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

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Date:   Mon, 23 Apr 2018 16:04:06 +0100
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Marc Zyngier <marc.zyngier@....com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, Allen Yan <yanwei@...vell.com>,
        Miquel Raynal <miquel.raynal@...e-electrons.com>,
        Gregory CLEMENT <gregory.clement@...e-electrons.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] serial: mvebu-uart: Fix local flags handling on termios
 update

On Sun, Apr 22, 2018 at 08:58:04PM +0100, Russell King - ARM Linux wrote:
> On Sun, Apr 22, 2018 at 06:07:28PM +0100, Marc Zyngier wrote:
> > 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.
> 
> Right, remember, I'm the author of the serial core layer (I took over
> from tytso back in the 2.x era.)
> 
> POSIX requirements are:
> 
>   The tcsetattr() function shall return successfully if it
>   was able to perform any of the requested actions, even if
>   some of the requested actions could not be performed. It
>   shall set all the attributes that the implementation
>   supports as requested and leave all the attributes not
>   supported by the implementation unchanged.
> 
> In other words, if the system does not support two stop bits, asking
> for CSTOPB to be set results in CSTOPB being ignored but all other
> supported modes being updated.  This is what the code in that commit
> is trying to implement, but it forgets that the TTY layer implements
> a whole load of termios modes that are not specific to the serial
> driver.

I've just been talking to Jon Nettleton about this, and it seems that
he reported the exact same bug that you've found to Marvell a while
back, yet it seems that the bug was still propagated into mainline.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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