I've never used a DMA before and I'm reading the datasheet for the Atmel SAM 4E This question does not necessarily have to pertain to this device, it can apply to any device that supports DMA if they all essentially work the same.
But how much data can I transfer ?
If the transmitter and the receiver have a 4 deep FIFO, then I can transmit 4 bytes safely, without having to worry too much about lost data. I'd then have to wait until I can get some indication that the fifo is clear and ready to accept more data.
This then brings me to using flow controlled UART.
If I setup both transmit side and receive side to support flow control, then will the transmitter pause transmitting, when it sees that CTS is not set from the receiver and resume when the CTS is set?
If this is true, then I would assume then its possible to send as much data as I want without having to directly read the UART receive registers and send as much data as I want.