linux-kernel - Re: [PATCH] n_tty: Remove LINEMODE support

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Message-ID: <54BD5D95.2060604@hurleysoftware.com>
Date:	Mon, 19 Jan 2015 14:40:05 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	Howard Chu <hyc@...as.com>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Jiri Slaby <jslaby@...e.cz>,
	Linux Kernel Mailing List <Linux-Kernel@...r.Kernel.ORG>,
	linux-serial@...r.kernel.org, Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH] n_tty: Remove LINEMODE support

On 01/19/2015 11:36 AM, Howard Chu wrote:
> Peter Hurley wrote:

[...]

>> A description of how the pty master uses EXTPROC to implement line mode would
>> be very helpful, especially to people working in the tty code (eg., me, although
>> I don't need it now).
> 
> The original patch submission carried this description
> http://lkml.iu.edu/hypermail/linux/kernel/1006.2/02428.html

[from linked source]

> Paraphrased from the 1989 BSD patch by David Borman @ cray.com:
> 
> These are the changes needed for the kernel to support
> LINEMODE in the server.
> 
> There is a new bit in the termios local flag word, EXTPROC.
> When this bit is set, several aspects of the terminal driver
> are disabled. Input line editing, character echo, and mapping
> of signals are all disabled. This allows the telnetd to turn
> off these functions when in linemode, but still keep track of
> what state the user wants the terminal to be in.

Ok, but this should go on to describe at least the basics of
transparent local/remote line editing. Even mentioning that
it should be transparent is important.

> New ioctl:
> TIOCSIG Generate a signal to processes in the
> current process group of the pty.

Yes, but documentation should describe why this is important/required/useful.
That is, how this supports client signal handling.

> There is a new mode for packet driver, the TIOCPKT_IOCTL bit.
> When packet mode is turned on in the pty, and the EXTPROC bit
> is set, then whenever the state of the pty is changed, the
> next read on the master side of the pty will have the TIOCPKT_IOCTL
> bit set. This allows the process on the server side of the pty
> to know when the state of the terminal has changed; it can then
> issue the appropriate ioctl to retrieve the new state.

... and go on to outline in a sentence or two what the pty master
process does with this information.

IOW, _how_ does this kernel interface support LINEMODE?

Regards,
Peter Hurley
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