1 """ Pipeline and BufferedPipeline implementation, conforming to the same API.
2 For multi-input and multi-output variants, see multipipe.
7 a strategically very important function that is identical in function
8 to nmigen's Signal.eq function, except it may take objects, or a list
9 of objects, or a tuple of objects, and where objects may also be
15 stage requires compliance with a strict API that may be
16 implemented in several means, including as a static class.
17 the methods of a stage instance must be as follows:
19 * ispec() - Input data format specification
20 returns an object or a list or tuple of objects, or
21 a Record, each object having an "eq" function which
22 takes responsibility for copying by assignment all
24 * ospec() - Output data format specification
25 requirements as for ospec
26 * process(m, i) - Processes an ispec-formatted object
27 returns a combinatorial block of a result that
28 may be assigned to the output, by way of the "eq"
30 * setup(m, i) - Optional function for setting up submodules
31 may be used for more complex stages, to link
32 the input (i) to submodules. must take responsibility
33 for adding those submodules to the module (m).
34 the submodules must be combinatorial blocks and
35 must have their inputs and output linked combinatorially.
37 Both StageCls (for use with non-static classes) and Stage (for use
38 by static classes) are abstract classes from which, for convenience
39 and as a courtesy to other developers, anything conforming to the
40 Stage API may *choose* to derive.
45 A useful combinatorial wrapper around stages that chains them together
46 and then presents a Stage-API-conformant interface. By presenting
47 the same API as the stages it wraps, it can clearly be used recursively.
52 A convenience class that takes an input shape, output shape, a
53 "processing" function and an optional "setup" function. Honestly
54 though, there's not much more effort to just... create a class
55 that returns a couple of Records (see ExampleAddRecordStage in
61 A convenience class that takes a single function as a parameter,
62 that is chain-called to create the exact same input and output spec.
63 It has a process() function that simply returns its input.
65 Instances of this class are completely redundant if handed to
66 StageChain, however when passed to UnbufferedPipeline they
67 can be used to introduce a single clock delay.
72 The base class for pipelines. Contains previous and next ready/valid/data.
73 Also has an extremely useful "connect" function that can be used to
74 connect a chain of pipelines and present the exact same prev/next
80 A simple stalling clock-synchronised pipeline that has no buffering
81 (unlike BufferedPipeline). Data flows on *every* clock cycle when
82 the conditions are right (this is nominally when the input is valid
83 and the output is ready).
85 A stall anywhere along the line will result in a stall back-propagating
86 down the entire chain. The BufferedPipeline by contrast will buffer
87 incoming data, allowing previous stages one clock cycle's grace before
90 An advantage of the UnbufferedPipeline over the Buffered one is
91 that the amount of logic needed (number of gates) is greatly
92 reduced (no second set of buffers basically)
94 The disadvantage of the UnbufferedPipeline is that the valid/ready
95 logic, if chained together, is *combinatorial*, resulting in
96 progressively larger gate delay.
101 A convenience class that, because UnbufferedPipeline introduces a single
102 clock delay, when its stage is a PassThroughStage, it results in a Pipeline
103 stage that, duh, delays its (unmodified) input by one clock cycle.
108 nmigen implementation of buffered pipeline stage, based on zipcpu:
109 https://zipcpu.com/blog/2017/08/14/strategies-for-pipelining.html
111 this module requires quite a bit of thought to understand how it works
112 (and why it is needed in the first place). reading the above is
113 *strongly* recommended.
115 unlike john dawson's IEEE754 FPU STB/ACK signalling, which requires
116 the STB / ACK signals to raise and lower (on separate clocks) before
117 data may proceeed (thus only allowing one piece of data to proceed
118 on *ALTERNATE* cycles), the signalling here is a true pipeline
119 where data will flow on *every* clock when the conditions are right.
121 input acceptance conditions are when:
122 * incoming previous-stage strobe (p.i_valid) is HIGH
123 * outgoing previous-stage ready (p.o_ready) is LOW
125 output transmission conditions are when:
126 * outgoing next-stage strobe (n.o_valid) is HIGH
127 * outgoing next-stage ready (n.i_ready) is LOW
129 the tricky bit is when the input has valid data and the output is not
130 ready to accept it. if it wasn't for the clock synchronisation, it
131 would be possible to tell the input "hey don't send that data, we're
132 not ready". unfortunately, it's not possible to "change the past":
133 the previous stage *has no choice* but to pass on its data.
