some processors UART interfaces have a very small HW Rx buffer size (Vybrid, imx6) in some cases only eight bytes! If the CPU is under heavy load and you additionally just receive some serial data there may bytes lost (call "ClearCommError" to get extented informations).
Enable DMA for UART prevents this behavior. Refer the Device Driver Document to enable DMA for UART.
In general it is done by:
reg open \Drivers\builtin\<Board Name>\<UARTx>
reg set value RXDMAEnable dword 1