lib.fifo.AsyncFIFO: fix incorrect latency of r_level.

Co-authored-by: Andrew Wygle <awygle@gmail.com>

nmigen/lib/fifo.py

@@ -369,7 +369,7 @@ class AsyncFIFO(Elaboratable, FIFOInterface):
         produce_dec = m.submodules.produce_dec = \
             GrayDecoder(self._ctr_bits)
         m.d.comb += produce_dec.i.eq(produce_r_gry),
-        m.d[self._r_domain] += produce_r_bin.eq(produce_dec.o)
+        m.d.comb += produce_r_bin.eq(produce_dec.o)
 
         w_full  = Signal()
         r_empty = Signal()
@@ -381,7 +381,7 @@ class AsyncFIFO(Elaboratable, FIFOInterface):
         ]
 
         m.d[self._w_domain] += self.w_level.eq((produce_w_bin - consume_w_bin)[:self._ctr_bits-1])
-        m.d[self._r_domain] += self.r_level.eq((produce_r_bin - consume_r_bin)[:self._ctr_bits-1])
+        m.d.comb += self.r_level.eq((produce_r_bin - consume_r_bin)[:self._ctr_bits-1])
 
         storage = Memory(width=self.width, depth=self.depth)
         w_port  = m.submodules.w_port = storage.write_port(domain=self._w_domain)
@@ -509,12 +509,13 @@ class AsyncFIFOBuffered(Elaboratable, FIFOInterface):
             self.w_level.eq(fifo.w_level),
         ]
 
+        m.d[self._r_domain] += self.r_level.eq(fifo.r_level + self.r_rdy - self.r_en)
+
         with m.If(self.r_en | ~self.r_rdy):
             m.d[self._r_domain] += [
                 self.r_data.eq(fifo.r_data),
                 self.r_rdy.eq(fifo.r_rdy),
                 self.r_rst.eq(fifo.r_rst),
-                self.r_level.eq(fifo.r_level),
             ]
             m.d.comb += [
                 fifo.r_en.eq(1)

tests/test_lib_fifo.py

@@ -280,3 +280,26 @@ class FIFOFormalCase(FHDLTestCase):
 
     def test_async_buffered(self):
         self.check_async_fifo(AsyncFIFOBuffered(width=8, depth=4))
+
+
+class AsyncFIFOSimCase(FHDLTestCase):
+    def test_async_fifo_r_level_latency(self):
+        fifo = AsyncFIFO(width=32, depth=10, r_domain="sync", w_domain="sync")
+
+        ff_syncronizer_latency = 2
+
+        def testbench():
+            for i in range(10):
+                yield fifo.w_data.eq(i)
+                yield fifo.w_en.eq(1)
+                yield
+
+                if (i - ff_syncronizer_latency) > 0:
+                    self.assertEqual((yield fifo.r_level), i - ff_syncronizer_latency)
+                else:
+                    self.assertEqual((yield fifo.r_level), 0)
+
+        simulator = Simulator(fifo)
+        simulator.add_clock(100e-6)
+        simulator.add_sync_process(testbench)
+        simulator.run()