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

[Joe Perches <joe@perches.com>]

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. 
> 
> It is inspired in part by the code in jiffies.h, but adapted to 16-bit
> types. The code you suggested (below) will not work in this case.
> 
> See e.g. http://en.wikipedia.org/wiki/Serial_number_arithmetic

No worries, I was just reading the comment as the
comment doesn't match the code.

Perhaps the comment should be updated to reflect the
wrapping test.