I thought I'd give people a chance to jump on this before I opened a PR on it.
I'm currently using a Cyclades board at 115200 to talk to my PII laptop with
a 16550A UART on board via a null modem cable. I'm trying to run PPP over the
link with crtscts set, although I see the same problems using programs
other than PPP.
I stuck a printf() at line 2113 of /usr/src/sys/i386/isa/sio.c to see if
CRTS_IFLOW was being turned on, whereas CRTS_IFLOW is the input half of
crtscts.
I managed to get the printf, so I'm assuming that sio is being advised to do
RTS input flow control.
However, when I start running data, I get silo overflows. Sometimes its more
frequent than others, but its enough to keep most of the packets from going
through the link.
I have a breakout box on the line, as well as detectors in the Cyclades driver
for sensing line state change, and RTS never drops on the 16550A. It just
happily overflows.
I realize that 99% of the time, sio is used with a modem. Therefore, also being
a modem tester, I realize that its rare to completely saturate a 115200bps link
between the modem and the UART in the PC (we have exactly one test case with
highly-compressible data where this happens reliably), so I suspect that this
problem may go unnoticed in many cases, simply because you don't fill the FIFO.
I also went so far as to check individual RS232 signals via TIOCMSET to make
sure that the port functioned as commanded. It did so.
So, I suspect the problem is that the driver is not correctly driving flow
control.
I will therefore start to dive in to the problem to isolate it. However, I
figured I'd give everyone a chance to comment before I filed a PR.
-Brian
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