From 8b1f314d3150b42787aaceea54ce06fb07b1a20e Mon Sep 17 00:00:00 2001
From: Luke Kenneth Casson Leighton <lkcl@lkcl.net>
Date: Wed, 27 Mar 2019 09:17:16 +0000
Subject: [PATCH] split out pipeline classes into singlepipe.py

---
 src/add/example_buf_pipe.py | 547 +----------------------------------
 src/add/singlepipe.py       | 549 ++++++++++++++++++++++++++++++++++++
 2 files changed, 555 insertions(+), 541 deletions(-)
 create mode 100644 src/add/singlepipe.py

diff --git a/src/add/example_buf_pipe.py b/src/add/example_buf_pipe.py
index 2e8e21d6..92575a73 100644
--- a/src/add/example_buf_pipe.py
+++ b/src/add/example_buf_pipe.py
@@ -1,106 +1,10 @@
-""" Pipeline and BufferedPipeline implementation, conforming to the same API.
-
-    eq:
-    --
-
-    a strategically very important function that is identical in function
-    to nmigen's Signal.eq function, except it may take objects, or a list
-    of objects, or a tuple of objects, and where objects may also be
-    Records.
-
-    Stage API:
-    ---------
-
-    stage requires compliance with a strict API that may be
-    implemented in several means, including as a static class.
-    the methods of a stage instance must be as follows:
-
-    * ispec() - Input data format specification
-                returns an object or a list or tuple of objects, or
-                a Record, each object having an "eq" function which
-                takes responsibility for copying by assignment all
-                sub-objects
-    * ospec() - Output data format specification
-                requirements as for ospec
-    * process(m, i) - Processes an ispec-formatted object
-                returns a combinatorial block of a result that
-                may be assigned to the output, by way of the "eq"
-                function
-    * setup(m, i) - Optional function for setting up submodules
-                may be used for more complex stages, to link
-                the input (i) to submodules.  must take responsibility
-                for adding those submodules to the module (m).
-                the submodules must be combinatorial blocks and
-                must have their inputs and output linked combinatorially.
-
-    StageChain:
-    ----------
-
-    A useful combinatorial wrapper around stages that chains them together
-    and then presents a Stage-API-conformant interface.
-
-    UnbufferedPipeline:
-    ------------------
-
-    A simple stalling clock-synchronised pipeline that has no buffering
-    (unlike BufferedPipeline).  A stall anywhere along the line will
-    result in a stall back-propagating down the entire chain.
-
-    The BufferedPipeline by contrast will buffer incoming data, allowing
-    previous stages one clock cycle's grace before also having to stall.
-
-    An advantage of the UnbufferedPipeline over the Buffered one is
-    that the amount of logic needed (number of gates) is greatly
-    reduced.
-
-    BufferedPipeline:
-    ----------------
-
-    nmigen implementation of buffered pipeline stage, based on zipcpu:
-    https://zipcpu.com/blog/2017/08/14/strategies-for-pipelining.html
-
-    this module requires quite a bit of thought to understand how it works
-    (and why it is needed in the first place).  reading the above is
-    *strongly* recommended.
-
-    unlike john dawson's IEEE754 FPU STB/ACK signalling, which requires
-    the STB / ACK signals to raise and lower (on separate clocks) before
-    data may proceeed (thus only allowing one piece of data to proceed
-    on *ALTERNATE* cycles), the signalling here is a true pipeline
-    where data will flow on *every* clock when the conditions are right.
-
-    input acceptance conditions are when:
-        * incoming previous-stage strobe (p.i_valid) is HIGH
-        * outgoing previous-stage ready   (p.o_ready) is LOW
-
-    output transmission conditions are when:
-        * outgoing next-stage strobe (n.o_valid) is HIGH
-        * outgoing next-stage ready   (n.i_ready) is LOW
-
-    the tricky bit is when the input has valid data and the output is not
-    ready to accept it.  if it wasn't for the clock synchronisation, it
-    would be possible to tell the input "hey don't send that data, we're
-    not ready".  unfortunately, it's not possible to "change the past":
-    the previous stage *has no choice* but to pass on its data.
-
-    therefore, the incoming data *must* be accepted - and stored: that
-    is the responsibility / contract that this stage *must* accept.
-    on the same clock, it's possible to tell the input that it must
-    not send any more data.  this is the "stall" condition.
-
-    we now effectively have *two* possible pieces of data to "choose" from:
-    the buffered data, and the incoming data.  the decision as to which
-    to process and output is based on whether we are in "stall" or not.
-    i.e. when the next stage is no longer ready, the output comes from
-    the buffer if a stall had previously occurred, otherwise it comes
-    direct from processing the input.
-
-    this allows us to respect a synchronous "travelling STB" with what
-    dan calls a "buffered handshake".
-
-    it's quite a complex state machine!
+""" Pipeline and BufferedPipeline examples
 """
 
+from singlepipe import (PrevControl, NextControl, ControlBase,
+                        StageCls, Stage, StageChain,
+                        BufferedPipeline, UnbufferedPipeline, eq)
+
 from nmigen import Signal, Cat, Const, Mux, Module
 from nmigen.cli import verilog, rtlil
 from nmigen.hdl.rec import Record, Layout
@@ -109,378 +13,6 @@ from abc import ABCMeta, abstractmethod
 from collections.abc import Sequence
 
 
-class PrevControl:
-    """ contains signals that come *from* the previous stage (both in and out)
-        * i_valid: previous stage indicating all incoming data is valid.
