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

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Date:	Tue, 17 Feb 2015 14:03:58 -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 02/16/2015 08:04 AM, Peter Hurley 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.
> 
> 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.

I'm not even seeing how this works as advertised.

In QEMU, the *master* end is the source/sink for the guest VM, while the
*slave* pty is sink/source for the host. Like this:

Host <--> slave pty <===> master pty <--> qemu <==> emulated serial port

So this patch enables the guest VM to transmit BREAK to the host.
Why is that useful?

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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