blit_setup(): Screen depth 15 not supported!

[Forest]

when I opened framebuffer with ARGB8888,then run the sample ./simpledecoration -wqs,the program run well.
but when I opened framebuffer with ARGB1555,run the sample ./simpledecoration -wqs again,
got the following error:
blit_setup(): Screen depth 15 not supported!
Aborted

I changed the -depths to -depths all,and configured again, the same error.
I even changed the qscreenlinuxfb_qws.cpp. change QScreen::setPixelFormat(format) to
QScreen::setPixelFormat(QImage::Format_RGB16),stil l got the same error.
the Qt version is 4.8.2

why? Thanks a lot.
Forest