135 therefore, the incoming data *must* be accepted - and stored: that
136 is the responsibility / contract that this stage *must* accept.
137 on the same clock, it's possible to tell the input that it must
138 not send any more data. this is the "stall" condition.
140 we now effectively have *two* possible pieces of data to "choose" from:
141 the buffered data, and the incoming data. the decision as to which
142 to process and output is based on whether we are in "stall" or not.
143 i.e. when the next stage is no longer ready, the output comes from
144 the buffer if a stall had previously occurred, otherwise it comes
145 direct from processing the input.
147 this allows us to respect a synchronous "travelling STB" with what
148 dan calls a "buffered handshake".
150 it's quite a complex state machine!
153 from nmigen
import Signal
, Cat
, Const
, Mux
, Module
, Value
154 from nmigen
.cli
import verilog
, rtlil
155 from nmigen
.hdl
.ast
import ArrayProxy
156 from nmigen
.hdl
.rec
import Record
, Layout
158 from abc
import ABCMeta
, abstractmethod
159 from collections
.abc
import Sequence
163 """ contains signals that come *from* the previous stage (both in and out)
164 * i_valid: previous stage indicating all incoming data is valid.
165 may be a multi-bit signal, where all bits are required
166 to be asserted to indicate "valid".
167 * o_ready: output to next stage indicating readiness to accept data
168 * i_data : an input - added by the user of this class
171 def __init__(self
, i_width
=1, stage_ctl
=False):
172 self
.stage_ctl
= stage_ctl
173 self
.i_valid
= Signal(i_width
, name
="p_i_valid") # prev >>in self
174 self
._o
_ready
= Signal(name
="p_o_ready") # prev <<out self
175 self
.i_data
= None # XXX MUST BE ADDED BY USER
177 self
.s_o_ready
= Signal(name
="p_s_o_rdy") # prev <<out self
181 """ public-facing API: indicates (externally) that stage is ready
184 return self
.s_o_ready
# set dynamically by stage
185 return self
._o
_ready
# return this when not under dynamic control
187 def _connect_in(self
, prev
):
188 """ internal helper function to connect stage to an input source.
189 do not use to connect stage-to-stage!
191 return [self
.i_valid
.eq(prev
.i_valid_test
),
192 prev
.o_ready
.eq(self
.o_ready
),
193 eq(self
.i_data
, prev
.i_data
),
197 def i_valid_test(self
):
198 vlen
= len(self
.i_valid
)
200 # multi-bit case: valid only when i_valid is all 1s
201 all1s
= Const(-1, (len(self
.i_valid
), False))
202 i_valid
= (self
.i_valid
== all1s
)
204 # single-bit i_valid case
205 i_valid
= self
.i_valid
207 # when stage indicates not ready, incoming data
208 # must "appear" to be not ready too
210 i_valid
= i_valid
& self
.s_o_ready
216 """ contains the signals that go *to* the next stage (both in and out)
217 * o_valid: output indicating to next stage that data is valid
218 * i_ready: input from next stage indicating that it can accept data
219 * o_data : an output - added by the user of this class
221 def __init__(self
, stage_ctl
=False):
222 self
.stage_ctl
= stage_ctl
223 self
.o_valid
= Signal(name
="n_o_valid") # self out>> next
224 self
.i_ready
= Signal(name
="n_i_ready") # self <<in next
225 self
.o_data
= None # XXX MUST BE ADDED BY USER
226 self
.d_valid
= Signal(reset
=1) # INTERNAL (data valid)
229 def i_ready_test(self
):
231 return self
.i_ready
& self
.d_valid
234 def connect_to_next(self
, nxt
):
235 """ helper function to connect to the next stage data/valid/ready.
236 data/valid is passed *TO* nxt, and ready comes *IN* from nxt.
