linux-kernel - Re: [PATCH 3.12 16/33] isdn/gigaset: reset tty->receive_room when attaching ser

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Message-ID: <55F951EA.1060001@imap.cc>
Date:	Wed, 16 Sep 2015 13:26:34 +0200
From:	Tilman Schmidt <tilman@...p.cc>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Jiri Slaby <jslaby@...e.cz>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>,
	Linux kernel mailing list <linux-kernel@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 3.12 16/33] isdn/gigaset: reset tty->receive_room when
 attaching ser_gigaset

Am 16.09.2015 um 03:18 schrieb Peter Hurley:
> On Tue, Sep 15, 2015 at 8:37 PM, Tilman Schmidt <tilman@...p.cc> wrote:
>> Am 16.09.2015 um 01:08 schrieb Peter Hurley:
>>> On Tue, Sep 15, 2015 at 10:22 AM, Jiri Slaby <jslaby@...e.cz> wrote:
>>>
>>>     From: Tilman Schmidt <tilman@...p.cc>
>>>
>>>     3.12-stable review patch.  If anyone has any objections, please let
>>>     me know.
>>>
>>>     ===============
>>>
>>>     [ Upstream commit fd98e9419d8d622a4de91f76b306af6aa627aa9c ]
>>>
>>>     Commit 79901317ce80 ("n_tty: Don't flush buffer when closing ldisc"),
>>>     first merged in kernel release 3.10, caused the following regression
>>>     in the Gigaset M101 driver:
>>>
>>>
>>> Again, I'll just note my objection to this commit log.
>>>
>>> This driver was always broken because it never initialized
>>> tty->receive_room,
>>> but rather relied on common but not guaranteed circumstances to
>>> function.
>>>
>>> The commit noted simply made the underlying bug more evident, but the
>>> root cause was from the original merge commit of this driver.
>>
>> I must admit I still don't understand that objection. The meaning of the
>> term "regression" is simply that something which previously worked
>> stopped working. It doesn't imply any statement about the root cause.
>>
>> The ser-gigaset driver worked before the introduction of commit
>> 79901317ce80. It didn't work anymore after the introduction of that
>> commit. So it is correct, and does not contradict your statements above
>> in any way, to state that commit introduced the described regression.
> 
> By asserting that commit 79901317ce80 caused the regression, you're
> claiming that this fix is unnecessary for kernel versions prior to 3.10

Correct.

> Are you certain that no other sequence of state leads to the same
> condition (and thus requiring the same fix) in earlier kernel versions?

Reasonably certain, yes, for three reasons:
- There where no reports of that problem before 3.10.
- My own tests did never encounter that condition, and even after being
made aware of it I was not able to come up with a test that would
provoke it with a kernel version before 3.10.
- The requirement for line disciplines to set receive_room wasn't (and
btw still isn't) documented anywhere, so it's unlikely anything actively
relied on it.

But if you want to propose that patch for inclusion in earlier stable
releases I won't oppose it.

-- 
Tilman Schmidt                              E-Mail: tilman@...p.cc
Bonn, Germany
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