src/soc/regfile/sram_wrapper.py

   1 # SPDX-License-Identifier: LGPLv3+
   2 # Copyright (C) 2022 Cesar Strauss <cestrauss@gmail.com>
   3 # Sponsored by NLnet and NGI POINTER under EU Grants 871528 and 957073
   4 # Part of the Libre-SOC Project.
   5
   6 """
   7 Wrapper around a single port (1R or 1W) SRAM, to make a multi-port regfile.
   8
   9 This SRAM primitive has one cycle delay for reads, and, after a write,
  10 it reads the value just written. The goal is to use it to make at least an
  11 1W2R regfile.
  12
  13 See https://bugs.libre-soc.org/show_bug.cgi?id=781 and
  14 https://bugs.libre-soc.org/show_bug.cgi?id=502
  15 """
  16
  17 import unittest
  18
  19 from nmigen import Elaboratable, Module, Memory, Signal
  20 from nmigen.back import rtlil
  21 from nmigen.sim import Simulator
  22 from nmigen.asserts import Assert, Past, AnyConst
  23
  24 from nmutil.formaltest import FHDLTestCase
  25 from nmutil.gtkw import write_gtkw
  26
  27
  28 class SinglePortSRAM(Elaboratable):
  29     """
  30     Model of a single port SRAM, which can be simulated, verified and/or
  31     synthesized to an FPGA.
  32
  33     :param addr_width: width of the address bus
  34     :param data_width: width of the data bus
  35     :param we_width: number of write enable lines
  36
  37     .. note:: The debug read port is meant only to assist in formal proofs!
  38     """
  39     def __init__(self, addr_width, data_width, we_width):
  40         self.addr_width = addr_width
  41         self.data_width = data_width
  42         self.we_width = we_width
  43         self.d = Signal(data_width)
  44         """ write data"""
  45         self.q = Signal(data_width)
  46         """read data"""
  47         self.a = Signal(addr_width)
  48         """ read/write address"""
  49         self.we = Signal(we_width)
  50         """write enable"""
  51         self.dbg_a = Signal(addr_width)
  52         """debug read port address"""
  53         self.dbg_q = Signal(data_width)
  54         """debug read port data"""
  55
  56     def elaborate(self, _):
  57         m = Module()
  58         # backing memory
  59         depth = 1 << self.addr_width
  60         granularity = self.data_width // self.we_width
  61         mem = Memory(width=self.data_width, depth=depth)
  62         # create read and write ports
  63         # By connecting the same address to both ports, they behave, in fact,
  64         # as a single, "half-duplex" port.
  65         # The transparent attribute means that, on a write, we read the new
  66         # value, on the next cycle
  67         # Note that nmigen memories have a one cycle delay, for reads,
  68         # by default
  69         m.submodules.rdport = rdport = mem.read_port(transparent=True)
  70         m.submodules.wrport = wrport = mem.write_port(granularity=granularity)
  71         # duplicate the address to both ports
  72         m.d.comb += wrport.addr.eq(self.a)
  73         m.d.comb += rdport.addr.eq(self.a)
  74         # write enable
  75         m.d.comb += wrport.en.eq(self.we)
  76         # read and write data
  77         m.d.comb += wrport.data.eq(self.d)
  78         m.d.comb += self.q.eq(rdport.data)
  79         # the debug port is an asynchronous read port, allowing direct access
  80         # to a given memory location by the formal engine
  81         m.submodules.dbgport = dbgport = mem.read_port(domain="comb")
  82         m.d.comb += dbgport.addr.eq(self.dbg_a)
  83         m.d.comb += self.dbg_q.eq(dbgport.data)
  84         return m
  85
  86     def ports(self):
  87         return [
  88             self.d,
  89             self.a,
  90             self.we,
  91             self.q
  92         ]
  93
  94
  95 def create_ilang(dut, ports, test_name):
  96     vl = rtlil.convert(dut, name=test_name, ports=ports)
  97     with open("%s.il" % test_name, "w") as f:
  98         f.write(vl)
  99
 100
 101 class SinglePortSRAMTestCase(FHDLTestCase):
 102     @staticmethod
 103     def test_simple_rtlil():
 104         """
 105         Generate a simple SRAM. Try ``read_rtlil mem_simple.il; proc; show``
 106         from a yosys prompt, to see the memory primitives, and
 107         ``read_rtlil mem_simple.il; synth; show`` to see it implemented as
 108         flip-flop RAM
 109         """
 110         dut = SinglePortSRAM(2, 4, 2)
 111         create_ilang(dut, dut.ports(), "mem_simple")
 112
 113     @staticmethod
 114     def test_blkram_rtlil():
 115         """
