Re: [PATCH 1/2] ARM: SMMU: add devices attached to the SMMU to an IOMMU group

Antonios Motakis
2013-10-14T15:13:17Z

On Mon, Oct 14, 2013 at 2:48 PM, Will Deacon <will.deacon@arm.com> wrote:
>
> Hi Antonios,
>
> On Fri, Oct 11, 2013 at 02:24:46PM +0100, Antonios Motakis wrote:
> > IOMMU groups are expected by certain users of the IOMMU API,
> > e.g. VFIO. Add new devices found by the SMMU driver to an IOMMU
> > group to satisfy those users.
>
> Cheers for looking at this: VFIO has been on my TODO list for some time.
>
> > Signed-off-by: Antonios Motakis <a.motakis@virtualopensystems.com>
> > ---
> > drivers/iommu/arm-smmu.c | 18 +++++++++++++++++-
> > 1 file changed, 17 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/iommu/arm-smmu.c b/drivers/iommu/arm-smmu.c
> > index 0f45a48..8b71332 100644
> > --- a/drivers/iommu/arm-smmu.c
> > +++ b/drivers/iommu/arm-smmu.c
> > @@ -1502,6 +1502,8 @@ static int arm_smmu_add_device(struct device *dev)
> > {
> > struct arm_smmu_device *child, *parent, *smmu;
> > struct arm_smmu_master *master = NULL;
> > + struct iommu_group *group;
> > + int ret;
> >
> > spin_lock(&arm_smmu_devices_lock);
> > list_for_each_entry(parent, &arm_smmu_devices, list) {
> > @@ -1534,13 +1536,27 @@ static int arm_smmu_add_device(struct device *dev)
> > if (!master)
> > return -ENODEV;
> >
> > + group = iommu_group_get(dev);
>
> I'm not especially familiar with IOMMU groups (I understand them as the
> minimum translation granularity, which would mean single StreamID for the
> ARM SMMU), but under what circumstances would you expect to receive a
> non-NULL group here? I can't see any other code adding devices to groups
> (outside of other drivers)...
>
Hi Will,

You are right, only other IOMMU drivers will add a device to a group.
There was a discussion about this when I posted a similar patch for
the Exynos System MMU driver, see
http://lists.infradead.org/pipermail/linux-arm-kernel/2013-July/185675.html

The idea is to check in the case of add_device() being called multiple
times, which is not the case most of the time, but still a sane
safeguard.

>
> Will