On Wed, Mar 22, 2017 at 04:27:50PM -0300, Guilherme G. Piccoli wrote:

The xmon parameter nobt was added long time ago, by commit 26c8af5f01df
("[POWERPC] print backtrace when entering xmon"). The problem that time
was that during a crash in a machine with USB keyboard, xmon wouldn't
respond to commands from the keyboard, so printing the backtrace wouldn't
be possible.

Idea then was to show automatically the backtrace on xmon crash for the
first time it's invoked (if it recovers, next time xmon won't show
backtrace automatically). The nobt parameter was added _only_ to prevent
this automatic trace show. Seems long time ago USB keyboards didn't work
that well!

Xmon still can't use a USB keyboard.  It's not a question of how well
USB keyboards work, it's that having a non-interrupt-driven USB stack
with (at least) an OHCI host controller driver and a keyboard driver
in xmon is impractical.

Paul.