1 """ Pipeline and BufferedPipeline implementation, conforming to the same API.
6 a strategically very important function that is identical in function
7 to nmigen's Signal.eq function, except it may take objects, or a list
8 of objects, or a tuple of objects, and where objects may also be
14 stage requires compliance with a strict API that may be
15 implemented in several means, including as a static class.
16 the methods of a stage instance must be as follows:
18 * ispec() - Input data format specification
19 returns an object or a list or tuple of objects, or
20 a Record, each object having an "eq" function which
21 takes responsibility for copying by assignment all
23 * ospec() - Output data format specification
24 requirements as for ospec
25 * process(m, i) - Processes an ispec-formatted object
26 returns a combinatorial block of a result that
27 may be assigned to the output, by way of the "eq"
29 * setup(m, i) - Optional function for setting up submodules
30 may be used for more complex stages, to link
31 the input (i) to submodules. must take responsibility
32 for adding those submodules to the module (m).
33 the submodules must be combinatorial blocks and
34 must have their inputs and output linked combinatorially.
39 A useful combinatorial wrapper around stages that chains them together
40 and then presents a Stage-API-conformant interface.
45 A simple stalling clock-synchronised pipeline that has no buffering
46 (unlike BufferedPipeline). A stall anywhere along the line will
47 result in a stall back-propagating down the entire chain.
49 The BufferedPipeline by contrast will buffer incoming data, allowing
50 previous stages one clock cycle's grace before also having to stall.
52 An advantage of the UnbufferedPipeline over the Buffered one is
53 that the amount of logic needed (number of gates) is greatly
59 nmigen implementation of buffered pipeline stage, based on zipcpu:
60 https://zipcpu.com/blog/2017/08/14/strategies-for-pipelining.html
62 this module requires quite a bit of thought to understand how it works
63 (and why it is needed in the first place). reading the above is
64 *strongly* recommended.
66 unlike john dawson's IEEE754 FPU STB/ACK signalling, which requires
67 the STB / ACK signals to raise and lower (on separate clocks) before
68 data may proceeed (thus only allowing one piece of data to proceed
69 on *ALTERNATE* cycles), the signalling here is a true pipeline
70 where data will flow on *every* clock when the conditions are right.
72 input acceptance conditions are when:
73 * incoming previous-stage strobe (p.i_valid) is HIGH
74 * outgoing previous-stage ready (p.o_ready) is LOW
76 output transmission conditions are when:
77 * outgoing next-stage strobe (n.o_valid) is HIGH
78 * outgoing next-stage ready (n.i_ready) is LOW
80 the tricky bit is when the input has valid data and the output is not
81 ready to accept it. if it wasn't for the clock synchronisation, it
82 would be possible to tell the input "hey don't send that data, we're
83 not ready". unfortunately, it's not possible to "change the past":
84 the previous stage *has no choice* but to pass on its data.
86 therefore, the incoming data *must* be accepted - and stored: that
87 is the responsibility / contract that this stage *must* accept.
88 on the same clock, it's possible to tell the input that it must
89 not send any more data. this is the "stall" condition.
91 we now effectively have *two* possible pieces of data to "choose" from:
92 the buffered data, and the incoming data. the decision as to which
93 to process and output is based on whether we are in "stall" or not.
94 i.e. when the next stage is no longer ready, the output comes from
95 the buffer if a stall had previously occurred, otherwise it comes
96 direct from processing the input.
98 this allows us to respect a synchronous "travelling STB" with what
99 dan calls a "buffered handshake".
101 it's quite a complex state machine!
104 from nmigen
import Signal
, Cat
, Const
, Mux
, Module
105 from nmigen
.cli
import verilog
, rtlil
106 from nmigen
.hdl
.rec
import Record
, Layout
108 from abc
import ABCMeta
, abstractmethod
109 from collections
.abc
import Sequence
113 """ contains signals that come *from* the previous stage (both in and out)
114 * i_valid: previous stage indicating all incoming data is valid.
115 may be a multi-bit signal, where all bits are required
116 to be asserted to indicate "valid".