-                   may be a multi-bit signal, where all bits are required
-                   to be asserted to indicate "valid".
-        * o_ready: output to next stage indicating readiness to accept data
-        * i_data : an input - added by the user of this class
-    """
-
-    def __init__(self, i_width=1):
-        self.i_valid = Signal(i_width, name="p_i_valid") # prev   >>in  self
-        self.o_ready = Signal(name="p_o_ready") # prev   <<out self
-
-    def _connect_in(self, prev):
-        """ internal helper function to connect stage to an input source.
-            do not use to connect stage-to-stage!
-        """
-        return [self.i_valid.eq(prev.i_valid),
-                prev.o_ready.eq(self.o_ready),
-                eq(self.i_data, prev.i_data),
-               ]
-
-    def i_valid_logic(self):
-        vlen = len(self.i_valid)
-        if vlen > 1: # multi-bit case: valid only when i_valid is all 1s
-            all1s = Const(-1, (len(self.i_valid), False))
-            return self.i_valid == all1s
-        # single-bit i_valid case
-        return self.i_valid
-
-
-class NextControl:
-    """ contains the signals that go *to* the next stage (both in and out)
-        * o_valid: output indicating to next stage that data is valid
-        * i_ready: input from next stage indicating that it can accept data
-        * o_data : an output - added by the user of this class
-    """
-    def __init__(self):
-        self.o_valid = Signal(name="n_o_valid") # self out>>  next
-        self.i_ready = Signal(name="n_i_ready") # self <<in   next
-
-    def connect_to_next(self, nxt):
-        """ helper function to connect to the next stage data/valid/ready.
-            data/valid is passed *TO* nxt, and ready comes *IN* from nxt.
-            use this when connecting stage-to-stage
-        """
-        return [nxt.i_valid.eq(self.o_valid),
-                self.i_ready.eq(nxt.o_ready),
-                eq(nxt.i_data, self.o_data),
-               ]
-
-    def _connect_out(self, nxt):
-        """ internal helper function to connect stage to an output source.
-            do not use to connect stage-to-stage!
-        """
-        return [nxt.o_valid.eq(self.o_valid),
-                self.i_ready.eq(nxt.i_ready),
-                eq(nxt.o_data, self.o_data),
-               ]
-
-
-def eq(o, i):
-    """ makes signals equal: a helper routine which identifies if it is being
-        passed a list (or tuple) of objects, or signals, or Records, and calls
-        the objects' eq function.
-
-        complex objects (classes) can be used: they must follow the
-        convention of having an eq member function, which takes the
-        responsibility of further calling eq and returning a list of
-        eq assignments
-
-        Record is a special (unusual, recursive) case, where the input may be
-        specified as a dictionary (which may contain further dictionaries,
-        recursively), where the field names of the dictionary must match
-        the Record's field spec.  Alternatively, an object with the same
-        member names as the Record may be assigned: it does not have to
-        *be* a Record.
-    """
-    if not isinstance(o, Sequence):
-        o, i = [o], [i]
-    res = []
-    for (ao, ai) in zip(o, i):
-        #print ("eq", ao, ai)
-        if isinstance(ao, Record):
-            for idx, (field_name, field_shape, _) in enumerate(ao.layout):
-                if isinstance(field_shape, Layout):
-                    val = ai.fields
-                else:
-                    val = ai
-                if hasattr(val, field_name): # check for attribute
-                    val = getattr(val, field_name)
-                else:
-                    val = val[field_name] # dictionary-style specification
-                rres = eq(ao.fields[field_name], val)
-                res += rres
-        else:
-            rres = ao.eq(ai)
-            if not isinstance(rres, Sequence):
-                rres = [rres]
-            res += rres
-    return res
-
-
-class StageCls(metaclass=ABCMeta):
-    """ Class-based "Stage" API.  requires instantiation (after derivation)
-
-        see "Stage API" above..  Note: python does *not* require derivation
-        from this class.  All that is required is that the pipelines *have*
-        the functions listed in this class.  Derivation from this class
-        is therefore merely a "courtesy" to maintainers.
-    """
-    @abstractmethod
-    def ispec(self): pass       # REQUIRED
-    @abstractmethod
-    def ospec(self): pass       # REQUIRED
-    #@abstractmethod
-    #def setup(self, m, i): pass # OPTIONAL
-    @abstractmethod
-    def process(self, i): pass  # REQUIRED
-
-
-class Stage(metaclass=ABCMeta):
-    """ Static "Stage" API.  does not require instantiation (after derivation)
-
-        see "Stage API" above.  Note: python does *not* require derivation
-        from this class.  All that is required is that the pipelines *have*
-        the functions listed in this class.  Derivation from this class
-        is therefore merely a "courtesy" to maintainers.
-    """
-    @staticmethod
-    @abstractmethod
-    def ispec(): pass
-
-    @staticmethod
-    @abstractmethod
-    def ospec(): pass
-
-    #@staticmethod
-    #@abstractmethod
-    #def setup(m, i): pass
-
-    @staticmethod
-    @abstractmethod
-    def process(i): pass
-
-
-class StageChain(StageCls):
-    """ pass in a list of stages, and they will automatically be
-        chained together via their input and output specs into a
-        combinatorial chain.
