gpu: host1x: Avoid trying to use GART on Tegra20

[ Upstream commit c2418f911a31a266af4fbaca998dc73d3676475a ]

Since commit c7e3ca515e ("iommu/tegra: gart: Do not register with
bus") quite some time ago, the GART driver has effectively disabled
itself to avoid issues with the GPU driver expecting it to work in ways
that it doesn't. As of commit 57365a04c921 ("iommu: Move bus setup to
IOMMU device registration") that bodge no longer works, but really the
GPU driver should be responsible for its own behaviour anyway. Make the
workaround explicit.

Reported-by: Jon Hunter <jonathanh@nvidia.com>
Suggested-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Tested-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/gpu/drm/tegra/drm.c

@@ -1042,6 +1042,10 @@ static bool host1x_drm_wants_iommu(struct host1x_device *dev)
 	struct host1x *host1x = dev_get_drvdata(dev->dev.parent);
 	struct iommu_domain *domain;
 
+	/* Our IOMMU usage policy doesn't currently play well with GART */
+	if (of_machine_is_compatible("nvidia,tegra20"))
+		return false;
+
 	/*
 	 * If the Tegra DRM clients are backed by an IOMMU, push buffers are
 	 * likely to be allocated beyond the 32-bit boundary if sufficient