117 * o_ready: output to next stage indicating readiness to accept data
118 * i_data : an input - added by the user of this class
121 def __init__(self
, i_width
=1):
122 self
.i_valid
= Signal(i_width
, name
="p_i_valid") # prev >>in self
123 self
.o_ready
= Signal(name
="p_o_ready") # prev <<out self
125 def _connect_in(self
, prev
):
126 """ helper function to connect stage to an input source. do not
127 use to connect stage-to-stage!
129 return [self
.i_valid
.eq(prev
.i_valid
),
130 prev
.o_ready
.eq(self
.o_ready
),
131 eq(self
.i_data
, prev
.i_data
),
134 def i_valid_logic(self
):
135 vlen
= len(self
.i_valid
)
136 if vlen
> 1: # multi-bit case: valid only when i_valid is all 1s
137 all1s
= Const(-1, (len(self
.i_valid
), False))
138 return self
.i_valid
== all1s
139 # single-bit i_valid case
144 """ contains the signals that go *to* the next stage (both in and out)
145 * o_valid: output indicating to next stage that data is valid
146 * i_ready: input from next stage indicating that it can accept data
147 * o_data : an output - added by the user of this class
150 self
.o_valid
= Signal(name
="n_o_valid") # self out>> next
151 self
.i_ready
= Signal(name
="n_i_ready") # self <<in next
153 def connect_to_next(self
, nxt
):
154 """ helper function to connect to the next stage data/valid/ready.
155 data/valid is passed *TO* nxt, and ready comes *IN* from nxt.
157 return [nxt
.i_valid
.eq(self
.o_valid
),
158 self
.i_ready
.eq(nxt
.o_ready
),
159 eq(nxt
.i_data
, self
.o_data
),
162 def _connect_out(self
, nxt
):
163 """ helper function to connect stage to an output source. do not
164 use to connect stage-to-stage!
166 return [nxt
.o_valid
.eq(self
.o_valid
),
167 self
.i_ready
.eq(nxt
.i_ready
),
168 eq(nxt
.o_data
, self
.o_data
),
173 """ makes signals equal: a helper routine which identifies if it is being
174 passed a list (or tuple) of objects, or signals, or Records, and calls
175 the objects' eq function.
177 complex objects (classes) can be used: they must follow the
178 convention of having an eq member function, which takes the
179 responsibility of further calling eq and returning a list of
182 Record is a special (unusual, recursive) case, where the input may be
183 specified as a dictionary (which may contain further dictionaries,
184 recursively), where the field names of the dictionary must match
185 the Record's field spec. Alternatively, an object with the same
186 member names as the Record may be assigned: it does not have to
189 if not isinstance(o
, Sequence
):
192 for (ao
, ai
) in zip(o
, i
):
193 #print ("eq", ao, ai)
194 if isinstance(ao
, Record
):
195 for idx
, (field_name
, field_shape
, _
) in enumerate(ao
.layout
):
196 if isinstance(field_shape
, Layout
):
200 if hasattr(val
, field_name
): # check for attribute
201 val
= getattr(val
, field_name
)
203 val
= val
[field_name
] # dictionary-style specification
204 rres
= eq(ao
.fields
[field_name
], val
)
208 if not isinstance(rres
, Sequence
):
214 class StageCls(metaclass
=ABCMeta
):
215 """ Class-based "Stage" API. requires instantiation (after derivation)
217 see "Stage API" above.. Note: python does *not* require derivation
218 from this class. All that is required is that the pipelines *have*
219 the functions listed in this class. Derivation from this class
220 is therefore merely a "courtesy" to maintainers.
223 def ispec(self
): pass # REQUIRED
225 def ospec(self
): pass # REQUIRED
227 #def setup(self, m, i): pass # OPTIONAL
229 def process(self
, i
): pass # REQUIRED
232 class Stage(metaclass
=ABCMeta
):
233 """ Static "Stage" API. does not require instantiation (after derivation)
235 see "Stage API" above. Note: python does *not* require derivation
236 from this class. All that is required is that the pipelines *have*
237 the functions listed in this class. Derivation from this class
238 is therefore merely a "courtesy" to maintainers.
