On Thu, May 03, 2018 at 08:08:48PM +0530, Muni Sekhar wrote:
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?
It all depends on which specific UART driver you are looking at, they all do it a bit different depending on the hardware. Which one are you looking at? Look at what the start_tx callback does for that specific driver, that should give you a hint as to how data starts flowing. Usually an interrupt is enabled that is used to flush the buffer out to the hardware. thanks, greg k-h