netdev - Re: [PATCH] tty: Fix WARNING in tty_set

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Date:   Thu, 31 Jan 2019 16:33:06 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Marcel Holtmann <marcel@...tmann.org>
Cc:     Johan Hovold <johan@...nel.org>, shuah <shuah@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        "open list:NFC SUBSYSTEM" <linux-wireless@...r.kernel.org>,
        chris@...-brannons.com, devel@...verdev.osuosl.org,
        Rob Herring <robh@...nel.org>,
        Samuel Ortiz <sameo@...ux.intel.com>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        Jiri Slaby <jslaby@...e.com>, santhameena13@...il.com,
        kirk@...sers.ca, Johan Hedberg <johan.hedberg@...il.com>,
        Arnd Bergmann <arnd@...db.de>, samuel.thibault@...-lyon.org,
        m.maya.nakamura@...il.com, zhongjiang@...wei.com,
        Greg KH <gregkh@...uxfoundation.org>,
        speakup@...ux-speakup.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Bluez mailing list <linux-bluetooth@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        nishka.dasgupta_ug18@...oka.edu.in,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] tty: Fix WARNING in tty_set_termios

On Thu, Jan 31, 2019 at 04:18:33PM +0100, Marcel Holtmann wrote:

> > I agree with Al that this change doesn't make much sense. The WARN_ON
> > is there to catch any bugs leading to the termios being changed for a
> > master side pty. Those should bugs should be fixed, and not worked
> > around in order to silence a WARN_ON.
> > 
> > The problem started with 7721383f4199 ("Bluetooth: hci_uart: Support
> > operational speed during setup") which introduced a new way for how
> > tty_set_termios() could end up being called for a master pty.
> > 
> > As Al hinted at, setting these ldiscs for a master pty really makes no
> > sense and perhaps that is what we should prevent unless simply making
> > sure they do not call tty_set_termios() is sufficient for the time
> > being.
> > 
> > Finally, note that serdev never operates on a pty, and that this is only
> > an issue for (the three) line disciplines.
> 
> I think for PTYs we should just fail setting the HCI line discipline.
> Fail early and just move on with life.

Sounds good to me. At least for the pty master. There may be some people
trying to use a bluetooth device connected to a remote serial port (I've
seen descriptions of such setups at least), and maybe we need not prevent
that.

Johan