kbuild: Disable -Wpointer-to-enum-cast

commit 82f2bc2fcc0160d6f82dd1ac64518ae0a4dd183f upstream.

Clang's -Wpointer-to-int-cast deviates from GCC in that it warns when
casting to enums. The kernel does this in certain places, such as device
tree matches to set the version of the device being used, which allows
the kernel to avoid using a gigantic union.

https://elixir.bootlin.com/linux/v5.5.8/source/drivers/ata/ahci_brcm.c#L428
https://elixir.bootlin.com/linux/v5.5.8/source/drivers/ata/ahci_brcm.c#L402
https://elixir.bootlin.com/linux/v5.5.8/source/include/linux/mod_devicetable.h#L264

To avoid a ton of false positive warnings, disable this particular part
of the warning, which has been split off into a separate diagnostic so
that the entire warning does not need to be turned off for clang. It
will be visible under W=1 in case people want to go about fixing these
easily and enabling the warning treewide.

Cc: [email protected]
Link: ClangBuiltLinux/linux#887
Link: llvm/llvm-project@2a41b31
Signed-off-by: Nathan Chancellor <[email protected]>
Signed-off-by: Masahiro Yamada <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>

Author: nathanchance

Diff for: scripts/Makefile.extrawarn

@@ -72,5 +72,6 @@ KBUILD_CFLAGS += $(call cc-disable-warning, format)
 KBUILD_CFLAGS += $(call cc-disable-warning, sign-compare)
 KBUILD_CFLAGS += $(call cc-disable-warning, format-zero-length)
 KBUILD_CFLAGS += $(call cc-disable-warning, uninitialized)
+KBUILD_CFLAGS += $(call cc-disable-warning, pointer-to-enum-cast)
 endif
 endif