Re: [PATCH V3 0/8] IOMMU probe deferral support

Will Deacon
2016-11-07T19:13:12Z

On Fri, Nov 04, 2016 at 08:46:06PM +0530, Sricharan wrote:
> >>>Yikes, on second look, that definitely shouldn't be happening.
> >>>Everything below is probably the resulting fallout.
> >>
> >>[ 40.206703] vfio-pci 0000:08:00.0: Failed to setup iommu ops
> >>
> >>I think the above print which says "failed to setup iommu_ops"
> >>because the call ops->add_device failed in of_pci_iommu_configure
> >>is the reason for the failure, in my case i simply do not get this even with
> >>your scripts. ops->add_device succeeds in the rebind as well. So still
> >>checking what could be happening in your case.
> >
> >I was looking at your code base from [1].The ops->add_device
> >callback from of_pci_iommu_configure on the rebind is the
> >one which is causing the failure. But not able to spot out
> >from code which point is causing the failure. It would be very helpful
> >if i can know which is the return value from the add_device callback
> >or point inside add_device callback which fails in your setup.
> >
> >
> >[1] git://linux-arm.org/linux-rm iommu/misc

So this also applies to mainline.

> With little more try, i saw an issue where i had an failure
> similar to what you reported. The issue happens when multiple
> devices fall in to same group due to matching sids. I ended up
> doing a fix like below and it would be nice to verify if it is the same
> that we are seeing in your setup and if the fix makes a difference ?
>
> From: Sricharan R <sricharan-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>
> Date: Fri, 4 Nov 2016 20:28:49 +0530
> Subject: [PATCH] iommu/arm-smmu: Fix group's reference counting
>
> iommu_group_get_for_dev which gets called in the add_device
> callback, increases the reference count of the iommu_group,
> so we do an iommu_group_put after that. iommu_group_get_for_dev
> inturn calls device_group callback and in the case of arm-smmu
> we call generic_device_group/pci_device_group which takes
> care of increasing the group's reference. But when we return
> an already existing group(when multiple devices have same group)
> the reference is not incremented, resulting in issues when the
> remove_device callback for the devices is invoked.
> Fixing the same here.
>
> Signed-off-by: Sricharan R <sricharan-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>
> ---
> drivers/iommu/arm-smmu.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/iommu/arm-smmu.c b/drivers/iommu/arm-smmu.c
> index 71ce4b6..a1d0b3c 100644
> --- a/drivers/iommu/arm-smmu.c
> +++ b/drivers/iommu/arm-smmu.c
> @@ -1516,8 +1516,10 @@ static struct iommu_group *arm_smmu_device_group(struct device *dev)
> group = smmu->s2crs[idx].group;
> }
>
> - if (group)
> + if (group) {
> + iommu_group_get_by_id(iommu_group_id(group));
> return group;
> + }

This is foul :(

I think we should either add a function for incrementing the refcount on a
group, or we should get a handle on the existing aliasing device and get
the group from that. As written, this patch is far too subtle.

Joerg -- do you have any preference?

Will