 116         Generates a bigger SRAM.
 117         Try ``read_rtlil mem_blkram.il; synth_ecp5; show`` from a yosys
 118         prompt, to see it implemented as block RAM
 119         """
 120         dut = SinglePortSRAM(10, 16, 2)
 121         create_ilang(dut, dut.ports(), "mem_blkram")
 122
 123     def test_sram_model(self):
 124         """
 125         Simulate some read/write/modify operations on the SRAM model
 126         """
 127         dut = SinglePortSRAM(7, 32, 4)
 128         sim = Simulator(dut)
 129         sim.add_clock(1e-6)
 130
 131         def process():
 132             # 1) write 0x12_34_56_78 to address 0
 133             yield dut.a.eq(0)
 134             yield dut.d.eq(0x12_34_56_78)
 135             yield dut.we.eq(0b1111)
 136             yield
 137             # 2) write 0x9A_BC_DE_F0 to address 1
 138             yield dut.a.eq(1)
 139             yield dut.d.eq(0x9A_BC_DE_F0)
 140             yield dut.we.eq(0b1111)
 141             yield
 142             # ... and read value just written to address 0
 143             self.assertEqual((yield dut.q), 0x12_34_56_78)
 144             # 3) prepare to read from address 0
 145             yield dut.d.eq(0)
 146             yield dut.we.eq(0b0000)
 147             yield dut.a.eq(0)
 148             yield
 149             # ... and read value just written to address 1
 150             self.assertEqual((yield dut.q), 0x9A_BC_DE_F0)
 151             # 4) prepare to read from address 1
 152             yield dut.a.eq(1)
 153             yield
 154             # ... and read value from address 0
 155             self.assertEqual((yield dut.q), 0x12_34_56_78)
 156             # 5) write 0x9A and 0xDE to bytes 1 and 3, leaving
 157             # bytes 0 and 2 unchanged
 158             yield dut.a.eq(0)
 159             yield dut.d.eq(0x9A_FF_DE_FF)
 160             yield dut.we.eq(0b1010)
 161             yield
 162             # ... and read value from address 1
 163             self.assertEqual((yield dut.q), 0x9A_BC_DE_F0)
 164             # 6) nothing more to do
 165             yield dut.d.eq(0)
 166             yield dut.we.eq(0)
 167             yield
 168             # ... other than confirm that bytes 1 and 3 were modified
 169             # correctly
 170             self.assertEqual((yield dut.q), 0x9A_34_DE_78)
 171
 172         sim.add_sync_process(process)
 173         traces = ['rdport.clk', 'a[6:0]', 'we[3:0]', 'd[31:0]', 'q[31:0]']
 174         write_gtkw('test_sram_model.gtkw', 'test_sram_model.vcd',
 175                    traces, module='top')
 176         sim_writer = sim.write_vcd('test_sram_model.vcd')
 177         with sim_writer:
 178             sim.run()
 179
 180     def test_model_sram_proof(self):
 181         """
 182         Formal proof of the single port SRAM model
 183         """
 184         m = Module()
 185         # 128 x 32-bit, 8-bit granularity
 186         m.submodules.dut = dut = SinglePortSRAM(7, 32, 4)
 187         gran = len(dut.d) // len(dut.we)  # granularity
 188         # choose a single random memory location to test
 189         a_const = AnyConst(dut.a.shape())
 190         # choose a single byte lane to test (one-hot encoding)
 191         we_mask = Signal.like(dut.we)
 192         # ... by first creating a random bit pattern
 193         we_const = AnyConst(dut.we.shape())
 194         # ... and zeroing all but the first non-zero bit
 195         m.d.comb += we_mask.eq(we_const & (-we_const))
 196         # holding data register
 197         d_reg = Signal(gran)
 198         # for some reason, simulated formal memory is not zeroed at reset
 199         # ... so, remember whether we wrote it, at least once.
 200         wrote = Signal()
 201         # if our memory location and byte lane is being written
 202         # ... capture the data in our holding register
 203         with m.If(dut.a == a_const):
 204             for i in range(len(dut.we)):
 205                 with m.If(we_mask[i] & dut.we[i]):
 206                     m.d.sync += d_reg.eq(dut.d[i*gran:i*gran+gran])
 207                     m.d.sync += wrote.eq(1)
 208         # if our memory location is being read
 209         # ... and the holding register has valid data
 210         # ... then its value must match the memory output, on the given lane
 211         with m.If((Past(dut.a) == a_const) & wrote):
 212             for i in range(len(dut.we)):
 213                 with m.If(we_mask[i]):
 214                     m.d.sync += Assert(d_reg == dut.q[i*gran:i*gran+gran])
 215
 216         # the following is needed for induction, where an unreachable state
 217         # (memory and holding register differ) is turned into an illegal one
 218         # first, get the value stored in our memory location, using its debug
 219         # port
 220         stored = Signal.like(dut.q)
 221         m.d.comb += dut.dbg_a.eq(a_const)
 222         m.d.comb += stored.eq(dut.dbg_q)
 223         # now, ensure that the value stored in memory is always in sync
 224         # with the holding register
 225         with m.If(wrote):
 226             for i in range(len(dut.we)):
 227                 with m.If(we_mask[i]):
 228                     m.d.sync += Assert(d_reg == stored[i*gran:i*gran+gran])
 229
 230         self.assertFormal(m, mode="prove", depth=2)
 231
 232
 233 if __name__ == "__main__":
 234     unittest.main()