On 06/06/2013 08:04 PM, Stephen Boyd wrote: > On 06/06, Daniel Lezcano wrote: >> On 06/03/2013 10:33 PM, Stephen Boyd wrote: >>> On an SMP system with only one global clockevent and a dummy >>> clockevent per CPU we run into problems. We want the dummy >>> clockevents to be registered as the per CPU tick devices, but >>> we can only achieve that if we register the dummy clockevents >>> before the global clockevent or if we artificially inflate the >>> rating of the dummy clockevents to be higher than the rating >>> of the global clockevent. Failure to do so leads to boot >>> hangs when the dummy timers are registered on all other CPUs >>> besides the CPU that accepted the global clockevent as its tick >>> device and there is no broadcast timer to poke the dummy >>> devices. >>> >>> If we're registering multiple clockevents and one clockevent is >>> global and the other is local to a particular CPU we should >>> choose to use the local clockevent regardless of the rating of >>> the device. This way, if the clockevent is a dummy it will take >>> the tick device duty as long as there isn't a higher rated tick >>> device and any global clockevent will be bumped out into >>> broadcast mode, fixing the problem described above. >> >> It is not clear the connection between the changelog, the patch and the >> comment. Could you clarify a bit ? >> > > There is one tick device per-cpu and one broadcast device. The > broadcast device can only be a global clockevent, whereas the > per-cpu tick device can be a global clockevent or a per-cpu > clockevent. The code tries hard to keep per-cpu clockevents in > the tick device slots but it has an ordering/rating requirement > that doesn't work when there are only dummy per-cpu devices and > one global device. > > Perhaps an example will help. Let's say you only have one global > clockevent such as the sp804, and you have SMP enabled. To > support SMP we have to register dummy clockevents on each CPU so > that the sp804 can go into broadcast mode. If we don't do this, > only the CPU that registered the sp804 will get interrupts while > the other CPUs will be left with no tick device and thus no > scheduling. To fix this we register dummy clockevents on all the > CPUs _before_ we register the sp804 to force the sp804 into the > broadcast slot. Or we give the dummy clockevents a higher rating > than the sp804 so that when we register them after the sp804 the > sp804 is bumped out to broadcast duty. > > If the dummy devices are registered before the sp804 we can give > the dummies a low rating and the sp804 will still go into the > broadcast slot due to this code: > > /* > * If we have a cpu local device already, do not replace it > * by a non cpu local device > */ > if (curdev && cpumask_equal(curdev->cpumask, cpumask_of(cpu))) > goto out_bc; > > If we register the sp804 before the dummies we're also fine as > long as the rating of the dummy is more than the sp804. Playing > games with the dummy rating is not very nice so this patch fixes > it by allowing the per-cpu device to replace the global device no > matter what the rating of the global device is. > > This fixes the sp804 case when the dummy is rated lower than > sp804 and it removes any ordering requirement from the > registration of clockevents. It also completes the logic above > where we prefer cpu local devices over non cpu local devices. Thanks for the detailed explanation. Did Thomas reacted to this patch ? -- <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs Follow Linaro: <http://www.facebook.com/pages/Linaro> Facebook | <http://twitter.com/#!/linaroorg> Twitter | <http://www.linaro.org/linaro-blog/> Blog -- To unsubscribe from this list: send the line "unsubscribe linux-arm-msm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
