Re: [PATCH v5 5/5] pci: Suspend iommu function prior to resetting a device

[Baolu Lu]

On 11/18/25 09:42, Nicolin Chen wrote:
> On Tue, Nov 18, 2025 at 12:29:43AM +0000, Tian, Kevin wrote:
> > > From: Nicolin Chen<nicolinc@xxxxxxxxxx>
> > > Sent: Tuesday, November 18, 2025 3:27 AM
> > >
> > > On Mon, Nov 17, 2025 at 04:52:05AM +0000, Tian, Kevin wrote:
> > > > > From: Nicolin Chen<nicolinc@xxxxxxxxxx>
> > > > > Sent: Saturday, November 15, 2025 2:01 AM
> > > > >
> > > > > On Fri, Nov 14, 2025 at 09:45:31AM +0000, Tian, Kevin wrote:
> > > > > > > From: Nicolin Chen<nicolinc@xxxxxxxxxx>
> > > > > > > Sent: Tuesday, November 11, 2025 1:13 PM
> > > > > > >
> > > > > > > +/*
> > > > > > > + * Per PCIe r6.3, sec 10.3.1 IMPLEMENTATION NOTE, software
> > > disables
> > > > > ATS
> > > > > > > before
> > > > > > > + * initiating a reset. Notify the iommu driver that enabled ATS.
> > > > > > > + */
> > > > > > > +int pci_reset_iommu_prepare(struct pci_dev *dev)
> > > > > > > +{
> > > > > > > +	if (pci_ats_supported(dev))
> > > > > > > +		return iommu_dev_reset_prepare(&dev->dev);
> > > > > > > +	return 0;
> > > > > > > +}
> > > > > > the comment says "driver that enabled ATS", but the code checks
> > > > > > whether ATS is supported.
> > > > > >
> > > > > > which one is desired?
> > > > > The comments says "the iommu driver that enabled ATS". It doesn't
> > > > > conflict with what the PCI core checks here?
> > > > actually this is sent to all IOMMU drivers. there is no check on whether
> > > > a specific driver has enabled ATS in this path.
> > > But the comment doesn't say "check"..
> > >
> > > How about "Notify the iommu driver that enables/disables ATS"?
> > >
> > > The point is that pci_enable_ats() is called in iommu drivers.
> > but in current way even an iommu driver which doesn't call
> > pci_enable_ats() will also be notified then I didn't see the
> > point of adding an attribute to "the iommu driver".
> Hmm, that's a fair point.
>
> Having looked closely, I see only AMD and ARM call that to enable
> ATs. How others (e.g. Intel) enable it?

The VT-d driver enables ATS in the iommu probe_finalize() path (for
scalable mode).

static void intel_iommu_probe_finalize(struct device *dev)
{

[...]
        if (sm_supported(iommu) && !dev_is_real_dma_subdevice(dev)) {
                iommu_enable_pci_ats(info);
                /* Assign a DEVTLB cache tag to the default domain. */
                if (info->ats_enabled && info->domain) {
                        u16 did = domain_id_iommu(info->domain, iommu);

                        if (cache_tag_assign(info->domain, did, dev,
                                             IOMMU_NO_PASID, 
CACHE_TAG_DEVTLB))
                                iommu_disable_pci_ats(info);
                }
        }

[...]
}

iommu_enable_pci_ats() will eventually call pci_enable_ats() after some
necessary checks.

Thanks,
baolu