250 #def setup(m, i): pass
257 class StageChain(StageCls
):
258 """ pass in a list of stages, and they will automatically be
259 chained together via their input and output specs into a
262 the end result basically conforms to the exact same Stage API.
264 * input to this class will be the input of the first stage
265 * output of first stage goes into input of second
266 * output of second goes into input into third (etc. etc.)
267 * the output of this class will be the output of the last stage
269 def __init__(self
, chain
):
273 return self
.chain
[0].ispec()
276 return self
.chain
[-1].ospec()
278 def setup(self
, m
, i
):
279 for (idx
, c
) in enumerate(self
.chain
):
280 if hasattr(c
, "setup"):
281 c
.setup(m
, i
) # stage may have some module stuff
282 o
= self
.chain
[idx
].ospec() # only the last assignment survives
283 m
.d
.comb
+= eq(o
, c
.process(i
)) # process input into "o"
284 if idx
!= len(self
.chain
)-1:
285 ni
= self
.chain
[idx
+1].ispec() # becomes new input on next loop
286 m
.d
.comb
+= eq(ni
, o
) # assign output to next input
288 self
.o
= o
# last loop is the output
290 def process(self
, i
):
295 """ Common functions for Pipeline API
297 def __init__(self
, in_multi
=None):
298 """ Base class containing ready/valid/data to previous and next stages
300 * p: contains ready/valid to the previous stage
301 * n: contains ready/valid to the next stage
304 * add i_data member to PrevControl (p) and
305 * add o_data member to NextControl (n)
308 # set up input and output IO ACK (prev/next ready/valid)
309 self
.p
= PrevControl(in_multi
)
310 self
.n
= NextControl()
312 def connect_to_next(self
, nxt
):
313 """ helper function to connect to the next stage data/valid/ready.
315 return self
.n
.connect_to_next(nxt
.p
)
317 def _connect_in(self
, prev
):
318 """ helper function to connect stage to an input source. do not
319 use to connect stage-to-stage!
321 return self
.p
._connect
_in
(prev
.p
)
323 def _connect_out(self
, nxt
):
324 """ helper function to connect stage to an output source. do not
325 use to connect stage-to-stage!
327 return self
.n
._connect
_out
(nxt
.n
)
329 def connect(self
, m
, pipechain
):
330 """ connects a chain (list) of Pipeline instances together and
331 links them to this ControlBase instance:
333 in <----> self <---> out
336 [pipe1, pipe2, pipe3, pipe4]
339 out---in out--in out---in
341 Also takes care of allocating i_data/o_data, by looking up
342 the data spec for each end of the pipechain.
344 Basically this function is the direct equivalent of StageChain,
345 except that unlike StageChain, the Pipeline logic is followed.
347 Just as StageChain presents an object that conforms to the
348 Stage API from a list of objects that also conform to the
349 Stage API, an object that calls this Pipeline connect function
350 has the exact same pipeline API as the list of pipline objects
353 Thus it becomes possible to build up larger chains recursively.
354 More complex chains (multi-input, multi-output) will have to be
357 eqs
= [] # collated list of assignment statements
359 # connect inter-chain
360 for i
in range(len(pipechain
)-1):
362 pipe2
= pipechain
[i
+1]
363 eqs
+= pipe1
.connect_to_next(pipe2
)
365 # connect front of chain to ourselves
367 self
.p
.i_data
= front
.stage
.ispec()
368 eqs
+= front
._connect
_in
(self
)
370 # connect end of chain to ourselves
372 self
.n
.o_data
= end
.stage
.ospec()
373 eqs
+= end
._connect
_out
(self
)
375 # activate the assignments
378 def set_input(self
, i
):
379 """ helper function to set the input data
381 return eq(self
.p
.i_data
, i
)
384 return [self
.p
.i_valid
, self
.n
.i_ready
,
385 self
.n
.o_valid
, self
.p
.o_ready
,
386 self
.p
.i_data
, self
.n
.o_data
# XXX need flattening!
390 class BufferedPipeline(ControlBase
):
391 """ buffered pipeline stage. data and strobe signals travel in sync.
