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Date: Mon, 06 Apr 2015 14:26:43 -0700 From: Kevin Hilman <khilman@...nel.org> To: Viresh Kumar <viresh.kumar@...aro.org> Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>, linaro-kernel@...ts.linaro.org, linux-kernel@...r.kernel.org, Daniel Lezcano <daniel.lezcano@...aro.org>, Preeti U Murthy <preeti@...ux.vnet.ibm.com>, Frederic Weisbecker <fweisbec@...il.com> Subject: Re: [PATCH 1/3] clockevents: Introduce CLOCK_EVT_STATE_ONESHOT_STOPPED state Viresh Kumar <viresh.kumar@...aro.org> writes: > When no timers/hrtimers are pending, the expiry time is set to a special value: > 'KTIME_MAX'. This normally happens with NO_HZ_{IDLE|FULL} in both LOWRES/HIGHRES > modes. > > When 'expiry == KTIME_MAX', we either cancel the 'tick-sched' hrtimer > (NOHZ_MODE_HIGHRES) or skip reprogramming clockevent device (NOHZ_MODE_LOWRES). > But, the clockevent device is already reprogrammed from the tick-handler for > next tick. > > As the clock event device is programmed in ONESHOT mode it will at least fire > one more time (unnecessarily). Timers on few implementations (like > arm_arch_timer, etc.) only support PERIODIC mode and their drivers emulate > ONESHOT over that. Which means that on these platforms we will get spurious > interrupts periodically (at last programmed interval rate, normally tick rate). > > In order to avoid spurious interrupts, the clockevent device should be stopped > or its interrupts should be masked. > > A simple (yet hacky) solution to get this fixed could be: update > hrtimer_force_reprogram() to always reprogram clockevent device and update > clockevent drivers to STOP generating events (or delay it to max time) when > 'expires' is set to KTIME_MAX. But the drawback here is that every clockevent > driver has to be hacked for this particular case and its very easy for new ones > to miss this. > > However, Thomas suggested to add an optional state ONESHOT_STOPPED to solve this > problem: lkml.org/lkml/2014/5/9/508. > > This patch adds support for ONESHOT_STOPPED state in clockevents core. It will > only be available to drivers that implement the state-specific callbacks instead > of the legacy ->set_mode() callback. > > Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org> Reviewed-by: Kevin Hilman <khilman@...aro.org> with a minor nit... [...] > diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c > index 73689df1e4b8..04f6c3433f8e 100644 > --- a/kernel/time/clockevents.c > +++ b/kernel/time/clockevents.c > @@ -138,6 +138,17 @@ static int __clockevents_set_state(struct clock_event_device *dev, > return -ENOSYS; > return dev->set_state_oneshot(dev); > > + case CLOCK_EVT_STATE_ONESHOT_STOPPED: > + /* Core internal bug */ This comment is not useful at all (nor are all the other ones already in this file.) IMO, the comment should say something like: "ONESHOT_STOPPED is only valid when currently in the ONESHOT state." or something similar. > + if (WARN_ONCE(dev->state != CLOCK_EVT_STATE_ONESHOT, > + "Current state: %d\n", dev->state)) Similarily this output will not be useful, and should say something like: "Can only enter ONESHOT_STOPPED from ONESHOT. Current state: %d\n". > + return -EINVAL; > + > + if (dev->set_state_oneshot_stopped) > + return dev->set_state_oneshot_stopped(dev); > + else > + return -ENOSYS; > + > default: > return -ENOSYS; > } Kevin -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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