serial flow control appears broken

Lee Howard
2007-07-26T02:13:57Z

Hello.

I have fax modems that will, in their proper behavior with certain
features, send up to 64 kilobytes of data to the host DTE all at once.
(So, the fax modem handles an incoming fax and periodically will send
between 256 bytes and 64 kilobytes of data in bursts.)

When the DCE-DTE (modem-to-host) communication rate is established at
115200 bps data loss occurs systems using at least Linux kernels 2.6.5
and 2.6.18 (and probably everything in-beween and then some more). This
is because the modem overflows the host's buffer. This is evidenced in
kernel logging:

Jul 23 14:01:30 gollum kernel: ttyS1: 1 input overrun(s)
Jul 23 17:09:45 gollum kernel: ttyS1: 1 input overrun(s)

Normally I would blame the modem itself for not honoring the host's flow
control signals. However, I have worked with the modem manufacturer
closely on this matter for over three months now. In that process they
have improved the responsiveness of the modem and have fixed other
problems, but the end result is that it truly does appear that the
serial tty driver is not using flow control. Whether software flow
control (XON/XOFF) or hardware flow control (RTS/CTS) is used the result
is the same.

This is evidenced in hardware flow control by a little LED labeled "RTS"
that is on the external modem. This LED lights up when pin 7 of the DB9
serial connection is given +12Vdc current (signalling "RTS" is on - that
the host can accept data). The LED goes dark when the current is
removed (signalling that the host cannot accept data). This "RTS" LED
never flickers at all, as it should, when receiving these bursts of data
- the LED stays lit as long as the serial cable is connected to the
host... and yet I will see those "input overrun" messages. Thus, it
seems quite clear that the Linux serial tty driver is not deasserting
RTS as it should in hardware flow control. (And probably the analogous
problem exists in software flow control, too.)

Please tell me what I can do to help you resove and/or remedy this
matter. Also, please let me know if I have contacted the wrong people.
(I have cross-posted to linux-kernel as a catch-all. I am not
subscribed to either linux-serial or linux-kernel mailing lists. So
please CC me in any list responses.)

If it is of any value to know (perhaps they have common code?), the same
error occurs on FreeBSD 6.2 as well. The problem does not occur on
Windows. The problem does not occur on RedHat 6.0 (kernel 2.2.5).

Thanks,

Lee.