linux-kernel - Re: [RFC] tty: Always allow tcflow(TCOON) to unwedge terminal

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Message-ID: <20140911135610.GD15632@thunk.org>
Date:	Thu, 11 Sep 2014 09:56:10 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] tty: Always allow tcflow(TCOON) to unwedge terminal

On Wed, Sep 10, 2014 at 08:45:01PM -0400, Peter Hurley wrote:
> > I'm all for working around broken hardware in the kernel, but this seems
> > like a very old issue, if it's even one at all, that we would be
> > changing for no one who has reported it (that I know of...)
> 
> How to unwedge a terminal comes up from time to time.

Are you trying to unwedge a terminal using hardware flow control, or
software flow control?

For software flow control, this is a guarantee that we can make wrt to
the behavior of tcflow().  For hardware flow control, we can't make
any guarantees, whether it's with tcflow(TCOON) or tcflow(TCOOF);
tcflow(TCOON). 

> It's possible that this may cause userspace breakage. Some app
> may call tcflow(TCOON) thus accidently overriding the flow control
> state when it would have had no effect before.

It's very likely that an application that would be using tcflow() at
all would first be sending a TCOOFF, and then sending a TCOON.  So
this doesn't worry me that much.

Indeed, given that the definition of how TCION and TCIOFF is worded
(send a START or STOP command), it's completely reasonable to
interpret TCOON and TCOOFF as emulating what would happen if the
system received a START or STOP command.  (Note though that part of
this is that Posix doesn't define CRTSCTS, so POSIX is entirely silent
on the subject of hardware flow control).

Cheers,

					- Ted
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