src/nmutil/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 should be combinatorial blocks (Moore FSMs).
  19     It would be ok to have input come in from sync'd sources
  20     (clock-driven, Mealy FSMs) however by doing so they would no longer
  21     be deterministic, and chaining such blocks with such side-effects
  22     together could result in unexpected, unpredictable, unreproduceable
  23     behaviour.
  24
  25     So generally to be avoided, then unless you know what you are doing.
  26     https://en.wikipedia.org/wiki/Moore_machine
  27     https://en.wikipedia.org/wiki/Mealy_machine
  28
  29     the methods of a stage instance must be as follows:
  30
  31     * ispec() - Input data format specification.  Takes a bit of explaining.
  32                 The requirements are: something that eventually derives from
  33                 nmigen Value must be returned *OR* an iterator or iterable
  34                 or sequence (list, tuple etc.) or generator must *yield*
  35                 thing(s) that (eventually) derive from the nmigen Value class.
  36
  37                 Complex to state, very simple in practice:
  38                 see test_buf_pipe.py for over 25 worked examples.
  39
  40     * ospec() - Output data format specification.
  41                 format requirements identical to ispec.
  42
  43     * process(m, i) - Optional function for processing ispec-formatted data.
  44                 returns a combinatorial block of a result that
  45                 may be assigned to the output, by way of the "nmoperator.eq"
  46                 function.  Note that what is returned here can be
  47                 extremely flexible.  Even a dictionary can be returned
  48                 as long as it has fields that match precisely with the
  49                 Record into which its values is intended to be assigned.
  50                 Again: see example unit tests for details.
  51
  52     * setup(m, i) - Optional function for setting up submodules.
  53                 may be used for more complex stages, to link
  54                 the input (i) to submodules.  must take responsibility
  55                 for adding those submodules to the module (m).
  56                 the submodules must be combinatorial blocks and
  57                 must have their inputs and output linked combinatorially.
  58
  59     Both StageCls (for use with non-static classes) and Stage (for use
  60     by static classes) are abstract classes from which, for convenience
  61     and as a courtesy to other developers, anything conforming to the
  62     Stage API may *choose* to derive.  See Liskov Substitution Principle:
  63     https://en.wikipedia.org/wiki/Liskov_substitution_principle
  64
  65     StageChain:
  66     ----------
  67
  68     A useful combinatorial wrapper around stages that chains them together
  69     and then presents a Stage-API-conformant interface.  By presenting
  70     the same API as the stages it wraps, it can clearly be used recursively.
  71
  72     StageHelper:
  73     ----------
  74
  75     A convenience wrapper around a Stage-API-compliant "thing" which
  76     complies with the Stage API and provides mandatory versions of
  77     all the optional bits.
  78 """
  79
  80 from nmigen import Elaboratable
  81 from abc import ABCMeta, abstractmethod
  82 import inspect
  83
  84 from nmutil import nmoperator
  85
  86
  87 def _spec(fn, name=None):
  88     """ useful function that determines if "fn" has an argument "name".
  89         if so, fn(name) is called otherwise fn() is called.
  90
  91         means that ispec and ospec can be declared with *or without*
  92         a name argument.  normally it would be necessary to have
  93         "ispec(name=None)" to achieve the same effect.
  94     """
  95     if name is None:
  96         return fn()
  97     varnames = dict(inspect.getmembers(fn.__code__))['co_varnames']
  98     if 'name' in varnames:
  99         return fn(name=name)
 100     return fn()
 101
 102
 103 class StageCls(metaclass=ABCMeta):
 104     """ Class-based "Stage" API.  requires instantiation (after derivation)
 105
 106         see "Stage API" above..  Note: python does *not* require derivation
 107         from this class.  All that is required is that the pipelines *have*
 108         the functions listed in this class.  Derivation from this class
 109         is therefore merely a "courtesy" to maintainers.
 110     """
 111     @abstractmethod
 112     def ispec(self): pass       # REQUIRED
 113     @abstractmethod
 114     def ospec(self): pass       # REQUIRED
 115     #@abstractmethod
 116     #def setup(self, m, i): pass # OPTIONAL
 117     #@abstractmethod
 118     #def process(self, i): pass  # OPTIONAL
 119
 120
 121 class Stage(metaclass=ABCMeta):
 122     """ Static "Stage" API.  does not require instantiation (after derivation)
 123
 124         see "Stage API" above.  Note: python does *not* require derivation
 125         from this class.  All that is required is that the pipelines *have*
 126         the functions listed in this class.  Derivation from this class
 127         is therefore merely a "courtesy" to maintainers.
 128     """
 129     @staticmethod
 130     @abstractmethod
 131     def ispec(): pass
 132
 133     @staticmethod
 134     @abstractmethod
 135     def ospec(): pass
 136
 137     #@staticmethod
 138     #@abstractmethod
 139     #def setup(m, i): pass
 140
 141     #@staticmethod
 142     #@abstractmethod
 143     #def process(i): pass
 144
 145
 146 class StageHelper(Stage):
 147     """ a convenience wrapper around something that is Stage-API-compliant.
