linux-kernel - Re: [PATCH] arm_pmu: Mark expected switch fall-through

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Message-ID: <20190726151825.GA12552@e121166-lin.cambridge.arm.com>
Date:   Fri, 26 Jul 2019 16:18:25 +0100
From:   Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Anders Roxell <anders.roxell@...aro.org>, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm_pmu: Mark expected switch fall-through

On Fri, Jul 26, 2019 at 01:29:56PM +0100, Mark Rutland wrote:
> On Fri, Jul 26, 2019 at 01:27:37PM +0200, Anders Roxell wrote:
> > When fall-through warnings was enabled by default the following warning
> > was starting to show up:
> > 
> > ../drivers/perf/arm_pmu.c: In function ‘cpu_pm_pmu_notify’:
> > ../drivers/perf/arm_pmu.c:726:3: warning: this statement may fall
> >  through [-Wimplicit-fallthrough=]
> >    cpu_pm_pmu_setup(armpmu, cmd);
> >    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > ../drivers/perf/arm_pmu.c:727:2: note: here
> >   case CPU_PM_ENTER_FAILED:
> >   ^~~~
> > 
> > Rework so that the compiler doesn't warn about fall-through.
> > 
> > Fixes: d93512ef0f0e ("Makefile: Globally enable fall-through warning")
> > Signed-off-by: Anders Roxell <anders.roxell@...aro.org>
> > ---
> > 
> > I'm not convinced that this is the correct patch to fix this issue.
> > However, I can't see why we do 'armpmu->start(armpmu);' only in 'case
> > CPU_PM_ENTER_FAILED' and why we not call function cpu_pm_pmu_setup()
> > there also, since in cpu_pm_pmu_setup() has a case prepared for
> > CPU_PM_ENTER_FAILED.
> 
> I agree, think that should be:
> 
> 	case CPU_PM_EXIT:
> 	case CPU_PM_ENTER_FAILED:
> 		cpu_pm_pmu_setup(armpmu, cmd);
> 		armpmu->start(armpmu);
> 		break;
> 
> ... so that we re-start the events before we start the PMU.
> 
> That would be a fix for commit:
> 
>   da4e4f18afe0f372 ("drivers/perf: arm_pmu: implement CPU_PM notifier")

Yes that's correct, apologies. Probably we did not hit it because CPU PM
notifier entry failures are a pretty rare event; regardless:

Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi@....com>

I can send the updated fix, just let me know.

Thanks,
Lorenzo

> Thanks,
> Mark.
> 
> > 
> >  drivers/perf/arm_pmu.c | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/drivers/perf/arm_pmu.c b/drivers/perf/arm_pmu.c
> > index 2d06b8095a19..465a15705bab 100644
> > --- a/drivers/perf/arm_pmu.c
> > +++ b/drivers/perf/arm_pmu.c
> > @@ -724,6 +724,7 @@ static int cpu_pm_pmu_notify(struct notifier_block *b, unsigned long cmd,
> >  		break;
> >  	case CPU_PM_EXIT:
> >  		cpu_pm_pmu_setup(armpmu, cmd);
> > +		/* Fall through */
> >  	case CPU_PM_ENTER_FAILED:
> >  		armpmu->start(armpmu);
> >  		break;
> > -- 
> > 2.20.1
> >