[Bug 205933] [boot] Loader hangs when connecting serial console

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=205933

            Bug ID: 205933
           Summary: [boot] Loader hangs when connecting serial console
           Product: Base System
           Version: 10.2-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: aram.h at mgk.ro

$ freebsd-version ; uname -a
10.2-RELEASE-p8
FreeBSD z800.mgk.ro 10.2-RELEASE-p7 FreeBSD 10.2-RELEASE-p7 #0: Mon Nov  2
14:19:39 UTC 2015    
root at amd64-builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC  amd64
$
$ cat /boot/loader.conf 
loader_color="NO"
loader_logo="beastiebw"
boot_multicons="YES"
boot_serial="YES"
comconsole_speed="115200"
console="comconsole,vidconsole"
kern.geom.label.gptid.enable="0"
zfs_load="YES"
$
$ grep -v '^#' /etc/ttys 
console none                            unknown off secure
ttyv0   "/usr/libexec/getty Pc"         xterm   on  secure
ttyv1   "/usr/libexec/getty Pc"         xterm   on  secure
ttyv2   "/usr/libexec/getty Pc"         xterm   on  secure
ttyv3   "/usr/libexec/getty Pc"         xterm   on  secure
ttyv4   "/usr/libexec/getty Pc"         xterm   on  secure
ttyv5   "/usr/libexec/getty Pc"         xterm   on  secure
ttyv6   "/usr/libexec/getty Pc"         xterm   on  secure
ttyv7   "/usr/libexec/getty Pc"         xterm   on  secure
ttyv8   "/usr/local/bin/xdm -nodaemon"  xterm   off secure
ttyu0   "/usr/libexec/getty std.115200" vt100   onifconsole secure
ttyu1   "/usr/libexec/getty std.9600"   dialup  off secure
ttyu2   "/usr/libexec/getty std.9600"   dialup  off secure
ttyu3   "/usr/libexec/getty std.9600"   dialup  off secure
dcons   "/usr/libexec/getty std.9600"   vt100   off secure

When I boot without a serial console attached, boot proceeds fine
(the loader autoboots after an interval). However, when I attach a
serial console, the loader hangs as if I had pressed a key to stop
the boot. The boot proceeds normally after I press Enter.

This seems to be correlated with whether I have a remote terminal
emulator on that serial line or not. If there's nobody listening,
so to speak, the boot proceeds fine, but if I connect with screen,
cu, or minicom, the boot pauses and waits for input.

Not sure if this matters or not, but the remote machine connects
to the serial console through an USB adapter.

This seems to be related, or perhaps the same problem as the one
described in this mailing list post:
https://lists.freebsd.org/pipermail/freebsd-stable/2014-April/078378.html.

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