drm/ttm, drm/amdgpu: Allow the driver some control over swapping

We are calling the eviction_valuable driver callback at eviction time to
determine whether we actually can evict a buffer object.
The upcoming i915 TTM backend needs the same functionality for swapout,
and that might actually be beneficial to other drivers as well.

Add an eviction_valuable call also in the swapout path. Try to keep the
current behaviour for all drivers by returning true if the buffer object
is already in the TTM_PL_SYSTEM placement. We change behaviour for the
case where a buffer object is in a TT backed placement when swapped out,
in which case the drivers normal eviction_valuable path is run.

Reviewed-by: Maarten Lankhorst <[e-mail]>
Cc: Christian König <[e-mail]>
Signed-off-by: Thomas Hellström <[e-mail]>
Acked-by: Christian König <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c

···
 	struct dma_fence *f;
 	int i;
 
+	/* Swapout? */
+	if (bo->resource->mem_type == TTM_PL_SYSTEM)
+		return true;
+
 	if (bo->type == ttm_bo_type_kernel &&
 	    !amdgpu_vm_evictable(ttm_to_amdgpu_bo(bo)))
 		return false;

drivers/gpu/drm/ttm/ttm_bo.c

···
 bool ttm_bo_eviction_valuable(struct ttm_buffer_object *bo,
 			      const struct ttm_place *place)
 {
+	dma_resv_assert_held(bo->base.resv);
+	if (bo->resource->mem_type == TTM_PL_SYSTEM)
+		return true;
+
 	/* Don't evict this BO if it's outside of the
 	 * requested placement range
 	 */
···
  * b. Otherwise, trylock it.
  */
 static bool ttm_bo_evict_swapout_allowable(struct ttm_buffer_object *bo,
-			struct ttm_operation_ctx *ctx, bool *locked, bool *busy)
+					   struct ttm_operation_ctx *ctx,
+					   const struct ttm_place *place,
+					   bool *locked, bool *busy)
 {
 	bool ret = false;
 
···
 		*locked = ret;
 		if (busy)
 			*busy = !ret;
+	}
+
+	if (ret && place && !bo->bdev->funcs->eviction_valuable(bo, place)) {
+		ret = false;
+		if (*locked) {
+			dma_resv_unlock(bo->base.resv);
+			*locked = false;
+		}
 	}
 
 	return ret;
···
 		list_for_each_entry(bo, &man->lru[i], lru) {
 			bool busy;
 
-			if (!ttm_bo_evict_swapout_allowable(bo, ctx, &locked,
-							    &busy)) {
+			if (!ttm_bo_evict_swapout_allowable(bo, ctx, place,
+							    &locked, &busy)) {
 				if (busy && !busy_bo && ticket !=
 				    dma_resv_locking_ctx(bo->base.resv))
 					busy_bo = bo;
 				continue;
 			}
 
-			if (place && !bdev->funcs->eviction_valuable(bo,
-								      place)) {
-				if (locked)
-					dma_resv_unlock(bo->base.resv);
-				continue;
-			}
 			if (!ttm_bo_get_unless_zero(bo)) {
 				if (locked)
 					dma_resv_unlock(bo->base.resv);
···
 int ttm_bo_swapout(struct ttm_buffer_object *bo, struct ttm_operation_ctx *ctx,
 		   gfp_t gfp_flags)
 {
+	struct ttm_place place;
 	bool locked;
 	int ret;
 
-	if (!ttm_bo_evict_swapout_allowable(bo, ctx, &locked, NULL))
+	/*
+	 * While the bo may already reside in SYSTEM placement, set
+	 * SYSTEM as new placement to cover also the move further below.
+	 * The driver may use the fact that we're moving from SYSTEM
+	 * as an indication that we're about to swap out.
+	 */
+	memset(&place, 0, sizeof(place));
+	place.mem_type = TTM_PL_SYSTEM;
+	if (!ttm_bo_evict_swapout_allowable(bo, ctx, &place, &locked, NULL))
 		return -EBUSY;
 
 	if (!ttm_bo_get_unless_zero(bo)) {
···
 	if (bo->resource->mem_type != TTM_PL_SYSTEM) {
 		struct ttm_operation_ctx ctx = { false, false };
 		struct ttm_resource *evict_mem;
-		struct ttm_place place, hop;
+		struct ttm_place hop;
 
-		memset(&place, 0, sizeof(place));
 		memset(&hop, 0, sizeof(hop));
-
-		place.mem_type = TTM_PL_SYSTEM;
-
 		ret = ttm_resource_alloc(bo, &place, &evict_mem);
 		if (unlikely(ret))
 			goto out;
···
 	if (bo->bdev->funcs->swap_notify)
 		bo->bdev->funcs->swap_notify(bo);
 
-	ret = ttm_tt_swapout(bo->bdev, bo->ttm, gfp_flags);
+	if (ttm_tt_is_populated(bo->ttm))
+		ret = ttm_tt_swapout(bo->bdev, bo->ttm, gfp_flags);
 out:
 
 	/*