For a few years I couldn't use my Banana Pi M3 for any serious work, because I couldn't compile the optimized Linux image for it.
Yesterday I've managed to create a Buildroot configuration that allows building of BR image for it. Thanks to use of 4.18.6 kernel the SMP is supported and all 8 cores are working.
The sources are available on alt.sources: https://groups.google.com/forum/#!topic/alt.sources/xgJPnIlJWiw
Of course I don't give warranty of any kind. It just works for me, and I hope that it may be useful for others.
With bestr regards,