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

[Robin Murphy]

On 2021-11-29 10:55, Will Deacon wrote:
> Hi Thomas,
>
> On Sat, Nov 27, 2021 at 02:20:59AM +0100, Thomas Gleixner wrote:
> > Let the core code fiddle with the MSI descriptor retrieval.
> >
> > Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > ---
> >   drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c |   19 +++----------------
> >   1 file changed, 3 insertions(+), 16 deletions(-)
> >
> > --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> > +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> > @@ -3154,7 +3154,6 @@ static void arm_smmu_write_msi_msg(struc
> >   
> >   static void arm_smmu_setup_msis(struct arm_smmu_device *smmu)
> >   {
> > -	struct msi_desc *desc;
> >   	int ret, nvec = ARM_SMMU_MAX_MSIS;
> >   	struct device *dev = smmu->dev;
> >   
> > @@ -3182,21 +3181,9 @@ static void arm_smmu_setup_msis(struct a
> >   		return;
> >   	}
> >   
> > -	for_each_msi_entry(desc, dev) {
> > -		switch (desc->msi_index) {
> > -		case EVTQ_MSI_INDEX:
> > -			smmu->evtq.q.irq = desc->irq;
> > -			break;
> > -		case GERROR_MSI_INDEX:
> > -			smmu->gerr_irq = desc->irq;
> > -			break;
> > -		case PRIQ_MSI_INDEX:
> > -			smmu->priq.q.irq = desc->irq;
> > -			break;
> > -		default:	/* Unknown */
> > -			continue;
> > -		}
> > -	}
> > +	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. 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?

Robin.