Re: Closing a serial device takes one second

[Marc Balmer <marc%msys.ch@localhost>]

Am 06.02.14 17:18, schrieb Marc Balmer:
> Closing a serial port (e.g. /dev/dtyXX) takes almost exactly one second.
>  Why is that so?  Why does the call not return immediately?
> 
> int fd;
> 
> fd = open("/dev/dty03", O_RDWR);  /* returns immediately */
> close(fd); /* returns after one second */
> 

So it is clear now that the delay is there for a specific application
case:  A modem on a tty line that hangs up when DTR is gone for one
second.  For probably all other use cases the delay is not necessary.

Removing the delay in the driver probably means that init will reopen
the tty more or less immediately after it closes the line.  And _if_
there is such a modem on that line, it will probably not hang up.

So the question is whether we want to add a 1 sec. delay to init?

The other proposed "solution", namely to add a delay to open in case the
device is reopened, does not help either in the use cases where we run
into trouble:

When talking to serial printers, we open a tty, send the data to the
printer and then close the tty again.  This is done several times in a
sequence, each time causing a one second delay.

Now adding softcar to the respective line /etc/ttys can fix the issue,
if the hardware does not rely on it (which we are currently testing).

In conclusion, the delay probably comes from times long gone when people
used dial-in modems.  For "modern" serial applications they are more a
nuisance.  Is it time to switch the default to no delay?