Clang enables -Wenum-enum-conversion and -Wenum-compare-conditional
under -Wenum-conversion. A recent change in Clang strengthened these
warnings and they appear frequently in common builds, primarily due to
several instances in common headers but there are quite a few drivers
that have individual instances as well.

  include/linux/vmstat.h:508:43: warning: arithmetic between different enumeration types ('enum zone_stat_item' and 'enum numa_stat_item') [-Wenum-enum-conversion]
    508 |         return vmstat_text[NR_VM_ZONE_STAT_ITEMS +
        |                            ~~~~~~~~~~~~~~~~~~~~~ ^
    509 |                            item];
        |                            ~~~~

  drivers/net/wireless/intel/iwlwifi/mvm/mac-ctxt.c:955:24: warning: conditional expression between different enumeration types ('enum iwl_mac_beacon_flags' and 'enum iwl_mac_beacon_flags_v1') [-Wenum-compare-conditional]
    955 |                 flags |= is_new_rate ? IWL_MAC_BEACON_CCK
        |                                      ^ ~~~~~~~~~~~~~~~~~~
    956 |                           : IWL_MAC_BEACON_CCK_V1;
        |                             ~~~~~~~~~~~~~~~~~~~~~
  drivers/net/wireless/intel/iwlwifi/mvm/mac-ctxt.c:1120:21: warning: conditional expression between different enumeration types ('enum iwl_mac_beacon_flags' and 'enum iwl_mac_beacon_flags_v1') [-Wenum-compare-conditional]
   1120 |                                                0) > 10 ?
        |                                                        ^
   1121 |                         IWL_MAC_BEACON_FILS :
        |                         ~~~~~~~~~~~~~~~~~~~
   1122 |                         IWL_MAC_BEACON_FILS_V1;
        |                         ~~~~~~~~~~~~~~~~~~~~~~

Doing arithmetic between or returning two different types of enums could
be a bug, so each of the instance of the warning needs to be evaluated.
Unfortunately, as mentioned above, there are many instances of this
warning in many different configurations, which can break the build when
CONFIG_WERROR is enabled.

To avoid introducing new instances of the warnings while cleaning up the
disruption for the majority of users, disable these warnings for the
default build while leaving them on for W=1 builds.

Cc: stable@vger.kernel.org
Closes: https://github.com/ClangBuiltLinux/linux/issues/2002
Link: https://github.com/llvm/llvm-project/commit/8c2ae42b3e1c6aa7c18f873edcebff7c0b45a37eAcked-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>