 148         (that "something" may be a static class, for example).
 149
 150         StageHelper happens to also be compliant with the Stage API,
 151         it differs from the stage that it wraps in that all the "optional"
 152         functions are provided (hence the designation "convenience wrapper")
 153     """
 154     def __init__(self, stage):
 155         self.stage = stage
 156         self._ispecfn = None
 157         self._ospecfn = None
 158         if stage is not None:
 159             self.set_specs(self, self)
 160
 161     def ospec(self, name=None):
 162         assert self._ospecfn is not None
 163         return _spec(self._ospecfn, name)
 164
 165     def ispec(self, name=None):
 166         assert self._ispecfn is not None
 167         return _spec(self._ispecfn, name)
 168
 169     def set_specs(self, p, n):
 170         """ sets up the ispecfn and ospecfn for getting input and output data
 171         """
 172         if hasattr(p, "stage"):
 173             p = p.stage
 174         if hasattr(n, "stage"):
 175             n = n.stage
 176         self._ispecfn = p.ispec
 177         self._ospecfn = n.ospec
 178
 179     def new_specs(self, name):
 180         """ allocates new ispec and ospec pair
 181         """
 182         return (_spec(self.ispec, "%s_i" % name),
 183                 _spec(self.ospec, "%s_o" % name))
 184
 185     def process(self, i):
 186         if self.stage and hasattr(self.stage, "process"):
 187             return self.stage.process(i)
 188         return i
 189
 190     def setup(self, m, i):
 191         if self.stage is not None and hasattr(self.stage, "setup"):
 192             self.stage.setup(m, i)
 193
 194     def _postprocess(self, i): # XXX DISABLED
 195         return i # RETURNS INPUT
 196         if hasattr(self.stage, "postprocess"):
 197             return self.stage.postprocess(i)
 198         return i
 199
 200
 201 class StageChain(StageHelper):
 202     """ pass in a list of stages, and they will automatically be
 203         chained together via their input and output specs into a
 204         combinatorial chain, to create one giant combinatorial block.
 205
 206         the end result basically conforms to the exact same Stage API.
 207
 208         * input to this class will be the input of the first stage
 209         * output of first stage goes into input of second
 210         * output of second goes into input into third
 211         * ... (etc. etc.)
 212         * the output of this class will be the output of the last stage
 213
 214         NOTE: whilst this is very similar to ControlBase.connect(), it is
 215         *really* important to appreciate that StageChain is pure
 216         combinatorial and bypasses (does not involve, at all, ready/valid
 217         signalling of any kind).
 218
 219         ControlBase.connect on the other hand respects, connects, and uses
 220         ready/valid signalling.
 221
 222         Arguments:
 223
 224         * :chain: a chain of combinatorial blocks conforming to the Stage API
 225                   NOTE: StageChain.ispec and ospect have to have something
 226                   to return (beginning and end specs of the chain),
 227                   therefore the chain argument must be non-zero length
 228
 229         * :specallocate: if set, new input and output data will be allocated
 230                          and connected (eq'd) to each chained Stage.
 231                          in some cases if this is not done, the nmigen warning
 232                          "driving from two sources, module is being flattened"
 233                          will be issued.
 234
 235         NOTE: do NOT use StageChain with combinatorial blocks that have
 236         side-effects (state-based / clock-based input) or conditional
 237         (inter-chain) dependencies, unless you really know what you are doing.
 238     """
 239     def __init__(self, chain, specallocate=False):
 240         assert len(chain) > 0, "stage chain must be non-zero length"
 241         self.chain = chain
 242         StageHelper.__init__(self, None)
 243         if specallocate:
 244             self.setup = self._sa_setup
 245         else:
 246             self.setup = self._na_setup
 247         self.set_specs(self.chain[0], self.chain[-1])
 248
 249     def _sa_setup(self, m, i):
 250         for (idx, c) in enumerate(self.chain):
 251             if hasattr(c, "setup"):
 252                 c.setup(m, i)               # stage may have some module stuff
 253             ofn = self.chain[idx].ospec     # last assignment survives
 254             cname = 'chainin%d' % idx
 255             o = _spec(ofn, cname)
 256             if isinstance(o, Elaboratable):
 257                 setattr(m.submodules, cname, o)
 258             m.d.comb += nmoperator.eq(o, c.process(i)) # process input into "o"
 259             if idx == len(self.chain)-1:
 260                 break
 261             ifn = self.chain[idx+1].ispec   # new input on next loop
 262             i = _spec(ifn, 'chainin%d' % (idx+1))
 263             m.d.comb += nmoperator.eq(i, o) # assign to next input
 264         self.o = o
 265         return self.o                       # last loop is the output
 266
 267     def _na_setup(self, m, i):
 268         for (idx, c) in enumerate(self.chain):
 269             if hasattr(c, "setup"):
 270                 c.setup(m, i)               # stage may have some module stuff
 271             i = o = c.process(i)            # store input into "o"
 272         self.o = o
 273         return self.o                       # last loop is the output
 274
 275     def process(self, i):
 276         return self.o # conform to Stage API: return last-loop output
 277
 278