linux-kernel - serial: start_tx & buffer handling

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Message-ID: <CAHhAz+iPbNjeHH72Omi7VCUVL_cOUnQjrqbyJKQstUSL6J1hCw@mail.gmail.com>
Date:   Thu, 3 May 2018 20:08:48 +0530
From:   Muni Sekhar <munisekharrms@...il.com>
To:     linux-serial <linux-serial@...r.kernel.org>,
        linux-kernel@...r.kernel.org,
        kernelnewbies <kernelnewbies@...nelnewbies.org>
Subject: 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