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>

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;
 	}

drivers/gpu/drm/tegra/plane.c

@@ -3,6 +3,8 @@
  * Copyright (C) 2017 NVIDIA CORPORATION.  All rights reserved.
  */
 
+#include <linux/iommu.h>
+
 #include <drm/drm_atomic.h>
 #include <drm/drm_atomic_helper.h>
 #include <drm/drm_fourcc.h>
@@ -107,21 +109,27 @@ const struct drm_plane_funcs tegra_plane_funcs = {
 
 static int tegra_dc_pin(struct tegra_dc *dc, struct tegra_plane_state *state)
 {
+	struct iommu_domain *domain = iommu_get_domain_for_dev(dc->dev);
 	unsigned int i;
 	int err;
 
 	for (i = 0; i < state->base.fb->format->num_planes; i++) {
 		struct tegra_bo *bo = tegra_fb_get_plane(state->base.fb, i);
-
-		if (!dc->client.group) {
+		dma_addr_t phys_addr, *phys;
 		struct sg_table *sgt;
 
-			sgt = host1x_bo_pin(dc->dev, &bo->base, NULL);
+		if (!domain || dc->client.group)
+			phys = &phys_addr;
+		else
+			phys = NULL;
+
+		sgt = host1x_bo_pin(dc->dev, &bo->base, phys);
 		if (IS_ERR(sgt)) {
 			err = PTR_ERR(sgt);
 			goto unpin;
 		}
 
+		if (sgt) {
 			err = dma_map_sg(dc->dev, sgt->sgl, sgt->nents,
 					 DMA_TO_DEVICE);
 			if (err == 0) {
@@ -143,7 +151,7 @@ static int tegra_dc_pin(struct tegra_dc *dc, struct tegra_plane_state *state)
 			state->iova[i] = sg_dma_address(sgt->sgl);
 			state->sgt[i] = sgt;
 		} else {
-			state->iova[i] = bo->iova;
+			state->iova[i] = phys_addr;
 		}
 	}
 
@@ -156,9 +164,11 @@ static int tegra_dc_pin(struct tegra_dc *dc, struct tegra_plane_state *state)
 		struct tegra_bo *bo = tegra_fb_get_plane(state->base.fb, i);
 		struct sg_table *sgt = state->sgt[i];
 
-		dma_unmap_sg(dc->dev, sgt->sgl, sgt->nents, DMA_TO_DEVICE);
-		host1x_bo_unpin(dc->dev, &bo->base, sgt);
+		if (sgt)
+			dma_unmap_sg(dc->dev, sgt->sgl, sgt->nents,
+				     DMA_TO_DEVICE);
 
+		host1x_bo_unpin(dc->dev, &bo->base, sgt);
 		state->iova[i] = DMA_MAPPING_ERROR;
 		state->sgt[i] = NULL;
 	}
@@ -172,17 +182,13 @@ static void tegra_dc_unpin(struct tegra_dc *dc, struct tegra_plane_state *state)
 
 	for (i = 0; i < state->base.fb->format->num_planes; i++) {
 		struct tegra_bo *bo = tegra_fb_get_plane(state->base.fb, i);
-
-		if (!dc->client.group) {
 		struct sg_table *sgt = state->sgt[i];
 
-			if (sgt) {
+		if (sgt)
 			dma_unmap_sg(dc->dev, sgt->sgl, sgt->nents,
 				     DMA_TO_DEVICE);
-				host1x_bo_unpin(dc->dev, &bo->base, sgt);
-			}
-		}
 
+		host1x_bo_unpin(dc->dev, &bo->base, sgt);
 		state->iova[i] = DMA_MAPPING_ERROR;
 		state->sgt[i] = NULL;
 	}

drivers/gpu/host1x/job.c

@@ -8,6 +8,7 @@
 #include <linux/dma-mapping.h>
 #include <linux/err.h>
 #include <linux/host1x.h>
+#include <linux/iommu.h>
 #include <linux/kref.h>
 #include <linux/module.h>
 #include <linux/scatterlist.h>
@@ -101,9 +102,11 @@ static unsigned int pin_job(struct host1x *host, struct host1x_job *job)
 {
 	struct host1x_client *client = job->client;
 	struct device *dev = client->dev;
+	struct iommu_domain *domain;
 	unsigned int i;
 	int err;
 
+	domain = iommu_get_domain_for_dev(dev);
 	job->num_unpins = 0;
 
 	for (i = 0; i < job->num_relocs; i++) {
@@ -117,7 +120,19 @@ static unsigned int pin_job(struct host1x *host, struct host1x_job *job)
 			goto unpin;
 		}
 
-		if (client->group)
+		/*
+		 * If the client device is not attached to an IOMMU, the
+		 * physical address of the buffer object can be used.
+		 *
+		 * Similarly, when an IOMMU domain is shared between all
+		 * host1x clients, the IOVA is already available, so no
+		 * need to map the buffer object again.
+		 *
+		 * XXX Note that this isn't always safe to do because it
+		 * relies on an assumption that no cache maintenance is
+		 * needed on the buffer objects.
+		 */
+		if (!domain || client->group)
 			phys = &phys_addr;
 		else
 			phys = NULL;
@@ -176,6 +191,7 @@ static unsigned int pin_job(struct host1x *host, struct host1x_job *job)
 		dma_addr_t phys_addr;
 		unsigned long shift;
 		struct iova *alloc;
+		dma_addr_t *phys;
 		unsigned int j;
 
 		g->bo = host1x_bo_get(g->bo);
@@ -184,7 +200,17 @@ static unsigned int pin_job(struct host1x *host, struct host1x_job *job)
 			goto unpin;
 		}
 
-		sgt = host1x_bo_pin(host->dev, g->bo, NULL);
+		/**
+		 * If the host1x is not attached to an IOMMU, there is no need
+		 * to map the buffer object for the host1x, since the physical
+		 * address can simply be used.
+		 */
+		if (!iommu_get_domain_for_dev(host->dev))
+			phys = &phys_addr;
+		else
+			phys = NULL;
+
+		sgt = host1x_bo_pin(host->dev, g->bo, phys);
 		if (IS_ERR(sgt)) {
 			err = PTR_ERR(sgt);
 			goto unpin;
@@ -214,7 +240,7 @@ static unsigned int pin_job(struct host1x *host, struct host1x_job *job)
 
 			job->unpins[job->num_unpins].size = gather_size;
 			phys_addr = iova_dma_addr(&host->iova, alloc);
-		} else {
+		} else if (sgt) {
 			err = dma_map_sg(host->dev, sgt->sgl, sgt->nents,
 					 DMA_TO_DEVICE);
 			if (!err) {