Re: [patch 33/37] iommu/arm-smmu-v3: Use msi_get_virq()

[Robin Murphy]

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