-
-        the end result basically conforms to the exact same Stage API.
-
-        * input to this class will be the input of the first stage
-        * output of first stage goes into input of second
-        * output of second goes into input into third (etc. etc.)
-        * the output of this class will be the output of the last stage
-    """
-    def __init__(self, chain):
-        self.chain = chain
-
-    def ispec(self):
-        return self.chain[0].ispec()
-
-    def ospec(self):
-        return self.chain[-1].ospec()
-
-    def setup(self, m, i):
-        for (idx, c) in enumerate(self.chain):
-            if hasattr(c, "setup"):
-                c.setup(m, i)               # stage may have some module stuff
-            o = self.chain[idx].ospec()     # only the last assignment survives
-            m.d.comb += eq(o, c.process(i)) # process input into "o"
-            if idx != len(self.chain)-1:
-                ni = self.chain[idx+1].ispec() # becomes new input on next loop
-                m.d.comb += eq(ni, o)          # assign output to next input
-                i = ni
-        self.o = o                             # last loop is the output
-
-    def process(self, i):
-        return self.o # conform to Stage API: return last-loop output
-
-
-class ControlBase:
-    """ Common functions for Pipeline API
-    """
-    def __init__(self, in_multi=None):
-        """ Base class containing ready/valid/data to previous and next stages
-
-            * p: contains ready/valid to the previous stage
-            * n: contains ready/valid to the next stage
-
-            User must also:
-            * add i_data member to PrevControl (p) and
-            * add o_data member to NextControl (n)
-        """
-
-        # set up input and output IO ACK (prev/next ready/valid)
-        self.p = PrevControl(in_multi)
-        self.n = NextControl()
-
-    def connect_to_next(self, nxt):
-        """ helper function to connect to the next stage data/valid/ready.
-        """
-        return self.n.connect_to_next(nxt.p)
-
-    def _connect_in(self, prev):
-        """ internal helper function to connect stage to an input source.
-            do not use to connect stage-to-stage!
-        """
-        return self.p._connect_in(prev.p)
-
-    def _connect_out(self, nxt):
-        """ internal helper function to connect stage to an output source.
-            do not use to connect stage-to-stage!
-        """
-        return self.n._connect_out(nxt.n)
-
-    def connect(self, m, pipechain):
-        """ connects a chain (list) of Pipeline instances together and
-            links them to this ControlBase instance:
-
-                      in <----> self <---> out
-                       |                   ^
-                       v                   |
-                    [pipe1, pipe2, pipe3, pipe4]
-                       |    ^  |    ^  |     ^
-                       v    |  v    |  v     |
-                     out---in out--in out---in
-
-            Also takes care of allocating i_data/o_data, by looking up
-            the data spec for each end of the pipechain.  i.e It is NOT
-            necessary to allocate self.p.i_data or self.n.o_data manually:
-            this is handled AUTOMATICALLY, here.
-
-            Basically this function is the direct equivalent of StageChain,
-            except that unlike StageChain, the Pipeline logic is followed.
-
-            Just as StageChain presents an object that conforms to the
-            Stage API from a list of objects that also conform to the
-            Stage API, an object that calls this Pipeline connect function
-            has the exact same pipeline API as the list of pipline objects
-            it is called with.
-
-            Thus it becomes possible to build up larger chains recursively.
-            More complex chains (multi-input, multi-output) will have to be
-            done manually.
-        """
-        eqs = [] # collated list of assignment statements
-
-        # connect inter-chain
-        for i in range(len(pipechain)-1):
-            pipe1 = pipechain[i]
-            pipe2 = pipechain[i+1]
-            eqs += pipe1.connect_to_next(pipe2)
-
-        # connect front of chain to ourselves
-        front = pipechain[0]
-        self.p.i_data = front.stage.ispec()
-        eqs += front._connect_in(self)
-
-        # connect end of chain to ourselves
-        end = pipechain[-1]
-        self.n.o_data = end.stage.ospec()
-        eqs += end._connect_out(self)
-
-        # activate the assignments
-        m.d.comb += eqs
-
-    def set_input(self, i):
-        """ helper function to set the input data
-        """
-        return eq(self.p.i_data, i)
-
-    def ports(self):
-        return [self.p.i_valid, self.n.i_ready,
-                self.n.o_valid, self.p.o_ready,
-                self.p.i_data, self.n.o_data   # XXX need flattening!
-               ]
-
-
-class BufferedPipeline(ControlBase):
-    """ buffered pipeline stage.  data and strobe signals travel in sync.
-        if ever the input is ready and the output is not, processed data
-        is shunted in a temporary register.
-
-        Argument: stage.  see Stage API above
-
-        stage-1   p.i_valid >>in   stage   n.o_valid out>>   stage+1
-        stage-1   p.o_ready <<out  stage   n.i_ready <<in    stage+1
-        stage-1   p.i_data  >>in   stage   n.o_data  out>>   stage+1
-                              |             |
-                            process --->----^
-                              |             |
-                              +-- r_data ->-+
-
-        input data p.i_data is read (only), is processed and goes into an
-        intermediate result store [process()].  this is updated combinatorially.
-
-        in a non-stall condition, the intermediate result will go into the
-        output (update_output).  however if ever there is a stall, it goes
-        into r_data instead [update_buffer()].
