Re: BeagleBone Black doesn't boot without serial console (10.0 release)

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

>> diff -u -p -u -r1.382 com.c
>> [...]

> As far (or as little) as i can understand, when i connect the UART to
> J1 by default "a signal" i generated which is the same signal that
> opens the debugger.

Yes (well, modulo the appropriateness of "open" as the verb).  This
signal is a break condition, which is defined as particular kind of
framing error: a valid start bit (the line going from idle mark to
space and staying that way for a full bit time) followed by the line
staying spacing for at least nine (I think) more consecutive bit times.
(In particular, it is not a character; I have seen people write of a
"break character", but that is, _at best_, a confusingly loose use of
the term.)

> With your change the signal generated is different so the debugger
> does not pop up.

No.  Nothing done in software on the BBB can affect what the hardware
does when you plug or unplug the cable (possibly excepting things like
software-controlled pullup/pulldown resistors).  You still usually get
various framing errors, probably including at least one break.  What
the change does do is cause the kernel to not respond to the break
condition by entering DDB.  (You could also do this with a sysctl,
ddb.fromconsole I think it is, but that's of less use; I think there's
a kernel config option that controls the default state of
ddb.fromconsole.  Also, turning that off makes it impossible to enter
DDB from the console deliberately, whereas the patch leaves it
possible, just requiring a different input gesture.)

> [...], i never [saw] any other device, on any tested OS, calling the
> debugger when i connect to the serial console.

I have, relatively often.  Probably either Suns or peecees, simply
because those are what I use most, but possibly also various other
hardware.  I certainly would never count on any hardware to _not_
produce a break condition on serial connect or disconnect.

What various hardware/OS combinations do in response to a break on
serial console varies.  Those that have anything like DDB typically
will invoke it.  Others vary - for example, SPARCstations under SunOS
would typically break to the ROM monitor.

/~\ The ASCII				  Mouse
\ / Ribbon Campaign
 X  Against HTML		mouse%rodents-montreal.org@localhost
/ \ Email!	     7D C8 61 52 5D E7 2D 39  4E F1 31 3E E8 B3 27 4B