codekipper

mainline should support slave mode
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/sound/soc/sunxi/sun4i-i2s.c?h=v4.15-rc2#n502
there is also a few patches which I haven't delivered yet here
https://github.com/codekipper/linux-sunxi/commits/sunxi-wip
however I still need to test these properly before pushing for mainline. I'm also trying to come up with a cleaner solution for when the codec expects 32bits frame width.