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     """ useful function that determines if "fn" has an argument "name".
  84         if so, fn(name) is called otherwise fn() is called.
  85
  86         means that ispec and ospec can be declared with *or without*
  87         a name argument.  normally it would be necessary to have
  88         "ispec(name=None)" to achieve the same effect.
  89     """
  90     if name is None:
  91         return fn()
  92     varnames = dict(inspect.getmembers(fn.__code__))['co_varnames']
  93     if 'name' in varnames:
  94         return fn(name=name)
  95     return fn()
  96
  97
  98 class StageCls(metaclass=ABCMeta):
  99     """ Class-based "Stage" API.  requires instantiation (after derivation)
 100
 101         see "Stage API" above..  Note: python does *not* require derivation
 102         from this class.  All that is required is that the pipelines *have*
 103         the functions listed in this class.  Derivation from this class
 104         is therefore merely a "courtesy" to maintainers.
 105     """
 106     @abstractmethod
 107     def ispec(self): pass       # REQUIRED
 108     @abstractmethod
 109     def ospec(self): pass       # REQUIRED
 110     #@abstractmethod
 111     #def setup(self, m, i): pass # OPTIONAL
 112     #@abstractmethod
 113     #def process(self, i): pass  # OPTIONAL
 114
 115
 116 class Stage(metaclass=ABCMeta):
 117     """ Static "Stage" API.  does not require instantiation (after derivation)
 118
 119         see "Stage API" above.  Note: python does *not* require derivation
 120         from this class.  All that is required is that the pipelines *have*
 121         the functions listed in this class.  Derivation from this class
 122         is therefore merely a "courtesy" to maintainers.
 123     """
 124     @staticmethod
 125     @abstractmethod
 126     def ispec(): pass
 127
 128     @staticmethod
 129     @abstractmethod
 130     def ospec(): pass
 131
 132     #@staticmethod
 133     #@abstractmethod
 134     #def setup(m, i): pass
 135
 136     #@staticmethod
 137     #@abstractmethod
 138     #def process(i): pass
 139
 140
 141 class StageHelper(Stage):
 142     """ a convenience wrapper around something that is Stage-API-compliant.
 143         (that "something" may be a static class, for example).
 144
 145         StageHelper happens to also be compliant with the Stage API,
 146         it differs from the stage that it wraps in that all the "optional"
 147         functions are provided (hence the designation "convenience wrapper")
 148     """
 149     def __init__(self, stage):
 150         self.stage = stage
 151         self._ispecfn = None
 152         self._ospecfn = None
 153         if stage is not None:
 154             self.set_specs(self, self)
 155
 156     def ospec(self, name):
 157         assert self._ospecfn is not None
 158         return _spec(self._ospecfn, name)
 159
 160     def ispec(self, name):
 161         assert self._ispecfn is not None
 162         return _spec(self._ispecfn, name)
 163
 164     def set_specs(self, p, n):
 165         """ sets up the ispecfn and ospecfn for getting input and output data
 166         """
 167         if hasattr(p, "stage"):
 168             p = p.stage
 169         if hasattr(n, "stage"):
 170             n = n.stage
 171         self._ispecfn = p.ispec
 172         self._ospecfn = n.ospec
 173
 174     def new_specs(self, name):
 175         """ allocates new ispec and ospec pair
 176         """
 177         return self.ispec("%s_i" % name), self.ospec("%s_o" % name)
 178
 179     def process(self, i):
 180         if self.stage and hasattr(self.stage, "process"):
 181             return self.stage.process(i)
 182         return i
 183
 184     def setup(self, m, i):
 185         if self.stage is not None and hasattr(self.stage, "setup"):
 186             self.stage.setup(m, i)
 187
 188     def _postprocess(self, i): # XXX DISABLED
 189         return i # RETURNS INPUT
 190         if hasattr(self.stage, "postprocess"):
 191             return self.stage.postprocess(i)
 192         return i
 193
 194
 195 class StageChain(StageHelper):
 196     """ pass in a list of stages, and they will automatically be
 197         chained together via their input and output specs into a
 198         combinatorial chain, to create one giant combinatorial block.
 199
 200         the end result basically conforms to the exact same Stage API.
 201
 202         * input to this class will be the input of the first stage
 203         * output of first stage goes into input of second
 204         * output of second goes into input into third
 205         * ... (etc. etc.)
 206         * the output of this class will be the output of the last stage
 207
 208         NOTE: whilst this is very similar to ControlBase.connect(), it is
 209         *really* important to appreciate that StageChain is pure
 210         combinatorial and bypasses (does not involve, at all, ready/valid
 211         signalling of any kind).
 212
 213         ControlBase.connect on the other hand respects, connects, and uses
 214         ready/valid signalling.
 215
 216         Arguments:
 217
 218         * :chain: a chain of combinatorial blocks conforming to the Stage API
 219                   NOTE: StageChain.ispec and ospect have to have something
 220                   to return (beginning and end specs of the chain),
 221                   therefore the chain argument must be non-zero length
 222
 223         * :specallocate: if set, new input and output data will be allocated
 224                          and connected (eq'd) to each chained Stage.
 225                          in some cases if this is not done, the nmigen warning
 226                          "driving from two sources, module is being flattened"
 227                          will be issued.
 228
 229         NOTE: do NOT use StageChain with combinatorial blocks that have
 230         side-effects (state-based / clock-based input) or conditional
 231         (inter-chain) dependencies, unless you really know what you are doing.
 232     """
 233     def __init__(self, chain, specallocate=False):
 234         assert len(chain) > 0, "stage chain must be non-zero length"
 235         self.chain = chain
 236         StageHelper.__init__(self, None)
 237         self.setup = self._sa_setup if specallocate else self._na_setup
 238         self.set_specs(self.chain[0], self.chain[-1])
 239
 240     def _sa_setup(self, m, i):
 241         for (idx, c) in enumerate(self.chain):
 242             if hasattr(c, "setup"):
 243                 c.setup(m, i)               # stage may have some module stuff
 244             ofn = self.chain[idx].ospec     # last assignment survives
 245             o = _spec(ofn, 'chainin%d' % idx)
 246             m.d.comb += nmoperator.eq(o, c.process(i)) # process input into "o"
 247             if idx == len(self.chain)-1:
 248                 break
 249             ifn = self.chain[idx+1].ispec   # new input on next loop
 250             i = _spec(ifn, 'chainin%d' % (idx+1))
 251             m.d.comb += nmoperator.eq(i, o) # assign to next input
 252         self.o = o
 253         return self.o                       # last loop is the output
 254
 255     def _na_setup(self, m, i):
 256         for (idx, c) in enumerate(self.chain):
 257             if hasattr(c, "setup"):
 258                 c.setup(m, i)               # stage may have some module stuff
 259             i = o = c.process(i)            # store input into "o"
 260         self.o = o
 261         return self.o                       # last loop is the output
 262
 263     def process(self, i):
 264         return self.o # conform to Stage API: return last-loop output
 265
 266