linux-kernel - Re: [PATCH v3 2/4] drm/i915/pmu: Use kstat

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Date:   Sun, 06 Dec 2020 17:38:44 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jerry Snitselaar <jsnitsel@...hat.com>,
        linux-kernel@...r.kernel.org
Cc:     linux-integrity@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Peter Huewe <peterhuewe@....de>,
        James Bottomley <James.Bottomley@...senPartnership.com>,
        Matthew Garrett <mjg59@...gle.com>,
        Hans de Goede <hdegoede@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 2/4] drm/i915/pmu: Use kstat_irqs to get interrupt count

On Fri, Dec 04 2020 at 18:43, Jerry Snitselaar wrote:

> Now that kstat_irqs is exported, get rid of count_interrupts in
> i915_pmu.c
> --- a/drivers/gpu/drm/i915/i915_pmu.c
> +++ b/drivers/gpu/drm/i915/i915_pmu.c
> @@ -423,22 +423,6 @@ static enum hrtimer_restart i915_sample(struct hrtimer *hrtimer)
>  	return HRTIMER_RESTART;
>  }
>  
> -static u64 count_interrupts(struct drm_i915_private *i915)
> -{
> -	/* open-coded kstat_irqs() */
> -	struct irq_desc *desc = irq_to_desc(i915->drm.pdev->irq);
> -	u64 sum = 0;
> -	int cpu;
> -
> -	if (!desc || !desc->kstat_irqs)
> -		return 0;
> -
> -	for_each_possible_cpu(cpu)
> -		sum += *per_cpu_ptr(desc->kstat_irqs, cpu);
> -
> -	return sum;
> -}

May I ask why this has been merged in the first place?

Nothing in a driver has ever to fiddle with the internals of an irq
descriptor. We have functions for properly accessing them. Just because
C allows to fiddle with everything is not a justification. If the
required function is not exported then adding the export with a proper
explanation is not asked too much.

Also this lacks protection or at least a comment why this can be called
safely and is not subject to a concurrent removal of the irq descriptor.
The same problem exists when calling kstat_irqs(). It's even documented
at the top of the function.

Thanks,

        tglx