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Date: Mon, 21 Oct 2002 21:32:55 +0900
From: Joosun Hahn <jshan@zeen.snu.ac.kr>
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To: linux-kernel@vger.kernel.org
Subject: Problem in writing to the serial port in 2.4.19
References: <20021021102622Z261330-32597+6184@vger.kernel.org>
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Hello, all.

I wrote a linux loadable kernel module 
that exchanges data with a device through /dev/ttyS0.
For this purpose, I set the serial tty's ldisc (tty->ldisc) 
to my own line discipline and it worked well in the kernel 2.4.2.

But, when I upgrade the kernel to 2.4.19,
it does not seem to send data as it was before.
If I load the module and send some data in succession,
the device attached to the serial port becomes down shortly after.

The following is my routine to send data to /dev/ttyS0:

--
while (len > 0) {
        int space, n_wr;

        if (tty->driver.write_room)
                space = tty->driver.write_room(tty);

        n_wr = (len < space)? len: space;

        if (tty->driver.flush_chars)
                tty->driver.flush_chars(tty);

        n_wr = tty->driver.write(tty, 0, buf, n_wr);

        buf += n_wr;
        len += n_wr;
}
--

In order to send data in succession, I call the above routine
within another while loop with different (buf, len) parameters. 

The scheme worked well in 2.4.2, but does not work in 2.4.19.
So, I re-wrote the routine like this:

--
tty->flags |= (1 << TTY_DO_WRITE_WAKEUP);

while (len > 0) {
        int space, n_wr;

        if (tty->driver.write_room)
                space = tty->driver.write_room(tty);

        n_wr = (len < space)? len: space;

        if (tty->driver.flush_chars)
                tty->driver.flush_chars(tty);

        n_wr = tty->driver.write(tty, 0, buf, n_wr);

        buf += n_wr;
        len += n_wr;
}
--

And, in the function void xx_write_wakeup (struct tty_struct *tty):

--
tty->flags &= ~(1 << TTY_DO_WRITE_WAKEUP);

/* and, re-send data */
--

The result is that both do not work. :-(

I believe the serial driver in 2.4.19 will work correctly.
If there is anything that I'm missing, please let me know.

Thank you in advance.


Joosun.

ps. UART(16550A), baudrate(115200), crtscts(ON).