[PATCH] bitfield: avoid gcc-8 -Wint-in-bool-context warning

[Arnd Bergmann]

Passing an enum into FIELD_GET() produces a long but harmless warning on
newer compilers:

                 from include/linux/linkage.h:7,
                 from include/linux/kernel.h:7,
                 from include/linux/skbuff.h:17,
                 from include/linux/if_ether.h:23,
                 from include/linux/etherdevice.h:25,
                 from drivers/net/wireless/intel/iwlwifi/mvm/rxmq.c:63:
drivers/net/wireless/intel/iwlwifi/mvm/rxmq.c: In function 'iwl_mvm_rx_mpdu_mq':
include/linux/bitfield.h:56:20: error: enum constant in boolean context [-Werror=int-in-bool-context]
   BUILD_BUG_ON_MSG(!(_mask), _pfx "mask is zero"); \
                    ^
...
include/linux/bitfield.h:103:3: note: in expansion of macro '__BF_FIELD_CHECK'
   __BF_FIELD_CHECK(_mask, _reg, 0U, "FIELD_GET: "); \
   ^~~~~~~~~~~~~~~~
drivers/net/wireless/intel/iwlwifi/mvm/rxmq.c:1025:21: note: in expansion of macro 'FIELD_GET'
    le16_encode_bits(FIELD_GET(IWL_RX_HE_PHY_SIBG_SYM_OR_USER_NUM_MASK,

The problem here is that the caller has no idea how the macro gets
expanding, leading to a false-positive. It can be trivially avoided
by doing a comparison against zero.

This only recently started appearing as the iwlwifi driver was patched
to use FIELD_GET.

Fixes: 514c30696fbc ("iwlwifi: add support for IEEE802.11ax")
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
---
 include/linux/bitfield.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
index 65a6981eef7b..3f1ef4450a7c 100644
--- a/include/linux/bitfield.h
+++ b/include/linux/bitfield.h
@@ -53,7 +53,7 @@
 	({								\
 		BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask),		\
 				 _pfx "mask is not constant");		\
-		BUILD_BUG_ON_MSG(!(_mask), _pfx "mask is zero");	\
+		BUILD_BUG_ON_MSG((_mask) == 0, _pfx "mask is zero");	\
 		BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ?		\
 				 ~((_mask) >> __bf_shf(_mask)) & (_val) : 0, \
 				 _pfx "value too large for the field"); \
-- 
2.18.0