-
-        when the non-stall condition is released, r_data is the first
-        to be transferred to the output [flush_buffer()], and the stall
-        condition cleared.
-
-        on the next cycle (as long as stall is not raised again) the
-        input may begin to be processed and transferred directly to output.
-
-    """
-    def __init__(self, stage):
-        ControlBase.__init__(self)
-        self.stage = stage
-
-        # set up the input and output data
-        self.p.i_data = stage.ispec() # input type
-        self.n.o_data = stage.ospec()
-
-    def elaborate(self, platform):
-        m = Module()
-
-        result = self.stage.ospec()
-        r_data = self.stage.ospec()
-        if hasattr(self.stage, "setup"):
-            self.stage.setup(m, self.p.i_data)
-
-        # establish some combinatorial temporaries
-        o_n_validn = Signal(reset_less=True)
-        i_p_valid_o_p_ready = Signal(reset_less=True)
-        p_i_valid = Signal(reset_less=True)
-        m.d.comb += [p_i_valid.eq(self.p.i_valid_logic()),
-                     o_n_validn.eq(~self.n.o_valid),
-                     i_p_valid_o_p_ready.eq(p_i_valid & self.p.o_ready),
-        ]
-
-        # store result of processing in combinatorial temporary
-        m.d.comb += eq(result, self.stage.process(self.p.i_data))
-
-        # if not in stall condition, update the temporary register
-        with m.If(self.p.o_ready): # not stalled
-            m.d.sync += eq(r_data, result) # update buffer
-
-        with m.If(self.n.i_ready): # next stage is ready
-            with m.If(self.p.o_ready): # not stalled
-                # nothing in buffer: send (processed) input direct to output
-                m.d.sync += [self.n.o_valid.eq(p_i_valid),
-                             eq(self.n.o_data, result), # update output
-                            ]
-            with m.Else(): # p.o_ready is false, and something is in buffer.
-                # Flush the [already processed] buffer to the output port.
-                m.d.sync += [self.n.o_valid.eq(1),      # declare reg empty
-                             eq(self.n.o_data, r_data), # flush buffer
-                             self.p.o_ready.eq(1),      # clear stall condition
-                            ]
-                # ignore input, since p.o_ready is also false.
-
-        # (n.i_ready) is false here: next stage is ready
-        with m.Elif(o_n_validn): # next stage being told "ready"
-            m.d.sync += [self.n.o_valid.eq(p_i_valid),
-                         self.p.o_ready.eq(1), # Keep the buffer empty
-                         eq(self.n.o_data, result), # set output data
-                        ]
-
-        # (n.i_ready) false and (n.o_valid) true:
-        with m.Elif(i_p_valid_o_p_ready):
-            # If next stage *is* ready, and not stalled yet, accept input
-            m.d.sync += self.p.o_ready.eq(~(p_i_valid & self.n.o_valid))
-
-        return m
-
-
 class ExampleAddStage(StageCls):
     """ an example of how to use the buffered pipeline, as a class instance
     """
@@ -553,75 +85,8 @@ class ExampleBufPipe(BufferedPipeline):
         BufferedPipeline.__init__(self, ExampleStage)
 
 
-class UnbufferedPipeline(ControlBase):
-    """ A simple pipeline stage with single-clock synchronisation
-        and two-way valid/ready synchronised signalling.
-
-        Note that a stall in one stage will result in the entire pipeline
-        chain stalling.
-
-        Also that unlike BufferedPipeline, the valid/ready signalling does NOT
-        travel synchronously with the data: the valid/ready signalling
-        combines in a *combinatorial* fashion.  Therefore, a long pipeline
-        chain will lengthen propagation delays.
-
-        Argument: stage.  see Stage API, above
-
-        stage-1   p.i_valid >>in   stage   n.o_valid out>>   stage+1
-        stage-1   p.o_ready <<out  stage   n.i_ready <<in    stage+1
-        stage-1   p.i_data  >>in   stage   n.o_data  out>>   stage+1
-                              |             |
-                            r_data        result
-                              |             |
-                              +--process ->-+
-
-        Attributes:
-        -----------
-        p.i_data : StageInput, shaped according to ispec
-            The pipeline input
-        p.o_data : StageOutput, shaped according to ospec
-            The pipeline output
-        r_data : input_shape according to ispec
-            A temporary (buffered) copy of a prior (valid) input.
-            This is HELD if the output is not ready.  It is updated
-            SYNCHRONOUSLY.
-        result: output_shape according to ospec
-            The output of the combinatorial logic.  it is updated
-            COMBINATORIALLY (no clock dependence).
-    """
-
-    def __init__(self, stage):
-        ControlBase.__init__(self)
-        self.stage = stage
-        self._data_valid = Signal()
-
-        # set up the input and output data
-        self.p.i_data = stage.ispec() # input type
-        self.n.o_data = stage.ospec() # output type
-
-    def elaborate(self, platform):
-        m = Module()
-
-        r_data = self.stage.ispec() # input type
-        result = self.stage.ospec() # output data
-        if hasattr(self.stage, "setup"):
-            self.stage.setup(m, r_data)
-
-        p_i_valid = Signal(reset_less=True)
-        m.d.comb += p_i_valid.eq(self.p.i_valid_logic())
-        m.d.comb += eq(result, self.stage.process(r_data))
-        m.d.comb += self.n.o_valid.eq(self._data_valid)
-        m.d.comb += self.p.o_ready.eq(~self._data_valid | self.n.i_ready)
-        m.d.sync += self._data_valid.eq(p_i_valid | \
-                                        (~self.n.i_ready & self._data_valid))
-        with m.If(self.p.i_valid & self.p.o_ready):
-            m.d.sync += eq(r_data, self.p.i_data)
-        m.d.comb += eq(self.n.o_data, result)
-        return m
-
-
 class ExamplePipeline(UnbufferedPipeline):
-    """ an example of how to use the combinatorial pipeline.
