Both address Plantinga's proper functionalism, as well as Michael Bergmann's more recent version of it. Also, both are accessible (and currently free!) in the Online First sections of the relevant journal sites.
Johnson, Daniel M. "Proper Function and Defeating Experiences", Synthese (forthcoming).
Long, Todd R. "Mentalist Evidentialism Vindicated (and a Super-Blooper
Epistemic Design Problem for Proper Function Justification)", Phil. Studies (forthcoming).