ahb: ignore hrdata on an AHB error

From the AHB spec:
 "A slave only has to provide valid data when a transfer completes with an OKAY
  response. ERROR responses do not require valid read data."

src/main/scala/amba/ahb/Test.scala

@@ -49,7 +49,7 @@ class AHBFuzzMaster(aFlow: Boolean, txns: Int)(implicit p: Parameters) extends L
 {
   val node  = AHBIdentityNode()
   val fuzz  = LazyModule(new TLFuzzer(txns, overrideAddress = Some(fuzzAddr)))
-  val model = LazyModule(new TLRAMModel("AHBFuzzMaster"))
+  val model = LazyModule(new TLRAMModel("AHBFuzzMaster", ignoreErrorData=true))
 
   (node
      := TLToAHB(aFlow)

src/main/scala/amba/ahb/ToTL.scala

@@ -106,7 +106,14 @@ class AHBToTL()(implicit p: Parameters) extends LazyModule
       out.a.bits.mask    := MaskGen(d_addr, d_size, beatBytes)
 
       out.d.ready  := d_recv // backpressure AccessAckData arriving faster than AHB beats
-      in.hrdata    := out.d.bits.data
+
+      // NOTE: on error, we present the read result on the hreadyout LOW cycle
+      // This means that if you latch hrdata from an error, the result is garbage.
+      // To fix this would require a bus-wide register, and the AHB spec says this:
+      // "A slave only has to provide valid data when a transfer completes with an OKAY
+      //  response. ERROR responses do not require valid read data."
+      // Therefore, we choose to accept this slight TL-AHB infidelity.
+      in.hrdata := out.d.bits.data
 
       // In a perfect world, we'd use these signals
       val hresp = d_error || (out.d.valid && out.d.bits.error)