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

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Date:   Fri, 26 Jul 2019 13:29:56 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Anders Roxell <anders.roxell@...aro.org>
Cc:     will@...nel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, lorenzo.pieralisi@....com
Subject: Re: [PATCH] arm_pmu: Mark expected switch fall-through

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")
 
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
>