From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Hurley Subject: Re: [RFC] tty: Always allow tcflow(TCOON) to unwedge terminal Date: Thu, 11 Sep 2014 11:40:20 -0400 Message-ID: <5411C264.8000105@hurleysoftware.com> References: <1410384499-4982-1-git-send-email-peter@hurleysoftware.com> <20140911000343.GA5328@kroah.com> <5410E8A2.5030802@hurleysoftware.com> <20140911002410.GA11732@kroah.com> <5410F08D.10707@hurleysoftware.com> <20140911135610.GD15632@thunk.org> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Return-path: Received: from mailout32.mail01.mtsvc.net ([216.70.64.70]:49738 "EHLO n23.mail01.mtsvc.net" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751146AbaIKPkZ (ORCPT ); Thu, 11 Sep 2014 11:40:25 -0400 In-Reply-To: <20140911135610.GD15632@thunk.org> Sender: linux-serial-owner@vger.kernel.org List-Id: linux-serial@vger.kernel.org To: Theodore Ts'o , Greg Kroah-Hartman , Jiri Slaby , One Thousand Gnomes , linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org 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