237 use this when connecting stage-to-stage
239 return [nxt
.i_valid
.eq(self
.o_valid
),
240 self
.i_ready
.eq(nxt
.o_ready
),
241 eq(nxt
.i_data
, self
.o_data
),
244 def _connect_out(self
, nxt
):
245 """ internal helper function to connect stage to an output source.
246 do not use to connect stage-to-stage!
248 return [nxt
.o_valid
.eq(self
.o_valid
),
249 self
.i_ready
.eq(nxt
.i_ready_test
),
250 eq(nxt
.o_data
, self
.o_data
),
255 """ makes signals equal: a helper routine which identifies if it is being
256 passed a list (or tuple) of objects, or signals, or Records, and calls
257 the objects' eq function.
259 complex objects (classes) can be used: they must follow the
260 convention of having an eq member function, which takes the
261 responsibility of further calling eq and returning a list of
264 Record is a special (unusual, recursive) case, where the input may be
265 specified as a dictionary (which may contain further dictionaries,
266 recursively), where the field names of the dictionary must match
267 the Record's field spec. Alternatively, an object with the same
268 member names as the Record may be assigned: it does not have to
271 ArrayProxy is also special-cased, it's a bit messy: whilst ArrayProxy
272 has an eq function, the object being assigned to it (e.g. a python
273 object) might not. despite the *input* having an eq function,
274 that doesn't help us, because it's the *ArrayProxy* that's being
275 assigned to. so.... we cheat. use the ports() function of the
276 python object, enumerate them, find out the list of Signals that way,
280 if isinstance(o
, dict):
281 for (k
, v
) in o
.items():
282 print ("d-eq", v
, i
[k
])
283 res
.append(v
.eq(i
[k
]))
286 if not isinstance(o
, Sequence
):
288 for (ao
, ai
) in zip(o
, i
):
289 #print ("eq", ao, ai)
290 if isinstance(ao
, Record
):
291 for idx
, (field_name
, field_shape
, _
) in enumerate(ao
.layout
):
292 if isinstance(field_shape
, Layout
):
296 if hasattr(val
, field_name
): # check for attribute
297 val
= getattr(val
, field_name
)
299 val
= val
[field_name
] # dictionary-style specification
300 rres
= eq(ao
.fields
[field_name
], val
)
302 elif isinstance(ao
, ArrayProxy
) and not isinstance(ai
, Value
):
304 op
= getattr(ao
, p
.name
)
305 #print (op, p, p.name)
307 if not isinstance(rres
, Sequence
):
312 if not isinstance(rres
, Sequence
):
318 class StageCls(metaclass
=ABCMeta
):
319 """ Class-based "Stage" API. requires instantiation (after derivation)
321 see "Stage API" above.. Note: python does *not* require derivation
322 from this class. All that is required is that the pipelines *have*
323 the functions listed in this class. Derivation from this class
324 is therefore merely a "courtesy" to maintainers.
327 def ispec(self
): pass # REQUIRED
329 def ospec(self
): pass # REQUIRED
331 #def setup(self, m, i): pass # OPTIONAL
333 def process(self
, i
): pass # REQUIRED
336 class Stage(metaclass
=ABCMeta
):
337 """ Static "Stage" API. does not require instantiation (after derivation)
339 see "Stage API" above. Note: python does *not* require derivation
340 from this class. All that is required is that the pipelines *have*
341 the functions listed in this class. Derivation from this class
342 is therefore merely a "courtesy" to maintainers.
354 #def setup(m, i): pass
361 class RecordBasedStage(Stage
):
362 """ convenience class which provides a Records-based layout.
363 honestly it's a lot easier just to create a direct Records-based
364 class (see ExampleAddRecordStage)
366 def __init__(self
, in_shape
, out_shape
, processfn
, setupfn
=None):
367 self
.in_shape
= in_shape
368 self
.out_shape
= out_shape
369 self
.__process
= processfn
370 self
.__setup
= setupfn
371 def ispec(self
): return Record(self
.in_shape
)
372 def ospec(self
): return Record(self
.out_shape
)
373 def process(seif
, i
): return self
.__process
(i
)
374 def setup(seif
, m
, i
): return self
.__setup
(m
, i
)
377 class StageChain(StageCls
):
378 """ pass in a list of stages, and they will automatically be
379 chained together via their input and output specs into a
382 the end result basically conforms to the exact same Stage API.
