On 09/24/2013 09:50 PM, Uwe Kleine-König wrote: > From: Thomas Gleixner <[email protected]> > > 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. > > 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. But this only works for the case where for > the scaled math mult/shift pair "mult <= 1 << shift" is true. For the > case where "mult > 1 << shift" we can apply the rounding add only for > the minimum delta value to make sure that the backward conversion is > not less than the given hardware limit. For the upper bound we need to > omit the rounding add, because the backwards conversion is always > larger than the original latch value. That would violate the upper > bound of the hardware device. > > 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; > > So we need to make sure, that neither the shift nor the rounding add > is overflowing the u64 boundary. > > Signed-off-by: Thomas Gleixner <[email protected]> > Cc: Russell King - ARM Linux <[email protected]> > Cc: Marc Kleine-Budde <[email protected]> > Cc: [email protected] > Cc: Marc Pignat <[email protected]> > Cc: [email protected] > Cc: [email protected] > Cc: Ronald Wahl <[email protected]> > Cc: LAK <[email protected]> > Cc: Ludovic Desroches <[email protected]> > [ukl: move assignment to rnd after eventually changing mult, fix build > issue and correct comment with the right math] > Signed-off-by: Uwe Kleine-König <[email protected]>
What's the status of this patch? Marc -- Pengutronix e.K. | Marc Kleine-Budde | Industrial Linux Solutions | Phone: +49-231-2826-924 | Vertretung West/Dortmund | Fax: +49-5121-206917-5555 | Amtsgericht Hildesheim, HRA 2686 | http://www.pengutronix.de |
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