kbuild: refactor scripts/Makefile.extrawarn

Instead of the warning-[123] magic, let's accumulate compiler options
to KBUILD_CFLAGS directly as the top Makefile does. I think this makes
it easier to understand what is going on in this file.

This commit slightly changes the behavior, I think all of which are OK.

[1] Currently, cc-option calls are needlessly evaluated. For example,
      warning-3 += $(call cc-option, -Wpacked-bitfield-compat)
    needs evaluating only when W=3, but it is actually evaluated for
    W=1, W=2 as well. With this commit, only relevant cc-option calls
    will be evaluated. This is a slight optimization.

[2] Currently, unsupported level like W=4 is checked by:
      $(error W=$(KBUILD_ENABLE_EXTRA_GCC_CHECKS) is unknown)
    This will no longer be checked, but I do not think it is a big
    deal.

[3] Currently, 4 Clang warnings (Winitializer-overrides, Wformat,
    Wsign-compare, Wformat-zero-length) are shown by any of W=1, W=2,
    and W=3. With this commit, they will be warned only by W=1. I
    think this is a more correct behavior since each warning belongs
    to only one group.

For understanding this commit correctly:

We have 3 warning groups, W=1, W=2, and W=3. You may think W=3 has a
higher level than W=1, but they are actually independent. If you like,
you can combine them like W=13. To enable all the warnings, you can
pass W=123. It is shown by 'make help', but not noticed much. Since we
support W= combination, there should not exist intersection among the
three groups. If we enable Winitializer-overrides for W=1, we do not
need to for W=2 or W=3. This is the reason why I think the change [3]
makes sense.

The documentation says -Winitializer-overrides is enabled by default.
(https://clang.llvm.org/docs/DiagnosticsReference.html#winitializer-overrides)
We negate it by passing -Wno-initializer-overrides for the normal
build, but we do not do that for W=1. This means, W=1 effectively
enables -Winitializer-overrides by the clang's default. The same for
the other three.

Add comments in case people are confused with the code.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Acked-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>

scripts/Makefile.extrawarn

 # SPDX-License-Identifier: GPL-2.0
 # ==========================================================================
-#
 # make W=... settings
 #
-# W=1 - warnings that may be relevant and does not occur too often
-# W=2 - warnings that occur quite often but may still be relevant
-# W=3 - the more obscure warnings, can most likely be ignored
-#
-# $(call cc-option, -W...) handles gcc -W.. options which
-# are not supported by all versions of the compiler
+# There are three warning groups enabled by W=1, W=2, W=3.
+# They are independent, and can be combined like W=12 or W=123.
 # ==========================================================================
 
 KBUILD_CFLAGS += $(call cc-disable-warning, packed-not-aligned)
@@ -17,58 +12,69 @@ ifeq ("$(origin W)", "command line")
   export KBUILD_ENABLE_EXTRA_GCC_CHECKS := $(W)
 endif
 
-ifdef KBUILD_ENABLE_EXTRA_GCC_CHECKS
-warning-  := $(empty)
+#
+# W=1 - warnings which may be relevant and do not occur too often
+#
+ifneq ($(findstring 1, $(KBUILD_ENABLE_EXTRA_GCC_CHECKS)),)
 
