I have problem with UART on I.MX6. I use Android 5.0.2, but i think trouble is in Linux kernel.
Serial port works in half-duplex mode, in additional driver wait ~50-100ms after end of receive data and then starts transmission. I must eliminate this delay, best is change port configuration to full-duplex mode.
Actually, from the hardware point of view, UART is the full duplex device by its nature, so, nothing special should be done to "switch" it to full duplex. Another point is how the upper level software controls data transmission and reception. So, this optimization is completely software-dependent.
Have a great day,
Artur