`roundrobinCollect Parallel` is not a round-robin arbiter, but a priority arbiter.

[christiaanb]

Looking at:

clash-protocols/clash-protocols/src/Protocols/Df.hs

Lines 857 to 870 in ec001d4

	roundrobinCollect Parallel =
	Circuit (B.first C.unbundle . C.unbundle . fmap go . C.bundle . B.first C.bundle)
	where
	go (dats0, ack) = (acks, dat1)
	where
	nacks = C.repeat (Ack False)
	acks = Maybe.fromMaybe nacks ((\i -> C.replace i ack nacks) <$> iM)
	dat1 = Maybe.fromMaybe NoData dat0
	(iM, dat0) = Data.List.NonEmpty.unzip dats1
	dats1 = C.fold @(n C.- 1) (<|>) (C.zipWith goDat C.indicesI dats0)
	goDat i dat
	| Maybe.isJust (dataToMaybe dat) = Just (i, dat)
	| otherwise = Nothing

it doesn't actually have the roundrobin properties that I would expect from an roundrobin arbiter: that regardless of the conditions, all inputs will get a chance to propagate. Instead, it's a priority arbiter which, from left-to-right, gives right the highest priority. I think the functions should be renamed to arbiter and highlight how the different scheduling "collect" strategies affect the arbitration.

[rowanG077]

Yes I agree, the PacketStream version is also called arbiter:

clash-protocols/clash-protocols/src/Protocols/PacketStream/Routing.hs

Lines 27 to 59 in ec001d4

	packetArbiterC ::
	forall dataWidth sources meta dom.
	(HiddenClockResetEnable dom) =>
	(KnownNat sources) =>
	(1 <= sources) =>
	-- | Determines the mode of arbitration. See `Df.CollectMode`
	Df.CollectMode ->
	Circuit
	(Vec sources (PacketStream dom dataWidth meta))
	(PacketStream dom dataWidth meta)
	packetArbiterC mode =
	Circuit (B.first unbundle . mealyB go (maxBound, True) . B.first bundle)
	where
	go (i, first) (fwds, bwd@(PacketStreamS2M ack)) = ((i', continue), (bwds, fwd))
	where
	bwds = replace i bwd (repeat (PacketStreamS2M False))
	fwd = fwds !! i
	-- We may only switch sources if we are not currently in the middle
	-- of forwarding a packet.
	continue = case (fwd, mode) of
	(Nothing, Df.NoSkip) -> False
	(Nothing, _) -> first
	(Just transferIn, _) -> isJust (_last transferIn) && ack
	i' = case (mode, continue) of
	(_, False) -> i
	(Df.NoSkip, _) -> satSucc SatWrap i
	(Df.Skip, _) -> satSucc SatWrap i
	(Df.Parallel, _) ->
	-- Index of last sink with data
	fromMaybe maxBound
	$ fold @(sources - 1) (<|>) (zipWith (<$) indicesI fwds)