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

[Zeng Heng <zengheng4@xxxxxxxxxx>]

Enable implicit enum-conversion warning option in kernel gcc build.
When it set enabled, it can detect implicit enum type conversion
and locate 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 in
compile runtime and call attention on potential mistakes, which is
firstly introduced from GNU gcc-10.

Although "-Wenum-conversion" could be enabled by "-Wextra"
when compiling with 'W=[123]' option, there are many warnings
generated by '-Wextra' that cause too much noise in the build.

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

Therefore, "-Wenum-conversion" warning option needs to be
explicitly requested for GCC when compilation process is only
companied with '-Wall'.

With clang, -Wenum-conversion is just default enabled, not even
behind -Wall.

Provide a couple examples for reference as below:

$ cat test.c
enum enum1 { A = 1 };
enum enum2 { B = 2 };

enum enum1 foo(enum enum2 bar)
{
    return bar;
}

$ gcc -Wall -fsyntax-only test.c

$ gcc -Wall -Wenum-conversion -fsyntax-only test.c
test.c: In function ‘foo’:
test.c:6:9: warning: implicit conversion from ‘enum enum2’ to ‘enum enum1’ [-Wenum-conversion]
    6 |  return bar;
      |         ^~~

$ gcc -Wextra -fsyntax-only test.c
test.c: In function ‘foo’:
test.c:6:9: warning: implicit conversion from ‘enum enum2’ to ‘enum enum1’ [-Wenum-conversion]
    6 |  return bar;
      |         ^~~

$ clang -fsyntax-only test.c
test.c:6:9: warning: implicit conversion from enumeration type 'enum enum2' to different enumeration type 'enum enum1' [-Wenum-conversion]
        return bar;
        ~~~~~~ ^~~
1 warning generated.

Signed-off-by: Zeng Heng <zengheng4@xxxxxxxxxx>
Suggested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Suggested-by: Nathan Chancellor <nathan@xxxxxxxxxx>
---
 Makefile | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/Makefile b/Makefile
index 8f6ed52fa08f..72103d22df23 100644
--- a/Makefile
+++ b/Makefile
@@ -880,6 +880,10 @@ endif
 KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
 KBUILD_CFLAGS += $(call cc-disable-warning, unused-const-variable)
 
+# implicit enum conversion checking is supported since from gcc-10
+# this warning option has to be explicitly requested for GCC
+KBUILD_CFLAGS += $(call cc-option, -Wenum-conversion)
+
 # These result in bogus false positives
 KBUILD_CFLAGS += $(call cc-disable-warning, dangling-pointer)
 
-- 
2.25.1