Re: [PATCH v1] mt76: mt7615: Fix build with older compilers

["Pablo Sebastián Greco" <pgreco@centosproject.org>]

On 2/12/19 06:25, Kalle Valo wrote:
> Pablo Greco <pgreco@centosproject.org> writes:
>
>> Some compilers (tested with 4.8.5 from CentOS 7) fail properly process
>> FIELD_GET inside an inline function, which ends up in a BUILD_BUG_ON.
>> Convert inline function to a macro.
>>
>> Fixes commit bf92e7685100 ("mt76: mt7615: add support for per-chain
>> signal strength reporting")
>> Reported in https://lkml.org/lkml/2019/9/21/146
>>
>> Reported-by: kbuild test robot <lkp@intel.com>
>> Signed-off-by: Pablo Greco <pgreco@centosproject.org>
>> ---
>>   drivers/net/wireless/mediatek/mt76/mt7615/mac.c | 5 +----
>>   1 file changed, 1 insertion(+), 4 deletions(-)
>>
>> diff --git a/drivers/net/wireless/mediatek/mt76/mt7615/mac.c b/drivers/net/wireless/mediatek/mt76/mt7615/mac.c
>> index c77adc5d2552..77e395ca2c6a 100644
>> --- a/drivers/net/wireless/mediatek/mt76/mt7615/mac.c
>> +++ b/drivers/net/wireless/mediatek/mt76/mt7615/mac.c
>> @@ -13,10 +13,7 @@
>>   #include "../dma.h"
>>   #include "mac.h"
>>   
>> -static inline s8 to_rssi(u32 field, u32 rxv)
>> -{
>> -	return (FIELD_GET(field, rxv) - 220) / 2;
>> -}
>> +#define to_rssi(field, rxv)		((FIELD_GET(field, rxv) - 220) / 2)
> What about u32_get_bits() instead of FIELD_GET(), would that work? I
> guess chances for that is slim, but it's always a shame to convert a
> function to a macro so we should try other methods first.
Anything that doesn't check field at build time should work, but between 
losing a check, or turning an inline into a macro, I'd rather use the macro.
> Or even better if we could fix FIELD_GET() to work with older compilers.
>
The problem is not FIELD_GET itself, is that the compiler is trying to 
use "field" as a variable, instead as the macro expansion of GENMASK, as 
if the function wasn't inline.
In the linked page you can see this message

BUILD_BUG_ON failed: (((field) + (1ULL << (__builtin_ffsll(field) - 1))) 
& (((field) + (1ULL << (__builtin_ffsll(field) - 1))) - 1)) != 0
      _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)

which is is not right, because "field" should never be used for that check.



Pablo.