linux-kernel - Re: [PATCH] serial: mvebu-uart: Fix local flags handling on termios update

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Date:   Sun, 22 Apr 2018 18:07:28 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Gregory CLEMENT <gregory.clement@...e-electrons.com>,
        Allen Yan <yanwei@...vell.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Miquel Raynal <miquel.raynal@...e-electrons.com>
Subject: Re: [PATCH] serial: mvebu-uart: Fix local flags handling on termios
 update

On Sun, 22 Apr 2018 16:55:16 +0100
Russell King - ARM Linux <linux@...linux.org.uk> wrote:

> On Sun, Apr 22, 2018 at 01:33:46PM +0100, Marc Zyngier wrote:
> > Commit 68a0db1d7da2 reworked the baud rate selection, but also added
> > a (not so) subtle change in the way the local flags (c_lflag in the
> > termios structure) are handled, forcing the new flags to always be the
> > same as the old ones.
> > 
> > The reason for that particular change is both obscure and undocumented.
> > It also completely breaks userspace. Something as trivial as getty is
> > unusable:
> > 
> > <example>
> > 	Debian GNU/Linux 9 sy-borg ttyMV0
> > 
> > 	sy-borg login: root
> > 	root
> > 	[timeout]
> > 
> > 	Debian GNU/Linux 9 sy-borg ttyMV0
> > </example>
> > 
> > which is quite obvious in retrospect: getty cannot get in control of
> > the echo mode, is stuck in canonical mode, and times out without ever
> > seeing anything valid. It also begs the question of how this change was
> > ever tested.
> > 
> > The fix is pretty obvious: stop messing with c_lflag, and the world
> > will be a happier place.  
> 
> The c_iflag code also looks suspicious as well.  Apparently, the driver
> only supports INPCK and IGNPAR, but things such as ISTRIP, INLCR, IGNCR,
> ICRNL, IUCLC, IMAXBEL and IUTF8 are all software things done by the TTY
> layer and have nothing to do with the driver.

Indeed. I stuck with the most glaring issue (well, the one that
prevented me from using this particular box), but the whole termios
massaging is quite odd. Someone with a good understanding of the
intricacies of the TTY layer should definitely have a look at this.

	M.
-- 
Without deviation from the norm, progress is not possible.