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

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Message-ID: <5410E8A2.5030802@hurleysoftware.com>
Date:	Wed, 10 Sep 2014 20:11:14 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:	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 09/10/2014 08:03 PM, Greg Kroah-Hartman wrote:
> On Wed, Sep 10, 2014 at 05:28:19PM -0400, Peter Hurley wrote:
>> This patch changes user-space behavior (for the better) but I'm not sure
>> that it's consequence-free. Also, it might not be enough to unwedge the
>> terminal if the driver got its own flow control state mangled.
>>
>> Thoughts?
>>
>> --- >% ---
>> Subject: [RFC] tty: Always allow tcflow(TCOON) to unwedge terminal
>>
>> If terminal flow has been stopped, the terminal can be unwedged
>> by:
>> 	tcflow(fd, TCOOFF);
>> 	tcflow(fd, TCOON);
>> This works because tcflow(TCOOFF) ensures that ->flow_stopped is set,
>> which allows tcflow(TCOON) to override the terminal flow state in
>> __start_tty().
>>
>> Instead, allow unwedging with only:
>>         tcflow(fd, TCOON);
>> by disregarding the existing ->flow_stopped state.
> 
> I don't see the benifit here, what are you trying to solve?  Sending one
> extra tcflow command?

It's not common knowledge (and its certainly counterintuitive) that
turning off output when output is already turned off (ie., tcflow(TCOOFF))
is the required trickery to unwedge a terminal.

Unwedging directly seems the straightforward approach.

Regards,
Peter Hurley

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