-warning-1 := -Wextra -Wunused -Wno-unused-parameter
-warning-1 += -Wmissing-declarations
-warning-1 += -Wmissing-format-attribute
-warning-1 += -Wmissing-prototypes
-warning-1 += -Wold-style-definition
-warning-1 += -Wmissing-include-dirs
-warning-1 += $(call cc-option, -Wunused-but-set-variable)
-warning-1 += $(call cc-option, -Wunused-const-variable)
-warning-1 += $(call cc-option, -Wpacked-not-aligned)
-warning-1 += $(call cc-option, -Wstringop-truncation)
+KBUILD_CFLAGS += -Wextra -Wunused -Wno-unused-parameter
+KBUILD_CFLAGS += -Wmissing-declarations
+KBUILD_CFLAGS += -Wmissing-format-attribute
+KBUILD_CFLAGS += -Wmissing-prototypes
+KBUILD_CFLAGS += -Wold-style-definition
+KBUILD_CFLAGS += -Wmissing-include-dirs
+KBUILD_CFLAGS += $(call cc-option, -Wunused-but-set-variable)
+KBUILD_CFLAGS += $(call cc-option, -Wunused-const-variable)
+KBUILD_CFLAGS += $(call cc-option, -Wpacked-not-aligned)
+KBUILD_CFLAGS += $(call cc-option, -Wstringop-truncation)
 # The following turn off the warnings enabled by -Wextra
-warning-1 += -Wno-missing-field-initializers
-warning-1 += -Wno-sign-compare
-
-warning-2 += -Wcast-align
-warning-2 += -Wdisabled-optimization
-warning-2 += -Wnested-externs
-warning-2 += -Wshadow
-warning-2 += $(call cc-option, -Wlogical-op)
-warning-2 += -Wmissing-field-initializers
-warning-2 += -Wsign-compare
-warning-2 += $(call cc-option, -Wmaybe-uninitialized)
-warning-2 += $(call cc-option, -Wunused-macros)
-
-warning-3 := -Wbad-function-cast
-warning-3 += -Wcast-qual
-warning-3 += -Wconversion
-warning-3 += -Wpacked
-warning-3 += -Wpadded
-warning-3 += -Wpointer-arith
-warning-3 += -Wredundant-decls
-warning-3 += -Wswitch-default
-warning-3 += $(call cc-option, -Wpacked-bitfield-compat)
-
-warning := $(warning-$(findstring 1, $(KBUILD_ENABLE_EXTRA_GCC_CHECKS)))
-warning += $(warning-$(findstring 2, $(KBUILD_ENABLE_EXTRA_GCC_CHECKS)))
-warning += $(warning-$(findstring 3, $(KBUILD_ENABLE_EXTRA_GCC_CHECKS)))
-
-ifeq ("$(strip $(warning))","")
-        $(error W=$(KBUILD_ENABLE_EXTRA_GCC_CHECKS) is unknown)
-endif
+KBUILD_CFLAGS += -Wno-missing-field-initializers
+KBUILD_CFLAGS += -Wno-sign-compare
 
-KBUILD_CFLAGS += $(warning)
 else
 
+# Some diagnostics enabled by default are noisy.
+# Suppress them by using -Wno... except for W=1.
+
 ifdef CONFIG_CC_IS_CLANG
 KBUILD_CFLAGS += -Wno-initializer-overrides
 KBUILD_CFLAGS += -Wno-format
 KBUILD_CFLAGS += -Wno-sign-compare
 KBUILD_CFLAGS += -Wno-format-zero-length
 endif
+
+endif
+
+#
+# W=2 - warnings which occur quite often but may still be relevant
+#
+ifneq ($(findstring 2, $(KBUILD_ENABLE_EXTRA_GCC_CHECKS)),)
+
+KBUILD_CFLAGS += -Wcast-align
+KBUILD_CFLAGS += -Wdisabled-optimization
+KBUILD_CFLAGS += -Wnested-externs
+KBUILD_CFLAGS += -Wshadow
+KBUILD_CFLAGS += $(call cc-option, -Wlogical-op)
+KBUILD_CFLAGS += -Wmissing-field-initializers
+KBUILD_CFLAGS += -Wsign-compare
+KBUILD_CFLAGS += $(call cc-option, -Wmaybe-uninitialized)
+KBUILD_CFLAGS += $(call cc-option, -Wunused-macros)
+
+endif
+
+#
+# W=3 - more obscure warnings, can most likely be ignored
+#
+ifneq ($(findstring 3, $(KBUILD_ENABLE_EXTRA_GCC_CHECKS)),)
+
+KBUILD_CFLAGS += -Wbad-function-cast
+KBUILD_CFLAGS += -Wcast-qual
+KBUILD_CFLAGS += -Wconversion
+KBUILD_CFLAGS += -Wpacked
+KBUILD_CFLAGS += -Wpadded
+KBUILD_CFLAGS += -Wpointer-arith
+KBUILD_CFLAGS += -Wredundant-decls
+KBUILD_CFLAGS += -Wswitch-default
+KBUILD_CFLAGS += $(call cc-option, -Wpacked-bitfield-compat)
+
 endif