Re: [PATCH v7 3/3] iommu/tegra-smmu: Add PCI support

[Robin Murphy]

On 2020-10-16 04:53, Nicolin Chen wrote:
> On Thu, Oct 15, 2020 at 10:55:52AM +0100, Robin Murphy wrote:
> > On 2020-10-15 05:13, Nicolin Chen wrote:
> > > On Wed, Oct 14, 2020 at 06:42:36PM +0100, Robin Murphy wrote:
> > > > On 2020-10-09 17:19, Nicolin Chen wrote:
> > > > > This patch simply adds support for PCI devices.
> > > > >
> > > > > Reviewed-by: Dmitry Osipenko <digetx@xxxxxxxxx>
> > > > > Tested-by: Dmitry Osipenko <digetx@xxxxxxxxx>
> > > > > Signed-off-by: Nicolin Chen <nicoleotsuka@xxxxxxxxx>
> > > > > ---
> > > > >
> > > > > Changelog
> > > > > v6->v7
> > > > >     * Renamed goto labels, suggested by Thierry.
> > > > > v5->v6
> > > > >     * Added Dmitry's Reviewed-by and Tested-by.
> > > > > v4->v5
> > > > >     * Added Dmitry's Reviewed-by
> > > > > v3->v4
> > > > >     * Dropped !iommu_present() check
> > > > >     * Added CONFIG_PCI check in the exit path
> > > > > v2->v3
> > > > >     * Replaced ternary conditional operator with if-else in .device_group()
> > > > >     * Dropped change in tegra_smmu_remove()
> > > > > v1->v2
> > > > >     * Added error-out labels in tegra_smmu_probe()
> > > > >     * Dropped pci_request_acs() since IOMMU core would call it.
> > > > >
> > > > >     drivers/iommu/tegra-smmu.c | 35 +++++++++++++++++++++++++----------
> > > > >     1 file changed, 25 insertions(+), 10 deletions(-)
> > > > >
> > > > > diff --git a/drivers/iommu/tegra-smmu.c b/drivers/iommu/tegra-smmu.c
> > > > > index be29f5977145..2941d6459076 100644
> > > > > --- a/drivers/iommu/tegra-smmu.c
> > > > > +++ b/drivers/iommu/tegra-smmu.c
> > > > > @@ -10,6 +10,7 @@
> > > > >     #include <linux/kernel.h>
> > > > >     #include <linux/of.h>
> > > > >     #include <linux/of_device.h>
> > > > > +#include <linux/pci.h>
> > > > >     #include <linux/platform_device.h>
> > > > >     #include <linux/slab.h>
> > > > >     #include <linux/spinlock.h>
> > > > > @@ -865,7 +866,11 @@ static struct iommu_group *tegra_smmu_device_group(struct device *dev)
> > > > >     	group->smmu = smmu;
> > > > >     	group->soc = soc;
> > > > > -	group->group = iommu_group_alloc();
> > > > > +	if (dev_is_pci(dev))
> > > > > +		group->group = pci_device_group(dev);
> > > >
> > > > Just to check, is it OK to have two or more swgroups "owning" the same
> > > > iommu_group if an existing one gets returned here? It looks like that might
> > > > not play nice with the use of iommu_group_set_iommudata().
> > >
> > > Do you mean by "gets returned here" the "IS_ERR" check below?
> >
> > I mean that unlike iommu_group_alloc()/generic_device_group(),
> > pci_device_group() may give you back a group that already contains another
> > device and has already been set up from that device's perspective. This can
> > happen for topological reasons like requester ID aliasing through a PCI-PCIe
> > bridge or lack of isolation between functions.
>
> Okay..but we don't really have two swgroups owning the same groups
> in case of PCI devices. For Tegra210, all PCI devices inherit the
> same swgroup from the PCI controller. And I'd think previous chips
> do the same. The only use case currently of 2+ swgroups owning the
> same iommu_group is for display controller.
>
> Or do you suggest we need an additional check for pci_device_group?

Ah, OK - I still don't have the best comprehension of what exactly 
swgroups are, and the path through .of_xlate looked like you might be 
using the PCI requester ID as the swgroup identifier, but I guess that 
ultimately depends on what your "iommu-map" is supposed to look like. If 
pci_device_group() will effectively only ever get called once regardless 
of how many endpoints exist, then indeed this won't be a concern 
(although if that's *guaranteed* to be the case then you may as well 
just stick with calling iommu_group_alloc() directly). Thanks for 
clarifying.

Robin.