384 * input to this class will be the input of the first stage
385 * output of first stage goes into input of second
386 * output of second goes into input into third (etc. etc.)
387 * the output of this class will be the output of the last stage
389 def __init__(self
, chain
, specallocate
=False):
391 self
.specallocate
= specallocate
394 return self
.chain
[0].ispec()
397 return self
.chain
[-1].ospec()
399 def setup(self
, m
, i
):
400 for (idx
, c
) in enumerate(self
.chain
):
401 if hasattr(c
, "setup"):
402 c
.setup(m
, i
) # stage may have some module stuff
403 if self
.specallocate
:
404 o
= self
.chain
[idx
].ospec() # last assignment survives
405 m
.d
.comb
+= eq(o
, c
.process(i
)) # process input into "o"
407 o
= c
.process(i
) # store input into "o"
408 if idx
!= len(self
.chain
)-1:
409 if self
.specallocate
:
410 ni
= self
.chain
[idx
+1].ispec() # new input on next loop
411 m
.d
.comb
+= eq(ni
, o
) # assign to next input
415 self
.o
= o
# last loop is the output
417 def process(self
, i
):
418 return self
.o
# conform to Stage API: return last-loop output
422 """ Common functions for Pipeline API
424 def __init__(self
, in_multi
=None, stage_ctl
=False):
425 """ Base class containing ready/valid/data to previous and next stages
427 * p: contains ready/valid to the previous stage
428 * n: contains ready/valid to the next stage
430 Except when calling Controlbase.connect(), user must also:
431 * add i_data member to PrevControl (p) and
432 * add o_data member to NextControl (n)
434 # set up input and output IO ACK (prev/next ready/valid)
435 self
.p
= PrevControl(in_multi
, stage_ctl
)
436 self
.n
= NextControl(stage_ctl
)
438 def connect_to_next(self
, nxt
):
439 """ helper function to connect to the next stage data/valid/ready.
441 return self
.n
.connect_to_next(nxt
.p
)
443 def _connect_in(self
, prev
):
444 """ internal helper function to connect stage to an input source.
445 do not use to connect stage-to-stage!
447 return self
.p
._connect
_in
(prev
.p
)
449 def _connect_out(self
, nxt
):
450 """ internal helper function to connect stage to an output source.
451 do not use to connect stage-to-stage!
453 return self
.n
._connect
_out
(nxt
.n
)
455 def connect(self
, pipechain
):
456 """ connects a chain (list) of Pipeline instances together and
457 links them to this ControlBase instance:
459 in <----> self <---> out
462 [pipe1, pipe2, pipe3, pipe4]
465 out---in out--in out---in
467 Also takes care of allocating i_data/o_data, by looking up
468 the data spec for each end of the pipechain. i.e It is NOT
469 necessary to allocate self.p.i_data or self.n.o_data manually:
470 this is handled AUTOMATICALLY, here.
472 Basically this function is the direct equivalent of StageChain,
473 except that unlike StageChain, the Pipeline logic is followed.
