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)

src/main/scala/tilelink/RAMModel.scala

@@ -22,7 +22,7 @@ import freechips.rocketchip.util._
 // put, get, getAck, putAck => ok: detected by getAck (it sees busy>0)		impossible for FIFO
 // If FIFO, the getAck should check data even if its validity was wiped
 
-class TLRAMModel(log: String = "")(implicit p: Parameters) extends LazyModule
+class TLRAMModel(log: String = "", ignoreErrorData: Boolean = false)(implicit p: Parameters) extends LazyModule
 {
   val node = TLAdapterNode()
 
@@ -288,6 +288,8 @@ class TLRAMModel(log: String = "")(implicit p: Parameters) extends LazyModule
                 printf(", undefined (concurrent incomplete puts #%d)\n", d_inc(i) - d_dec(i))
               } .elsewhen (!d_fifo && !d_valid) {
                 printf(", undefined (concurrent completed put)\n")
+              } .elsewhen (Bool(ignoreErrorData) && d.error) {
+                printf(", undefined (error result)\n")
               } .otherwise {
                 printf("\n")
                 when (shadow.value =/= got) { printf("EXPECTED: 0x%x\n", shadow.value) }
@@ -303,8 +305,9 @@ class TLRAMModel(log: String = "")(implicit p: Parameters) extends LazyModule
           when ((Cat(race.reverse) & d_mask).orR) { d_no_race := Bool(false) }
           when (d_last) {
             val must_match = d_crc_valid && (d_fifo || (d_valid && d_no_race))
+            val error = Bool(ignoreErrorData) && d.error
             printf(log + " crc = 0x%x %d\n", d_crc, must_match.asUInt)
-            when (must_match && d_crc =/= d_crc_check) { printf("EXPECTED: 0x%x\n", d_crc_check) }
+            when (!error && must_match && d_crc =/= d_crc_check) { printf("EXPECTED: 0x%x\n", d_crc_check) }
             assert (!must_match || d_crc === d_crc_check)
           }
         }