Formal proof of the phased write dual port memory wrapper

[soc.git] / src / soc / regfile / sram_wrapper.py

diff --git a/src/soc/regfile/sram_wrapper.py b/src/soc/regfile/sram_wrapper.py
index f80c43236da3eac96325af6e5dd3ff2196fd42b2..89e5ff40c1d6b81698ac246aab688f69b5255574 100644 (file)
--- a/src/soc/regfile/sram_wrapper.py
+++ b/src/soc/regfile/sram_wrapper.py
@@ -19,7 +19,7 @@ import unittest
 from nmigen import Elaboratable, Module, Memory, Signal
 from nmigen.back import rtlil
 from nmigen.sim import Simulator
-from nmigen.asserts import Assert, Past, AnyConst
+from nmigen.asserts import Assert, Assume, Past, AnyConst
 
 from nmutil.formaltest import FHDLTestCase
 from nmutil.gtkw import write_gtkw
@@ -433,6 +433,49 @@ class PhasedDualPortRegfileTestCase(FHDLTestCase):
         with self.subTest("writes happen on phase 1 (transparent reads)"):
             self.do_test_phased_dual_port_regfile(1, True)
 
+    def test_phased_dual_port_regfile_proof(self):
+        """
+        Formal proof of the pseudo 1W/1R regfile
+        """
+        m = Module()
+        # 128 x 32-bit, 8-bit granularity
+        m.submodules.dut = dut = PhasedDualPortRegfile(7, 32, 4, 0, True)
+        gran = dut.data_width // dut.we_width  # granularity
+        # choose a single random memory location to test
+        a_const = AnyConst(dut.addr_width)
+        # choose a single byte lane to test (one-hot encoding)
+        we_mask = Signal(dut.we_width)
+        # ... by first creating a random bit pattern
+        we_const = AnyConst(dut.we_width)
+        # ... and zeroing all but the first non-zero bit
+        m.d.comb += we_mask.eq(we_const & (-we_const))
+        # drive alternating phases
+        m.d.comb += Assume(dut.phase != Past(dut.phase))
+        # holding data register
+        d_reg = Signal(gran)
+        # for some reason, simulated formal memory is not zeroed at reset
+        # ... so, remember whether we wrote it, at least once.
+        wrote = Signal()
+        # if our memory location and byte lane is being written,
+        # capture the data in our holding register
+        with m.If((dut.wr_addr_i == a_const)
+                  & (dut.phase == dut.write_phase)):
+            for i in range(dut.we_width):
+                with m.If(we_mask[i] & dut.wr_we_i[i]):
+                    m.d.sync += d_reg.eq(
+                        dut.wr_data_i[i * gran:i * gran + gran])
+                    m.d.sync += wrote.eq(1)
+        # if our memory location is being read,
+        # and the holding register has valid data,
+        # then its value must match the memory output, on the given lane
+        with m.If((Past(dut.rd_addr_i) == a_const) & wrote):
+            for i in range(dut.we_width):
+                with m.If(we_mask[i]):
+                    m.d.sync += Assert(
+                        d_reg == dut.rd_data_o[i * gran:i * gran + gran])
+
+        self.assertFormal(m, mode="bmc", depth=10)
+
 
 if __name__ == "__main__":
     unittest.main()