392 if ever the input is ready and the output is not, processed data
393 is stored in a temporary register.
395 Argument: stage. see Stage API above
397 stage-1 p.i_valid >>in stage n.o_valid out>> stage+1
398 stage-1 p.o_ready <<out stage n.i_ready <<in stage+1
399 stage-1 p.i_data >>in stage n.o_data out>> stage+1
405 input data p.i_data is read (only), is processed and goes into an
406 intermediate result store [process()]. this is updated combinatorially.
408 in a non-stall condition, the intermediate result will go into the
409 output (update_output). however if ever there is a stall, it goes
410 into r_data instead [update_buffer()].
412 when the non-stall condition is released, r_data is the first
413 to be transferred to the output [flush_buffer()], and the stall
416 on the next cycle (as long as stall is not raised again) the
417 input may begin to be processed and transferred directly to output.
420 def __init__(self
, stage
):
421 ControlBase
.__init
__(self
)
424 # set up the input and output data
425 self
.p
.i_data
= stage
.ispec() # input type
426 self
.n
.o_data
= stage
.ospec()
428 def elaborate(self
, platform
):
431 result
= self
.stage
.ospec()
432 r_data
= self
.stage
.ospec()
433 if hasattr(self
.stage
, "setup"):
434 self
.stage
.setup(m
, self
.p
.i_data
)
436 # establish some combinatorial temporaries
437 o_n_validn
= Signal(reset_less
=True)
438 i_p_valid_o_p_ready
= Signal(reset_less
=True)
439 p_i_valid
= Signal(reset_less
=True)
440 m
.d
.comb
+= [p_i_valid
.eq(self
.p
.i_valid_logic()),
441 o_n_validn
.eq(~self
.n
.o_valid
),
442 i_p_valid_o_p_ready
.eq(p_i_valid
& self
.p
.o_ready
),
445 # store result of processing in combinatorial temporary
446 m
.d
.comb
+= eq(result
, self
.stage
.process(self
.p
.i_data
))
448 # if not in stall condition, update the temporary register
449 with m
.If(self
.p
.o_ready
): # not stalled
450 m
.d
.sync
+= eq(r_data
, result
) # update buffer
452 with m
.If(self
.n
.i_ready
): # next stage is ready
453 with m
.If(self
.p
.o_ready
): # not stalled
454 # nothing in buffer: send (processed) input direct to output
455 m
.d
.sync
+= [self
.n
.o_valid
.eq(p_i_valid
),
456 eq(self
.n
.o_data
, result
), # update output
458 with m
.Else(): # p.o_ready is false, and something is in buffer.
459 # Flush the [already processed] buffer to the output port.
460 m
.d
.sync
+= [self
.n
.o_valid
.eq(1), # declare reg empty
461 eq(self
.n
.o_data
, r_data
), # flush buffer
462 self
.p
.o_ready
.eq(1), # clear stall condition
464 # ignore input, since p.o_ready is also false.
466 # (n.i_ready) is false here: next stage is ready
467 with m
.Elif(o_n_validn
): # next stage being told "ready"
468 m
.d
.sync
+= [self
.n
.o_valid
.eq(p_i_valid
),
469 self
.p
.o_ready
.eq(1), # Keep the buffer empty
470 eq(self
.n
.o_data
, result
), # set output data
473 # (n.i_ready) false and (n.o_valid) true:
474 with m
.Elif(i_p_valid_o_p_ready
):
475 # If next stage *is* ready, and not stalled yet, accept input
476 m
.d
.sync
+= self
.p
.o_ready
.eq(~
(p_i_valid
& self
.n
.o_valid
))
481 class ExampleAddStage(StageCls
):
482 """ an example of how to use the buffered pipeline, as a class instance
486 """ returns a tuple of input signals which will be the incoming data
488 return (Signal(16), Signal(16))
491 """ returns an output signal which will happen to contain the sum
496 def process(self
, i
):
497 """ process the input data (sums the values in the tuple) and returns it
502 class ExampleBufPipeAdd(BufferedPipeline
):
503 """ an example of how to use the buffered pipeline, using a class instance
507 addstage
= ExampleAddStage()
508 BufferedPipeline
.__init
__(self
, addstage
)
511 class ExampleStage(Stage
):
512 """ an example of how to use the buffered pipeline, in a static class
517 return Signal(16, name
="example_input_signal")
520 return Signal(16, name
="example_output_signal")
523 """ process the input data and returns it (adds 1)
528 class ExampleStageCls(StageCls
):
529 """ an example of how to use the buffered pipeline, in a static class
534 return Signal(16, name
="example_input_signal")
537 return Signal(16, name
="example_output_signal")
539 def process(self
, i
):
540 """ process the input data and returns it (adds 1)
545 class ExampleBufPipe(BufferedPipeline
):
546 """ an example of how to use the buffered pipeline.