475 Just as StageChain presents an object that conforms to the
476 Stage API from a list of objects that also conform to the
477 Stage API, an object that calls this Pipeline connect function
478 has the exact same pipeline API as the list of pipline objects
481 Thus it becomes possible to build up larger chains recursively.
482 More complex chains (multi-input, multi-output) will have to be
485 eqs
= [] # collated list of assignment statements
487 # connect inter-chain
488 for i
in range(len(pipechain
)-1):
490 pipe2
= pipechain
[i
+1]
491 eqs
+= pipe1
.connect_to_next(pipe2
)
493 # connect front of chain to ourselves
495 self
.p
.i_data
= front
.stage
.ispec()
496 eqs
+= front
._connect
_in
(self
)
498 # connect end of chain to ourselves
500 self
.n
.o_data
= end
.stage
.ospec()
501 eqs
+= end
._connect
_out
(self
)
505 def set_input(self
, i
):
506 """ helper function to set the input data
508 return eq(self
.p
.i_data
, i
)
511 res
= [self
.p
.i_valid
, self
.n
.i_ready
,
512 self
.n
.o_valid
, self
.p
.o_ready
,
514 if hasattr(self
.p
.i_data
, "ports"):
515 res
+= self
.p
.i_data
.ports()
518 if hasattr(self
.n
.o_data
, "ports"):
519 res
+= self
.n
.o_data
.ports()
524 def _elaborate(self
, platform
):
525 """ handles case where stage has dynamic ready/valid functions
528 if not self
.p
.stage_ctl
:
531 # intercept the previous (outgoing) "ready", combine with stage ready
532 m
.d
.comb
+= self
.p
.s_o_ready
.eq(self
.p
._o
_ready
& self
.stage
.p_o_ready
)
534 # intercept the next (incoming) "ready" and combine it with data valid
535 m
.d
.comb
+= self
.n
.d_valid
.eq(self
.n
.i_ready
& self
.stage
.d_valid
)
540 class BufferedPipeline(ControlBase
):
541 """ buffered pipeline stage. data and strobe signals travel in sync.
542 if ever the input is ready and the output is not, processed data
543 is shunted in a temporary register.
545 Argument: stage. see Stage API above
547 stage-1 p.i_valid >>in stage n.o_valid out>> stage+1
548 stage-1 p.o_ready <<out stage n.i_ready <<in stage+1
549 stage-1 p.i_data >>in stage n.o_data out>> stage+1
555 input data p.i_data is read (only), is processed and goes into an
556 intermediate result store [process()]. this is updated combinatorially.
558 in a non-stall condition, the intermediate result will go into the
559 output (update_output). however if ever there is a stall, it goes
560 into r_data instead [update_buffer()].
562 when the non-stall condition is released, r_data is the first
563 to be transferred to the output [flush_buffer()], and the stall
566 on the next cycle (as long as stall is not raised again) the
567 input may begin to be processed and transferred directly to output.
570 def __init__(self
, stage
, stage_ctl
=False):
571 ControlBase
.__init
__(self
, stage_ctl
=stage_ctl
)
574 # set up the input and output data
575 self
.p
.i_data
= stage
.ispec() # input type
576 self
.n
.o_data
= stage
.ospec()
578 def elaborate(self
, platform
):
580 self
.m
= ControlBase
._elaborate
(self
, platform
)
582 result
= self
.stage
.ospec()
583 r_data
= self
.stage
.ospec()
584 if hasattr(self
.stage
, "setup"):
585 self
.stage
.setup(self
.m
, self
.p
.i_data
)
587 # establish some combinatorial temporaries
588 o_n_validn
= Signal(reset_less
=True)
589 i_p_valid_o_p_ready
= Signal(reset_less
=True)
590 p_i_valid
= Signal(reset_less
=True)
591 self
.m
.d
.comb
+= [p_i_valid
.eq(self
.p
.i_valid_test
),
592 o_n_validn
.eq(~self
.n
.o_valid
),
593 i_p_valid_o_p_ready
.eq(p_i_valid
& self
.p
.o_ready
),
596 # store result of processing in combinatorial temporary
597 self
.m
.d
.comb
+= eq(result
, self
.stage
.process(self
.p
.i_data
))
599 # if not in stall condition, update the temporary register
600 with self
.m
.If(self
.p
.o_ready
): # not stalled
601 self
.m
.d
.sync
+= eq(r_data
, result
) # update buffer
603 with self
.m
.If(self
.n
.i_ready_test
): # next stage is ready
604 with self
.m
.If(self
.p
._o
_ready
): # not stalled
605 # nothing in buffer: send (processed) input direct to output
606 self
.m
.d
.sync
+= [self
.n
.o_valid
.eq(p_i_valid
),
607 eq(self
.n
.o_data
, result
), # update output
609 with self
.m
.Else(): # p.o_ready is false, and something in buffer
610 # Flush the [already processed] buffer to the output port.
611 self
.m
.d
.sync
+= [self
.n
.o_valid
.eq(1), # reg empty
612 eq(self
.n
.o_data
, r_data
), # flush buffer
613 self
.p
._o
_ready
.eq(1), # clear stall
615 # ignore input, since p.o_ready is also false.
