src/add/example_buf_pipe.py

   1 """ nmigen implementation of buffered pipeline stage, based on zipcpu:
   2     https://zipcpu.com/blog/2017/08/14/strategies-for-pipelining.html
   3
   4     this module requires quite a bit of thought to understand how it works
   5     (and why it is needed in the first place).  reading the above is
   6     *strongly* recommended.
   7
   8     unlike john dawson's IEEE754 FPU STB/ACK signalling, which requires
   9     the STB / ACK signals to raise and lower (on separate clocks) before
  10     data may proceeed (thus only allowing one piece of data to proceed
  11     on *ALTERNATE* cycles), the signalling here is a true pipeline
  12     where data will flow on *every* clock when the conditions are right.
  13
  14     input acceptance conditions are when:
  15         * incoming previous-stage strobe (p.i_valid) is HIGH
  16         * outgoing previous-stage ready   (p.o_ready) is LOW
  17
  18     output transmission conditions are when:
  19         * outgoing next-stage strobe (n.o_valid) is HIGH
  20         * outgoing next-stage ready   (n.i_ready) is LOW
  21
  22     the tricky bit is when the input has valid data and the output is not
  23     ready to accept it.  if it wasn't for the clock synchronisation, it
  24     would be possible to tell the input "hey don't send that data, we're
  25     not ready".  unfortunately, it's not possible to "change the past":
  26     the previous stage *has no choice* but to pass on its data.
  27
  28     therefore, the incoming data *must* be accepted - and stored: that
  29     is the responsibility / contract that this stage *must* accept.
  30     on the same clock, it's possible to tell the input that it must
  31     not send any more data.  this is the "stall" condition.
  32
  33     we now effectively have *two* possible pieces of data to "choose" from:
  34     the buffered data, and the incoming data.  the decision as to which
  35     to process and output is based on whether we are in "stall" or not.
  36     i.e. when the next stage is no longer ready, the output comes from
  37     the buffer if a stall had previously occurred, otherwise it comes
  38     direct from processing the input.
  39
  40     this allows us to respect a synchronous "travelling STB" with what
  41     dan calls a "buffered handshake".
  42
  43     it's quite a complex state machine!
  44 """
  45
  46 from nmigen import Signal, Cat, Const, Mux, Module
  47 from nmigen.cli import verilog, rtlil
  48 from collections.abc import Sequence
  49
  50
  51 class PrevControl:
  52     """ contains signals that come *from* the previous stage (both in and out)
  53         * i_valid: input from previous stage indicating incoming data is valid
  54         * o_ready: output to next stage indicating readiness to accept data
  55         * i_data : an input - added by the user of this class
  56     """
  57
  58     def __init__(self):
  59         self.i_valid = Signal(name="p_i_valid") # >>in
  60         self.o_ready = Signal(name="p_o_ready") # <<out
  61
  62     def connect_in(self, prev):
  63         """ helper function to connect stage to an input source.  do not
  64             use to connect stage-to-stage!
  65         """
  66         return [self.i_valid.eq(prev.i_valid),
  67                 prev.o_ready.eq(self.o_ready),
  68                 eq(self.i_data, prev.i_data),
  69                ]
  70
  71
  72 class NextControl:
  73     """ contains the signals that go *to* the next stage (both in and out)
  74         * o_valid: output indicating to next stage that data is valid
  75         * i_ready: input from next stage indicating that it can accept data
  76         * o_data : an output - added by the user of this class
  77     """
  78     def __init__(self):
  79         self.o_valid = Signal(name="n_o_valid") # out>>
  80         self.i_ready = Signal(name="n_i_ready") # <<in
  81
  82     def connect_to_next(self, nxt):
  83         """ helper function to connect to the next stage data/valid/ready.
  84             data/valid is passed *TO* nxt, and ready comes *IN* from nxt.
  85         """
  86         return [nxt.i_valid.eq(self.o_valid),
  87                 self.i_ready.eq(nxt.o_ready),
  88                 eq(nxt.i_data, self.o_data),
  89                ]
  90
  91     def connect_out(self, nxt):
  92         """ helper function to connect stage to an output source.  do not
  93             use to connect stage-to-stage!
  94         """
  95         return [nxt.o_valid.eq(self.o_valid),
  96                 self.i_ready.eq(nxt.i_ready),
  97                 eq(nxt.o_data, self.o_data),
  98                ]
  99
 100
 101 def eq(o, i):
 102     """ makes signals equal: a helper routine which identifies if it is being
 103         passsed a list (or tuple) of objects, and calls the objects' eq
 104         function.
