Linux MMC framework consists of MMC core+ SD protocol implementation(Google is your friend) +MMC host controller + Host controller driver(for detecting the card and notifying host controller).

Host controller :SDHCI(it is there is open source)+ if it is qualcomm it is i guess proprietary.
Right now i am not able to think of any open source host controller driver for detecting the card.Probably someone more experienced can give a helping hand to you.