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

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Date:   Mon, 29 Nov 2021 14:54:18 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Thomas Gleixner <tglx@...utronix.de>, 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 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.

Cheers,
Robin.

>> 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
>