[PATCH -next] Makefile: add implicit enum-conversion check for compile build

[Zeng Heng]

Provide implicit enum-conversion warning option
in general build. When it set enabled, it can
detect implicit enum type conversion and find
potential conversion errors like below
(use "allmodconfig"):

drivers/gpu/drm/amd/amdgpu/../display/dc/dml/dcn20/display_mode_vba_20.c:3904:46:
error: implicit conversion from ‘enum <anonymous>’ to ‘enum odm_combine_mode’ [-Werror=enum-conversion]
 3904 |       locals->ODMCombineEnablePerState[i][k] = true;
      |                                              ^

The '-Wenum-conversion' could be regarded as
effective check on compile runtime and
call attention on potential mistakes.

Anothor practical example could be referred to:
https://lore.kernel.org/all/CADnq5_OE0yZvEYGu82QJHL9wvVcTFZrmeTgX7URgh7FVA=jqYg@xxxxxxxxxxxxxx

"-Wenum-conversion" was firstly introduced from
GNU gcc-10.

Although "-Wenum-conversion" could be enabled
by "-Wextra" when compiling with 'W=1' option,
there are many warnings generated by '-Wextra'
that cause too much noise in a build.

Seeing the details from the following link:
https://gcc.gnu.org/onlinedocs/gcc-11.3.0/gcc/Warning-Options.html

Because there are still some concerned warnings
exist, the patch marks the option disabled in default
for avoiding compile failed like using "allmodconfig".

Signed-off-by: Zeng Heng <zengheng4@xxxxxxxxxx>
---
 Makefile          | 5 +++++
 lib/Kconfig.debug | 7 +++++++
 2 files changed, 12 insertions(+)

diff --git a/Makefile b/Makefile
index ebd48fc956a3..1790a3624358 100644
--- a/Makefile
+++ b/Makefile
@@ -880,6 +880,11 @@ endif
 KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
 KBUILD_CFLAGS += $(call cc-disable-warning, unused-const-variable)
 
+# check implicit enum conversion
+ifdef CONFIG_ENUM_CONVERSION
+KBUILD_CFLAGS += -Wenum-conversion
+endif
+
 # These result in bogus false positives
 KBUILD_CFLAGS += $(call cc-disable-warning, dangling-pointer)
 
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 4f2b81229a2f..a64e06a747d8 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -417,6 +417,13 @@ config FRAME_WARN
 	  Setting this too low will cause a lot of warnings.
 	  Setting it to 0 disables the warning.
 
+config ENUM_CONVERSION
+	bool "Warn for implicit enum conversion"
+	depends on GCC_VERSION >= 100300
+	default n
+	help
+	  Tell gcc to warn at build time for implicit enum conversion.
+
 config STRIP_ASM_SYMS
 	bool "Strip assembler-generated symbols during link"
 	default n
-- 
2.25.1