Re: [PATCH v3 3/3] iommu/arm-smmu-v3: Fix ATS state tracking via ats_prepared gate

[Pranjal Shrivastava <praan@google.com>]

On Tue, May 19, 2026 at 11:59:47AM -0300, Jason Gunthorpe wrote:
> On Tue, May 19, 2026 at 02:55:54PM +0000, Pranjal Shrivastava wrote:
> > On Tue, May 19, 2026 at 11:44:30AM -0300, Jason Gunthorpe wrote:
> > > On Tue, May 19, 2026 at 01:53:22PM +0000, Pranjal Shrivastava wrote:
> > > > @@ -4450,7 +4450,8 @@ static struct iommu_device *arm_smmu_probe_device(struct device *dev)
> > > >  	if (dev_is_pci(dev)) {
> > > >  		unsigned int stu = __ffs(smmu->pgsize_bitmap);
> > > >  
> > > > -		pci_prepare_ats(to_pci_dev(dev), stu);
> > > > +		if (!pci_prepare_ats(to_pci_dev(dev), stu))
> > > > +			master->ats_prepared = true;
> > > >  	}
> > > 
> > > This should fail not keep going, it is a kernel bug if
> > > pci_prepare_ats() fails.
> > 

[...]
 
> > If ats isn't supported or if there's a STU mismatch, shuoldn't we just
> > ensure that we don't try enabling ATS for that device?
> 
> No, the kernel is broken somehow.
> 
> > Returning an error here would fail the device probe.. which seems like a
> > big hammer for devices that can't reliably use ATS.. 
> 
> It has nothing to do with devices, the kernel is broken if it fails.

Ack. I see your point.. none of these should happen if the kernel isn't
broken somewhere.

> 
> > We have a special case in SMMUv3 where we pre-allocate stuff in prepare
> > but actually enable ATS in attach_commit, which IMO neeeds to be gated 
> > based on client device's ATS capabilities?
> 
> Yes, all of this should be gated by pci_ats_supported()
> 
> But also failing to enable should translate into a failed attach.

Ack. I guess we already check pci_ats_supported during attach_prepare()
while setting the state->ats_enable. Based on this state->ats_enable we
call pci_enable_ats() in attach_commit().

However, I'm thinking what's the right thing to do if pci_enable_ats() 
fails in attach_commit():

1. Fail to attach but unmerging the invs_array entry is complicated.
2. Move the pci_enable_ats() to attach_prepare()
3. Continue without *really* enabling ATS.. 

Thanks,
Praan