drm/ttm: Don't override vm_ops callbacks, if set

Drivers may want to set their own callbacks for a VM area. Only set
TTM's callbacks if the vm_ops field is clear.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210525151055.8174-2-tzimmermann@suse.de

drivers/gpu/drm/ttm/ttm_bo_vm.c

@@ -586,7 +586,12 @@ static struct ttm_buffer_object *ttm_bo_vm_lookup(struct ttm_device *bdev,
 
 static void ttm_bo_mmap_vma_setup(struct ttm_buffer_object *bo, struct vm_area_struct *vma)
 {
-	vma->vm_ops = &ttm_bo_vm_ops;
+	/*
+	 * Drivers may want to override the vm_ops field. Otherwise we
+	 * use TTM's default callbacks.
+	 */
+	if (!vma->vm_ops)
+		vma->vm_ops = &ttm_bo_vm_ops;
 
 	/*
 	 * Note: We're transferring the bo reference to