+    """ an example of how to use the unbuffered pipeline.
     """
 
     def __init__(self):
diff --git a/src/add/singlepipe.py b/src/add/singlepipe.py
new file mode 100644
index 00000000..38175dd2
--- /dev/null
+++ b/src/add/singlepipe.py
@@ -0,0 +1,549 @@
+""" Pipeline and BufferedPipeline implementation, conforming to the same API.
+
+    eq:
+    --
+
+    a strategically very important function that is identical in function
+    to nmigen's Signal.eq function, except it may take objects, or a list
+    of objects, or a tuple of objects, and where objects may also be
+    Records.
+
+    Stage API:
+    ---------
+
+    stage requires compliance with a strict API that may be
+    implemented in several means, including as a static class.
+    the methods of a stage instance must be as follows:
+
+    * ispec() - Input data format specification
+                returns an object or a list or tuple of objects, or
+                a Record, each object having an "eq" function which
+                takes responsibility for copying by assignment all
+                sub-objects
+    * ospec() - Output data format specification
+                requirements as for ospec
+    * process(m, i) - Processes an ispec-formatted object
+                returns a combinatorial block of a result that
+                may be assigned to the output, by way of the "eq"
+                function
+    * setup(m, i) - Optional function for setting up submodules
+                may be used for more complex stages, to link
+                the input (i) to submodules.  must take responsibility
+                for adding those submodules to the module (m).
+                the submodules must be combinatorial blocks and
+                must have their inputs and output linked combinatorially.
+
+    StageChain:
+    ----------
+
+    A useful combinatorial wrapper around stages that chains them together
+    and then presents a Stage-API-conformant interface.
+
+    UnbufferedPipeline:
+    ------------------
+
+    A simple stalling clock-synchronised pipeline that has no buffering
+    (unlike BufferedPipeline).  A stall anywhere along the line will
+    result in a stall back-propagating down the entire chain.
+
+    The BufferedPipeline by contrast will buffer incoming data, allowing
+    previous stages one clock cycle's grace before also having to stall.
+
+    An advantage of the UnbufferedPipeline over the Buffered one is
+    that the amount of logic needed (number of gates) is greatly
+    reduced.
+
+    BufferedPipeline:
+    ----------------
+
+    nmigen implementation of buffered pipeline stage, based on zipcpu:
+    https://zipcpu.com/blog/2017/08/14/strategies-for-pipelining.html
+
+    this module requires quite a bit of thought to understand how it works
+    (and why it is needed in the first place).  reading the above is
+    *strongly* recommended.
+
+    unlike john dawson's IEEE754 FPU STB/ACK signalling, which requires
+    the STB / ACK signals to raise and lower (on separate clocks) before
+    data may proceeed (thus only allowing one piece of data to proceed
+    on *ALTERNATE* cycles), the signalling here is a true pipeline
+    where data will flow on *every* clock when the conditions are right.
+
+    input acceptance conditions are when:
+        * incoming previous-stage strobe (p.i_valid) is HIGH
+        * outgoing previous-stage ready   (p.o_ready) is LOW
+
+    output transmission conditions are when:
+        * outgoing next-stage strobe (n.o_valid) is HIGH
+        * outgoing next-stage ready   (n.i_ready) is LOW
+
+    the tricky bit is when the input has valid data and the output is not
+    ready to accept it.  if it wasn't for the clock synchronisation, it
+    would be possible to tell the input "hey don't send that data, we're
+    not ready".  unfortunately, it's not possible to "change the past":
+    the previous stage *has no choice* but to pass on its data.
+
+    therefore, the incoming data *must* be accepted - and stored: that
+    is the responsibility / contract that this stage *must* accept.
+    on the same clock, it's possible to tell the input that it must
+    not send any more data.  this is the "stall" condition.
+
+    we now effectively have *two* possible pieces of data to "choose" from:
+    the buffered data, and the incoming data.  the decision as to which
+    to process and output is based on whether we are in "stall" or not.
+    i.e. when the next stage is no longer ready, the output comes from
+    the buffer if a stall had previously occurred, otherwise it comes
+    direct from processing the input.
+
+    this allows us to respect a synchronous "travelling STB" with what
+    dan calls a "buffered handshake".
+
+    it's quite a complex state machine!
+"""
+
+from nmigen import Signal, Cat, Const, Mux, Module
+from nmigen.cli import verilog, rtlil
+from nmigen.hdl.rec import Record, Layout
+
+from abc import ABCMeta, abstractmethod
+from collections.abc import Sequence
+
+
+class PrevControl:
+    """ contains signals that come *from* the previous stage (both in and out)
+        * i_valid: previous stage indicating all incoming data is valid.
+                   may be a multi-bit signal, where all bits are required
+                   to be asserted to indicate "valid".
