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List:       freebsd-arm
Subject:    Re: Looking for FreeBSD u-boot/kernel debugging help (BeagleBone Black)
From:       Warner Losh <imp () bsdimp ! com>
Date:       2014-04-27 18:30:29
Message-ID: 35FCA9EC-1569-494D-B193-90926D802219 () bsdimp ! com
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On Apr 27, 2014, at 11:04 AM, Ian Lepore <ian@FreeBSD.org> wrote:

> On Sat, 2014-04-26 at 16:34 -0700, Bakul Shah wrote:
> > On Sat, 26 Apr 2014 18:29:35 EDT Winston Smith <smith.winston.101@gmail.com> \
> > wrote:
> > > I also discovered that if I disconnect my
> > > serial terminal and reconnect it, it seems to bring the FreeBSD kernel
> > > to a debug prompt of sorts -- is there any documentation on this?
> > 
> > I suspect disconnecting/reconnecting the serial cable looks like
> > a break to the kernel and that drops it into the debugger "ddb".
> > [Though I can't seem to send a real break to it using kermit!]
> > 
> > Type c and hit return to continue. 
> > 
> > To disable this behavior I think you can do
> > 	sysctl debug.kbd.break_to_debugger=0
> > or add
> > 	debug.kbd.break_to_debugger=0
> > to /etc/sysctl.conf
> > 
> > man 1 ddb -- for debugger commands
> > man 4 ddb -- for kernel config options to control ddb behavior
> 
> That's exactly correct -- connecting the cable sometimes leads to a
> spurious break being asserted on the line (a break is just a long
> sequence of zeroes with no start/stop/data bit transitions).
> 
> It's possible to configure a kernel without BREAK_TO_DEBUGGER and with
> ALT_BREAK_TO_DEBUGGER.  That eliminates most line-noise spurious breaks
> but still allows the <CR> ~ ^b break sequence (which could theoretically
> happen in a burst of line noise, but not likely).  A few of our kernels
> have BREAK_TO_DEBUGGER in the config, and I think that's probably a
> mistake.  Most folks these days don't know anything about breaks or how
> to generate one on purpose.  
> 
> Any objections to removing them and using only the safer ALT_BREAK
> option?

Please leave it in the ATMEL config, but I think you can remove it from the rest \
(including the specific atmel SoC boards).

Warner
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