drm/ttm: add ttm_bo_pipeline_gutting

Allows us to gut a BO of it's backing store when the driver says that it
isn't needed any more.
Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Roger He <Hongbo.He@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/ttm/ttm_bo.c

@@ -622,14 +622,23 @@ static int ttm_bo_evict(struct ttm_buffer_object *bo,
 
 	reservation_object_assert_held(bo->resv);
 
+	placement.num_placement = 0;
+	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);
+	}
+
 	evict_mem = bo->mem;
 	evict_mem.mm_node = NULL;
 	evict_mem.bus.io_reserved_vm = false;
 	evict_mem.bus.io_reserved_count = 0;
 
-	placement.num_placement = 0;
-	placement.num_busy_placement = 0;
-	bdev->driver->evict_flags(bo, &placement);
 	ret = ttm_bo_mem_space(bo, &placement, &evict_mem, ctx);
 	if (ret) {
 		if (ret != -ERESTARTSYS) {

drivers/gpu/drm/ttm/ttm_bo_util.c

@@ -801,3 +801,27 @@ int ttm_bo_pipeline_move(struct ttm_buffer_object *bo,
 	return 0;
 }
 EXPORT_SYMBOL(ttm_bo_pipeline_move);
+
+int ttm_bo_pipeline_gutting(struct ttm_buffer_object *bo)
+{
+	struct ttm_buffer_object *ghost;
+	int ret;
+
+	ret = ttm_buffer_object_transfer(bo, &ghost);
+	if (ret)
+		return ret;
+
+	ret = reservation_object_copy_fences(ghost->resv, bo->resv);
+	/* Last resort, wait for the BO to be idle when we are OOM */
+	if (ret)
+		ttm_bo_wait(bo, false, false);
+
+	memset(&bo->mem, 0, sizeof(bo->mem));
+	bo->mem.mem_type = TTM_PL_SYSTEM;
+	bo->ttm = NULL;
+
+	ttm_bo_unreserve(ghost);
+	ttm_bo_unref(&ghost);
+
+	return 0;
+}

include/drm/ttm/ttm_bo_driver.h

@@ -849,6 +849,15 @@ int ttm_bo_pipeline_move(struct ttm_buffer_object *bo,
 			 struct dma_fence *fence, bool evict,
 			 struct ttm_mem_reg *new_mem);
 
+/**
+ * ttm_bo_pipeline_gutting.
+ *
+ * @bo: A pointer to a struct ttm_buffer_object.
+ *
+ * Pipelined gutting a BO of it's backing store.
+ */
+int ttm_bo_pipeline_gutting(struct ttm_buffer_object *bo);
+
 /**
  * ttm_io_prot
  *