seemingly reasonable stopping point

bs/TagEngine.bs

@@ -5,40 +5,41 @@ package TagEngine(
 
 import Vector
 import Util
+import FIFO
+import FIFOF
 
 #define UIntLog2N(n) (UInt (TLog n))
 
 interface (TagEngine :: # -> *) numTags =
-    requestTag :: ActionValue (Maybe UIntLog2N(numTags))
-    retireTag  :: UIntLog2N(numTags) -> ActionValue BasicResult
+    requestTag :: ActionValue UIntLog2N(numTags)
+    retireTag  :: UIntLog2N(numTags) -> Action
 
 -- The tag engine returns a tag that is unique for the duration of
 -- the lifetime of the tag. Useful when you need to tag transactions
 -- on a bus for example.
--- This implementation is stack based.
+-- This implementation is FIFO based.
 mkTagEngine :: Module (TagEngine numTags)
 mkTagEngine =
     do
-
         let reifiedNumTags = fromInteger |> valueOf numTags
 
-        freeStackVec :: Vector numTags (Reg UIntLog2N(numTags))
-        freeStackVec <- mapM (\i -> mkReg |> fromInteger i) genVector
-
         inUseVec :: Vector numTags (Reg Bool)
         inUseVec <- replicateM |> mkReg False
 
-        stackPtr :: (Reg (Maybe(UIntLog2N(numTags))))
-        stackPtr <- mkReg |> Just |> reifiedNumTags - 1
+        -- Since Bluespec doesn't allow us to initialize FIFOs with values at
+        -- reset, we can pretend as if the buffer within our tagFIFO is populated
+        -- with sequentially incrementing values(starting from 0) on reset
+        -- by having our tag engine effectively return the value of a decrementing
+        -- counter initialized to (numTags - 1) for the first n tag requests made 
+        -- to TagEngine where `n := numTags`.
+        resetTagCounter :: (Reg (Maybe UIntLog2N(numTags)))
+        resetTagCounter <- mkReg |> Just |> reifiedNumTags - 1
 
-        methodRequestTagCalled :: PulseWire
-        methodRequestTagCalled <- mkPulseWire
+        retireQueue ::  FIFOF UIntLog2N(numTags)
+        retireQueue <- mkSizedFIFOF reifiedNumTags
 
-        methodRetireTagCalledValid :: RWire UIntLog2N(numTags)
-        methodRetireTagCalledValid <- mkUnsafeRWire
-
-        -- computedTagResult :: BypassWire (Maybe UIntLog2N(numTags))
-        -- methodRetireTagCalledValid <- mkBypassWire
+        freeQueue ::  FIFOF UIntLog2N(numTags)
+        freeQueue <- mkSizedFIFOF reifiedNumTags
 
         debugOnce <- mkReg True
 
@@ -46,71 +47,46 @@ mkTagEngine =
             rules
                 "display": when (debugOnce == True) ==>
                     do
-                        $display "freeStackVec : " (fshow |> readVReg freeStackVec)
                         $display "inUseVec : " (fshow |> readVReg inUseVec)
-                        $display "stackPtr : " (fshow stackPtr)
                         debugOnce := False
-
-                "update stack pointer": when True ==>
+            
+                -- The "retire_tags" rule can't fire when the `requestTag` method is called.
+                -- In practice, this is Ok since:
+                --    1. the `requestTag` method should take priority over the "retire_tags" rule
+                --    2. the `requestTag` method handles the case where there are some tags to retire.
+                "retire_tags": when True ==>
                     do
-                        stackPtr :=
-                            case (methodRequestTagCalled, methodRetireTagCalledValid.wget) of
-                                (True, Just _) -> stackPtr
-                                (True, Nothing) ->
-                                    case stackPtr of
-                                        Just 0 -> Nothing
-                                        Just sampledStackPtr -> Just |> sampledStackPtr - 1
-                                        Nothing -> Nothing
-                                (False, Just _) ->
-                                    case stackPtr of
-                                        Just sampledStackPtr -> Just |> sampledStackPtr + 1
-                                        Nothing -> Nothing
-                                (False, Nothing) -> stackPtr
-
-                "update free stack": when True ==>
-                    do
-                        case (methodRequestTagCalled, methodRetireTagCalledValid.wget) of
-                            (True, Just _) -> do action {}
-                            (True, Nothing) -> do action {}
-                            (False, Just tag) -> do
-                                case stackPtr of
-                                    Just sampledStackPtr -> do
-                                        (select freeStackVec (sampledStackPtr + 1)) := tag
-                                    Nothing -> do
-                                        (select freeStackVec 0) := tag
-                            (False, Nothing) -> do action {}
-
-                "update in use": when True ==>
-                    do
-                        case (methodRequestTagCalled, methodRetireTagCalledValid.wget) of
-                            (True, Just _) -> do action {}
-                            (True, Nothing) ->
-                                case stackPtr of
-                                    Just sampledStackPtr -> do
-                                        (select inUseVec sampledStackPtr) := True
-                                    Nothing -> do action {}
-                            (False, Just tag) -> do
-                                (select inUseVec tag) := False
-                            (False, Nothing) -> do action {}
+                        let tag = retireQueue.first
+                        $display "firing retire_tags" (fshow tag)
+                        retireQueue.deq
+                        freeQueue.enq tag
+                        (select inUseVec tag) := False
 
         return $
           interface TagEngine
 
-            requestTag :: ActionValue (Maybe UIntLog2N(numTags))
+            requestTag :: ActionValue UIntLog2N(numTags)
             requestTag =
                 do
-                    methodRequestTagCalled.send
-                    case methodRetireTagCalledValid.wget of
+                    case resetTagCounter of
+                        Just 0 -> do
+                            resetTagCounter := Nothing
+                            (select inUseVec 0) := True
+                            return 0
                         Just tag -> do
-                            return |> Just tag
+                            resetTagCounter := Just |> tag - 1
+                            (select inUseVec tag) := True
+                            return tag
                         Nothing -> do
-                            return |>
-                                case stackPtr of
-                                    Just sampledStackPtr ->
-                                        Just |> readReg (select freeStackVec sampledStackPtr)
-                                    Nothing -> Nothing
+                            let tag = freeQueue.first
+                            freeQueue.deq
+                            return tag
 
-            retireTag :: UIntLog2N(numTags) -> ActionValue BasicResult
+            -- `retireTag` isn't guarded on tag validity(this would break Bluespec's safety model) 
+            -- so it is advisable that the caller of `retireTag` only attempt to retire valid tags.
+            -- Internally, the tagEngine will keep a correct and consistent state since TagEngine
+            -- validates tags before attempting to retire them.
+            retireTag :: UIntLog2N(numTags) -> Action
             retireTag tag =
                 do
                     let
@@ -118,7 +94,6 @@ mkTagEngine =
                         tagInUse = readReg (select inUseVec tag)
                     if (tagValid && tagInUse)
                         then do
-                            methodRetireTagCalledValid.wset tag
-                            return Success
+                            retireQueue.enq tag
                         else do
-                            return Failure
+                            action {}

bs/Tests/TagEngineTester.bs

@@ -44,6 +44,7 @@ mkTagEngineTester = do
       |> do requestTagAction
       |> do retireTagAction 1
       |> do requestTagAction
+      |> do $finish
 
   addRules $
     rules