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Date: Mon, 23 Jan 2012 02:44:52 -0800
From: Andres Salomon <dilinger@queued.net>
To: Jens Rottmann <JRottmann@LiPPERTEmbedded.de>
Cc: Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org,
        linux-geode@lists.infradead.org
Subject: Re: [PATCH] cs5535-clockevt: don't ignore MFGPT on SMP-capable
 kernels
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On Thu, 19 Jan 2012 13:57:31 +0100
Jens Rottmann <JRottmann@LiPPERTEmbedded.de> wrote:

> Hi Andres,
> 
> and a happy new year to you. Sorry for the delay.
> 
> Andres Salomon wrote:
> > Jens Rottmann <JRottmann@LiPPERTEmbedded.de> wrote:
> >> cs5535-clockevt: don't ignore MFGPT on SMP-capable kernels
> >>
> >> On SMP-capable kernels (e.g. generic distro kernel) the
> >> cs5535-clockevt driver loads but is not actually used.
> >>
> >> Setting cpumask to cpu_all_mask works for UP-only kernels, but if
> >> compiled for SMP - though still running on the same UP hardware -
> >> kernel/time/tick-common.c:tick_check_new_device() reads this as
> >> "non-cpu-local" and silently ignores the device.
> >>
> >> If we leave cpumask unset clockevents_register_device() will
> >> initialize it and the cs5535-clockevt driver will be used no matter
> >> how the kernel was compiled. Should anyone ever manage to stick a
> >> CS553x in an SMP system (is this even possible?) then a warning
> >> will be printed.  This is fine as the cs5535-clockevt driver was
> >> never written/tested for SMP.
> >>
> >> If bisecting led you here this patch may have exposed a
> >> pre-existing MFGPT problem.  Configure for UP-only and re-check.
> >>
> >> Signed-off-by: Jens Rottmann <JRottmann@LiPPERTEmbedded.de>
> >> ---
> >>
> >> --- linux-3.2-rc6/drivers/clocksource/cs5535-clockevt.c
> >> +++ use_mfgpt_on_smp_kernels/drivers/clocksource/cs5535-clockevt.c
> >> @@ -100,7 +100,6 @@ static struct clock_event_device cs5535_
> >>  	.set_mode = mfgpt_set_mode,
> >>  	.set_next_event = mfgpt_next_event,
> >>  	.rating = 250,
> >> -	.cpumask = cpu_all_mask,
> >>  	.shift = 32
> >>  };
> >>
> >> _
> >
> > Hm, have you tried setting cpumask to cpumask_of(0), like a bunch of
> > the other clock_event_device drivers do?
> 
> Yes, I've seen that, and it was my initial approach. cpumask_of(0)
> isn't regarded constant, so moved it from the struct init to
> cs5535_mfgpt_init() ... but then I saw
> 
> clockevents.c:clockevents_register_device():
> 	if (!dev->cpumask) {
> 		WARN_ON(num_possible_cpus() > 1);
> 		dev->cpumask = cpumask_of(smp_processor_id());
> 	}
> 
> Looks to me like meant exactly for this purpose. This will set
> cpumask to cpumask_of(0), i.e. does exactly what you're suggesting.
> 

Ah, great!  In that case,

Acked-by: Andres Salomon <dilinger@queued.net>