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, Assume, 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); """ write data"""
  44         self.q = Signal(data_width); """read data"""
  45         self.a = Signal(addr_width); """ read/write address"""
  46         self.we = Signal(we_width); """write enable"""
  47         self.dbg_a = Signal(addr_width); """debug read port address"""
  48         self.dbg_q = Signal(data_width); """debug read port data"""
  49
  50     def elaborate(self, _):
  51         m = Module()
  52         # backing memory
  53         depth = 1 << self.addr_width
  54         granularity = self.data_width // self.we_width
  55         mem = Memory(width=self.data_width, depth=depth)
  56         # create read and write ports
  57         # By connecting the same address to both ports, they behave, in fact,
  58         # as a single, "half-duplex" port.
  59         # The transparent attribute means that, on a write, we read the new
  60         # value, on the next cycle
  61         # Note that nmigen memories have a one cycle delay, for reads,
  62         # by default
  63         m.submodules.rdport = rdport = mem.read_port(transparent=True)
  64         m.submodules.wrport = wrport = mem.write_port(granularity=granularity)
  65         # duplicate the address to both ports
  66         m.d.comb += wrport.addr.eq(self.a)
  67         m.d.comb += rdport.addr.eq(self.a)
  68         # write enable
  69         m.d.comb += wrport.en.eq(self.we)
  70         # read and write data
  71         m.d.comb += wrport.data.eq(self.d)
  72         m.d.comb += self.q.eq(rdport.data)
  73         # the debug port is an asynchronous read port, allowing direct access
  74         # to a given memory location by the formal engine
  75         m.submodules.dbgport = dbgport = mem.read_port(domain="comb")
  76         m.d.comb += dbgport.addr.eq(self.dbg_a)
  77         m.d.comb += self.dbg_q.eq(dbgport.data)
  78         return m
  79
  80     def ports(self):
  81         return [
  82             self.d,
  83             self.a,
  84             self.we,
  85             self.q
  86         ]
  87
  88
  89 def create_ilang(dut, ports, test_name):
  90     vl = rtlil.convert(dut, name=test_name, ports=ports)
  91     with open("%s.il" % test_name, "w") as f:
  92         f.write(vl)
  93
  94
  95 class SinglePortSRAMTestCase(FHDLTestCase):
  96     @staticmethod
  97     def test_simple_rtlil():
  98         """
  99         Generate a simple SRAM. Try ``read_rtlil mem_simple.il; proc; show``
 100         from a yosys prompt, to see the memory primitives, and
 101         ``read_rtlil mem_simple.il; synth; show`` to see it implemented as
 102         flip-flop RAM
 103         """
 104         dut = SinglePortSRAM(2, 4, 2)
 105         create_ilang(dut, dut.ports(), "mem_simple")
 106
 107     @staticmethod
 108     def test_blkram_rtlil():
 109         """
 110         Generates a bigger SRAM.
 111         Try ``read_rtlil mem_blkram.il; synth_ecp5; show`` from a yosys
 112         prompt, to see it implemented as block RAM
 113         """
 114         dut = SinglePortSRAM(10, 16, 2)
 115         create_ilang(dut, dut.ports(), "mem_blkram")
 116
 117     def test_sram_model(self):
 118         """
 119         Simulate some read/write/modify operations on the SRAM model
 120         """
 121         dut = SinglePortSRAM(7, 32, 4)
 122         sim = Simulator(dut)
 123         sim.add_clock(1e-6)
 124
 125         def process():
 126             # 1) write 0x12_34_56_78 to address 0
 127             yield dut.a.eq(0)
 128             yield dut.d.eq(0x12_34_56_78)
 129             yield dut.we.eq(0b1111)
 130             yield
 131             # 2) write 0x9A_BC_DE_F0 to address 1
 132             yield dut.a.eq(1)
 133             yield dut.d.eq(0x9A_BC_DE_F0)
 134             yield dut.we.eq(0b1111)
 135             yield
 136             # ... and read value just written to address 0
 137             self.assertEqual((yield dut.q), 0x12_34_56_78)
 138             # 3) prepare to read from address 0
 139             yield dut.d.eq(0)
 140             yield dut.we.eq(0b0000)
 141             yield dut.a.eq(0)
 142             yield
 143             # ... and read value just written to address 1
 144             self.assertEqual((yield dut.q), 0x9A_BC_DE_F0)
 145             # 4) prepare to read from address 1
 146             yield dut.a.eq(1)
 147             yield
 148             # ... and read value from address 0
