[PATCH v3 4/4] drm/amdgpu: Optimize DMABuf attachment with XGMI

[Felix Kuehling]

When peer memory is accessed through XGMI, it does not need to be visible
in the BAR and there is no need for SG-tables or DMA mappings.

Signed-off-by: Felix Kuehling <felix.kuehling at amd.com>
Tested-by: Hao (Claire) Zhou <hao.zhou at amd.com>
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
index e6913fcf2c7b..266aa1434471 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
@@ -146,6 +146,11 @@ static void amdgpu_dma_buf_unpin(struct dma_buf_attachment *attach)
 	amdgpu_bo_unpin(bo);
 }
 
+/* Dummy SG table for XGMI attachments. It should never be dereferenced. If it
+ * is, it will be caught as a kernel oops.
+ */
+#define SGT_DUMMY ((struct sg_table *)1)
+
 /**
  * amdgpu_dma_buf_map - &dma_buf_ops.map_dma_buf implementation
  * @attach: DMA-buf attachment
@@ -162,9 +167,11 @@ static void amdgpu_dma_buf_unpin(struct dma_buf_attachment *attach)
 static struct sg_table *amdgpu_dma_buf_map(struct dma_buf_attachment *attach,
 					   enum dma_data_direction dir)
 {
+	struct amdgpu_device *attach_adev = dma_buf_attach_adev(attach);
 	struct dma_buf *dma_buf = attach->dmabuf;
 	struct drm_gem_object *obj = dma_buf->priv;
 	struct amdgpu_bo *bo = gem_to_amdgpu_bo(obj);
+	bool is_xgmi = amdgpu_dmabuf_is_xgmi_accessible(attach_adev, bo);
 	struct amdgpu_device *adev = amdgpu_ttm_adev(bo->tbo.bdev);
 	struct sg_table *sgt;
 	long r;
@@ -176,7 +183,8 @@ static struct sg_table *amdgpu_dma_buf_map(struct dma_buf_attachment *attach,
 
 		if (bo->preferred_domains & AMDGPU_GEM_DOMAIN_VRAM &&
 		    attach->peer2peer) {
-			bo->flags |= AMDGPU_GEM_CREATE_CPU_ACCESS_REQUIRED;
+			if (!is_xgmi)
+				bo->flags |= AMDGPU_GEM_CREATE_CPU_ACCESS_REQUIRED;
 			domains |= AMDGPU_GEM_DOMAIN_VRAM;
 		}
 		amdgpu_bo_placement_from_domain(bo, domains);
@@ -199,6 +207,9 @@ static struct sg_table *amdgpu_dma_buf_map(struct dma_buf_attachment *attach,
 		break;
 
 	case TTM_PL_VRAM:
+		if (is_xgmi)
+			return SGT_DUMMY;
+
 		r = amdgpu_vram_mgr_alloc_sgt(adev, bo->tbo.resource, 0,
 					      bo->tbo.base.size, attach->dev,
 					      dir, &sgt);
@@ -230,6 +241,9 @@ static void amdgpu_dma_buf_unmap(struct dma_buf_attachment *attach,
 				 struct sg_table *sgt,
 				 enum dma_data_direction dir)
 {
+	if (sgt == SGT_DUMMY)
+		return;
+
 	if (sg_page(sgt->sgl)) {
 		dma_unmap_sgtable(attach->dev, sgt, dir, 0);
 		sg_free_table(sgt);
-- 
2.34.1