Remove CUSTOM_FIRRTL_PASS support, SFC passes are discouraged

common.mk

@@ -16,7 +16,6 @@ HELP_COMPILATION_VARIABLES += \
 "   EXTRA_SIM_LDFLAGS         = additional LDFLAGS for building simulators" \
 "   EXTRA_SIM_SOURCES         = additional simulation sources needed for simulator" \
 "   EXTRA_SIM_REQS            = additional make requirements to build the simulator" \
-"   ENABLE_CUSTOM_FIRRTL_PASS = if set, enable custom firrtl passes (SFC lowers to LowFIRRTL & MFC converts to Verilog)" \
 "   ENABLE_YOSYS_FLOW         = if set, add compilation flags to enable the vlsi flow for yosys(tutorial flow)" \
 "   EXTRA_CHISEL_OPTIONS      = additional options to pass to the Chisel compiler" \
 "   EXTRA_BASE_FIRRTL_OPTIONS = additional options to pass to the Scala FIRRTL compiler" \
@@ -200,20 +199,13 @@ MFC_BASE_LOWERING_OPTIONS ?= emittedLineLength=2048,noAlwaysComb,disallowLocalVa
 # compiles Chisel to CHIRRTL, and MFC compiles CHIRRTL to Verilog. Otherwise,
 # when custom FIRRTL transforms are included
 # SFC compiles Chisel to LowFIRRTL and MFC compiles it to Verilog.
-# Users can indicate to the Makefile of custom FIRRTL transforms by setting the
-# "ENABLE_CUSTOM_FIRRTL_PASS" variable.
 #
 # hack: when using dontTouch, io.cpu annotations are not removed by SFC,
 # hence we remove them manually by using jq before passing them to firtool
 
 $(SFC_LEVEL) $(EXTRA_FIRRTL_OPTIONS) &: $(FIRRTL_FILE)
-ifeq (,$(ENABLE_CUSTOM_FIRRTL_PASS))
 	echo none > $(SFC_LEVEL)
 	echo "$(EXTRA_BASE_FIRRTL_OPTIONS)" > $(EXTRA_FIRRTL_OPTIONS)
-else
-	echo low > $(SFC_LEVEL)
-	echo "$(EXTRA_BASE_FIRRTL_OPTIONS)" "$(SFC_REPL_SEQ_MEM)" > $(EXTRA_FIRRTL_OPTIONS)
-endif
 
 $(MFC_LOWERING_OPTIONS):
 	mkdir -p $(dir $@)

docs/Customization/Firrtl-Transforms.rst

@@ -12,8 +12,7 @@ The Scala FIRRTL Compiler and the MLIR FIRRTL Compiler
 ------------------------------------------------------
 In Chipyard, two FIRRTL compilers work together to compile Chisel into Verilog. The Scala FIRRTL compiler (SFC) and the MLIR FIRRTL compiler (MFC).
 They are basically doing the same thing, except that MFC is written in C++ which makes compilation much faster (the generated Verilog will be different). In the default setting, the SFC will compile Chisel into CHIRRTL and MFC will
-compile CHIRRTL into Verilog. By setting the ``ENABLE_CUSTOM_FIRRTL_PASS`` env variable to a non-zero value,
-we can make the SFC compile Chisel into LowFIRRTL so that our custom FIRRTL passes are applied.
+compile CHIRRTL into Verilog.
 
 For more information on MLIR FIRRTL Compiler, please visit https://mlir.llvm.org/ and https://circt.llvm.org/.