lkml.org 
[lkml]   [2018]   [Apr]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [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@armlinux.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/
FTTC broadband for 0.8mile line in suburbia: sync at 8.8Mbps down 630kbps up
According to speedtest.net: 8.21Mbps down 510kbps up

\
 
 \ /
  Last update: 2018-04-23 17:05    [W:0.036 / U:0.432 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site