RE: Linux-2.4.20 modem control

[Ed Vance <EdV@macrolink.com>]

On Mon, Mar 17, 2003 at 8:19 AM, Richard B. Johnson wrote:
> 
> If a modem is connected to /dev/ttyS0 and a getty (actually agetty)
> is associated with that device, one can log-in using that modem.
> 
> This is how we've operated for many years. But, Linux version 2.4.20
> presents a new problem.
> 
> When a logged-in caller logs out, it is mandatory for the modem
> to disconnect. This has previously been done automatically when
> the terminal is closed. The closing of the tasks file-descriptors
> will eventually call tty_hangup() and the modem would (previously)
> hang up.
> 
> Something has changed so that the hang-up sequence doesn't happen if
> agetty has already opened the terminal for another possible 
> connection.
> It used to be that the caller, calling close(), did not get control
> back until the modem had been hung up. This prevented another agetty
> from opening that terminal for I/O because the previous task had not
> completed its exit procedure until the terminal was hung up.
> 
> Now, the hang-up sequence appears to be queued. It can (and does)
> happen after the previous terminal owner has expired and another
> owner has opened the device. This makes /dev/ttyS0 useless for remote
> log-ins.
> 
> It needs to be, that a 'close()' of a terminal, configured as a modem,
> cannot return to the caller until after the DTR has been lowered, and
> preferably, after waiting a few hundred milliseconds. Without this,
> once logged in, the modem will never disconnect so a new caller
> can't log in.
> 
> With faster machines, it is not sufficient to just lower DTR. One
> needs to lower DTR and then wait. This is because the next task
> can open that terminal in a few hundred microseconds, raising
> DTR again. This is not enough time for the modem to hang up because
> there is "glitch-filtering" on all modem-control leads. The hang-up
> event won't even be seen by the modem.
> 
> So, either the modem control needs to be reverted to its previous
> functionality or `agetty` needs to hang up its terminal when it
> starts, which seems backwards. In other words, the user of kernel
> services should not have to compensate for a defect in the logic
> of that service.
> 
> I have temporarily "fixed" this problem by modifying `agetty`.
> Can the kernel please be fixed instead?
> 
Hi Richard,

The following patch to serial.c in 2.4.20 is a brute-force addition 
of a hang-up delay of 0.5 sec just before close returns to the user, 
if the hupcl flag is set. Please try this to determine if there are 
any other issues with the remote login. If it works, I'll write a 
better patch that does not duplicate other delays, etc.

Cheers,
Ed

diff -urN -X dontdiff.txt linux-2.4.20/drivers/char/serial.c
patched-2.4.20/drivers/char/serial.c
--- linux-2.4.20/drivers/char/serial.c	Thu Nov 28 15:53:12 2002
+++ patched-2.4.20/drivers/char/serial.c	Tue Mar 18 16:03:43 2003
@@ -2848,6 +2848,10 @@
 		tty->driver.flush_buffer(tty);
 	if (tty->ldisc.flush_buffer)
 		tty->ldisc.flush_buffer(tty);
+	if (tty->termios->c_cflag & HUPCL) {
+		set_current_state(TASK_INTERRUPTIBLE);
+		schedule_timeout(HZ/2);	/* 0.5 sec to disconnect modem */
+	}
 	tty->closing = 0;
 	info->event = 0;
 	info->tty = 0;