+        * o_ready: output to next stage indicating readiness to accept data
+        * i_data : an input - added by the user of this class
+    """
+
+    def __init__(self, i_width=1):
+        self.i_valid = Signal(i_width, name="p_i_valid") # prev   >>in  self
+        self.o_ready = Signal(name="p_o_ready") # prev   <<out self
+
+    def _connect_in(self, prev):
+        """ internal helper function to connect stage to an input source.
+            do not use to connect stage-to-stage!
+        """
+        return [self.i_valid.eq(prev.i_valid),
+                prev.o_ready.eq(self.o_ready),
+                eq(self.i_data, prev.i_data),
+               ]
+
+    def i_valid_logic(self):
+        vlen = len(self.i_valid)
+        if vlen > 1: # multi-bit case: valid only when i_valid is all 1s
+            all1s = Const(-1, (len(self.i_valid), False))
+            return self.i_valid == all1s
+        # single-bit i_valid case
+        return self.i_valid
+
+
+class NextControl:
+    """ contains the signals that go *to* the next stage (both in and out)
+        * o_valid: output indicating to next stage that data is valid
+        * i_ready: input from next stage indicating that it can accept data
+        * o_data : an output - added by the user of this class
+    """
+    def __init__(self):
+        self.o_valid = Signal(name="n_o_valid") # self out>>  next
+        self.i_ready = Signal(name="n_i_ready") # self <<in   next
+
+    def connect_to_next(self, nxt):
+        """ helper function to connect to the next stage data/valid/ready.
+            data/valid is passed *TO* nxt, and ready comes *IN* from nxt.
+            use this when connecting stage-to-stage
+        """
+        return [nxt.i_valid.eq(self.o_valid),
+                self.i_ready.eq(nxt.o_ready),
+                eq(nxt.i_data, self.o_data),
+               ]
+
+    def _connect_out(self, nxt):
+        """ internal helper function to connect stage to an output source.
+            do not use to connect stage-to-stage!
+        """
+        return [nxt.o_valid.eq(self.o_valid),
+                self.i_ready.eq(nxt.i_ready),
+                eq(nxt.o_data, self.o_data),
+               ]
+
+
+def eq(o, i):
+    """ makes signals equal: a helper routine which identifies if it is being
+        passed a list (or tuple) of objects, or signals, or Records, and calls
+        the objects' eq function.
+
+        complex objects (classes) can be used: they must follow the
+        convention of having an eq member function, which takes the
+        responsibility of further calling eq and returning a list of
+        eq assignments
+
+        Record is a special (unusual, recursive) case, where the input may be
+        specified as a dictionary (which may contain further dictionaries,
+        recursively), where the field names of the dictionary must match
+        the Record's field spec.  Alternatively, an object with the same
+        member names as the Record may be assigned: it does not have to
+        *be* a Record.
+    """
+    if not isinstance(o, Sequence):
+        o, i = [o], [i]
+    res = []
+    for (ao, ai) in zip(o, i):
+        #print ("eq", ao, ai)
+        if isinstance(ao, Record):
+            for idx, (field_name, field_shape, _) in enumerate(ao.layout):
+                if isinstance(field_shape, Layout):
+                    val = ai.fields
+                else:
+                    val = ai
+                if hasattr(val, field_name): # check for attribute
+                    val = getattr(val, field_name)
+                else:
+                    val = val[field_name] # dictionary-style specification
+                rres = eq(ao.fields[field_name], val)
+                res += rres
+        else:
+            rres = ao.eq(ai)
+            if not isinstance(rres, Sequence):
+                rres = [rres]
+            res += rres
+    return res
+
+
+class StageCls(metaclass=ABCMeta):
+    """ Class-based "Stage" API.  requires instantiation (after derivation)
+
+        see "Stage API" above..  Note: python does *not* require derivation
+        from this class.  All that is required is that the pipelines *have*
+        the functions listed in this class.  Derivation from this class
+        is therefore merely a "courtesy" to maintainers.
+    """
+    @abstractmethod
+    def ispec(self): pass       # REQUIRED
+    @abstractmethod
+    def ospec(self): pass       # REQUIRED
+    #@abstractmethod
+    #def setup(self, m, i): pass # OPTIONAL
+    @abstractmethod
+    def process(self, i): pass  # REQUIRED
+
+
+class Stage(metaclass=ABCMeta):
+    """ Static "Stage" API.  does not require instantiation (after derivation)
+
+        see "Stage API" above.  Note: python does *not* require derivation
+        from this class.  All that is required is that the pipelines *have*
+        the functions listed in this class.  Derivation from this class
+        is therefore merely a "courtesy" to maintainers.
+    """
+    @staticmethod
+    @abstractmethod
+    def ispec(): pass
+
+    @staticmethod
+    @abstractmethod
+    def ospec(): pass
+
+    #@staticmethod
+    #@abstractmethod
+    #def setup(m, i): pass
+
+    @staticmethod
+    @abstractmethod
+    def process(i): pass
+
+
+class StageChain(StageCls):
+    """ pass in a list of stages, and they will automatically be
+        chained together via their input and output specs into a
+        combinatorial chain.
+
+        the end result basically conforms to the exact same Stage API.
+
+        * input to this class will be the input of the first stage
+        * output of first stage goes into input of second
+        * output of second goes into input into third (etc. etc.)
