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

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Date:   Tue, 30 Nov 2021 09:36:07 +0000
From:   Will Deacon <will@...nel.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, 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 02:54:18PM +0000, Robin Murphy wrote:
> On 2021-11-29 14:42, Thomas Gleixner wrote:
> > 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.
> 
> Oh, -1 for my reading comprehension but +1 for my confidence in the patch
> then :)
> 
> I'll let Will have the final say over how cautious we really want to be
> here, but as far as I'm concerned it's a welcome cleanup as-is. Ditto for
> patch #32 based on the same reasoning, although I don't have a suitable test
> platform on-hand to sanity-check that one.

If, as it appears, msi_get_virq() isn't going to fail meaningfully after
we've successfully called platform_msi_domain_alloc_irqs() then it sounds
like the patch is fine. Just wanted to check though, as Spring cleaning at
the end of November raised an eyebrow over here :)

Will