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 (i_p_stb) is HIGH
  16         * outgoing previous-stage busy   (o_p_busy) is LOW
  17
  18     output transmission conditions are when:
  19         * outgoing next-stage strobe (o_n_stb) is HIGH
  20         * outgoing next-stage busy   (i_n_busy) 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.
  29     on the same clock, it's possible to tell the input that it must
  30     not send any more data.  this is the "stall" condition.
  31
  32     we now effectively have *two* possible pieces of data to "choose" from:
  33     the buffered data, and the incoming data.  the decision as to which
  34     to process and output is based on whether we are in "stall" or not.
  35     i.e. when the next stage is no longer busy, the output comes from
  36     the buffer if a stall had previously occurred, otherwise it comes
  37     direct from processing the input.
  38
  39     it's quite a complex state machine!
  40 """
  41
  42 from nmigen import Signal, Cat, Const, Mux, Module
  43 from nmigen.cli import verilog, rtlil
  44
  45 class BufPipe:
  46     """ buffered pipeline stage
  47
  48         stage-1   i_p_stb  >>in   stage   o_n_stb  out>>   stage+1
  49         stage-1   o_p_busy <<out  stage   i_n_busy <<in    stage+1
  50         stage-1   i_data   >>in   stage   o_data   out>>   stage+1
  51                               |             |
  52                               +------->  process
  53                               |             |
  54                               +-- r_data ---+
  55     """
  56     def __init__(self):
  57         # input
  58         #self.i_p_rst = Signal()    # >>in - comes in from PREVIOUS stage
  59         self.i_p_stb = Signal()    # >>in - comes in from PREVIOUS stage
  60         self.i_n_busy = Signal()   # in<< - comes in from the NEXT stage
  61         self.i_data = Signal(16) # >>in - comes in from the PREVIOUS stage
  62         #self.i_rst = Signal()
  63
  64         # buffered
  65         self.r_data = Signal(16)
  66
  67         # output
  68         self.o_n_stb = Signal()    # out>> - goes out to the NEXT stage
  69         self.o_p_busy = Signal()   # <<out - goes out to the PREVIOUS stage
  70         self.o_data = Signal(16) # out>> - goes out to the NEXT stage
  71
  72     def pre_process(self, d_in):
  73         return d_in | 0xf0000
  74
  75     def process(self, d_in):
  76         return d_in + 1
  77
  78     def elaborate(self, platform):
  79         m = Module()
  80
  81         # establish some combinatorial temporaries
  82         o_p_busyn = Signal(reset_less=True)
  83         o_n_stbn = Signal(reset_less=True)
  84         i_n_busyn = Signal(reset_less=True)
  85         i_p_stb_o_p_busyn = Signal(reset_less=True)
  86         m.d.comb += [i_n_busyn.eq(~self.i_n_busy),
  87                      o_n_stbn.eq(~self.o_n_stb),
  88                      o_p_busyn.eq(~self.o_p_busy),
  89                      i_p_stb_o_p_busyn.eq(self.i_p_stb & o_p_busyn),
  90         ]
  91
  92         # store result of processing in combinatorial temporary
  93         result = Signal(16)
  94         with m.If(self.i_p_stb): # input is valid: process it
  95             m.d.comb += result.eq(self.process(self.i_data))
  96         with m.If(o_p_busyn): # not stalled
  97             m.d.sync += self.r_data.eq(result)
  98
  99         #with m.If(self.i_p_rst): # reset
 100         #    m.d.sync += self.o_n_stb.eq(0)
 101         #    m.d.sync += self.o_p_busy.eq(0)
 102         with m.If(i_n_busyn): # next stage is not busy
 103             with m.If(o_p_busyn): # not stalled
 104                 # nothing in buffer: send input direct to output
 105                 m.d.sync += [self.o_n_stb.eq(self.i_p_stb),
 106                              self.o_data.eq(result),
 107                             ]
 108             with m.Else(): # o_p_busy is true, and something is in our buffer.
 109                 # Flush the [already processed] buffer to the output port.
 110                 m.d.sync += [self.o_n_stb.eq(1),
 111                              self.o_data.eq(self.r_data),
 112                              # clear stall condition, declare register empty.
 113                              self.o_p_busy.eq(0),
 114                             ]
 115                 # ignore input, since o_p_busy is also true.
 116
 117         # (i_n_busy) is true here: next stage is busy
 118         with m.Elif(o_n_stbn): # next stage being told "not busy"
 119             m.d.sync += [self.o_n_stb.eq(self.i_p_stb),
 120                          self.o_p_busy.eq(0), # Keep the buffer empty
 121                          # set the output data (from comb result)
 122                          self.o_data.eq(result),
 123                         ]
 124         # (i_n_busy) and (o_n_stb) both true:
 125         with m.Elif(i_p_stb_o_p_busyn):
 126             # If next stage *is* busy, and not stalled yet, accept input
 127             m.d.sync += self.o_p_busy.eq(self.i_p_stb & self.o_n_stb)
 128
 129         with m.If(o_p_busyn): # not stalled
 130             # turns out that from all of the above conditions, just
 131             # always put result into buffer if not busy
 132             m.d.sync += self.r_data.eq(result)
 133
 134         return m
 135
 136     def ports(self):
 137         return [self.i_p_stb, self.i_n_busy, self.i_data,
 138                 self.r_data,
 139                 self.o_n_stb, self.o_p_busy, self.o_data
 140                ]
 141
 142
 143 if __name__ == '__main__':
 144     dut = BufPipe()
 145     vl = rtlil.convert(dut, ports=dut.ports())
 146     with open("test_bufpipe.il", "w") as f:
 147         f.write(vl)
 148