Hello Thomas, 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). What am I missing? Best regards Uwe -- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | http://www.pengutronix.de/ | -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/