Re: [PATCH v6 3/3] iommu/arm-smmu-v3: Allow ATS to be always on

[Jason Gunthorpe]

On Thu, May 28, 2026 at 04:32:23PM +0000, Pranjal Shrivastava wrote:
> On Thu, May 28, 2026 at 12:29:30PM -0300, Jason Gunthorpe wrote:
> > On Thu, May 28, 2026 at 03:24:40PM +0000, Pranjal Shrivastava wrote:
> > 
> > > > +	master->ats_always_on = true;
> > > > +
> > > > +	return arm_smmu_alloc_cd_tables(master);
> > > 
> > > Nit: I'm not sure if I'm getting this right, are we saying we *need* to
> > > allocate CDs for CXL.cache cases in the probe itself because STE.EATS
> > > requires Config=Translate with S1DSS in bypass?
> > 
> > Yeah, basically.
> > 
> > Effectively the ARM rules for enabling ATS and Bypass together require
> > a valid CD table pointer.
> 
> Ack. But do we expect transactions before attach_dev? What'd happen if
> we don't allocate CDs here?

It can be done at any point before a non-abort STE is formed.

The core code pretty much does this immediately after probe so there
isn't much point in moving it later.

> Am I missing the fact that CXL hardware could be already active and
> using ATS after the device_probe succeeds?

The STEs are preloaded with abort when the driver starts, so that
should not be possible.

Jason