[PATCH v2 09/22] drm/tegra: Don't use IOMMU on Tegra20

[Erik Faye-Lund]

On Wed, Jun 14, 2017 at 1:15 AM, Dmitry Osipenko <digetx at gmail.com> wrote:
> There is no IOMMU on Tegra20, instead a GART would be picked as an IOMMU
> provider.
>
> Signed-off-by: Dmitry Osipenko <digetx at gmail.com>
> Acked-by: Joerg Roedel <jroedel at suse.de>
> ---
>  drivers/gpu/drm/tegra/drm.c | 3 ++-
>  drivers/gpu/host1x/dev.c    | 3 ++-
>  2 files changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/tegra/drm.c b/drivers/gpu/drm/tegra/drm.c
> index e999391aedc9..aa7988dcc28f 100644
> --- a/drivers/gpu/drm/tegra/drm.c
> +++ b/drivers/gpu/drm/tegra/drm.c
> @@ -131,7 +131,8 @@ static int tegra_drm_load(struct drm_device *drm, unsigned long flags)
>         if (!tegra)
>                 return -ENOMEM;
>
> -       if (iommu_present(&platform_bus_type)) {
> +       if (iommu_present(&platform_bus_type) &&
> +           !of_machine_is_compatible("nvidia,tegra20")) {
>                 u64 carveout_start, carveout_end, gem_start, gem_end;
>                 struct iommu_domain_geometry *geometry;
>                 unsigned long order;
> diff --git a/drivers/gpu/host1x/dev.c b/drivers/gpu/host1x/dev.c
> index f05ebb14fa63..6a805ed908c3 100644
> --- a/drivers/gpu/host1x/dev.c
> +++ b/drivers/gpu/host1x/dev.c
> @@ -177,7 +177,8 @@ static int host1x_probe(struct platform_device *pdev)
>                 return err;
>         }
>
> -       if (iommu_present(&platform_bus_type)) {
> +       if (iommu_present(&platform_bus_type) &&
> +           !of_machine_is_compatible("nvidia,tegra20")) {
>                 struct iommu_domain_geometry *geometry;
>                 unsigned long order;
>

This doesn't feel great... The commit message says there's no IOMMU,
but iommu_present says otherwise. I know there's some more subtleties
here, and the commit message does hint at this. But...

If we don't want to treat the GART as an IOMMU, shouldn't we somehow
make sure iommu_present() doesn't return true in these cases (or
perhaps make something like tegra_use_iommu(), with a comment
explaining why we don't want to allow the GART to be treated like a
proper IOMMU)? These seems to be the only Tegra-specific calls to
iommu_present()...

That being said, the patch seems to have the right effect...