Date: Sun, 27 Apr 2014 11:04:56 -0600 From: Ian Lepore <ian@FreeBSD.org> To: Bakul Shah <bakul@bitblocks.com> Cc: FreeBSD ARM <freebsd-arm@FreeBSD.org> Subject: Re: Looking for FreeBSD u-boot/kernel debugging help (BeagleBone Black) Message-ID: <1398618296.61646.161.camel@revolution.hippie.lan> In-Reply-To: <20140426233438.49123B827@mail.bitblocks.com> References: <CADH-AwHN0=p4z0RJZNcoRdoDJTMVAtK5F3TNCh5qdMWessQ0KQ@mail.gmail.com> <20140426233438.49123B827@mail.bitblocks.com>
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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? -- Ian
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