Re: [PATCH] [media] mceusb: Optimize DIV_ROUND_CLOSEST call

[Jean Delvare <khali@linux-fr.org>]

Hi Mauro,

On Tue, 18 Sep 2012 12:49:53 -0300, Mauro Carvalho Chehab wrote:
> Em 01-09-2012 15:53, Jean Delvare escreveu:
> > DIV_ROUND_CLOSEST is faster if the compiler knows it will only be
> > dealing with unsigned dividends.
> > 
> > Signed-off-by: Jean Delvare <khali@linux-fr.org>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Guenter Roeck <linux@roeck-us.net>
> > Cc: Mauro Carvalho Chehab <mchehab@infradead.org>
> > ---
> >  drivers/media/rc/mceusb.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > --- linux-3.6-rc3.orig/drivers/media/rc/mceusb.c	2012-08-04 21:49:27.000000000 +0200
> > +++ linux-3.6-rc3/drivers/media/rc/mceusb.c	2012-09-01 18:53:32.053042123 +0200
> > @@ -627,7 +627,7 @@ static void mceusb_dev_printdata(struct
> >  			break;
> >  		case MCE_RSP_EQIRCFS:
> >  			period = DIV_ROUND_CLOSEST(
> > -					(1 << data1 * 2) * (data2 + 1), 10);
> > +					(1U << data1 * 2) * (data2 + 1), 10);
> >  			if (!period)
> >  				break;
> >  			carrier = (1000 * 1000) / period;
>
> Hmm... this generates the following warning with "W=1":
> 
> drivers/media/rc/mceusb.c:629:4: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
> drivers/media/rc/mceusb.c:629:4: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]

I doubt this is the only warning of that kind. There must be a reason
why -Wextra isn't enabled by default.

> Perhaps it makes sense to use an optimized version for unsigned, or to
> change the macro to take the data types into account.

This was discussed before, but Andrew said he preferred a single macro.
And I agree with him, having two macros would induce a risk of the
wrong one being called.

If you can come up with a variant of DIV_ROUND_CLOSEST which performs
the same and doesn't trigger the warning above, we'll be happy to see
it, but neither Guenter nor myself could come up with one.

-- 
Jean Delvare