+        * the output of this class will be the output of the last stage
+    """
+    def __init__(self, chain):
+        self.chain = chain
+
+    def ispec(self):
+        return self.chain[0].ispec()
+
+    def ospec(self):
+        return self.chain[-1].ospec()
+
+    def setup(self, m, i):
+        for (idx, c) in enumerate(self.chain):
+            if hasattr(c, "setup"):
+                c.setup(m, i)               # stage may have some module stuff
+            o = self.chain[idx].ospec()     # only the last assignment survives
+            m.d.comb += eq(o, c.process(i)) # process input into "o"
+            if idx != len(self.chain)-1:
+                ni = self.chain[idx+1].ispec() # becomes new input on next loop
+                m.d.comb += eq(ni, o)          # assign output to next input
+                i = ni
+        self.o = o                             # last loop is the output
+
+    def process(self, i):
+        return self.o # conform to Stage API: return last-loop output
+
+
+class ControlBase:
+    """ Common functions for Pipeline API
+    """
+    def __init__(self, in_multi=None):
+        """ Base class containing ready/valid/data to previous and next stages
+
+            * p: contains ready/valid to the previous stage
+            * n: contains ready/valid to the next stage
+
+            User must also:
+            * add i_data member to PrevControl (p) and
+            * add o_data member to NextControl (n)
+        """
+
+        # set up input and output IO ACK (prev/next ready/valid)
+        self.p = PrevControl(in_multi)
+        self.n = NextControl()
+
+    def connect_to_next(self, nxt):
+        """ helper function to connect to the next stage data/valid/ready.
+        """
+        return self.n.connect_to_next(nxt.p)
+
+    def _connect_in(self, prev):
+        """ internal helper function to connect stage to an input source.
+            do not use to connect stage-to-stage!
+        """
+        return self.p._connect_in(prev.p)
+
+    def _connect_out(self, nxt):
+        """ internal helper function to connect stage to an output source.
+            do not use to connect stage-to-stage!
+        """
+        return self.n._connect_out(nxt.n)
+
+    def connect(self, m, pipechain):
+        """ connects a chain (list) of Pipeline instances together and
+            links them to this ControlBase instance:
+
+                      in <----> self <---> out
+                       |                   ^
+                       v                   |
+                    [pipe1, pipe2, pipe3, pipe4]
+                       |    ^  |    ^  |     ^
+                       v    |  v    |  v     |
+                     out---in out--in out---in
+
+            Also takes care of allocating i_data/o_data, by looking up
+            the data spec for each end of the pipechain.  i.e It is NOT
+            necessary to allocate self.p.i_data or self.n.o_data manually:
+            this is handled AUTOMATICALLY, here.
+
+            Basically this function is the direct equivalent of StageChain,
+            except that unlike StageChain, the Pipeline logic is followed.
+
+            Just as StageChain presents an object that conforms to the
+            Stage API from a list of objects that also conform to the
+            Stage API, an object that calls this Pipeline connect function
+            has the exact same pipeline API as the list of pipline objects
+            it is called with.
+
+            Thus it becomes possible to build up larger chains recursively.
+            More complex chains (multi-input, multi-output) will have to be
+            done manually.
+        """
+        eqs = [] # collated list of assignment statements
+
+        # connect inter-chain
+        for i in range(len(pipechain)-1):
+            pipe1 = pipechain[i]
+            pipe2 = pipechain[i+1]
+            eqs += pipe1.connect_to_next(pipe2)
+
+        # connect front of chain to ourselves
+        front = pipechain[0]
+        self.p.i_data = front.stage.ispec()
+        eqs += front._connect_in(self)
+
+        # connect end of chain to ourselves
+        end = pipechain[-1]
+        self.n.o_data = end.stage.ospec()
+        eqs += end._connect_out(self)
+
+        # activate the assignments
+        m.d.comb += eqs
+
+    def set_input(self, i):
+        """ helper function to set the input data
+        """
+        return eq(self.p.i_data, i)
+
+    def ports(self):
+        return [self.p.i_valid, self.n.i_ready,
+                self.n.o_valid, self.p.o_ready,
+                self.p.i_data, self.n.o_data   # XXX need flattening!
+               ]
+
+
+class BufferedPipeline(ControlBase):
+    """ buffered pipeline stage.  data and strobe signals travel in sync.
+        if ever the input is ready and the output is not, processed data
+        is shunted in a temporary register.
+
+        Argument: stage.  see Stage API above
+
+        stage-1   p.i_valid >>in   stage   n.o_valid out>>   stage+1
+        stage-1   p.o_ready <<out  stage   n.i_ready <<in    stage+1
+        stage-1   p.i_data  >>in   stage   n.o_data  out>>   stage+1
+                              |             |
+                            process --->----^
+                              |             |
+                              +-- r_data ->-+
+
+        input data p.i_data is read (only), is processed and goes into an
+        intermediate result store [process()].  this is updated combinatorially.
+
+        in a non-stall condition, the intermediate result will go into the
+        output (update_output).  however if ever there is a stall, it goes
+        into r_data instead [update_buffer()].
+
+        when the non-stall condition is released, r_data is the first
+        to be transferred to the output [flush_buffer()], and the stall
+        condition cleared.
+
+        on the next cycle (as long as stall is not raised again) the
+        input may begin to be processed and transferred directly to output.
