Qt 4.7.0 QWSServer::setBackground(const QBrush &brush); doesn't work

I port qt-4.7.0 and qt-4.7.1 to embedded system.
I use -plugin-gfx-linuxfb to display (board's directfb version is 1.4.3)
I want to set the brush used as background in the absence of obscuring windows, to be the given brush.

QWSServer::setBackground(QBrush(Qt::white));

but it doesn't work. When I run the code, the screen shows default green background color.
Is this a bug ?