Directfb don't draw correctly when using opacity propterty (CSS) in a webpage

I build qtwebkit base on qt5.1.0rc1/directfb on a arm contex-a9 platform, css optacity property don't show the same as when I use qt4.8.2

the css is:

#layerbg {
position:absolute;
z-index:21;
top:0px;
left:0px;
width:1280px;
height:720px;
background-color:#CCC;
opacity:0.6;
}

html is:
<div id="detaillayer">
<div id="layerbg"></div>
<div id="detailbg"></div>
</div>

I compare the code with qt4.8.2, find some difference, so
I change
blittingFlags = DFBSurfaceBlittingFlags(blittingFlags | DSBLIT_BLEND_COLORALPHA | (m_premult ? DSBLIT_SRC_PREMULTCOLOR : 0));
to
blittingFlags = DFBSurfaceBlittingFlags(blittingFlags | DSBLIT_BLEND_COLORALPHA | (m_premult ? DSBLIT_SRC_PREMULTCOLOR : DSBLIT_SRC_PREMULTIPLY));

in
qtbase/src/plugins/platforms/directfb/qdirectfbblitter.cpp

the webpage draw correctly

but I am not sure whether i do the right thing.