I have a BeagleBone Black running a patched Linux 3.16.1 kernel which includes the Botic Audio Driver (http://bbb.ieero.com/) and a dummy codec configured to support both capture and playback streams. The mcasp0 is providing the clock (e.g. generated internally) and I have configured two McASP serializers ( 0-1) to support the transmit of two I2S streams (4 32-bit slots). The third serializer is configured for RX of a single I2S stream (e.g. two 32-bit slots). The fourth serializer on mcasp0 is inactive (unused). All serializer pins of mcasp0 are brought out through the P9 header via modifications to the device tree.
For my application, I need to support simultaneous capture/playback from the single mcasp0. Both streams have identical sample rates (e.g. the streams are symmetric). So I have one application (aplay) that plays a stream and a second (arecord) that records from the same ALSA PCM stream using the asym/dsnoop/dmix, plugins i.e.
/etc/asound.rc