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From: Jia-Shiun Li <jiashiun@gmail.com>
Date: Wed, 16 Oct 2013 02:38:32 +0800
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Subject: Re: BBB triggered trap when disconnecting uart
To: Ian Lepore <ian@freebsd.org>
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On Wed, Oct 16, 2013 at 2:28 AM, Ian Lepore <ian@freebsd.org> wrote:
> Is the usb adapter you're talking about your console cable?  If so, I
> wonder if unplugging it is sending an rs232 break (a long string of 0
> bits) and that's dropping into the debugger?  If that's the case, you
> should be able to type "continue" at the db> prompt and everything will
> resume running.
>
> It used to be possible to disable an rs232 break but enable the
> alt-break sequence.  I'm not sure if that's still the case (too busy to
> go code-spelunking at the moment).
>

Unplugging cables on the BBB header side (3 wires) cause the same
behavior. So it seems not term or dongle. And the term prints the kdb
message before completely disconnected:

login: KDB: enter: Break to debugger
[ thread pid 838 tid 100053 ]
Stopped at      $d:     ldrb    r15, [r15, r15, ror r15]!
db> ^R

so I tried turn off _break_to_debugger sysctls, and then it can
survive without being interrupted. Indeed this looks like what you
said, sending break rs232 signal.

Guess I need to have a proper sequence disabling break before
disconnecting from BBB and turning off my PC. ;)

Jia-Shiun.