drm/ttm: make TT creation purely optional v3

We only need the page array when the BO is about to be accessed.

So not only populate, but also create it on demand.

v2: move NULL check into ttm_tt_create()
v3: fix the occurrence in ttm_bo_kmap_ttm as well

Signed-off-by: Christian König <[e-mail]>
Reviewed-by: Michael J. Ruhl <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/373182/

drivers/gpu/drm/ttm/ttm_bo.c

···
 	 */
 
 	if (!(new_man->flags & TTM_MEMTYPE_FLAG_FIXED)) {
-		if (bo->ttm == NULL) {
-			bool zero = !(old_man->flags & TTM_MEMTYPE_FLAG_FIXED);
-			ret = ttm_tt_create(bo, zero);
-			if (ret)
-				goto out_err;
-		}
+		bool zero = !(old_man->flags & TTM_MEMTYPE_FLAG_FIXED);
+
+		ret = ttm_tt_create(bo, zero);
+		if (ret)
+			goto out_err;
 
 		ret = ttm_tt_set_placement_caching(bo->ttm, mem->placement);
 		if (ret)
···
 	placement.num_busy_placement = 0;
 	bdev->driver->evict_flags(bo, &placement);
 
-	if (!placement.num_placement && !placement.num_busy_placement) {
-		ret = ttm_bo_pipeline_gutting(bo);
-		if (ret)
-			return ret;
-
-		return ttm_tt_create(bo, false);
-	}
+	if (!placement.num_placement && !placement.num_busy_placement)
+		return ttm_bo_pipeline_gutting(bo);
 
 	evict_mem = bo->mem;
 	evict_mem.mm_node = NULL;
···
 	/*
 	 * Remove the backing store if no placement is given.
 	 */
-	if (!placement->num_placement && !placement->num_busy_placement) {
-		ret = ttm_bo_pipeline_gutting(bo);
-		if (ret)
-			return ret;
-
-		return ttm_tt_create(bo, false);
-	}
+	if (!placement->num_placement && !placement->num_busy_placement)
+		return ttm_bo_pipeline_gutting(bo);
 
 	/*
 	 * Check whether we need to move buffer.
···
 		 */
 		ttm_flag_masked(&bo->mem.placement, new_flags,
 				~TTM_PL_MASK_MEMTYPE);
-	}
-	/*
-	 * We might need to add a TTM.
-	 */
-	if (bo->mem.mem_type == TTM_PL_SYSTEM && bo->ttm == NULL) {
-		ret = ttm_tt_create(bo, true);
-		if (ret)
-			return ret;
 	}
 	return 0;
 }

drivers/gpu/drm/ttm/ttm_bo_util.c

···
 		.interruptible = false,
 		.no_wait_gpu = false
 	};
-	struct ttm_tt *ttm = bo->ttm;
+	struct ttm_tt *ttm;
 	pgprot_t prot;
 	int ret;
 
-	BUG_ON(!ttm);
+	ret = ttm_tt_create(bo, true);
+	if (ret)
+		return ret;
 
+	ttm = bo->ttm;
 	ret = ttm_tt_populate(ttm, &ctx);
 	if (ret)
 		return ret;

drivers/gpu/drm/ttm/ttm_bo_vm.c

···
 
 		};
 
+		if (ttm_tt_create(bo, true)) {
+			ret = VM_FAULT_OOM;
+			goto out_io_unlock;
+		}
+
 		ttm = bo->ttm;
 		if (ttm_tt_populate(bo->ttm, &ctx)) {
 			ret = VM_FAULT_OOM;

drivers/gpu/drm/ttm/ttm_tt.c

···
 
 	dma_resv_assert_held(bo->base.resv);
 
+	if (bo->ttm)
+		return 0;
+
 	if (bdev->need_dma32)
 		page_flags |= TTM_PAGE_FLAG_DMA32;
 
···
 		page_flags |= TTM_PAGE_FLAG_SG;
 		break;
 	default:
-		bo->ttm = NULL;
 		pr_err("Illegal buffer object type\n");
 		return -EINVAL;
 	}