Hello Thomas, On Tue, Sep 17, 2013 at 11:15:20PM +0200, Thomas Gleixner wrote: > Marc Kleine-Budde pointed out, that commit 77cc982 "clocksource: use > clockevents_config_and_register() where possible" caused a regression > for some of the converted subarchs. > > The reason is, that the clockevents core code converts the minimal > hardware tick delta to a nanosecond value for core internal > usage. This conversion is affected by integer math rounding loss, so > the backwards conversion to hardware ticks will likely result in a > value which is less than the configured hardware limitation. The > affected subarchs used their own workaround (SIGH!) which got lost in > the conversion. > > Now instead of fixing the underlying core code problem, Marcs patch s/Marcs/Marc's/
> tried to work around the core code issue by increasing the minimal > tick delta at clockevents registration time so the resulting limit in > the core code backwards conversion did not violate the hardware > limits. More SIGH! > > The solution for the issue at hand is simple: adding evt->mult - 1 to > the shifted value before the integer divison in the core conversion > function takes care of it. > > Though looking closer at the details of that function reveals another > bogosity: The upper bounds check is broken as well. Checking for a > resulting "clc" value greater than KTIME_MAX after the conversion is > pointless. The conversion does: > > u64 clc = (latch << evt->shift) / evt->mult; > > So there is no sanity check for (latch << evt->shift) exceeding the > 64bit boundary. The latch argument is "unsigned long", so on a 64bit > arch the handed in argument could easily lead to an unnoticed shift > overflow. With the above rounding fix applied the calculation before > the divison is: > > u64 clc = (latch << evt->shift) + evt->mult - 1; > > Now we can easily verify whether the whole equation fits into the > 64bit boundary. Shifting the "clc" result back by evt->shift MUST > result in "latch". If that's not the case, we have a clear indicator But this is only the case if evt->mult is <= (1 << evt->shift). Is this always given? Is it more sensible to adjust dev->max_delta_ns once at register time and so save the often recurrent overflow check in clockevents_program_event? Another doubt I have is: You changed clockevent_delta2ns to round up now unconditionally. For the numbers on at91 that doesn't matter, but I wonder if there are situations that make the timer core violate the max_delta_ticks condition now. > for boundary violation and can limit "clc" to (1 << 63) - 1 before the Where does this magic constant come from? Best regards Uwe > divison by evt->mult. The resulting nsec * evt->mult in the > programming path will therefor always be in the 64bit boundary. > > Signed-off-by: Thomas Gleixner <t...@linutronix.de> > --- > diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c > index 38959c8..4fc4826 100644 > --- a/kernel/time/clockevents.c > +++ b/kernel/time/clockevents.c > @@ -49,13 +49,25 @@ u64 clockevent_delta2ns(unsigned long latch, struct > clock_event_device *evt) > WARN_ON(1); > } > > + /* > + * Prevent integer rounding loss, otherwise the backward > + * conversion from nsec to ticks could result in a value less > + * than evt->min_delta_ticks. > + */ > + clc += evt->mult - 1; > + > + /* > + * Upper bound sanity check. If the backwards conversion is > + * not equal latch, we know that the above (shift + rounding > + * correction) exceeded the 64 bit boundary. > + */ > + if ((clc >> evt->shift) != (u64)latch) > + clc = ((u64)1 << 63) - 1; > + > do_div(clc, evt->mult); > - if (clc < 1000) > - clc = 1000; > - if (clc > KTIME_MAX) > - clc = KTIME_MAX; > > - return clc; > + /* Deltas less than 1usec are pointless noise */ > + return clc > 1000 ? clc : 1000; > } > EXPORT_SYMBOL_GPL(clockevent_delta2ns); > -- 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/