When "dfb_gfxcard_stretchblit" will go the software render flow at cobalt 19?

Question

1. My platform doesn't have gpu, so it uses directfb to draw UI and handle event. It works fine at Cobalt 11.153049. But after I upgrade it from cobalt 11 to cobalt19.lts.3, the directfb will go the software render flow. The API dfb_gfxcard_stretchblit of DFB1.5.3 will go the software render flow.

2. When the issue happens, it can't show the UI correctly and can't getEvent from dfb by the getEvent API. My starboard DirectFb code is the same between Cobalt 11 and cobalt 19. So I am curious that why the API will go the software render flow ?