'port forced close' after failed ppp

[Dan Williams]

On Wed, 2017-03-15 at 15:36 +0000, Colin Helliwell wrote:
> > On 14 March 2017 at 17:55 Dan Williams <dcbw at redhat.com> wrote:
> > 
> > On Tue, 2017-03-14 at 14:18 +0000, Colin Helliwell wrote:
> > 
> > > Still trying to get to the bottom of this problem.
> > > What are the events which can cause the G_IO_HUP (in MM)?
> > 
> > G_IO_HUP == POLLHUP from the standard C library (see
> > /usr/include/bits/poll.h). It's coming directly from the kernel fd
> > for
> > the serial port, which in turn comes directly from the driver and
> > the
> > TTY subsystem. See drivers/tty/n_tty.c which is the default serial
> > line discipline on Linux (IIRC).
> > 
> > The places n_tty sets POLLHUP are both in n_tty_poll():
> > 
> > if (test_bit(TTY_OTHER_CLOSED, &tty->flags))
> >  mask |= POLLHUP;
> > ** this one doesn't apply, since it's only set by drivers/tty/pty.c
> > and
> > we don't use pseudo-ttys for modem stuff.
> > 
> > if (tty_hung_up_p(file))
> >  mask |= POLLHUP;
> > 
> > tty_hung_up_p() will only return true if __tty_hangup() has been
> > called, which can happen from:
> > 
> > 1) tty_vhangup(): called on USB device disconnect, UART port
> > removal
> > (called from a lot of UART drivers, maybe yours?), from
> > TIOCVHANGUP,
> > and a couple other places. Could be the culprit.
> > 
> > 2) tty_hangup(): called from uart_handle_dcd_change() (which
> > shouldn't
> > affect anything that sets CLOCAL, but who knows...), and
> > tty_port_tty_hangup(). This last one is called mostly from serial
> > drivers when the DCD has changed.
> > 
> > Check your mux driver, does it do any calls to
> > tty_port_tty_hangup() or
> >  tty_hangup()?
> > 
> > Dan
> > 
> 
> No sign in the source of any calls to those functions.
> Could the signal be being sent from a hardware and 'physical uart
> driver' level (e.g. due to h/w handshake lines changing), to MM,
> without going through the mux driver...?

Yes, that's a possibility.  I assume the MUX driver is not open-source? 
Otherwise I'm kinda walking around in the dark trying to feel out
what's going on.

Dan