Advanced Users can post their questions and comments here for the not so Newbie crowd.
not sure where this one is being pulled in from, but quite possibly it's in the I2S code, as that would be the only one you're not using
There is an option to ld to have it generate a cross-reference as well, that would (in theory) show what file/function is using the function. It could indeed very well be some obscure SDK function.
I did a little research on the I2S stuff. Unluckily (for us) very little is revealed about the direct (i.e. register level) access to this module. So we end up calling SDK functions that do mysterious things under the hood. I'm shivering already again. I have no real use for I2C at the moment (other then a possible replacement for led pixel protocol output, instead of abusing one of the UARTS), so I am lucky I guess. If someone has more information about "bare metal" access to I2S, that would be great.
What are you using I2S for, actually recording or playing audio?