Re: [PATCH 3/4 V2] mac80211: add last beacon time in scan list

[Pavel Roskin <proski@gnu.org>]

On Thu, 2008-06-26 at 01:26 +0300, Tomas Winkler wrote:
> On Wed, Jun 25, 2008 at 7:12 PM, Johannes Berg
> <johannes@sipsolutions.net> wrote:
> > On Wed, 2008-06-25 at 11:58 -0400, Pavel Roskin wrote:
> >
> >> > +                   unsigned long mid_range = (-1) / 2 + 1;
> >>
> >> What is that?  I guess that's where Riemann's zeta function has its
> >> non-trivial roots :-)
> 
> >
> > The Pavel hypothesis: "The Riemann hypothesis is correct when calculated
> > in 32-bit integers" is obviously wrong ;)
> >
> Welcome to the Desert of the Real

Welcome to C.  mid_range is 1, as you can easily check.

> >> > +                   time_diff = jiffies - bss->last_update > mid_range ?
> >> > +                           jiffies - bss->last_update :
> >> > +                           bss->last_update - jiffies;
> >>
> >> That's pretty hairy.  Do we really lack a function to calculate time
> >> difference?
> 
> This is simplest as you can get under assumption that times are not
> apart more then 1/2 of UL.
>  It would be overkill to translate it to timeval or anything we have
> diff function for.

Why would jiffies ever lag behind bss->last_update?  When would
(bss->last_update - jiffies) make sense?  If it makes sense, how about
indicating it somehow?  It would be a value that decreases over time.  

> > Actually, come to think of it, isn't just doing the difference as in teh
> > original patch correct in 32-bit unsigned integers? It'll wrap around a
> > bit but that's ok, no?
> 
> It would make a very recent beacon to be like half an hour old on
> wraps, wouldn't it?

No.  Why would it?  Can you give an example?  If you are not increasing
precision, wraparounds should make no difference.

-- 
Regards,
Pavel Roskin