linux-kernel - Re: Uartlite

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Date:	Wed, 19 Jan 2011 16:27:02 +0100
From:	Peter Korsgaard <jacmet@...site.dk>
To:	monstr@...str.eu
Cc:	linux-serial@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: Uartlite - ulite_transmit

>>>>> "Michal" == Michal Simek <monstr@...str.eu> writes:

Hi,

 >> The problem is the uart_flush_buffer() call we see after uart_open() in
 >> 4. If doesn't seem to come from serial_core (only called from uart_close
 >> / uart_hangup), so presumably it comes from the TTY core or
 >> userspace. Could you add a bit more debug to figure out where exactly it
 >> comes from?

 Michal> Last week I have also implemented simple ioctl function for
 Michal> uartlite - just printk to see what happen. Look at the second
 Michal> log below.

 Michal> uart_open(0) called
 Michal> ulite_ioctl 5401 bff76a18
 Michal> n_tty_ioctl
 Michal> ulite_ioctl 540b 2
 Michal> n_tty_ioctl
 Michal> n_tty_ioctl_helper TCFLSH
 Michal> tty_perform_flush TCIOFLUSH
 Michal> tty_perform_flush TCOFLUSH
 Michal> tty_driver_flush_buffer
 Michal> uart_flush_buffer(0) called

So your userspace is calling tcflush, and serial_core responds by
flushing (dropping) the buffer. That seems like expected behaviour to
me.

This is presumably from the busybox getty applet, which does:

static void termios_init(struct termios *tp, int speed, struct options *op)
{
	speed_t ispeed, ospeed;
	/*
	 * Initial termios settings: 8-bit characters, raw-mode, blocking i/o.
	 * Special characters are set after we have read the login name; all
	 * reads will be done in raw mode anyway. Errors will be dealt with
	 * later on.
	 */
	/* flush input and output queues, important for modems! */
	tcflush(0, TCIOFLUSH);

You could change that to a tcdrain() if that's not what you want.

-- 
Bye, Peter Korsgaard
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