On 06/07, Daniel Lezcano wrote: > 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 ? > So far there has been no response from Thomas. -- Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation -- 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
