linux-kernel - Re: [RFC][PATCH] clocksource: avoid unnecessary overflow in cyclecounter

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Date:	Tue, 04 Mar 2014 08:36:28 +0100
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Henrik Austad <henrik@...tad.us>
Cc:	John Stultz <john.stultz@...aro.org>,
	Salman Qazi <sqazi@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] clocksource: avoid unnecessary overflow in
 cyclecounter_cyc2ns()

(boing boing boing... hell with it, today doesn't exist;)

On Tue, 2014-03-04 at 08:31 +0100, Mike Galbraith wrote: 
> On Tue, 2014-03-04 at 08:20 +0100, Henrik Austad wrote: 
> > On Tue, Mar 04, 2014 at 06:20:09AM +0100, Mike Galbraith wrote:
> > > Greetings,
> > > 
> > > While rummaging around looking for HTH a gaggle of weird a$$ machines
> > > can manage to timewarp back and forth by exactly 208 days, I stumbled
> > > across $subject which looks like it may want to borrow Salman's fix.
> > > 
> > > clocksource: avoid unnecessary overflow in cyclecounter_cyc2ns()
> > > 
> > > As per 4cecf6d401a "sched, x86: Avoid unnecessary overflow in sched_clock",
> > > cycles * mult >> shift is overflow prone. so give it the same treatment.
> > > 
> > > Cc: Salman Qazi <sqazi@...gle.com>
> > > Cc: John Stultz <johnstul@...ibm.com>
> > > Signed-off-by: Mike Galbraith <bitbucket@...ine.de>
> > > ---
> > >  include/linux/clocksource.h |   11 ++++++++---
> > >  1 file changed, 8 insertions(+), 3 deletions(-)
> > > 
> > > --- a/include/linux/clocksource.h
> > > +++ b/include/linux/clocksource.h
> > > @@ -77,13 +77,18 @@ struct timecounter {
> > >   *
> > >   * XXX - This could use some mult_lxl_ll() asm optimization. Same code
> > >   * as in cyc2ns, but with unsigned result.
> > > + *
> > > + * Because it is the same as x86 __cycles_2_ns, give it the same treatment as
> > > + * commit 4cecf6d401a "sched, x86: Avoid unnecessary overflow in sched_clock"
> > > + * to avoid a potential cycles * mult overflow.
> > 
> > Do we normally reference a particular commit in a comment? Why not just 
> > grab the same comment and add a "this is grabbed from arch/x86/... ?
> 
> Fewer '+' signs?  History doesn't go away, so seems fine to me.
> 
> -Mike
> 
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