 149             self.assertEqual((yield dut.q), 0x12_34_56_78)
 150             # 5) write 0x9A and 0xDE to bytes 1 and 3, leaving
 151             # bytes 0 and 2 unchanged
 152             yield dut.a.eq(0)
 153             yield dut.d.eq(0x9A_FF_DE_FF)
 154             yield dut.we.eq(0b1010)
 155             yield
 156             # ... and read value from address 1
 157             self.assertEqual((yield dut.q), 0x9A_BC_DE_F0)
 158             # 6) nothing more to do
 159             yield dut.d.eq(0)
 160             yield dut.we.eq(0)
 161             yield
 162             # ... other than confirm that bytes 1 and 3 were modified
 163             # correctly
 164             self.assertEqual((yield dut.q), 0x9A_34_DE_78)
 165
 166         sim.add_sync_process(process)
 167         traces = ['rdport.clk', 'a[6:0]', 'we[3:0]', 'd[31:0]', 'q[31:0]']
 168         write_gtkw('test_sram_model.gtkw', 'test_sram_model.vcd',
 169                    traces, module='top')
 170         sim_writer = sim.write_vcd('test_sram_model.vcd')
 171         with sim_writer:
 172             sim.run()
 173
 174     def test_model_sram_proof(self):
 175         """
 176         Formal proof of the single port SRAM model
 177         """
 178         m = Module()
 179         # 128 x 32-bit, 8-bit granularity
 180         m.submodules.dut = dut = SinglePortSRAM(7, 32, 4)
 181         gran = len(dut.d) // len(dut.we)  # granularity
 182         # choose a single random memory location to test
 183         a_const = AnyConst(dut.a.shape())
 184         # choose a single byte lane to test (one-hot encoding)
 185         we_mask = Signal.like(dut.we)
 186         # ... by first creating a random bit pattern
 187         we_const = AnyConst(dut.we.shape())
 188         # ... and zeroing all but the first non-zero bit
 189         m.d.comb += we_mask.eq(we_const & (-we_const))
 190         # holding data register
 191         d_reg = Signal(gran)
 192         # for some reason, simulated formal memory is not zeroed at reset
 193         # ... so, remember whether we wrote it, at least once.
 194         wrote = Signal()
 195         # if our memory location and byte lane is being written
 196         # ... capture the data in our holding register
 197         with m.If(dut.a == a_const):
 198             for i in range(len(dut.we)):
 199                 with m.If(we_mask[i] & dut.we[i]):
 200                     m.d.sync += d_reg.eq(dut.d[i*gran:i*gran+gran])
 201                     m.d.sync += wrote.eq(1)
 202         # if our memory location is being read
 203         # ... and the holding register has valid data
 204         # ... then its value must match the memory output, on the given lane
 205         with m.If((Past(dut.a) == a_const) & wrote):
 206             for i in range(len(dut.we)):
 207                 with m.If(we_mask[i]):
 208                     m.d.sync += Assert(d_reg == dut.q[i*gran:i*gran+gran])
 209
 210         # the following is needed for induction, where an unreachable state
 211         # (memory and holding register differ) is turned into an illegal one
 212         # first, get the value stored in our memory location, using its debug
 213         # port
 214         stored = Signal.like(dut.q)
 215         m.d.comb += dut.dbg_a.eq(a_const)
 216         m.d.comb += stored.eq(dut.dbg_q)
 217         # now, ensure that the value stored in memory is always in sync
 218         # with the holding register
 219         with m.If(wrote):
 220             for i in range(len(dut.we)):
 221                 with m.If(we_mask[i]):
 222                     m.d.sync += Assert(d_reg == stored[i*gran:i*gran+gran])
 223
 224         self.assertFormal(m, mode="prove", depth=2)
 225
 226
 227 class PhasedDualPortRegfile(Elaboratable):
 228     """
 229     Builds, from a pair of 1RW blocks, a pseudo 1W/1R RAM, where the
 230     read port works every cycle, but the write port is only available on
 231     either even (1eW/1R) or odd (1oW/1R) cycles.
 232
 233     :param addr_width: width of the address bus
 234     :param data_width: width of the data bus
 235     :param we_width: number of write enable lines
 236     :param write_phase: indicates on which phase the write port will
 237                         accept data
 238     :param transparent: whether a simultaneous read and write returns the
 239                         new value (True) or the old value (False)
 240
 241     .. note:: The debug read port is meant only to assist in formal proofs!
