src/add/stageapi.py

   1 """ Stage API
   2
   3     Associated development bugs:
   4     * http://bugs.libre-riscv.org/show_bug.cgi?id=64
   5     * http://bugs.libre-riscv.org/show_bug.cgi?id=57
   6
   7     Stage API:
   8     ---------
   9
  10     stage requires compliance with a strict API that may be
  11     implemented in several means, including as a static class.
  12
  13     Stages do not HOLD data, and they definitely do not contain
  14     signalling (ready/valid).  They do however specify the FORMAT
  15     of the incoming and outgoing data, and they provide a means to
  16     PROCESS that data (from incoming format to outgoing format).
  17
  18     Stage Blocks really must be combinatorial blocks.  It would be ok
  19     to have input come in from sync'd sources (clock-driven) however by
  20     doing so they would no longer be deterministic, and chaining such
  21     blocks with such side-effects together could result in unexpected,
  22     unpredictable, unreproduceable behaviour.
  23     So generally to be avoided, then unless you know what you are doing.
  24
  25     the methods of a stage instance must be as follows:
  26
  27     * ispec() - Input data format specification.  Takes a bit of explaining.
  28                 The requirements are: something that eventually derives from
  29                 nmigen Value must be returned *OR* an iterator or iterable
  30                 or sequence (list, tuple etc.) or generator must *yield*
  31                 thing(s) that (eventually) derive from the nmigen Value class.
  32
  33                 Complex to state, very simple in practice:
  34                 see test_buf_pipe.py for over 25 worked examples.
  35
  36     * ospec() - Output data format specification.
  37                 format requirements identical to ispec.
  38
  39     * process(m, i) - Optional function for processing ispec-formatted data.
  40                 returns a combinatorial block of a result that
  41                 may be assigned to the output, by way of the "nmoperator.eq"
  42                 function.  Note that what is returned here can be
  43                 extremely flexible.  Even a dictionary can be returned
  44                 as long as it has fields that match precisely with the
  45                 Record into which its values is intended to be assigned.
  46                 Again: see example unit tests for details.
  47
  48     * setup(m, i) - Optional function for setting up submodules.
  49                 may be used for more complex stages, to link
  50                 the input (i) to submodules.  must take responsibility
  51                 for adding those submodules to the module (m).
  52                 the submodules must be combinatorial blocks and
  53                 must have their inputs and output linked combinatorially.
  54
  55     Both StageCls (for use with non-static classes) and Stage (for use
  56     by static classes) are abstract classes from which, for convenience
  57     and as a courtesy to other developers, anything conforming to the
  58     Stage API may *choose* to derive.  See Liskov Substitution Principle:
  59     https://en.wikipedia.org/wiki/Liskov_substitution_principle
  60
  61     StageChain:
  62     ----------
  63
  64     A useful combinatorial wrapper around stages that chains them together
  65     and then presents a Stage-API-conformant interface.  By presenting
  66     the same API as the stages it wraps, it can clearly be used recursively.
  67
  68     StageHelper:
  69     ----------
  70
  71     A convenience wrapper around a Stage-API-compliant "thing" which
  72     complies with the Stage API and provides mandatory versions of
  73     all the optional bits.
  74 """
  75
  76 from abc import ABCMeta, abstractmethod
  77 import inspect
  78
  79 import nmoperator
  80
  81
  82 def _spec(fn, name=None):
  83     if name is None:
  84         return fn()
  85     varnames = dict(inspect.getmembers(fn.__code__))['co_varnames']
  86     if 'name' in varnames:
  87         return fn(name=name)
  88     return fn()
  89
  90
  91 class StageCls(metaclass=ABCMeta):
  92     """ Class-based "Stage" API.  requires instantiation (after derivation)
  93
  94         see "Stage API" above..  Note: python does *not* require derivation
  95         from this class.  All that is required is that the pipelines *have*
  96         the functions listed in this class.  Derivation from this class
  97         is therefore merely a "courtesy" to maintainers.
  98     """
  99     @abstractmethod
 100     def ispec(self): pass       # REQUIRED
 101     @abstractmethod
 102     def ospec(self): pass       # REQUIRED
 103     #@abstractmethod
 104     #def setup(self, m, i): pass # OPTIONAL
 105     #@abstractmethod
 106     #def process(self, i): pass  # OPTIONAL
 107
 108
 109 class Stage(metaclass=ABCMeta):
 110     """ Static "Stage" API.  does not require instantiation (after derivation)
 111
 112         see "Stage API" above.  Note: python does *not* require derivation
 113         from this class.  All that is required is that the pipelines *have*
 114         the functions listed in this class.  Derivation from this class
 115         is therefore merely a "courtesy" to maintainers.
 116     """
 117     @staticmethod
 118     @abstractmethod
 119     def ispec(): pass
 120
 121     @staticmethod
 122     @abstractmethod
 123     def ospec(): pass
 124
 125     #@staticmethod
 126     #@abstractmethod
 127     #def setup(m, i): pass
 128
 129     #@staticmethod
 130     #@abstractmethod
 131     #def process(i): pass
 132
 133
 134 class StageChain(StageCls):
 135     """ pass in a list of stages, and they will automatically be
 136         chained together via their input and output specs into a
 137         combinatorial chain, to create one giant combinatorial block.