 105     """
 106     if not isinstance(o, Sequence):
 107         o, i = [o], [i]
 108     res = []
 109     for (ao, ai) in zip(o, i):
 110         res.append(ao.eq(ai))
 111     return res
 112
 113
 114 class PipelineBase:
 115     """ Common functions for Pipeline API
 116     """
 117     def __init__(self, stage):
 118         """ pass in a "stage" which may be either a static class or a class
 119             instance, which has three functions:
 120             * ispec: returns input signals according to the input specification
 121             * ispec: returns output signals to the output specification
 122             * process: takes an input instance and returns processed data
 123
 124             User must also:
 125             * add i_data member to PrevControl and
 126             * add o_data member to NextControl
 127         """
 128         self.stage = stage
 129
 130         # set up input and output IO ACK (prev/next ready/valid)
 131         self.p = PrevControl()
 132         self.n = NextControl()
 133
 134     def connect_to_next(self, nxt):
 135         """ helper function to connect to the next stage data/valid/ready.
 136         """
 137         return self.n.connect_to_next(nxt.p)
 138
 139     def connect_in(self, prev):
 140         """ helper function to connect stage to an input source.  do not
 141             use to connect stage-to-stage!
 142         """
 143         return self.p.connect_in(prev.p)
 144
 145     def connect_out(self, nxt):
 146         """ helper function to connect stage to an output source.  do not
 147             use to connect stage-to-stage!
 148         """
 149         return self.n.connect_out(nxt.n)
 150
 151     def set_input(self, i):
 152         """ helper function to set the input data
 153         """
 154         return eq(self.p.i_data, i)
 155
 156     def ports(self):
 157         return [self.p.i_valid, self.n.i_ready,
 158                 self.n.o_valid, self.p.o_ready,
 159                 self.p.i_data, self.n.o_data
 160                ]
 161
 162
 163 class BufferedPipeline(PipelineBase):
 164     """ buffered pipeline stage.  data and strobe signals travel in sync.
 165         if ever the input is ready and the output is not, processed data
 166         is stored in a temporary register.
 167
 168         stage-1   p.i_valid >>in   stage   n.o_valid out>>   stage+1
 169         stage-1   p.o_ready <<out  stage   n.i_ready <<in    stage+1
 170         stage-1   p.i_data  >>in   stage   n.o_data  out>>   stage+1
 171                               |             |
 172                             process --->----^
 173                               |             |
 174                               +-- r_data ->-+
 175
 176         input data p.i_data is read (only), is processed and goes into an
 177         intermediate result store [process()].  this is updated combinatorially.
 178
 179         in a non-stall condition, the intermediate result will go into the
 180         output (update_output).  however if ever there is a stall, it goes
 181         into r_data instead [update_buffer()].
 182
 183         when the non-stall condition is released, r_data is the first
 184         to be transferred to the output [flush_buffer()], and the stall
 185         condition cleared.
 186
 187         on the next cycle (as long as stall is not raised again) the
 188         input may begin to be processed and transferred directly to output.
 189     """
 190     def __init__(self, stage):
 191         PipelineBase.__init__(self, stage)
 192
 193         # set up the input and output data
 194         self.p.i_data = stage.ispec() # input type
 195         self.r_data   = stage.ospec() # all these are output type
 196         self.result   = stage.ospec()
 197         self.n.o_data = stage.ospec()
 198
 199     def update_buffer(self):
 200         """ copies the result into the intermediate register r_data,
 201             which will need to be outputted on a subsequent cycle
 202             prior to allowing "normal" operation.
 203         """
 204         return eq(self.r_data, self.result)
 205
 206     def update_output(self):
 207         """ copies the (combinatorial) result into the output
 208         """
 209         return eq(self.n.o_data, self.result)
 210
 211     def flush_buffer(self):
 212         """ copies the *intermediate* register r_data into the output
 213         """
 214         return eq(self.n.o_data, self.r_data)
 215
 216     def elaborate(self, platform):
 217         m = Module()
 218         if hasattr(self.stage, "setup"):
 219             self.stage.setup(m, self.p.i_data)
 220
 221         # establish some combinatorial temporaries
 222         o_n_validn = Signal(reset_less=True)
 223         i_p_valid_o_p_ready = Signal(reset_less=True)
 224         m.d.comb += [o_n_validn.eq(~self.n.o_valid),
 225                      i_p_valid_o_p_ready.eq(self.p.i_valid & self.p.o_ready),
 226         ]
 227
 228         # store result of processing in combinatorial temporary
 229         with m.If(self.p.i_valid): # input is valid: process it
 230             m.d.comb += eq(self.result, self.stage.process(self.p.i_data))
 231         # if not in stall condition, update the temporary register
 232         with m.If(self.p.o_ready): # not stalled
 233             m.d.sync += self.update_buffer()
 234
 235         #with m.If(self.p.i_rst): # reset
 236         #    m.d.sync += self.n.o_valid.eq(0)
 237         #    m.d.sync += self.p.o_ready.eq(0)
 238         with m.If(self.n.i_ready): # next stage is ready
 239             with m.If(self.p.o_ready): # not stalled
 240                 # nothing in buffer: send (processed) input direct to output
 241                 m.d.sync += [self.n.o_valid.eq(self.p.i_valid),
 242                              self.update_output(),
 243                             ]
 244             with m.Else(): # p.o_ready is false, and something is in buffer.
 245                 # Flush the [already processed] buffer to the output port.