 242     """
 243
 244     def __init__(self, addr_width, data_width, we_width, write_phase,
 245                  transparent=False):
 246         self.addr_width = addr_width
 247         self.data_width = data_width
 248         self.we_width = we_width
 249         self.write_phase = write_phase
 250         self.transparent = transparent
 251         self.wr_addr_i = Signal(addr_width); """write port address"""
 252         self.wr_data_i = Signal(data_width); """write port data"""
 253         self.wr_we_i = Signal(we_width); """write port enable"""
 254         self.rd_addr_i = Signal(addr_width); """read port address"""
 255         self.rd_data_o = Signal(data_width); """read port data"""
 256         self.phase = Signal(); """even/odd cycle indicator"""
 257         self.dbg_a = Signal(addr_width); """debug read port address"""
 258         self.dbg_q1 = Signal(data_width); """debug read port data (1st mem)"""
 259         self.dbg_q2 = Signal(data_width); """debug read port data (2nd mem)"""
 260
 261     def elaborate(self, _):
 262         m = Module()
 263         # instantiate the two 1RW memory blocks
 264         mem1 = SinglePortSRAM(self.addr_width, self.data_width, self.we_width)
 265         mem2 = SinglePortSRAM(self.addr_width, self.data_width, self.we_width)
 266         m.submodules.mem1 = mem1
 267         m.submodules.mem2 = mem2
 268         # wire write port to first memory, and its output to the second
 269         m.d.comb += mem1.d.eq(self.wr_data_i)
 270         m.d.comb += mem2.d.eq(mem1.q)
 271         # holding registers for the write port of the second memory
 272         last_wr_addr = Signal(self.addr_width)
 273         last_wr_we = Signal(self.we_width)
 274         # do the read and write address coincide?
 275         same_read_write = Signal()
 276         with m.If(self.phase == self.write_phase):
 277             # write phase, start a write on the first memory
 278             m.d.comb += mem1.a.eq(self.wr_addr_i)
 279             m.d.comb += mem1.we.eq(self.wr_we_i)
 280             # save write address and write select for repeating the write
 281             # on the second memory, later
 282             m.d.sync += last_wr_we.eq(self.wr_we_i)
 283             m.d.sync += last_wr_addr.eq(self.wr_addr_i)
 284             # start a read on the second memory
 285             m.d.comb += mem2.a.eq(self.rd_addr_i)
 286             # output previously read data from the first memory
 287             m.d.comb += self.rd_data_o.eq(mem1.q)
 288             if self.transparent:
 289                 # remember whether we are reading from the same location we are
 290                 # writing
 291                 m.d.sync += same_read_write.eq(self.rd_addr_i == self.wr_addr_i)
 292         with m.Else():
 293             # read phase, write last written data on second memory
 294             m.d.comb += mem2.a.eq(last_wr_addr)
 295             m.d.comb += mem2.we.eq(last_wr_we)
 296             # start a read on the first memory
 297             m.d.comb += mem1.a.eq(self.rd_addr_i)
 298             if self.transparent:
 299                 with m.If(same_read_write):
 300                     # when transparent, and read and write addresses coincide,
 301                     # output the data just written
 302                     m.d.comb += self.rd_data_o.eq(mem1.q)
 303                 with m.Else():
 304                     # otherwise, output previously read data
 305                     # from the second memory
 306                     m.d.comb += self.rd_data_o.eq(mem2.q)
 307             else:
 308                 # always output the read data from the second memory,
 309                 # if not transparent
 310                 m.d.comb += self.rd_data_o.eq(mem2.q)
 311         # our debug port allow the formal engine to inspect the content of
 312         # a fixed, arbitrary address, on our memory blocks.
 313         # wire it to their debug ports.
 314         m.d.comb += mem1.dbg_a.eq(self.dbg_a)
 315         m.d.comb += mem2.dbg_a.eq(self.dbg_a)
 316         m.d.comb += self.dbg_q1.eq(mem1.dbg_q)
 317         m.d.comb += self.dbg_q2.eq(mem2.dbg_q)
 318
 319         return m
 320
 321
 322 class PhasedDualPortRegfileTestCase(FHDLTestCase):
 323
 324     def do_test_phased_dual_port_regfile(self, write_phase, transparent):
 325         """
 326         Simulate some read/write/modify operations on the phased write memory
 327         """
 328         dut = PhasedDualPortRegfile(7, 32, 4, write_phase, transparent)
 329         sim = Simulator(dut)
 330         sim.add_clock(1e-6)
 331
 332         # compare read data with previously written data
 333         # and start a new read
 334         def read(rd_addr_i, expected=None):
 335             if expected is not None:
 336                 self.assertEqual((yield dut.rd_data_o), expected)
 337             yield dut.rd_addr_i.eq(rd_addr_i)
 338
 339         # start a write, and set write phase
 340         def write(wr_addr_i, wr_we_i, wr_data_i):
 341             yield dut.wr_addr_i.eq(wr_addr_i)
 342             yield dut.wr_we_i.eq(wr_we_i)
 343             yield dut.wr_data_i.eq(wr_data_i)
 344             yield dut.phase.eq(write_phase)
 345
 346         # disable writes, and start read phase
 347         def skip_write():
 348             yield dut.wr_addr_i.eq(0)
 349             yield dut.wr_we_i.eq(0)
 350             yield dut.wr_data_i.eq(0)
 351             yield dut.phase.eq(~write_phase)
 352
 353         # writes a few values on the write port, and read them back
 354         # ... reads can happen every cycle
 355         # ... writes, only every two cycles.
