When Qt uses Directfb with a driver (RasterPaintEngine), calculation of widgets region is incorrectly

when using Qt with directfb driver (using SIS 315 driver for Directfb), calculates region of widgets incorrectly. i.e. in first load it will show correctly but in the later paints it will get height and width 2 pixel less than correct size, so we can not repaint the widget correctly; This is same as Creating a widget with a adjusted size(0,0,-2,-2).
when we use QT with software rasterizer or when using it with directfb with software rasterizer (Directfb with no hardware driver) no problem exists but no hardware acceleration is employed.
Thanks all, Please help me.