Subject: Re: [PATCH] ath9k: Add cast to u8 to FREQ2FBIN macro

From: Joe Perches <[email-masked]>
Date: Thu, 06 Apr 2017 19:36:41 -0700

On Thu, 2017-04-06 at 16:54 -0700, Matthias Kaehlcke wrote:
> Hi Joe,
> 
> El Thu, Apr 06, 2017 at 02:29:20PM -0700 Joe Perches ha dit:
> 
> > On Thu, 2017-04-06 at 14:21 -0700, Matthias Kaehlcke wrote:
> > > The macro results are assigned to u8 variables/fields. Adding the cast
> > > fixes plenty of clang warnings about "implicit conversion from 'int' to
> > > 'u8'".
> > > 
> > > Signed-off-by: Matthias Kaehlcke <[email-masked]>
> > > ---
> > >  drivers/net/wireless/ath/ath9k/eeprom.h | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/net/wireless/ath/ath9k/eeprom.h b/drivers/net/wireless/ath/ath9k/eeprom.h
> > > index 30bf722e33ed..31390af6c33e 100644
> > > --- a/drivers/net/wireless/ath/ath9k/eeprom.h
> > > +++ b/drivers/net/wireless/ath/ath9k/eeprom.h
> > > @@ -106,7 +106,7 @@
> > >  #define AR9285_RDEXT_DEFAULT    0x1F
> > >  
> > >  #define ATH9K_POW_SM(_r, _s)	(((_r) & 0x3f) << (_s))
> > > -#define FREQ2FBIN(x, y)		((y) ? ((x) - 2300) : (((x) - 4800) / 5))
> > > +#define FREQ2FBIN(x, y)		(u8)((y) ? ((x) - 2300) : (((x) - 4800) / 5))
> > 
> > Maybe better to use:
> > 
> > static inline u8 FREQ2FBIN(int x, int y)
> > {
> > 	if (y)
> > 		return x - 2300;
> > 	return (x - 4800) / 5;
> > }
> 
> Thanks for your suggestion! Unfortunately in this case an inline
> function is not suitable since FREQ2FBIN() is mostly used for
> structure initialization:
> 
> static const struct ar9300_eeprom ar9300_default = {
> ...
>         .calFreqPier2G = {
>                 FREQ2FBIN(2412, 1),
>                 FREQ2FBIN(2437, 1),
>                 FREQ2FBIN(2472, 1)
>         },
> ...

Maybe it's better to remove the second argument and write
something like:

#define FREQ2FBIN(x) \
	(u8)(((x) >= 2300 && (x) <= 2555) ? (x) - 2300 : \
	     ((x) >= 4800 && (x) <= 4800 + (256 * 5) ? ((x) - 4800) / 5) : \
	     __builtin_const_p(x) ? BUILD_BUG_ON(1) : 0)