linux-kernel - Re: [PATCH 1/1] pty: BREAK for pseudoterminals

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Message-ID: <20150216142244.33378c18@hananiah.suse.cz>
Date:	Mon, 16 Feb 2015 14:22:44 +0100
From:	Petr Tesarik <ptesarik@...e.cz>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Nan Li <nli@...e.com>, gregkh@...uxfoundation.org, jslaby@...e.cz,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] pty: BREAK for pseudoterminals

On Mon, 16 Feb 2015 08:04:02 -0500
Peter Hurley <peter@...leysoftware.com> wrote:

> On 02/05/2015 02:11 PM, Nan Li wrote:
> > This will greatly enhance the usefulness of QEMU virtual serial ports, because the Linux kernel interprets a break on the serial console as a SysRq, but there is currently no way to pass this signal over a pseudo-terminal. This patch will work for transmitting BREAK from master to slave through pseudo-terminal.
> 
> pty is an IPC mechanism, not a virtualization driver.

No, but it can be used as a TTY. Teletypes have always had the capacity
to send and receive BREAK.

> Those programs that know they're consuming from a slave pty may not
> have bothered to set termios for BREAKs, and may not be prepared
> for NULLs in the input stream.

Well, the default termios includes IGNBRK, so unless they bothered to
do anything about BREAKs, they won't see any change.

Anyway, the current kernel behaviour is clearly suboptimal. Calling
tcsendbreak() on a pty descriptor does nothing but reports success.
There are obviously two ways to fix it: either report an error, or
deliver the BREAK for real.

This patch implements the latter, adding at least one valid use case
to explain why it is better than the former.

Regards,
Petr Tesarik

> > Signed-off-by: Nan Li <nli@...e.com>
> > ---
> >  drivers/tty/pty.c | 19 +++++++++++++++++++
> >  1 file changed, 19 insertions(+)
> > 
> > diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
> > index e72ee62..ac8893a 100644
> > --- a/drivers/tty/pty.c
> > +++ b/drivers/tty/pty.c
> > @@ -31,6 +31,7 @@
> >  static struct tty_driver *ptm_driver;
> >  static struct tty_driver *pts_driver;
> >  static DEFINE_MUTEX(devpts_mutex);
> > +#define BREAK_STRING '\0'
> >  #endif
> >  
> >  static void pty_close(struct tty_struct *tty, struct file *filp)
> > @@ -674,6 +675,23 @@ static void pty_unix98_shutdown(struct
> > tty_struct *tty) devpts_kill_index(tty->driver_data, tty->index);
> >  }
> >  
> > +static int pty_unix98_break_ctl(struct tty_struct *tty, int
> > break_state) +{
> > +	int c;
> > +	struct tty_struct *to = tty->link;
> > +
> > +	if (break_state) {
> > +		/* Stuff the break data into the input queue of
> > the other end */
> > +		c = tty_insert_flip_char(to->port, BREAK_STRING,
> > TTY_BREAK);
> > +		/* And shovel */
> > +		if (c)
> > +			tty_flip_buffer_push(to->port);
> > +		else
> > +			return -ENOMEM;
> > +	}
> > +	return 0;
> > +}
> > +
> >  static const struct tty_operations ptm_unix98_ops = {
> >  	.lookup = ptm_unix98_lookup,
> >  	.install = pty_unix98_install,
> > @@ -686,6 +704,7 @@ static const struct tty_operations
> > ptm_unix98_ops = { .chars_in_buffer = pty_chars_in_buffer,
> >  	.unthrottle = pty_unthrottle,
> >  	.ioctl = pty_unix98_ioctl,
> > +	.break_ctl = pty_unix98_break_ctl,
> >  	.resize = pty_resize,
> >  	.shutdown = pty_unix98_shutdown,
> >  	.cleanup = pty_cleanup
> > 
> 

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