Comment # 4 on bug 69670 from Michel Dänzer

As you can see in the kwin stdout output, radeonsi only supports OpenGL 2.1 in
Mesa 9.2. I suspect the problem happens when kwin tries to create an OpenGL 3.x
context, but I wonder if this is radeonsi specific — does it not happen e.g. if
you run kwin with LIBGL_ALWAYS_SOFTWARE=1 ?