550 BufferedPipeline
.__init
__(self
, ExampleStage
)
553 class UnbufferedPipeline(ControlBase
):
554 """ A simple pipeline stage with single-clock synchronisation
555 and two-way valid/ready synchronised signalling.
557 Note that a stall in one stage will result in the entire pipeline
560 Also that unlike BufferedPipeline, the valid/ready signalling does NOT
561 travel synchronously with the data: the valid/ready signalling
562 combines in a *combinatorial* fashion. Therefore, a long pipeline
563 chain will lengthen propagation delays.
565 Argument: stage. see Stage API, above
567 stage-1 p.i_valid >>in stage n.o_valid out>> stage+1
568 stage-1 p.o_ready <<out stage n.i_ready <<in stage+1
569 stage-1 p.i_data >>in stage n.o_data out>> stage+1
577 p.i_data : StageInput, shaped according to ispec
579 p.o_data : StageOutput, shaped according to ospec
581 r_data : input_shape according to ispec
582 A temporary (buffered) copy of a prior (valid) input.
583 This is HELD if the output is not ready. It is updated
585 result: output_shape according to ospec
586 The output of the combinatorial logic. it is updated
587 COMBINATORIALLY (no clock dependence).
590 def __init__(self
, stage
):
591 ControlBase
.__init
__(self
)
593 self
._data
_valid
= Signal()
595 # set up the input and output data
596 self
.p
.i_data
= stage
.ispec() # input type
597 self
.n
.o_data
= stage
.ospec() # output type
599 def elaborate(self
, platform
):
602 r_data
= self
.stage
.ispec() # input type
603 result
= self
.stage
.ospec() # output data
604 if hasattr(self
.stage
, "setup"):
605 self
.stage
.setup(m
, r_data
)
607 p_i_valid
= Signal(reset_less
=True)
608 m
.d
.comb
+= p_i_valid
.eq(self
.p
.i_valid_logic())
609 m
.d
.comb
+= eq(result
, self
.stage
.process(r_data
))
610 m
.d
.comb
+= self
.n
.o_valid
.eq(self
._data
_valid
)
611 m
.d
.comb
+= self
.p
.o_ready
.eq(~self
._data
_valid | self
.n
.i_ready
)
612 m
.d
.sync
+= self
._data
_valid
.eq(p_i_valid | \
613 (~self
.n
.i_ready
& self
._data
_valid
))
614 with m
.If(self
.p
.i_valid
& self
.p
.o_ready
):
615 m
.d
.sync
+= eq(r_data
, self
.p
.i_data
)
616 m
.d
.comb
+= eq(self
.n
.o_data
, result
)
620 class ExamplePipeline(UnbufferedPipeline
):
621 """ an example of how to use the combinatorial pipeline.
625 UnbufferedPipeline
.__init
__(self
, ExampleStage
)
628 if __name__
== '__main__':
629 dut
= ExampleBufPipe()
630 vl
= rtlil
.convert(dut
, ports
=dut
.ports())
631 with
open("test_bufpipe.il", "w") as f
:
634 dut
= ExamplePipeline()
635 vl
= rtlil
.convert(dut
, ports
=dut
.ports())
636 with
open("test_combpipe.il", "w") as f
: