Re: [RFC][PATCH] clocksource: avoid unnecessary overflow in cyclecounter_cyc2ns()

[Henrik Austad <henrik@austad.us>]

On Tue, Mar 04, 2014 at 08:36:28AM +0100, Mike Galbraith wrote:
> (boing boing boing... hell with it, today doesn't exist;)

you lost me at boing.. :)

> 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@google.com>
> > > > Cc: John Stultz <johnstul@us.ibm.com>
> > > > Signed-off-by: Mike Galbraith <bitbucket@online.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.

I wasn't thinking about the number of +'s in the code, but rather 
referencing other parts of the code from the code and particular commits in 
the commit-msg itself. It was the code<->commitmsg interface I was 
pondering.

Besides, it wasn't meant as "you shouldn't do that", but more "is it ok to 
do that?" :)

-- 
Henrik Austad