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

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Message-ID: <87im9ehaim.fsf@redhat.com>
Date:   Sun, 06 Dec 2020 14:47:45 -0700
From:   Jerry Snitselaar <jsnitsel@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, 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


Thomas Gleixner @ 2020-12-06 09:38 MST:

> 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

I don't know the history behind this bit. I stumbled across it in cscope
when looking for places using kstat_irqs.