103 lines
4.2 KiB
Haskell
103 lines
4.2 KiB
Haskell
package TagEngine(
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TagEngine(..),
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Util.BasicResult(..),
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mkTagEngine) where
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import Vector
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import Util
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import FIFO
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import FIFOF
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#define UIntLog2N(n) (UInt (TLog n))
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interface (TagEngine :: # -> *) numTags =
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requestTag :: ActionValue UIntLog2N(numTags)
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retireTag :: UIntLog2N(numTags) -> Action
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-- The tag engine returns a tag that is unique for the duration of
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-- the lifetime of the tag. Useful when you need to tag transactions
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-- on a bus for example.
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-- This implementation is FIFO based.
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mkTagEngine :: Module (TagEngine numTags)
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mkTagEngine =
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do
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let reifiedNumTags = fromInteger |> valueOf numTags
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-- we really only use inUseVec after emptying out our `resetTagCounter` buffer
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-- perhaps rename this variable to better communicate that
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inUseVec :: Vector numTags (Reg Bool)
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inUseVec <- replicateM |> mkReg True
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-- Since Bluespec doesn't allow us to initialize FIFOs with values at
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-- reset, we can pretend as if the buffer within our tagFIFO is populated
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-- with sequentially incrementing values(starting from 0) on reset
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-- by having our tag engine effectively return the value of a decrementing
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-- counter initialized to (numTags - 1) for the first n tag requests made
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-- to TagEngine where `n := numTags`.
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-- Maybe resetTagCounter isn't the greatest name?
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resetTagCounter :: (Reg (Maybe UIntLog2N(numTags)))
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resetTagCounter <- mkReg |> Just |> reifiedNumTags - 1
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retireTag :: Wire UIntLog2N(numTags)
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retireTag <- mkWire
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freeQueue :: FIFOF UIntLog2N(numTags)
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freeQueue <- mkSizedFIFOF reifiedNumTags
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debugOnce <- mkReg True
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addRules $
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rules
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"display": when (debugOnce == True) ==>
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do
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$display "inUseVec : " (fshow |> readVReg inUseVec)
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debugOnce := False
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-- The "retire_tags" rule can't fire when the `requestTag` method is called.
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-- In practice, this is Ok since:
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-- 1. the `requestTag` method should take priority over the "retire_tags" rule
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-- 2. the `requestTag` method handles the case where there are some tags to retire.
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"retire_tags": when True ==>
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do
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$display "firing retire_tags" (fshow retireTag)
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freeQueue.enq retireTag
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(select inUseVec retireTag) := False
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return $
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interface TagEngine
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requestTag :: ActionValue UIntLog2N(numTags)
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requestTag =
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do
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case resetTagCounter of
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Just 0 -> do
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resetTagCounter := Nothing
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return 0
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Just tag -> do
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resetTagCounter := Just |> tag - 1
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return tag
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Nothing -> do
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let tag = freeQueue.first
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freeQueue.deq
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return tag
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-- `retireTag` isn't guarded on tag validity(this would break Bluespec's safety model)
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-- so it is advisable that the caller of `retireTag` only attempt to retire valid tags.
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-- Internally, the tagEngine will keep a correct and consistent state since TagEngine
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-- validates tags before attempting to retire them.
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-- Also worth noting the TagEngin will ignore attempts to retire a tag you just
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-- requested in the same cycle. This is enforce with `tag > tagCounter` below.
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retireTag :: UIntLog2N(numTags) -> Action
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retireTag tag =
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do
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let
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tagValid = tag < reifiedNumTags
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tagInUse = case resetTagCounter of
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Just tagCounter -> tag > tagCounter
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Nothing -> readReg (select inUseVec tag)
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if (tagValid && tagInUse)
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then do
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retireTag := tag
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else do
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action {}
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