serial: start_tx & buffer handling

Hi All, I’m trying to understand how user mode buffer is written to low level serial hardware registers. For this I read the kernel code and I came to know that from user mode write() API lands into kernel’s tty_write() ("drivers/tty/tty_io.c") and then it calls a uart_write() ("drivers/tty/serial/serial_core.c"). In uart_write(), the buffer is copied to circ_buf and then it calls low level serial hardware driver’s start_tx() (struct uart_ops .start_tx). But here I could not find how the buffer kept in circ_buf is copied to serial port’s TX_FIFO registers? Can someone take a moment to explain me on this? -- Thanks, Sekhar