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

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Date:	Mon, 16 Feb 2015 08:04:02 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	Nan Li <nli@...e.com>
CC:	gregkh@...uxfoundation.org, jslaby@...e.cz,
	linux-kernel@...r.kernel.org, ptesarik@...e.cz
Subject: Re: [PATCH 1/1] pty: BREAK for pseudoterminals

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.

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.

Regards,
Peter Hurley


> 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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