From owner-cvs-sys  Sun Dec  1 04:56:21 1996
Return-Path: owner-cvs-sys
Received: (from root@localhost)
          by freefall.freebsd.org (8.7.5/8.7.3) id EAA13937
          for cvs-sys-outgoing; Sun, 1 Dec 1996 04:56:21 -0800 (PST)
Received: from godzilla.zeta.org.au (godzilla.zeta.org.au [203.2.228.19])
          by freefall.freebsd.org (8.7.5/8.7.3) with ESMTP id EAA13931;
          Sun, 1 Dec 1996 04:56:12 -0800 (PST)
Received: (from bde@localhost) by godzilla.zeta.org.au (8.8.3/8.6.9) id XAA19611; Sun, 1 Dec 1996 23:35:47 +1100
Date: Sun, 1 Dec 1996 23:35:47 +1100
From: Bruce Evans <bde@zeta.org.au>
Message-Id: <199612011235.XAA19611@godzilla.zeta.org.au>
To: bde@zeta.org.au, pst@shockwave.com
Subject: Re: cvs commit: src/sys/i386/isa sio.c
Cc: bde@freefall.freebsd.org, cvs-all@freefall.freebsd.org,
        CVS-committers@freefall.freebsd.org, cvs-sys@freefall.freebsd.org
Sender: owner-cvs-sys@FreeBSD.ORG
X-Loop: FreeBSD.org
Precedence: bulk

>OK, we're in perfect agreement there.  So now, the $24 question is:
>
>"If I set comdefaultrate to 115200 and don't open /dev/ttyd0, exactly what
> is going to happen every time we output a character?"

The speed will be 9600 to begin with.  It will be changed to 115200.
Then the character will be written.  Then the output routine will spin
until the output is incomplete.  Then the speed will be restored to
9600.

Context switching the speed (and other h/w state) is bad iff input is
arriving while you're switching and if input arrives while you're not in
the spinloop waiting for it.  This is bad for gdb of course.  There is
a simple workaround: set the initial speed of /dev/ttyd0 to 115200 and
don't neglect to open it (stty 115200 </dev/ttyid0; stty -f /dev/ttyd0).

There may be other problems at 115200.  It's not clear how input at any
speed works - if you do a block of output large than the input fifo size,
then input is certain to be lost while the output routine is spinning.
gdb-remote apparently doesn't do this often.  I wrote some better
buffering code a few months ago but got confused by a hardware bug and
never finished.

Bruce