RE: [PATCH] iommu/exynos: add devices attached to the System MMU to anIOMMU group

[Inki Dae]

> -----Original Message-----
> From: Antonios Motakis [mailto:a.motakis@xxxxxxxxxxxxxxxxxxxxxx]
> Sent: Tuesday, July 23, 2013 8:00 PM
> To: Inki Dae
> Cc: Linux ARM Kernel; Linux IOMMU; Linux Samsung SOC; kvm-arm; Cho
KyongHo;
> Joerg Roedel; Sachin Kamat; Jiri Kosina; Wei Yongjun; open list
> Subject: Re: [PATCH] iommu/exynos: add devices attached to the System MMU
> to an IOMMU group
> 
> On Tue, Jul 23, 2013 at 12:31 PM, Inki Dae <inki.dae@xxxxxxxxxxx> wrote:
> >
> >
> >
> > > -----Original Message-----
> > > From: linux-samsung-soc-owner@xxxxxxxxxxxxxxx [mailto:linux-samsung-
> soc-
> > > owner@xxxxxxxxxxxxxxx] On Behalf Of Antonios Motakis
> > > Sent: Tuesday, July 23, 2013 7:02 PM
> > > To: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx;
> > iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx;
> > > linux-samsung-soc@xxxxxxxxxxxxxxx
> > > Cc: kvmarm@xxxxxxxxxxxxxxxxxxxxx; Antonios Motakis; Cho KyongHo; Joerg
> > > Roedel; Sachin Kamat; Jiri Kosina; Wei Yongjun; open list
> > > Subject: [PATCH] iommu/exynos: add devices attached to the System MMU
> to
> > > an IOMMU group
> > >
> > > IOMMU groups are expected by certain users of the IOMMU API,
> > > e.g. VFIO. Since each device is behind its own System MMU, we
> > > can allocate a new IOMMU group for each device.
> > >
> > > This patch depends on Cho KyongHo's patch series titled "[PATCH v7
> 00/12]
> > > iommu/exynos: Fixes and Enhancements of System MMU driver with DT",
> > > applied on a Linux 3.10.1 kernel. It has been tested on the Arndale
> board.
> > >
> > > Signed-off-by: Antonios Motakis <a.motakis@xxxxxxxxxxxxxxxxxxxxxx>
> > > ---
> > >  drivers/iommu/exynos-iommu.c | 24 ++++++++++++++++++++++++
> > >  1 file changed, 24 insertions(+)
> > >
> > > diff --git a/drivers/iommu/exynos-iommu.c b/drivers/iommu/exynos-
> iommu.c
> > > index 51d43bb..9f39eaa 100644
> > > --- a/drivers/iommu/exynos-iommu.c
> > > +++ b/drivers/iommu/exynos-iommu.c
> > > @@ -1134,6 +1134,28 @@ static phys_addr_t
> exynos_iommu_iova_to_phys(struct
> > > iommu_domain *domain,
> > >       return phys;
> > >  }
> > >
> > > +static int exynos_iommu_add_device(struct device *dev)
> > > +{
> > > +     struct iommu_group *group;
> > > +     int ret;
> > > +
> > > +     group = iommu_group_alloc();
> >
> > Is that correct? I don't see why you allocate a group object every time
> > add_device callback is called. That doesn't have any meaning we have to
> use
> > iommu group feature. I think the implementation should be one more
> devices
> > per a group. So I guess a given device object should be wrapped by
> higher
> > device object than the given device object. For a good example, you can
> > refer to intel-iommu.c file.
> 
> With an Intel IOMMU it can be the case that 2 devices have to share
> the same IOMMU mappings (i.e. you can't program them separately). With
> the Exynos System MMU, there is always one System MMU per device, so
> there is nothing stopping you from programming any 2 devices' mappings
> differently. So yes, the right thing to do here is to have a one to
> one relationship between devices and IOMMU groups.

In case of Exynos drm driver, a unified iommu mapping table is used for all
devices (fimd, g2d, hdmi, fimc, gsc, rotator) based on drm so they use the
same iommu mapping table even though they have each iommu hardware unit. And
the iommu mapping table is just logical data structure for hardware
translation process by each DMA. Actually, I am considering using iommu
group feature for more generic implementation.

And one question. Why do you allocate a iommu group object if we should have
one to one relationship between devices and iommu groups? In this case, is
there any reason you have to use the iommu group object?

Thanks,
Inki Dae

> 
> (resending because of html mail)
> 
> Cheers,
> Antonios
> 
> >
> >
> > Thanks,
> > Inki Dae
> >
> > > +     if (IS_ERR(group)) {
> > > +             dev_err(dev, "Failed to allocate IOMMU group\n");
> > > +             return PTR_ERR(group);
> > > +     }
> > > +
> > > +     ret = iommu_group_add_device(group, dev);
> > > +     iommu_group_put(group);
> > > +
> > > +     return ret;
> > > +}
> > > +
> > > +static void exynos_iommu_remove_device(struct device *dev)
> > > +{
> > > +     iommu_group_remove_device(dev);
> > > +}
> > > +
> > >  static struct iommu_ops exynos_iommu_ops = {
> > >       .domain_init = &exynos_iommu_domain_init,
> > >       .domain_destroy = &exynos_iommu_domain_destroy,
> > > @@ -1142,6 +1164,8 @@ static struct iommu_ops exynos_iommu_ops = {
> > >       .map = &exynos_iommu_map,
> > >       .unmap = &exynos_iommu_unmap,
> > >       .iova_to_phys = &exynos_iommu_iova_to_phys,
> > > +     .add_device     = exynos_iommu_add_device,
> > > +     .remove_device  = exynos_iommu_remove_device,
> > >       .pgsize_bitmap = SECT_SIZE | LPAGE_SIZE | SPAGE_SIZE,
> > >  };
> > >
> > > --
> > > 1.8.1.2
> > >
> > > --
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> >

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