On Thu, 19 Sep 2013, Uwe Kleine-König wrote: > On Thu, Sep 19, 2013 at 12:15:10PM +0200, Thomas Gleixner wrote: > > On Thu, 19 Sep 2013, Uwe Kleine-König wrote: > > > On Thu, Sep 19, 2013 at 12:01:25AM +0200, Thomas Gleixner wrote: > > > > Versus the 64bit overflow check, we need to be even more careful. We > > > > need to check for overflowing (1 << 63) - 1 (i.e. the max positive > > > > value which fits into a s64). See clockevents_program_event(). > > > > > > That is because you interpret times < 0 as in the past, right? But note > > > that the interim result we're talking about here is still to be divided > > > by evt->mult. So assuming mult > 1, that check is too strict unless you > > > move it below the do_div in clockevent_delta2ns. For sure it makes sense > > > to use the same value for a and b in the handling: > > > > No, it's not too strict. > > > > nsec = (latch << shift) / mult; > > > > Now the backwards conversion does: > > > > latch = (nsec * mult) >> shift; > > > > So we want nsec * mult to be in the positive range of s64. Which > > means, that latch << shift must be in that range as well. > The backwards conversion is in clockevents_program_event(), right? There > is: > > clc = ((unsigned long long) delta * dev->mult) >> dev->shift; > > So I don't see a problem if nsec * mult overflows (1 << 63) - 1 as long > as it still fits into an unsigned long long (i.e. a 64 bit value).
Right. It doesn't matter.
