Comment 3 for bug 1410618

So it looks as if the bug that generated that commit was due to no sound coming out of the air's speakers and there's really not much talk about the TRRS mic-in functionality:
https://bugzilla.kernel.org/show_bug.cgi?id=60811

That commit may be responsible for the working sound and headphone detection - but there may be more work to do in order to get the TRRS mic working. Likely why installing the latest ALSA driver isn't changing anything with regard to the mic.

sudo hdajacksensetest -c 1 prints out:
Pin 0x10 (Grey Headphone, Left side): present = Yes

Anyway it sounds as if the pin configuration introduced in that patch was gleaned from a Windows inf driver file back in 2013. Makes me wonder if the TRRS functionality is absent on Windows as well...