 138
 139         the end result basically conforms to the exact same Stage API.
 140
 141         * input to this class will be the input of the first stage
 142         * output of first stage goes into input of second
 143         * output of second goes into input into third
 144         * ... (etc. etc.)
 145         * the output of this class will be the output of the last stage
 146
 147         NOTE: whilst this is very similar to ControlBase.connect(), it is
 148         *really* important to appreciate that StageChain is pure
 149         combinatorial and bypasses (does not involve, at all, ready/valid
 150         signalling of any kind).
 151
 152         ControlBase.connect on the other hand respects, connects, and uses
 153         ready/valid signalling.
 154
 155         Arguments:
 156
 157         * :chain: a chain of combinatorial blocks conforming to the Stage API
 158                   NOTE: StageChain.ispec and ospect have to have something
 159                   to return (beginning and end specs of the chain),
 160                   therefore the chain argument must be non-zero length
 161
 162         * :specallocate: if set, new input and output data will be allocated
 163                          and connected (eq'd) to each chained Stage.
 164                          in some cases if this is not done, the nmigen warning
 165                          "driving from two sources, module is being flattened"
 166                          will be issued.
 167
 168         NOTE: do NOT use StageChain with combinatorial blocks that have
 169         side-effects (state-based / clock-based input) or conditional
 170         (inter-chain) dependencies, unless you really know what you are doing.
 171     """
 172     def __init__(self, chain, specallocate=False):
 173         assert len(chain) > 0, "stage chain must be non-zero length"
 174         self.chain = chain
 175         self.setup = self._sa_setup if specallocate else self._na_setup
 176
 177     def ispec(self):
 178         """ returns the ispec of the first of the chain
 179         """
 180         return _spec(self.chain[0].ispec, "chainin")
 181
 182     def ospec(self):
 183         """ returns the ospec of the last of the chain
 184         """
 185         return _spec(self.chain[-1].ospec, "chainout")
 186
 187     def _sa_setup(self, m, i):
 188         for (idx, c) in enumerate(self.chain):
 189             if hasattr(c, "setup"):
 190                 c.setup(m, i)               # stage may have some module stuff
 191             ofn = self.chain[idx].ospec     # last assignment survives
 192             o = _spec(ofn, 'chainin%d' % idx)
 193             m.d.comb += nmoperator.eq(o, c.process(i)) # process input into "o"
 194             if idx == len(self.chain)-1:
 195                 break
 196             ifn = self.chain[idx+1].ispec   # new input on next loop
 197             i = _spec(ifn, 'chainin%d' % (idx+1))
 198             m.d.comb += nmoperator.eq(i, o) # assign to next input
 199         self.o = o
 200         return self.o                       # last loop is the output
 201
 202     def _na_setup(self, m, i):
 203         for (idx, c) in enumerate(self.chain):
 204             if hasattr(c, "setup"):
 205                 c.setup(m, i)               # stage may have some module stuff
 206             i = o = c.process(i)            # store input into "o"
 207         self.o = o
 208         return self.o                       # last loop is the output
 209
 210     def process(self, i):
 211         return self.o # conform to Stage API: return last-loop output
 212
 213
 214 class StageHelper(Stage):
 215     """ a convenience wrapper around something that is Stage-API-compliant.
 216         (that "something" may be a static class, for example).
 217
 218         StageHelper happens to also be compliant with the Stage API,
 219         it differs from the stage that it wraps in that all the "optional"
 220         functions are provided (hence the designation "convenience wrapper")
 221     """
 222     def __init__(self, stage):
 223         self.stage = stage
 224         self._ispecfn = None
 225         self._ospecfn = None
 226         if stage is not None:
 227             self.set_specs(self, self)
 228
 229     def ospec(self, name):
 230         assert self._ospecfn is not None
 231         return _spec(self._ospecfn, name)
 232
 233     def ispec(self, name):
 234         assert self._ispecfn is not None
 235         return _spec(self._ispecfn, name)
 236
 237     def set_specs(self, p, n):
 238         self._ispecfn = p.stage.ispec
 239         self._ospecfn = n.stage.ospec
 240
 241     def new_specs(self, name):
 242         """ allocates new ispec and ospec pair
 243         """
 244         return self.ispec("%s_i" % name), self.ospec("%s_o" % name)
 245
 246     def process(self, i):
 247         if self.stage and hasattr(self.stage, "process"):
 248             return self.stage.process(i)
 249         return i
 250
 251     def setup(self, m, i):
 252         if self.stage is not None and hasattr(self.stage, "setup"):
 253             self.stage.setup(m, i)
 254
 255     def _postprocess(self, i): # XXX DISABLED
 256         return i # RETURNS INPUT
 257         if hasattr(self.stage, "postprocess"):
 258             return self.stage.postprocess(i)
 259         return i
 260