lib.fifo.AsyncFIFOBuffered: fix FFSynchronizer latency

Author: rroohhh

nmigen/lib/fifo.py

@@ -513,7 +513,7 @@ def elaborate(self, platform):
         m.d[self._r_domain] += self.r_level.eq(fifo.r_level + r_consume_buffered)
 
         w_consume_buffered = Signal()
-        m.submodules.consume_buffered_cdc = FFSynchronizer(r_consume_buffered, w_consume_buffered, o_domain=self._w_domain)
+        m.submodules.consume_buffered_cdc = FFSynchronizer(r_consume_buffered, w_consume_buffered, o_domain=self._w_domain, stages=4)
         m.d.comb += self.w_level.eq(fifo.w_level + w_consume_buffered)
 
         with m.If(self.r_en | ~self.r_rdy):

tests/test_lib_fifo.py

@@ -344,6 +344,10 @@ def test_async_buffered_fifo_level(self):
         fifo = AsyncFIFOBuffered(width=32, depth=9, r_domain="read", w_domain="write")
         self.check_async_fifo_level(fifo, fill_in=5, expected_level=5)
 
+    def test_async_buffered_fifo_level_only_three(self):
+        fifo = AsyncFIFOBuffered(width=32, depth=9, r_domain="read", w_domain="write")
+        self.check_async_fifo_level(fifo, fill_in=3, expected_level=3)
+
     def test_async_buffered_fifo_level_full(self):
         fifo = AsyncFIFOBuffered(width=32, depth=9, r_domain="read", w_domain="write")
         self.check_async_fifo_level(fifo, fill_in=10, expected_level=9)