drm/tegra: Reuse IOVA mapping where possible

This partially reverts the DMA API support that was recently merged
because it was causing performance regressions on older Tegra devices.
Unfortunately, the cache maintenance performed by dma_map_sg() and
dma_unmap_sg() causes performance to drop by a factor of 10.

The right solution for this would be to cache mappings for buffers per
consumer device, but that's a bit involved. Instead, we simply revert to
the old behaviour of sharing IOVA mappings when we know that devices can
do so (i.e. they share the same IOMMU domain).

Cc: <stable@vger.kernel.org> # v5.5
Reported-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Tested-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>

1 files changed, 9 insertions, 1 deletions

diff --git a/drivers/gpu/drm/tegra/gem.c b/drivers/gpu/drm/tegra/gem.c
index bc15b430156d..c46b4d4190ac 100644
--- a/drivers/gpu/drm/tegra/gem.c
+++ b/drivers/gpu/drm/tegra/gem.c
@@ -60,8 +60,16 @@ static struct sg_table *tegra_bo_pin(struct device *dev, struct host1x_bo *bo,
 	/*
 	 * If we've manually mapped the buffer object through the IOMMU, make
 	 * sure to return the IOVA address of our mapping.
+	 *
+	 * Similarly, for buffers that have been allocated by the DMA API the
+	 * physical address can be used for devices that are not attached to
+	 * an IOMMU. For these devices, callers must pass a valid pointer via
+	 * the @phys argument.
+	 *
+	 * Imported buffers were also already mapped at import time, so the
+	 * existing mapping can be reused.
 	 */
-	if (phys && obj->mm) {
+	if (phys) {
 		*phys = obj->iova;
 		return NULL;
 	}