linux-kernel - Re: Writing a console/tty driver -- how to use tty

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Date:	Thu, 28 Oct 2010 15:34:30 -0500
From:	Timur Tabi <timur@...escale.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	gregkh <gregkh@...e.de>, lkml <linux-kernel@...r.kernel.org>
Subject: Re: Writing a console/tty driver -- how to use tty_port?

Alan Cox wrote:
> drivers/mmc/card/sdio_uart.c
> 
> is in some ways the best example, it handles all the horrible cases
> including unloading of hardware v open races, its way more than you'd
> need but does illustrate it all.

I see something weird in sdio_uart_install():

	int ret = tty_init_termios(tty);

	if (ret == 0) {
		tty_driver_kref_get(driver);
		tty->count++;
		/* This is the ref sdio_uart_port get provided */
		tty->driver_data = port;
		driver->ttys[idx] = tty;

This function is called by tty_driver_install_tty(), which also does this:

	if (tty_init_termios(tty) == 0) {
		lock_kernel();
		tty_driver_kref_get(driver);
		tty->count++;
		driver->ttys[idx] = tty;
		unlock_kernel();
		return 0;
	}

Assuming that both calls to tty_init_termios(tty) return 0, these three lines
will be executed twice:

		tty_driver_kref_get(driver);
		tty->count++;
		driver->ttys[idx] = tty;

Is that right?

-- 
Timur Tabi
Linux kernel developer at Freescale

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