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

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Message-ID: <5411C264.8000105@hurleysoftware.com>
Date:	Thu, 11 Sep 2014 11:40:20 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Theodore Ts'o <tytso@....edu>,
	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 09/11/2014 09:56 AM, Theodore Ts'o wrote:
> 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 unwedging software flow control.

Like you point out, unwedging hardware flow control would be more complicated
and less predictable.

> 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).

This is the basic interpretation I assumed, and most of what the tty core
already did.

Regards,
Peter Hurley
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