 246                 m.d.sync += [self.n.o_valid.eq(1),
 247                              self.flush_buffer(),
 248                              # clear stall condition, declare register empty.
 249                              self.p.o_ready.eq(1),
 250                             ]
 251                 # ignore input, since p.o_ready is also false.
 252
 253         # (n.i_ready) is false here: next stage is ready
 254         with m.Elif(o_n_validn): # next stage being told "ready"
 255             m.d.sync += [self.n.o_valid.eq(self.p.i_valid),
 256                          self.p.o_ready.eq(1), # Keep the buffer empty
 257                          # set the output data (from comb result)
 258                          self.update_output(),
 259                         ]
 260         # (n.i_ready) false and (n.o_valid) true:
 261         with m.Elif(i_p_valid_o_p_ready):
 262             # If next stage *is* ready, and not stalled yet, accept input
 263             m.d.sync += self.p.o_ready.eq(~(self.p.i_valid & self.n.o_valid))
 264
 265         return m
 266
 267
 268 class ExampleAddStage:
 269     """ an example of how to use the buffered pipeline, as a class instance
 270     """
 271
 272     def ispec(self):
 273         """ returns a tuple of input signals which will be the incoming data
 274         """
 275         return (Signal(16), Signal(16))
 276
 277     def ospec(self):
 278         """ returns an output signal which will happen to contain the sum
 279             of the two inputs
 280         """
 281         return Signal(16)
 282
 283     def process(self, i):
 284         """ process the input data (sums the values in the tuple) and returns it
 285         """
 286         return i[0] + i[1]
 287
 288
 289 class ExampleBufPipeAdd(BufferedPipeline):
 290     """ an example of how to use the buffered pipeline, using a class instance
 291     """
 292
 293     def __init__(self):
 294         addstage = ExampleAddStage()
 295         BufferedPipeline.__init__(self, addstage)
 296
 297
 298 class ExampleStage:
 299     """ an example of how to use the buffered pipeline, in a static class
 300         fashion
 301     """
 302
 303     def ispec():
 304         return Signal(16)
 305
 306     def ospec():
 307         return Signal(16)
 308
 309     def process(i):
 310         """ process the input data and returns it (adds 1)
 311         """
 312         return i + 1
 313
 314
 315 class ExampleBufPipe(BufferedPipeline):
 316     """ an example of how to use the buffered pipeline.
 317     """
 318
 319     def __init__(self):
 320         BufferedPipeline.__init__(self, ExampleStage)
 321
 322
 323 class CombPipe(PipelineBase):
 324     """A simple pipeline stage containing combinational logic that can execute
 325     completely in one clock cycle.
 326
 327     Parameters:
 328     -----------
 329     input_shape : int or tuple or None
 330         the shape of ``input.data`` and ``comb_input``
 331     output_shape : int or tuple or None
 332         the shape of ``output.data`` and ``comb_output``
 333     name : str
 334         the name
 335
 336     Attributes:
 337     -----------
 338     input : StageInput
 339         The pipeline input
 340     output : StageOutput
 341         The pipeline output
 342     comb_input : Signal, input_shape
 343         The input to the combinatorial logic
 344     comb_output: Signal, output_shape
 345         The output of the combinatorial logic
 346     """
 347
 348     def __init__(self, stage):
 349         PipelineBase.__init__(self, stage)
 350         self._data_valid = Signal()
 351
 352         # set up the input and output data
 353         self.p.i_data = stage.ispec() # input type
 354         self.n.o_data = stage.ospec() # output type
 355
 356     def elaborate(self, platform):
 357         m = Module()
 358         r_data = self.stage.ispec() # input type
 359         result = self.stage.ospec() # output data
 360         if hasattr(self.stage, "setup"):
 361             self.stage.setup(m, r_data)
 362
 363         m.d.comb += eq(result, self.stage.process(r_data))
 364         m.d.comb += self.n.o_valid.eq(self._data_valid)
 365         m.d.comb += self.p.o_ready.eq(~self._data_valid | self.n.i_ready)
 366         m.d.sync += self._data_valid.eq(self.p.i_valid | \
 367                                         (~self.n.i_ready & self._data_valid))
 368         with m.If(self.p.i_valid & self.p.o_ready):
 369             m.d.sync += eq(r_data, self.p.i_data)
 370         m.d.comb += eq(self.n.o_data, result)
 371         return m
 372
 373
 374 class ExampleCombPipe(CombPipe):
 375     """ an example of how to use the combinatorial pipeline.
 376     """
 377
 378     def __init__(self):
 379         CombPipe.__init__(self, ExampleStage)
 380
 381
 382 if __name__ == '__main__':
 383     dut = ExampleBufPipe()
 384     vl = rtlil.convert(dut, ports=dut.ports())
 385     with open("test_bufpipe.il", "w") as f:
 386         f.write(vl)
 387
 388     dut = ExampleCombPipe()
 389     vl = rtlil.convert(dut, ports=dut.ports())
 390     with open("test_combpipe.il", "w") as f:
 391         f.write(vl)