronizerShiftReg(io.in, sync, Some(desc))
 }
 
 class AsyncQueueSource[T <: Data](gen: T, depth: Int, sync: Int, safe: Boolean = true, narrowData: Boolean = false) extends Module {
@@ -55,7 +40,7 @@ class AsyncQueueSource[T <: Data](gen: T, depth: Int, sync: Int, safe: Boolean =
   val sink_ready = Wire(init = Bool(true))
   val mem = Reg(Vec(depth, gen)) // This does NOT need to be reset at all.
   val widx = GrayCounter(bits+1, io.enq.fire(), !sink_ready, "widx_bin")
-  val ridx = UIntSyncChain(io.ridx, sync, "ridx_gray")
+  val ridx = AsyncResetSynchronizerShiftReg(io.ridx, sync, Some("ridx_gray"))
   val ready = sink_ready && widx =/= (ridx ^ UInt(depth | depth >> 1))
 
   val index = if (depth == 1) UInt(0) else io.widx(bits-1, 0) ^ (io.widx(bits, bits) << (bits-1))
@@ -112,7 +97,7 @@ class AsyncQueueSink[T <: Data](gen: T, depth: Int, sync: Int, safe: Boolean = t
 
   val source_ready = Wire(init = Bool(true))
   val ridx = GrayCounter(bits+1, io.deq.fire(), !source_ready, "ridx_bin")
-  val widx = UIntSyncChain(io.widx, sync, "widx_gray")
+  val widx = AsyncResetSynchronizerShiftReg(io.widx, sync, Some("widx_gray"))
   val valid = source_ready && ridx =/= widx
 
   // The mux is safe because timing analysis ensures ridx has reached the register
@@ -125,7 +110,8 @@ class AsyncQueueSink[T <: Data](gen: T, depth: Int, sync: Int, safe: Boolean = t
   // be considered unless the asynchronously reset deq valid register is set.
   // It is possible that bits latches when the source domain is reset / has power cut
   // This is safe, because isolation gates brought mem low before the zeroed widx reached us
-  io.deq.bits  := RegEnable(io.mem(if(narrowData) UInt(0) else index), valid)
+  val deq_bits_nxt = Mux(valid, io.mem(if(narrowData) UInt(0) else index), io.deq.bits)
+  io.deq.bits  := SynchronizerShiftReg(deq_bits_nxt, sync = 1, name = Some("deq_bits_reg"))
 
   val valid_reg = AsyncResetReg(valid.asUInt, "valid_reg")(0)
   io.deq.valid := valid_reg && source_ready

src/main/scala/util/AsyncResetReg.scala

@@ -63,6 +63,9 @@ class AsyncResetRegVec(val w: Int, val init: BigInt) extends Module {
   }
 
   io.q := q.asUInt
+
+  override def desiredName = s"AsyncResetRegVec_w${w}_i${init}"
+
 }
 
 object AsyncResetReg {
@@ -103,3 +106,4 @@ object AsyncResetReg {
   def apply(updateData: UInt): UInt = apply(updateData, resetData=BigInt(0), enable=Bool(true))
   def apply(updateData: UInt, name:String): UInt = apply(updateData, resetData=BigInt(0), enable=Bool(true), Some(name))
 }
+

src/main/scala/util/Crossing.scala

@@ -99,7 +99,7 @@ object LevelSyncCrossing {
       val out = Bool(OUTPUT)
     }
 
-    io.out := ShiftRegister(io.in, sync)
+    io.out := SynchronizerShiftReg(io.in, sync)
   }
 
   class SynchronizerFrontend(_clock: Clock) extends Module(Some(_clock)) {

src/main/scala/util/ResetCatchAndSync.scala

@@ -15,12 +15,7 @@ class ResetCatchAndSync (sync: Int = 3) extends Module {
     val sync_reset = Bool(OUTPUT)
   }
 
-  val reset_n_catch_reg = Module (new AsyncResetRegVec(sync, 0))
+  io.sync_reset := ~AsyncResetSynchronizerShiftReg(Bool(true), sync)
-
-  reset_n_catch_reg.io.en := Bool(true)
-  reset_n_catch_reg.io.d  := Cat(Bool(true), reset_n_catch_reg.io.q >> 1)
-
-  io.sync_reset := ~reset_n_catch_reg.io.q(0)
 
