frontend/wishbone: simplify/review and get FSM back (ease comprehension).

litedram/frontend/wishbone.py

@@ -36,37 +36,44 @@ class LiteDRAMWishbone2Native(Module):
 
         adr_offset = base_address >> log2_int(port.data_width//8)
 
-        # latch ready signals of cmd/wdata and then wait until all are ready
-        cmd_consumed   = Signal()
-        wdata_consumed = Signal()
-        self.sync += [
-            If(wishbone.ack,
-                cmd_consumed.eq(0),
-                wdata_consumed.eq(0),
-            ).Else(
-                If(port.cmd.valid   & port.cmd.ready,   cmd_consumed.eq(1)),
-                If(port.wdata.valid & port.wdata.ready, wdata_consumed.eq(1)),
-            ),
-        ]
-
-        ack_cmd   = Signal()
-        ack_wdata = Signal()
-        ack_rdata = Signal()
+        # Write Datapath ---------------------------------------------------------------------------
         self.comb += [
-            port.cmd.addr.eq(wishbone.adr - adr_offset),
-            port.cmd.we.eq(wishbone.we),
             port.wdata.data.eq(wishbone.dat_w),
             port.wdata.we.eq(wishbone.sel),
-            wishbone.dat_r.eq(port.rdata.data),
-            # always wait for reads, flush write when transaction ends
-            port.flush.eq(~wishbone.cyc),
-            port.cmd.last.eq(~wishbone.we),
-            # make sure cmd/wdata won't stay valid after it is consumed
-            port.cmd.valid.eq(wishbone.cyc & wishbone.stb & ~cmd_consumed),
-            port.wdata.valid.eq((port.cmd.valid | cmd_consumed) & port.cmd.we & ~wdata_consumed),
-            port.rdata.ready.eq((port.cmd.valid | cmd_consumed) & ~port.cmd.we),
-            wishbone.ack.eq(ack_cmd & ((wishbone.we & ack_wdata) | (~wishbone.we & ack_rdata))),
-            ack_cmd.eq((port.cmd.valid & port.cmd.ready) | cmd_consumed),
-            ack_wdata.eq((port.wdata.valid & port.wdata.ready) | wdata_consumed),
-            ack_rdata.eq(port.rdata.valid & port.rdata.ready),
         ]
+
+        # Read Datapath ----------------------------------------------------------------------------
+        self.comb += [
+            wishbone.dat_r.eq(port.rdata.data),
+        ]
+
+        # Control ----------------------------------------------------------------------------------
+        self.submodules.fsm = fsm = FSM(reset_state="CMD")
+        fsm.act("CMD",
+            port.flush.eq(~wishbone.cyc),   # Flush write when transaction ends.
+            port.cmd.last.eq(~wishbone.we), # Always wait for reads.
+            port.cmd.valid.eq(wishbone.cyc & wishbone.stb),
+            port.cmd.we.eq(wishbone.we),
+            port.cmd.addr.eq(wishbone.adr - adr_offset),
+            If(port.cmd.valid & port.cmd.ready,
+                If(wishbone.we,
+                    NextState("WAIT-WRITE")
+                ).Else(
+                    NextState("WAIT-READ")
+                )
+            )
+        )
+        fsm.act("WAIT-WRITE",
+            port.wdata.valid.eq(1),
+            If(port.wdata.ready,
+                wishbone.ack.eq(1),
+                NextState("CMD")
+            )
+        )
+        fsm.act("WAIT-READ",
+            port.rdata.ready.eq(1),
+            If(port.rdata.valid,
+               wishbone.ack.eq(1),
+               NextState("CMD")
+            )
+        )