Re: [RFC] iommu: Fix virtio-iommu probing

[Robin Murphy]

On 14/08/2025 4:19 pm, Eric Auger wrote:
> Hi Robin
>
> On 8/14/25 4:53 PM, Robin Murphy wrote:
> > On 14/08/2025 3:17 pm, Eric Auger wrote:
> > > Commit bcb81ac6ae3c ("iommu: Get DT/ACPI parsing into the proper
> > > probe path") broke virtio-iommu probing and no iommu group are
> > > produced anymore.
> > >
> > > When probe_iommu_group() gets called viommu_probe_device() fails
> > > because viommu_get_by_fwnode(fwspec->iommu_fwnode) returns NULL.
> >
> > ...which it's not supposed to. And *now* I remember, we never got this
> > finished, did we?
> Seems we did not ;-)
> > https://lore.kernel.org/linux-iommu/9beaed48da83a0882dba153e65e6cfd0a8e21482.1742484773.git.robin.murphy@xxxxxxx/
>
> Unfortunately it does not fix my issue. Still no iommu group when
> booting with ACPI.

Indeed the evidence at the time suggested the patch isn't quite right 
as-is, but that is definitely the place which needs fixing. Since 
Jean-Philippe's occupied with more exciting things at the moment, do you 
happen to have an easy recipe for testing virtio-iommu so I can try 
debugging it myself?

Cheers,
Robin.

> Thanks
>
> Eric
> > Thanks,
> > Robin.
> >
> > > So it seems we need to restore the original iommu_probe_device
> > > call site in acpi_iommu_configure_id() to get a chance to probe
> > > the device again.
> > >
> > > Maybe this defeats the whole purpose of the original commit but
> > > at least it fixes the virtio-iommu probing.
> > >
> > > Fixes: bcb81ac6ae3c ("iommu: Get DT/ACPI parsing into the proper
> > > probe path")
> > > Cc: stable@xxxxxxxxxxxxxxx # v6.15+
> > > Signed-off-by: Eric Auger <eric.auger@xxxxxxxxxx>
> > >
> > > ---
> > >
> > > I also tested smmu probing and this seems to work fine.
> > > ---
> > >    drivers/acpi/scan.c | 7 +++++++
> > >    1 file changed, 7 insertions(+)
> > >
> > > diff --git a/drivers/acpi/scan.c b/drivers/acpi/scan.c
> > > index fb1fe9f3b1a3..9f4efa8f75a6 100644
> > > --- a/drivers/acpi/scan.c
> > > +++ b/drivers/acpi/scan.c
> > > @@ -1632,6 +1632,13 @@ static int acpi_iommu_configure_id(struct
> > > device *dev, const u32 *id_in)
> > >            err = viot_iommu_configure(dev);
> > >        mutex_unlock(&iommu_probe_device_lock);
> > >    +    /*
> > > +     * If we have reason to believe the IOMMU driver missed the initial
> > > +     * iommu_probe_device() call for dev, replay it to get things in
> > > order.
> > > +     */
> > > +    if (!err && dev->bus)
> > > +        err = iommu_probe_device(dev);
> > > +
> > >        return err;
> > >    }
> > >