 }
 

src/main/scala/util/ShiftReg.scala

@@ -0,0 +1,144 @@
+// See LICENSE.SiFive for license details.
+
+package freechips.rocketchip.util
+
+import Chisel._
+
+// Similar to the Chisel ShiftRegister but allows the user to suggest a
+// name to the registers that get instantiated, and
+// to provide a reset value.
+object ShiftRegInit {
+  def apply[T <: Data](in: T, n: Int, init: T, name: Option[String] = None): T =
+
+  (0 until n).foldRight(in) {
+    case (i, next) => {
+      val r = Reg(next, next = next, init = init)
+      name.foreach { na => r.suggestName(s"${na}_${i}") }
+      r
+    }
+  }
+}
+
+/** These wrap behavioral
+  *  shift registers  into specific modules to allow for
+  *  backend flows to replace or constrain
+  *  them properly when used for CDC synchronization,
+  *  rather than buffering.
+  *  
+  *  The different types vary in their reset behavior:
+  *  AsyncResetShiftReg -- This is identical to the AsyncResetSynchronizerShiftReg, 
+  *      it is just named differently to distinguish its use case.
+  *      This is an async ShiftRegister meant for timing,
+  *      not for synchronization.
+  *  AsyncResetSynchronizerShiftReg -- asynchronously reset to specific value.
+  *  SyncResetSynchronizerShiftReg  -- reset to specific value.
+  *  SynchronizerShiftReg           -- no reset, pipeline only.
+  */
+
+abstract class AbstractPipelineReg(w: Int = 1) extends Module {
+  val io = new Bundle {
+    val d = UInt(INPUT, width = w)
+    val q = UInt(OUTPUT, width = w)
+  }
+}
+
+object AbstractPipelineReg {
+  def apply [T <: Chisel.Data](gen: => AbstractPipelineReg, in: T, name: Option[String] = None): T = {
+    val chain = Module(gen)
+    name.foreach{ chain.suggestName(_) }
+    chain.io.d := in.asUInt
+    chain.io.q.asTypeOf(in)
+  }
+}
+
+class AsyncResetShiftReg(w: Int = 1, depth: Int = 1, init: Int = 0, name: String = "pipe") extends AbstractPipelineReg(w) {
+  require(depth > 0, "Depth must be greater than 0.")
+
+  override def desiredName = s"AsyncResetShiftReg_w${w}_d${depth}_i${init}"
+
+  val chain = List.tabulate(depth) { i =>
+    Module (new AsyncResetRegVec(w, init)).suggestName(s"${name}_${i}")
+  }
+
+  chain.last.io.d := io.d
+  chain.last.io.en := Bool(true)
+
+  (chain.init zip chain.tail).foreach { case (sink, source) =>
+    sink.io.d := source.io.q
+    sink.io.en := Bool(true)
+  }
+  io.q := chain.head.io.q
+}
+
+object AsyncResetShiftReg {
+  def apply [T <: Chisel.Data](in: T, depth: Int, init: Int  = 0, name: Option[String] = None): T =
+    AbstractPipelineReg(new AsyncResetShiftReg(in.getWidth, depth, init), in, name)
+
+  def apply [T <: Chisel.Data](in: T, depth: Int, name: Option[String]): T =
+    apply(in, depth, 0, name)
+
+  def apply [T <: Chisel.Data](in: T, depth: Int, init: T, name: Option[String]): T =
+    apply(in, depth, init.litValue.toInt, name)
+
+  def apply [T <: Chisel.Data](in: T, depth: Int, init: T): T =
+    apply (in, depth, init.litValue.toInt, None)
+}
+
+// Note that it is important to ovveride "name" in order to ensure that the Chisel dedup does
+// not try to merge instances of this with instances of the superclass.
+class AsyncResetSynchronizerShiftReg(w: Int = 1, sync: Int = 3, init: Int = 0) extends AsyncResetShiftReg(w, depth = sync, init, name = "sync") {
+  require(sync > 0, "Sync must be greater than 0.")
+  override def desiredName = s"AsyncResetSynchronizerShiftReg_w${w}_d${sync}_i${init}"
+}
+
+object AsyncResetSynchronizerShiftReg {
+  def apply [T <: Chisel.Data](in: T, depth: Int, init: Int  = 0, name: Option[String] = None): T =
+    AbstractPipelineReg(new AsyncResetSynchronizerShiftReg(in.getWidth, depth, init), in, name)
+
+  def apply [T <: Chisel.Data](in: T, depth: Int, name: Option[String]): T =
+    apply(in, depth, 0, name)
+
+  def apply [T <: Chisel.Data](in: T, depth: Int, init: T, name: Option[String]): T =
+    apply(in, depth, init.litValue.toInt, name)
+
+  def apply [T <: Chisel.Data](in: T, depth: Int, init: T): T =
+    apply (in, depth, init.litValue.toInt, None)
+}
+
+class SynchronizerShiftReg(w: Int = 1, sync: Int = 3) extends AbstractPipelineReg(w) {
+  require(sync > 0, "Sync must be greater than 0.")
+
+  override def desiredName = s"SynchronizerShiftReg_w${w}_d${sync}"
+
+  val syncv = List.tabulate(sync) { i =>
+    val r = Reg(UInt(width = w))
+    r.suggestName(s"sync_${i}")
+  }
+
+  syncv.last := io.d
+
+  (syncv.init zip syncv.tail).foreach { case (sink, source) =>
+    sink := source
+  }
+  io.q := syncv.head
+}
+
+
+object SynchronizerShiftReg {
+  def apply [T <: Chisel.Data](in: T, sync: Int = 3, name: Option[String] = None): T =
+    AbstractPipelineReg(new SynchronizerShiftReg(in.getWidth, sync), in, name)
+}
+
+class SyncResetSynchronizerShiftReg(w: Int = 1, sync: Int = 3, init: Int = 0) extends AbstractPipelineReg(w) {
+  require (sync >= 0, "Sync must be greater than or equal to 0")
+
+  override def desiredName = s"SyncResetSynchronizerShiftReg_w${w}_d${sync}_i${init}"
+
+  io.q := ShiftRegInit(io.d, n = sync, init = init.U, name = Some("sync"))
+
+}
+
+object SyncResetSynchronizerShiftReg {
+  def apply [T <: Chisel.Data](in: T, sync: Int = 3, init: T, name: Option[String] = None): T =
+    AbstractPipelineReg(new SyncResetSynchronizerShiftReg(in.getWidth, sync, init.litValue.toInt), in, name)
+}