netdev - RE: [PATCH v5] net/hsr: Add support for the High-availability Seamless Redundancy protocol (HSRv0)

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Date:	Mon, 28 Oct 2013 09:17:06 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Joe Perches" <joe@...ches.com>,
	"Arvid Brodin" <arvid.brodin@...n.com>
Cc:	<netdev@...r.kernel.org>, "David Miller" <davem@...emloft.net>,
	"Stephen Hemminger" <shemminger@...tta.com>,
	"Javier Boticario" <jboticario@...il.com>,
	<balferreira@...glemail.com>,
	Elías Molina Muñoz <elias.molina@....es>
Subject: RE: [PATCH v5] net/hsr: Add support for the High-availability Seamless Redundancy protocol (HSRv0)

> Subject: Re: [PATCH v5] net/hsr: Add support for the High-availability Seamless Redundancy protocol
> (HSRv0)
> 
> On Fri, 2013-10-25 at 20:20 +0200, Arvid Brodin wrote:
> > On 2013-10-23 18:52, Joe Perches wrote:
> > > On Wed, 2013-10-23 at 18:09 +0200, Arvid Brodin wrote:
> []
> > >> +/* above(a, b) - return 1 if a > b, 0 otherwise.
> > >> + */
> > >> +static bool above(u16 a, u16 b)
> > >> +{
> > >> +	/* Remove inconsistency where above(a, b) == below(a, b) */
> > >> +	if ((int) b - a == 32768)
> > >> +		return 0;
> > >> +
> > >> +	return (((s16) (b - a)) < 0);
> > >> +}
> > >> +#define below(a, b)		above((b), (a))
> > >> +#define above_or_eq(a, b)	(!below((a), (b)))
> > >> +#define below_or_eq(a, b)	(!above((a), (b)))
> > >
> > > This looks odd.
> >
> > It relies on unsigned arithmetic to compare two values that may wrap. I.e.,
> > it doesn't care about the absolute sizes, but only about the distance
> > between the numbers.

I'd have thought it wasn't really worth worrying about what happens
when a ^ b == 0x8000. Anything using modulo 16 distances shouldn't
really see such values.
Perhaps just document the effect.

Also, if these definitions are in a .h file they need better names

	David



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