On a big-endian ppc64 host with llvmpipe providing OpenGL software rendering support, all Qt applications show incorrect colors (see attached).  Note that only Qt applications are affected; all other applications tested (including mesa test applications such as glxgears and the OpenGL xscreensavers) operate normally.

This appears to be the result of a clarification at some point to the OpenGL specification for 8888 formats [1], which now states the format is host endian agnostic.  Qt on the other hand tries to swap the channels, causing the problem.

Changing QImage::rgbSwapped_inplace() from

*p = ((c << 16) & 0xff000000) | ((c >> 16) & 0xff00) | (c & 0x00ff00ff);

to

*p = ((c << 24) & 0xff000000) | ((c >> 8) & 0x00ff0000) | ((c >> 8) & 0x0000ff00) | ((c >> 8) & 0x000000ff);

resolves the color issue entirely.  This effectively treats the outgoing OpenGL buffer as ARGB instead of BGRA, per the updated OpenGL format specifications.

Note this only affects OpenGL visuals; passing a standard X11 visual to the Qt application or disabling the X11 glx extension results in normal colors both with and without this patch.

[1] https://narkive.com/OP4sVuiK.4