[PATCH wireless-next v3 2/2] wifi: rtlwifi: rtl8821ae: phy: fix an undefined bitwise shift behavior

[Su Hui <suhui@nfschina.com>]

Clang staic checker warning:
drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c:184:49:
	The result of the left shift is undefined due to shifting by '32',
	which is greater or equal to the width of type 'u32'.
	[core.UndefinedBinaryOperatorResult]

If the value of the right operand is negative or is greater than or
equal to the width of the promoted left operand, the behavior is
undefined.[1][2]

For example, when using different gcc's compilation optimizaation options
(-O0 or -O2), the result of '(u32)data << 32' is different. One is 0, the
other is old value of data. Let _rtl8821ae_phy_calculate_bit_shift()'s
return value less than 32 to fix this problem. Warn if bitmask is zero.

[1]:https://stackoverflow.com/questions/11270492/what-does-the-c-
standard-say-about-bitshifting-more-bits-than-the-width-of-type
[2]:https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf

Fixes: 21e4b0726dc6 ("rtlwifi: rtl8821ae: Move driver from staging to regular tree")
Signed-off-by: Su Hui <suhui@nfschina.com>
---
v3:
 - use __ffs() and make code briefer. (Thanks to Ping-Ke)

v2:
 - fix the subject prefix problem
 - silence the warning by not return 32 bits rather than adding a type cast.(Thanks to Dan and Ping-Ke)

 drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c b/drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c
index 6df270e29e66..68c3fb0395ce 100644
--- a/drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c
+++ b/drivers/net/wireless/realtek/rtlwifi/rtl8821ae/phy.c
@@ -29,9 +29,10 @@ static void _rtl8821ae_phy_rf_serial_write(struct ieee80211_hw *hw,
 					   u32 data);
 static u32 _rtl8821ae_phy_calculate_bit_shift(u32 bitmask)
 {
-	u32 i = ffs(bitmask);
+	if (WARN_ON_ONCE(!bitmask))
+		return 0;
 
-	return i ? i - 1 : 32;
+	return __ffs(bitmask);
 }
 static bool _rtl8821ae_phy_bb8821a_config_parafile(struct ieee80211_hw *hw);
 /*static bool _rtl8812ae_phy_config_mac_with_headerfile(struct ieee80211_hw *hw);*/
-- 
2.30.2