Fix instruction cache refilling bug (kuznia-rdzeni/coreblocks#636)

transactron/lib/fifo.py

@@ -13,6 +13,9 @@ class BasicFifo(Elaboratable):
     read: Method
         Reads from the FIFO. Accepts an empty argument, returns a structure.
         Ready only if the FIFO is not empty.
+    peek: Method
+        Returns the element at the front (but not delete). Ready only if the FIFO
+        is not empty. The method is nonexclusive.
     write: Method
         Writes to the FIFO. Accepts a structure, returns empty result.
         Ready only if the FIFO is not full.
@@ -40,6 +43,7 @@ def __init__(self, layout: MethodLayout, depth: int, *, src_loc: int | SrcLoc =
 
         src_loc = get_src_loc(src_loc)
         self.read = Method(o=self.layout, src_loc=src_loc)
+        self.peek = Method(o=self.layout, nonexclusive=True, src_loc=src_loc)
         self.write = Method(i=self.layout, src_loc=src_loc)
         self.clear = Method(src_loc=src_loc)
         self.head = Signal(from_method_layout(layout))
@@ -93,6 +97,10 @@ def _() -> ValueLike:
             m.d.sync += self.read_idx.eq(next_read_idx)
             return self.head
 
+        @def_method(m, self.peek, self.read_ready)
+        def _() -> ValueLike:
+            return self.head
+
         @def_method(m, self.clear)
         def _() -> None:
             m.d.sync += self.read_idx.eq(0)

transactron/lib/reqres.py

@@ -1,7 +1,7 @@
 from amaranth import *
 from ..core import *
 from ..utils import SrcLoc, get_src_loc, MethodLayout
-from .connectors import Forwarder, FIFO
+from .connectors import Forwarder
 from transactron.lib import BasicFifo
 from amaranth.utils import *
 
@@ -39,6 +39,8 @@ class ArgumentsToResultsZipper(Elaboratable):
 
     Attributes
     ----------
+    peek_arg: Method
+        A nonexclusive method to read (but not delete) the head of the arg queue.
     write_args: Method
         Method to write arguments with `args_layout` format to 2-FIFO.
     write_results: Method
@@ -65,14 +67,15 @@ def __init__(self, args_layout: MethodLayout, results_layout: MethodLayout, src_
         self.args_layout = args_layout
         self.output_layout = [("args", self.args_layout), ("results", results_layout)]
 
+        self.peek_arg = Method(o=self.args_layout, nonexclusive=True, src_loc=self.src_loc)
         self.write_args = Method(i=self.args_layout, src_loc=self.src_loc)
         self.write_results = Method(i=self.results_layout, src_loc=self.src_loc)
         self.read = Method(o=self.output_layout, src_loc=self.src_loc)
 
     def elaborate(self, platform):
         m = TModule()
 
-        fifo = FIFO(self.args_layout, depth=2, src_loc=self.src_loc)
+        fifo = BasicFifo(self.args_layout, depth=2, src_loc=self.src_loc)
         forwarder = Forwarder(self.results_layout, src_loc=self.src_loc)
 
         m.submodules.fifo = fifo
@@ -92,6 +95,8 @@ def _():
             results = forwarder.read(m)
             return {"args": args, "results": results}
 
+        self.peek_arg.proxy(m, fifo.peek)
+
         return m