617 # (n.i_ready) is false here: next stage is ready
618 with self
.m
.Elif(o_n_validn
): # next stage being told "ready"
619 self
.m
.d
.sync
+= [self
.n
.o_valid
.eq(p_i_valid
),
620 self
.p
._o
_ready
.eq(1), # Keep the buffer empty
621 eq(self
.n
.o_data
, result
), # set output data
624 # (n.i_ready) false and (n.o_valid) true:
625 with self
.m
.Elif(i_p_valid_o_p_ready
):
626 # If next stage *is* ready, and not stalled yet, accept input
627 self
.m
.d
.sync
+= self
.p
._o
_ready
.eq(~
(p_i_valid
& self
.n
.o_valid
))
632 class UnbufferedPipeline(ControlBase
):
633 """ A simple pipeline stage with single-clock synchronisation
634 and two-way valid/ready synchronised signalling.
636 Note that a stall in one stage will result in the entire pipeline
639 Also that unlike BufferedPipeline, the valid/ready signalling does NOT
640 travel synchronously with the data: the valid/ready signalling
641 combines in a *combinatorial* fashion. Therefore, a long pipeline
642 chain will lengthen propagation delays.
644 Argument: stage. see Stage API, above
646 stage-1 p.i_valid >>in stage n.o_valid out>> stage+1
647 stage-1 p.o_ready <<out stage n.i_ready <<in stage+1
648 stage-1 p.i_data >>in stage n.o_data out>> stage+1
656 p.i_data : StageInput, shaped according to ispec
658 p.o_data : StageOutput, shaped according to ospec
660 r_data : input_shape according to ispec
661 A temporary (buffered) copy of a prior (valid) input.
662 This is HELD if the output is not ready. It is updated
664 result: output_shape according to ospec
665 The output of the combinatorial logic. it is updated
666 COMBINATORIALLY (no clock dependence).
669 def __init__(self
, stage
, stage_ctl
=False):
670 ControlBase
.__init
__(self
, stage_ctl
=stage_ctl
)
673 # set up the input and output data
674 self
.p
.i_data
= stage
.ispec() # input type
675 self
.n
.o_data
= stage
.ospec() # output type
677 def elaborate(self
, platform
):
678 self
.m
= ControlBase
._elaborate
(self
, platform
)
680 data_valid
= Signal() # is data valid or not
681 r_data
= self
.stage
.ispec() # input type
682 if hasattr(self
.stage
, "setup"):
683 self
.stage
.setup(self
.m
, r_data
)
686 p_i_valid
= Signal(reset_less
=True)
687 pv
= Signal(reset_less
=True)
688 self
.m
.d
.comb
+= p_i_valid
.eq(self
.p
.i_valid_test
)
689 self
.m
.d
.comb
+= pv
.eq(self
.p
.i_valid
& self
.p
.o_ready
)
691 self
.m
.d
.comb
+= self
.n
.o_valid
.eq(data_valid
)
692 self
.m
.d
.comb
+= self
.p
._o
_ready
.eq(~data_valid | self
.n
.i_ready_test
)
693 self
.m
.d
.sync
+= data_valid
.eq(p_i_valid | \
694 (~self
.n
.i_ready_test
& data_valid
))
696 self
.m
.d
.sync
+= eq(r_data
, self
.p
.i_data
)
697 self
.m
.d
.comb
+= eq(self
.n
.o_data
, self
.stage
.process(r_data
))
701 class PassThroughStage(StageCls
):
702 """ a pass-through stage which has its input data spec equal to its output,
703 and "passes through" its data from input to output.
705 def __init__(self
, iospecfn
):
706 self
.iospecfn
= iospecfn
707 def ispec(self
): return self
.iospecfn()
708 def ospec(self
): return self
.iospecfn()
709 def process(self
, i
): return i
712 class RegisterPipeline(UnbufferedPipeline
):
713 """ A pipeline stage that delays by one clock cycle, creating a
714 sync'd latch out of o_data and o_valid as an indirect byproduct
715 of using PassThroughStage
717 def __init__(self
, iospecfn
):
718 UnbufferedPipeline
.__init
__(self
, PassThroughStage(iospecfn
))