 356         # since reads have a one cycle delay, the expected value on
 357         # each read refers to the last read performed, not the
 358         # current one, which is in progress.
 359         def process():
 360             yield from read(0)
 361             yield from write(0x42, 0b1111, 0x12345678)
 362             yield
 363             yield from read(0x42)
 364             yield from skip_write()
 365             yield
 366             yield from read(0x42)
 367             yield from write(0x43, 0b1111, 0x9ABCDEF0)
 368             yield
 369             yield from read(0x43, 0x12345678)
 370             yield from skip_write()
 371             yield
 372             yield from read(0x42, 0x12345678)
 373             yield from write(0x43, 0b1001, 0xF0FFFF9A)
 374             yield
 375             yield from read(0x43, 0x9ABCDEF0)
 376             yield from skip_write()
 377             yield
 378             yield from read(0x43, 0x12345678)
 379             yield from write(0x42, 0b0110, 0xFF5634FF)
 380             yield
 381             yield from read(0x42, 0xF0BCDE9A)
 382             yield from skip_write()
 383             yield
 384             yield from read(0, 0xF0BCDE9A)
 385             yield from write(0, 0, 0)
 386             yield
 387             yield from read(0, 0x12563478)
 388             yield from skip_write()
 389             yield
 390             # try reading and writing to the same location, simultaneously
 391             yield from read(0x42)
 392             yield from write(0x42, 0b1111, 0x55AA9966)
 393             yield
 394             # ... and read again
 395             yield from read(0x42)
 396             yield from skip_write()
 397             yield
 398             if transparent:
 399                 # returns the value just written
 400                 yield from read(0, 0x55AA9966)
 401             else:
 402                 # returns the old value
 403                 yield from read(0, 0x12563478)
 404             yield from write(0, 0, 0)
 405             yield
 406             # after a cycle, always returns the new value
 407             yield from read(0, 0x55AA9966)
 408             yield from skip_write()
 409
 410         sim.add_sync_process(process)
 411         debug_file = f'test_phased_dual_port_{write_phase}'
 412         if transparent:
 413             debug_file += '_transparent'
 414         traces = ['clk', 'phase',
 415                   'wr_addr_i[6:0]', 'wr_we_i[3:0]', 'wr_data_i[31:0]',
 416                   'rd_addr_i[6:0]', 'rd_data_o[31:0]']
 417         write_gtkw(debug_file + '.gtkw',
 418                    debug_file + '.vcd',
 419                    traces, module='top', zoom=-22)
 420         sim_writer = sim.write_vcd(debug_file + '.vcd')
 421         with sim_writer:
 422             sim.run()
 423
 424     def test_phased_dual_port_regfile(self):
 425         """test both types (odd and even write ports) of phased write memory"""
 426         with self.subTest("writes happen on phase 0"):
 427             self.do_test_phased_dual_port_regfile(0, False)
 428         with self.subTest("writes happen on phase 1"):
 429             self.do_test_phased_dual_port_regfile(1, False)
 430         """test again, with a transparent read port"""
 431         with self.subTest("writes happen on phase 0 (transparent reads)"):
 432             self.do_test_phased_dual_port_regfile(0, True)
 433         with self.subTest("writes happen on phase 1 (transparent reads)"):
 434             self.do_test_phased_dual_port_regfile(1, True)
 435
 436     def do_test_phased_dual_port_regfile_proof(self, write_phase, transparent):
 437         """
 438         Formal proof of the pseudo 1W/1R regfile
 439         """
 440         m = Module()
 441         # 128 x 32-bit, 8-bit granularity
 442         dut = PhasedDualPortRegfile(7, 32, 4, write_phase, transparent)
 443         m.submodules.dut = dut
 444         gran = dut.data_width // dut.we_width  # granularity
 445         # choose a single random memory location to test
 446         a_const = AnyConst(dut.addr_width)
 447         # choose a single byte lane to test (one-hot encoding)
 448         we_mask = Signal(dut.we_width)
 449         # ... by first creating a random bit pattern
 450         we_const = AnyConst(dut.we_width)
 451         # ... and zeroing all but the first non-zero bit
 452         m.d.comb += we_mask.eq(we_const & (-we_const))
 453         # drive alternating phases
 454         m.d.comb += Assume(dut.phase != Past(dut.phase))
 455         # holding data register
 456         d_reg = Signal(gran)
 457         # for some reason, simulated formal memory is not zeroed at reset
 458         # ... so, remember whether we wrote it, at least once.
