linux-kernel - Re: [PATCH 1/2] isdn/gigaset: reset tty->receive_room when attaching ser

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Date:	Tue, 14 Jul 2015 21:01:20 +0200
From:	Paul Bolle <pebolle@...cali.nl>
To:	Tilman Schmidt <tilman@...p.cc>,
	Peter Hurley <peter@...leysoftware.com>
Cc:	netdev@...r.kernel.org, David Miller <davem@...emloft.net>,
	Hansjoerg Lipp <hjlipp@....de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] isdn/gigaset: reset tty->receive_room when
 attaching ser_gigaset

On di, 2015-07-14 at 01:58 +0200, Tilman Schmidt wrote:
> Am 14.07.2015 um 01:14 schrieb Peter Hurley:
> > That commit didn't cause the problem; it was a bug all along.
> 
> Sure. That's why it is correctly fixed in the Gigaset driver.
> But before that commit the bug was never actually triggered.
> So that commit defines the point in the commit history from
> which the fix is needed, and therefore needs to be mentioned
> in order to decide which stable releases will need the fix.

Yes, this seems a classic example of a bugfix that reveals another bug.
So the Fixes: tag, which does sound a bit awkward, really is
appropriate.

For ser-gigaset about the only line discipline change that will be
triggered, in practice, is from N_TTY to N_GIGASET_M101. Until commit
79901317ce80 ("n_tty: Don't flush buffer when closing ldisc") that
change would set receive_room to N_TTY_BUF_SIZE (ie, 4096). This patch
will set receive_room for ser-gigaset to RBUFSIZE/2 (ie, again 4096). So
we're back at the pre v3.10 behavior.

I'm really thankful that Tilman managed to bisect this and subsequently
saw how it could be properly fixed. I hope to forward this patch in a
few weeks so that it might finally be fixed in v4.3.

Applied.

Paul Bolle
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