Re: [bug-ncurses] Reset clears CLOCAL flag breaking some hardware serial ports

[Thomas Dickey]

On Wed, Oct 18, 2023 at 10:19:25AM +0200, Dr. Werner Fink wrote:
> On 2023/10/05 03:59:21 -0400, Thomas Dickey wrote:
> > On Thu, Oct 05, 2023 at 09:35:04AM +0200, Dr. Werner Fink wrote:
> > > On 2023/10/04 10:40:35 +0000, Michal Suchanek wrote:
> > > > Hello,
> > > > 
> > > > on some development boards and server BMCs the hardware apparently
> > > > lacks the modem control lines.
> > > > 
> > > > The OS correctly sets the CLOCAL flag and the terminal works. However,
> > > > when there is a need to reset the terminal after adjusting the TERM
> > > > variable the reset tool clears the CLOCAL flag. This is not documented
> > > > in the man page nor can I see any reason in the git history why this
> > > > functionality was added (in ncurses 4.1 AFAICS).
> > > 
> > > That is back more than 25 years ... ncurses-4.1/progs/tset.c
> > 
> > "more than 25 years" actually goes back to 1995 (28 years), as mentioned 
> > here
> > 
> > https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=60377
> > 
> > call that ncurses 1.9.6:
> > 
> > https://invisible-island.net/ncurses/ncurses-license.html#ncurses_1_9_6
> > 
> > Just changing it may be disruptive, which prompts caution.
> > 
> > In my response on the Debian bug report, I had in mind making a table for
> > the manual to explain the change, and perhaps make it an option rather
> > than surprise everyone.
> >  
> > >  static void
> > >  reset_mode(void)
> > > 
> > >  [...]
> > > 
> > >  mode.c_cflag &= ~(CSIZE | CSTOPB | PARENB | PARODD | CLOCAL);
> > > 
> > > 
> > > > 
> > > > On the affected devices (such as Raspberry Pi) the symptom is that
> > > > terminal applications using the alternate screen such as less or vim
> > > > keep waiting in the kernel and never start.
> > > > 
> > > > I don't think that the CLOCAL flag is part of what the reset tool
> > > > should change much like the terminal baud rate - this is property of
> > > > how the terminal is connected, and not the terminal type.
> > > 
> > > I suggest to check if the open file descriptor of the the tty line has
> > > set O_NONBLOCK via fcntl/F_GETFL and if so not to clear CLOCAL.
> > > Normally opening a a port to a modem blocks until the modem reports
> > > carrier detection, therefore O_NONBLOCK is used.  Then the first read
> > > or write call will wait for carrier to be established.
> > > 
> > > Werner
> 
> I can remember an old tool I had written for blogd called isserial.
> It simply check if the current input file descriptor has the Modem
> Control ioctl TIOCMGET ... means if the ioctl() returns -1 with error
> ENOTTY it is a standard terminal line whereas serials lines do not
> show an error ENOTTY.

That sounds helpful - but I don't know whether Michal's original comment
indicates that it may/may not work:

        on some development boards and server BMCs the hardware apparently
        lacks the modem control lines.

-- 
Thomas E. Dickey <dickey@invisible-island.net>
https://invisible-island.net

signature.asc
Description: PGP signature