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

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Date:   Sun, 06 Dec 2020 22:33:09 +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 Sun, Dec 06 2020 at 17:38, Thomas Gleixner wrote:
> 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
>
> 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.

And as pointed out vs. that TPM thing this really could have been a
trivial

    i915->irqs++;

in the interrupt handler and a read of that instead of iterating over
all possible cpus and summing it up. Oh well...

Thanks,

        tglx