linux-kernel - Re: [PATCH] detour TTY driver

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Message-ID: <20100611134455.3fa7d563@lxorguk.ukuu.org.uk>
Date:	Fri, 11 Jun 2010 13:44:55 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Samo Pogacnik <samo_pogacnik@....net>
Cc:	linux-embedded <linux-embedded@...r.kernel.org>,
	linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] detour TTY driver - now ttyprintk


> +/*
> + * TTY operations open function.
> + */
> +static int tpk_open(struct tty_struct *tty, struct file *filp)
> +{
> +	struct ttyprintk_port *port = &tpk_port;

You can replace the rest with

	return tty_port_open(port, tty, filp);
}

> + * TTY operations close function.
> + */
> +static void tpk_close(struct tty_struct *tty, struct file *filp)
> +{
> +	struct ttyprintk_port *port = tty->driver_data;

and
	tty_port_close(port, tty, filp);

> +}

which saves a lot of work

> +
> +/*
> + * TTY operations write function.
> + */
> +int tpk_write(struct tty_struct *tty,
> +		const unsigned char *buf, int count)
> +{
> +	struct ttyprintk_port *port;
> +	int ret;
> +
> +	port = tty->driver_data;
> +
> +	if (!port)
> +		return 0;
> +
> +	if (!(port->port.flags & ASYNC_INITIALIZED))
> +		return 0;

These two can't happen if you use tty_port_* so it is better to blow up.
If you think you may be seeing it occur then use WARN_ON() or similar

> +
> +	/* exclusive use of tpk_printk within this tty */
> +	mutex_lock(&port->port_write_mutex);
> +	ret = tpk_printk(buf, count);
> +	mutex_unlock(&port->port_write_mutex);

And this is serialized by the caller (not that having your own lock is
any harm)


> +#define	TPKRLEV	(('e'<<8) | 0) /* Wait for ttyprintk ratelimiting event*/
> +static int tpk_ioctl(struct tty_struct *tty, struct file *file,
> +			unsigned int cmd, unsigned long arg)
> +{
> +	struct ttyprintk_port *port;
> +
> +	port = tty->driver_data;
> +
> +	if (!port)
> +		return -EINVAL;
> +
> +	switch (cmd) {
> +	case TPKRLEV:
> +		wait_event_interruptible(ttyprintk_ratelimit_wq,
> +			(ttyprintk_ratelimit_event != 0));

Ok that wasn't quite what I had in mind. 

What I was thinking was needed was this

	/* Stop TIOCCONS */
	case TIOCCONS:
		return -EOPNOTSUPP;

only it won't work that way. I'll sort that out in tty_io.c once the
driver is happy. That way anything trying to mis-redirect the console
will get stopped early which is probably more reliable than a ratelimit ?


> +	memset(&tpk_port, 0, sizeof(tpk_port));

It's static so that isn't needed

> +	tty_port_init(&tpk_port.port);
> +	spin_lock_init(&tpk_port.lock);

The one other bit you will need to use the helpers is

struct tty_port_operations null_ops = { };

	tpk_port.port->ops = &null_ops;


Alan
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