noop,fu,MDU: make mulLatency configurable

src/main/scala/noop/fu/MDU.scala

@@ -34,6 +34,7 @@ object MDUInstr extends HasDecodeConst {
 
 class MDU extends Module with HasMDUOpType {
   val io = IO(new FunctionUnitIO)
+  val mulLatency = 4
 
   val (valid, src1, src2, func) = (io.in.valid, io.in.bits.src1, io.in.bits.src2, io.in.bits.func)
   def access(valid: Bool, src1: UInt, src2: UInt, func: UInt): UInt = {
@@ -45,7 +46,7 @@ class MDU extends Module with HasMDUOpType {
   }
 
   val mulRes = (src1.asSInt * src2.asSInt).asUInt
-  val mulPipeOut = Pipe(io.in.fire(), mulRes, 4)
+  val mulPipeOut = Pipe(io.in.fire(), mulRes, mulLatency)
   io.out.bits := LookupTree(func, 0.U, List(
     MduMul  -> mulPipeOut.bits(31, 0),
     MduMulh -> mulPipeOut.bits(63, 32)
@@ -59,6 +60,6 @@ class MDU extends Module with HasMDUOpType {
   when (io.in.valid && !busy) { busy := true.B }
   when (mulPipeOut.valid) { busy := false.B }
 
-  io.in.ready := !busy
+  io.in.ready := (if (mulLatency == 0) true.B else !busy)
   io.out.valid := mulPipeOut.valid
 }