linux-kernel - Re: [patch 33/37] iommu/arm-smmu-v3: Use msi_get

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Message-ID: <87czmjdnw9.ffs@tglx>
Date:   Mon, 29 Nov 2021 15:42:14 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Robin Murphy <robin.murphy@....com>, Will Deacon <will@...nel.org>
Cc:     Nishanth Menon <nm@...com>, Mark Rutland <mark.rutland@....com>,
        Stuart Yoder <stuyoder@...il.com>, linux-pci@...r.kernel.org,
        Ashok Raj <ashok.raj@...el.com>, Marc Zygnier <maz@...nel.org>,
        x86@...nel.org, Sinan Kaya <okaya@...nel.org>,
        iommu@...ts.linux-foundation.org,
        Bjorn Helgaas <helgaas@...nel.org>,
        Megha Dey <megha.dey@...el.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Santosh Shilimkar <ssantosh@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Tero Kristo <kristo@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Vinod Koul <vkoul@...nel.org>, dmaengine@...r.kernel.org
Subject: Re: [patch 33/37] iommu/arm-smmu-v3: Use msi_get_virq()

On Mon, Nov 29 2021 at 13:13, Robin Murphy wrote:
> On 2021-11-29 10:55, Will Deacon wrote:
>>> -	}
>>> +	smmu->evtq.q.irq = msi_get_virq(dev, EVTQ_MSI_INDEX);
>>> +	smmu->gerr_irq = msi_get_virq(dev, GERROR_MSI_INDEX);
>>> +	smmu->priq.q.irq = msi_get_virq(dev, PRIQ_MSI_INDEX);
>> 
>> Prviously, if retrieval of the MSI failed then we'd fall back to wired
>> interrupts. Now, I think we'll clobber the interrupt with 0 instead. Can
>> we make the assignments to smmu->*irq here conditional on the MSI being
>> valid, please?
>
> I was just looking at that too, but reached the conclusion that it's 
> probably OK, since consumption of this value later is gated on 
> ARM_SMMU_FEAT_PRI, so the fact that it changes from 0 to an error value 
> in the absence of PRI should make no practical difference.

It's actually 0 when the vector cannot be found.

> If we don't have MSIs at all, we'd presumably still fail earlier
> either at the dev->msi_domain check or upon trying to allocate the
> vectors, so we'll still fall back to any previously-set wired values
> before getting here.  The only remaining case is if we've
> *successfully* allocated the expected number of vectors yet are then
> somehow unable to retrieve one or more of them - presumably the system
> has to be massively borked for that to happen, at which point do we
> really want to bother trying to reason about anything?

Probably not. At that point something is going to explode sooner than
later in colorful ways.

Thanks,

        tglx