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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Jerry Snitselaar <jsnitsel@redhat.com>,
        linux-kernel@vger.kernel.org
Cc:     linux-integrity@vger.kernel.org, intel-gfx@lists.freedesktop.org,
        dri-devel@lists.freedesktop.org,
        Jani Nikula <jani.nikula@linux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Jarkko Sakkinen <jarkko@kernel.org>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Peter Huewe <peterhuewe@gmx.de>,
        James Bottomley <James.Bottomley@HansenPartnership.com>,
        Matthew Garrett <mjg59@google.com>,
        Hans de Goede <hdegoede@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v3 2/4] drm/i915/pmu: Use kstat_irqs to get interrupt count
In-Reply-To: <20201205014340.148235-3-jsnitsel@redhat.com>
References: <20201205014340.148235-1-jsnitsel@redhat.com> <20201205014340.148235-3-jsnitsel@redhat.com>
Date:   Sun, 06 Dec 2020 17:38:44 +0100
Message-ID: <875z5e99ez.fsf@nanos.tec.linutronix.de>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

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