frontend/wishbone: add efficient wishbone downconvert, improve DRAM a…

…ccess efficiency from CPU on boards with small native data_width.

litedram/frontend/wishbone.py

@@ -1,4 +1,4 @@
-# This file is Copyright (c) 2016-2018 Florent Kermarrec <florent@enjoy-digital.fr>
+# This file is Copyright (c) 2016-2020 Florent Kermarrec <florent@enjoy-digital.fr>
 # License: BSD
 
 """Wishbone frontend for LiteDRAM"""
@@ -12,52 +12,69 @@
 
 class LiteDRAMWishbone2Native(Module):
     def __init__(self, wishbone, port, base_address=0x00000000):
-        assert len(wishbone.dat_w) == len(port.wdata.data)
+        wishbone_data_width = len(wishbone.dat_w)
+        port_data_width     = len(port.wdata.data)
+        assert wishbone_data_width >= port_data_width
 
         # # #
 
         adr_offset = base_address >> log2_int(port.data_width//8)
 
-        # Write data buffer-------------------------------------------------------------------------
-        wdata_buffer = stream.Buffer([("data", port.data_width), ("we", port.data_width//8)])
-        self.submodules += wdata_buffer
+        # Write Datapath ---------------------------------------------------------------------------
+        wdata_converter = stream.StrideConverter(
+            [("data", wishbone_data_width), ("we", wishbone_data_width//8)],
+            [("data", port_data_width),     ("we", port_data_width//8)],
+        )
+        self.submodules += wdata_converter
+        self.comb += [
+            wdata_converter.sink.valid.eq(wishbone.cyc & wishbone.stb & wishbone.we),
+            wdata_converter.sink.data.eq(wishbone.dat_w),
+            wdata_converter.sink.we.eq(wishbone.sel),
+            wdata_converter.source.connect(port.wdata)
+        ]
+
+        # Read Datapath ----------------------------------------------------------------------------
+        rdata_converter = stream.StrideConverter(
+            [("data", port_data_width)],
+            [("data", wishbone_data_width)],
+        )
+        self.submodules += rdata_converter
+        self.comb += [
+            port.rdata.connect(rdata_converter.sink),
+            rdata_converter.source.ready.eq(1),
+            wishbone.dat_r.eq(rdata_converter.source.data),
+        ]
 
         # Control ----------------------------------------------------------------------------------
+        ratio = wishbone_data_width//port_data_width
+        count = Signal(max=max(ratio, 2))
         self.submodules.fsm = fsm = FSM(reset_state="CMD")
         fsm.act("CMD",
             port.cmd.valid.eq(wishbone.cyc & wishbone.stb),
             port.cmd.we.eq(wishbone.we),
+            port.cmd.addr.eq(wishbone.adr*ratio + count - adr_offset),
             If(port.cmd.valid & port.cmd.ready,
-                If(wishbone.we,
-                    NextState("WRITE")
-                ).Else(
-                    NextState("READ")
+                NextValue(count, count + 1),
+                If(count == (ratio - 1),
+                    If(wishbone.we,
+                        NextValue(count, 0),
+                        NextState("WAIT-WRITE")
+                    ).Else(
+                        NextValue(count, 0),
+                        NextState("WAIT-READ")
+                    )
                 )
             )
         )
-        fsm.act("WRITE",
-            wdata_buffer.sink.valid.eq(1),
-            If(wdata_buffer.sink.ready,
+        fsm.act("WAIT-WRITE",
+            If(wdata_converter.sink.ready,
                 wishbone.ack.eq(1),
                 NextState("CMD")
             )
         )
-        fsm.act("READ",
-            port.rdata.ready.eq(1),
-            If(port.rdata.valid,
-                wishbone.ack.eq(1),
-                NextState("CMD")
+        fsm.act("WAIT-READ",
+            If(rdata_converter.source.valid,
+               wishbone.ack.eq(1),
+               NextState("CMD")
             )
         )
-
-        # Datapath ---------------------------------------------------------------------------------
-        self.comb += [
-            # Cmd
-            port.cmd.addr.eq(wishbone.adr - adr_offset),
-            # Write
-            wdata_buffer.sink.data.eq(wishbone.dat_w),
-            wdata_buffer.sink.we.eq(wishbone.sel),
-            wdata_buffer.source.connect(port.wdata),
-            # Read
-            wishbone.dat_r.eq(port.rdata.data),
-        ]