+
+    """
+    def __init__(self, stage):
+        ControlBase.__init__(self)
+        self.stage = stage
+
+        # set up the input and output data
+        self.p.i_data = stage.ispec() # input type
+        self.n.o_data = stage.ospec()
+
+    def elaborate(self, platform):
+        m = Module()
+
+        result = self.stage.ospec()
+        r_data = self.stage.ospec()
+        if hasattr(self.stage, "setup"):
+            self.stage.setup(m, self.p.i_data)
+
+        # establish some combinatorial temporaries
+        o_n_validn = Signal(reset_less=True)
+        i_p_valid_o_p_ready = Signal(reset_less=True)
+        p_i_valid = Signal(reset_less=True)
+        m.d.comb += [p_i_valid.eq(self.p.i_valid_logic()),
+                     o_n_validn.eq(~self.n.o_valid),
+                     i_p_valid_o_p_ready.eq(p_i_valid & self.p.o_ready),
+        ]
+
+        # store result of processing in combinatorial temporary
+        m.d.comb += eq(result, self.stage.process(self.p.i_data))
+
+        # if not in stall condition, update the temporary register
+        with m.If(self.p.o_ready): # not stalled
+            m.d.sync += eq(r_data, result) # update buffer
+
+        with m.If(self.n.i_ready): # next stage is ready
+            with m.If(self.p.o_ready): # not stalled
+                # nothing in buffer: send (processed) input direct to output
+                m.d.sync += [self.n.o_valid.eq(p_i_valid),
+                             eq(self.n.o_data, result), # update output
+                            ]
+            with m.Else(): # p.o_ready is false, and something is in buffer.
+                # Flush the [already processed] buffer to the output port.
+                m.d.sync += [self.n.o_valid.eq(1),      # declare reg empty
+                             eq(self.n.o_data, r_data), # flush buffer
+                             self.p.o_ready.eq(1),      # clear stall condition
+                            ]
+                # ignore input, since p.o_ready is also false.
+
+        # (n.i_ready) is false here: next stage is ready
+        with m.Elif(o_n_validn): # next stage being told "ready"
+            m.d.sync += [self.n.o_valid.eq(p_i_valid),
+                         self.p.o_ready.eq(1), # Keep the buffer empty
+                         eq(self.n.o_data, result), # set output data
+                        ]
+
+        # (n.i_ready) false and (n.o_valid) true:
+        with m.Elif(i_p_valid_o_p_ready):
+            # If next stage *is* ready, and not stalled yet, accept input
+            m.d.sync += self.p.o_ready.eq(~(p_i_valid & self.n.o_valid))
+
+        return m
+
+
+class UnbufferedPipeline(ControlBase):
+    """ A simple pipeline stage with single-clock synchronisation
+        and two-way valid/ready synchronised signalling.
+
+        Note that a stall in one stage will result in the entire pipeline
+        chain stalling.
+
+        Also that unlike BufferedPipeline, the valid/ready signalling does NOT
+        travel synchronously with the data: the valid/ready signalling
+        combines in a *combinatorial* fashion.  Therefore, a long pipeline
+        chain will lengthen propagation delays.
+
+        Argument: stage.  see Stage API, above
+
+        stage-1   p.i_valid >>in   stage   n.o_valid out>>   stage+1
+        stage-1   p.o_ready <<out  stage   n.i_ready <<in    stage+1
+        stage-1   p.i_data  >>in   stage   n.o_data  out>>   stage+1
+                              |             |
+                            r_data        result
+                              |             |
+                              +--process ->-+
+
+        Attributes:
+        -----------
+        p.i_data : StageInput, shaped according to ispec
+            The pipeline input
+        p.o_data : StageOutput, shaped according to ospec
+            The pipeline output
+        r_data : input_shape according to ispec
+            A temporary (buffered) copy of a prior (valid) input.
+            This is HELD if the output is not ready.  It is updated
+            SYNCHRONOUSLY.
+        result: output_shape according to ospec
+            The output of the combinatorial logic.  it is updated
+            COMBINATORIALLY (no clock dependence).
+    """
+
+    def __init__(self, stage):
+        ControlBase.__init__(self)
+        self.stage = stage
+        self._data_valid = Signal()
+
+        # set up the input and output data
+        self.p.i_data = stage.ispec() # input type
+        self.n.o_data = stage.ospec() # output type
+
+    def elaborate(self, platform):
+        m = Module()
+
+        r_data = self.stage.ispec() # input type
+        result = self.stage.ospec() # output data
+        if hasattr(self.stage, "setup"):
+            self.stage.setup(m, r_data)
+
+        p_i_valid = Signal(reset_less=True)
+        m.d.comb += p_i_valid.eq(self.p.i_valid_logic())
+        m.d.comb += eq(result, self.stage.process(r_data))
+        m.d.comb += self.n.o_valid.eq(self._data_valid)
+        m.d.comb += self.p.o_ready.eq(~self._data_valid | self.n.i_ready)
+        m.d.sync += self._data_valid.eq(p_i_valid | \
+                                        (~self.n.i_ready & self._data_valid))
+        with m.If(self.p.i_valid & self.p.o_ready):
+            m.d.sync += eq(r_data, self.p.i_data)
+        m.d.comb += eq(self.n.o_data, result)
+        return m
+
-- 
2.30.2