 459         wrote = Signal()
 460         # if our memory location and byte lane is being written,
 461         # capture the data in our holding register
 462         with m.If((dut.wr_addr_i == a_const)
 463                   & (dut.phase == dut.write_phase)):
 464             for i in range(dut.we_width):
 465                 with m.If(we_mask[i] & dut.wr_we_i[i]):
 466                     m.d.sync += d_reg.eq(
 467                         dut.wr_data_i[i * gran:i * gran + gran])
 468                     m.d.sync += wrote.eq(1)
 469         # if our memory location is being read,
 470         # and the holding register has valid data,
 471         # then its value must match the memory output, on the given lane
 472         with m.If(Past(dut.rd_addr_i) == a_const):
 473             if transparent:
 474                 with m.If(wrote):
 475                     for i in range(dut.we_width):
 476                         rd_lane = dut.rd_data_o.word_select(i, gran)
 477                         with m.If(we_mask[i]):
 478                             m.d.sync += Assert(d_reg == rd_lane)
 479             else:
 480                 # with a non-transparent read port, the read value depends
 481                 # on whether there is a simultaneous write, or not
 482                 with m.If((Past(dut.wr_addr_i) == a_const)
 483                           & Past(dut.phase) == dut.write_phase):
 484                     # simultaneous write -> check against last written value
 485                     with m.If(Past(wrote)):
 486                         for i in range(dut.we_width):
 487                             rd_lane = dut.rd_data_o.word_select(i, gran)
 488                             with m.If(we_mask[i]):
 489                                 m.d.sync += Assert(Past(d_reg) == rd_lane)
 490                 with m.Else():
 491                     # otherwise, check against current written value
 492                     with m.If(wrote):
 493                         for i in range(dut.we_width):
 494                             rd_lane = dut.rd_data_o.word_select(i, gran)
 495                             with m.If(we_mask[i]):
 496                                 m.d.sync += Assert(d_reg == rd_lane)
 497
 498         # the following is needed for induction, where an unreachable state
 499         # (memory and holding register differ) is turned into an illegal one
 500         # first, get the values stored in our memory location, using its
 501         # debug port
 502         stored1 = Signal(dut.data_width)
 503         stored2 = Signal(dut.data_width)
 504         m.d.comb += dut.dbg_a.eq(a_const)
 505         m.d.comb += stored1.eq(dut.dbg_q1)
 506         m.d.comb += stored2.eq(dut.dbg_q2)
 507         # now, ensure that the value stored in the first memory is always
 508         # in sync with the holding register
 509         with m.If(wrote):
 510             for i in range(dut.we_width):
 511                 with m.If(we_mask[i]):
 512                     m.d.comb += Assert(
 513                         d_reg == stored1[i * gran:i * gran + gran])
 514         # same for the second memory, but one cycle later
 515         with m.If(Past(wrote)):
 516             for i in range(dut.we_width):
 517                 with m.If(we_mask[i]):
 518                     m.d.comb += Assert(
 519                         Past(d_reg) == stored2[i * gran:i * gran + gran])
 520
 521         self.assertFormal(m, mode="prove", depth=2)
 522
 523     def test_phased_dual_port_regfile_proof(self):
 524         """test both types (odd and even write ports) of phased write memory"""
 525         with self.subTest("writes happen on phase 0"):
 526             self.do_test_phased_dual_port_regfile_proof(0, False)
 527         with self.subTest("writes happen on phase 1"):
 528             self.do_test_phased_dual_port_regfile_proof(1, False)
 529         # test again, with transparent read ports
 530         with self.subTest("writes happen on phase 0 (transparent reads)"):
 531             self.do_test_phased_dual_port_regfile_proof(0, True)
 532         with self.subTest("writes happen on phase 1 (transparent reads)"):
 533             self.do_test_phased_dual_port_regfile_proof(1, True)
 534
 535
 536 if __name__ == "__main__":
 537     unittest.main()