core/udp: Simplify LiteEthUDPRX and make sure to drop exceeding payload.

liteeth/core/udp.py

@@ -155,46 +155,46 @@ class LiteEthUDPRX(Module):
 
         # # #
 
+        # Depacketizer.
         self.submodules.depacketizer = depacketizer = LiteEthUDPDepacketizer(dw)
-        self.comb += sink.connect(depacketizer.sink)
 
+        # Data-Path.
+        self.comb += [
+            sink.connect(depacketizer.sink),
+            depacketizer.source.connect(source, keep={
+                "src_port",
+                "dst_port",
+                "data",
+                "error"}),
+            source.ip_address.eq(sink.ip_address),
+            source.length.eq(depacketizer.source.length - udp_header.length),
+        ]
+
+        # Control-Path (FSM).
+        count = Signal(16)
         self.submodules.fsm = fsm = FSM(reset_state="IDLE")
         fsm.act("IDLE",
-            depacketizer.source.ready.eq(1),
+            NextValue(count, 0),
             If(depacketizer.source.valid,
-                depacketizer.source.ready.eq(0),
-                NextState("CHECK")
+                NextState("DROP"),
+                If(sink.protocol == udp_protocol,
+                    NextState("RECEIVE")
                 )
             )
-        valid = Signal(reset_less=True)
-        self.sync += valid.eq(
-            depacketizer.source.valid &
-            (sink.protocol == udp_protocol)
         )
-
-        fsm.act("CHECK",
-            If(valid,
-                NextState("PRESENT")
-            ).Else(
+        fsm.act("RECEIVE",
+            depacketizer.source.connect(source, keep={"valid", "ready"}),
+            source.last.eq(depacketizer.source.last | (count == (source.length - dw//8))),
+            If(source.valid & source.ready,
+                NextValue(count, count + dw//8),
+                If(depacketizer.source.last,
+                    NextState("IDLE")
+                ).Elif(source.last,
                     NextState("DROP")
                 )
             )
-        self.comb += [
-            source.last.eq(depacketizer.source.last),
-            source.src_port.eq(depacketizer.source.src_port),
-            source.dst_port.eq(depacketizer.source.dst_port),
-            source.ip_address.eq(sink.ip_address),
-            source.length.eq(depacketizer.source.length - udp_header.length),
-            source.data.eq(depacketizer.source.data),
-            source.error.eq(depacketizer.source.error)
-        ]
-        fsm.act("PRESENT",
-            source.valid.eq(depacketizer.source.valid),
-            depacketizer.source.ready.eq(source.ready),
-            If(source.valid & source.last & source.ready,
-                NextState("IDLE")
-            )
         )
+
         fsm.act("DROP",
             depacketizer.source.ready.eq(1),
             If(depacketizer.source.valid &