Comment # 15 on bug 930015 from Steffen Klein

What you have found is the alsa firewire support by takashi Sakamoto.
While I'm not sure, Idon't think it's permissions related.
In my case I see the same beahviour with firewire (Edirol FA-101) as well as
with ALSA, with RT or with RT disabled, as a normal user as well as root. 
I'm not even able to show the help info:

> /usr/bin/jackd -v -P70 -dalsa -dhw:0 -r96000 -p256 -n3 -m -H -M -Xseq
jackdmp 1.9.10
Copyright 2001-2005 Paul Davis and others.
Copyright 2004-2014 Grame.
jackdmp comes with ABSOLUTELY NO WARRANTY
This is free software, and you are welcome to redistribute it
under certain conditions; see the file COPYING for details
^C
> /usr/bin/jackd -r
jackdmp 1.9.10
Copyright 2001-2005 Paul Davis and others.
Copyright 2004-2014 Grame.
jackdmp comes with ABSOLUTELY NO WARRANTY
This is free software, and you are welcome to redistribute it
under certain conditions; see the file COPYING for details
^C
> /usr/bin/jackd
jackdmp 1.9.10
Copyright 2001-2005 Paul Davis and others.
Copyright 2004-2014 Grame.
jackdmp comes with ABSOLUTELY NO WARRANTY
This is free software, and you are welcome to redistribute it
under certain conditions; see the file COPYING for details
^C
> 
> /usr/bin/jackd -h
jackdmp 1.9.10
Copyright 2001-2005 Paul Davis and others.
Copyright 2004-2014 Grame.
jackdmp comes with ABSOLUTELY NO WARRANTY
This is free software, and you are welcome to redistribute it
under certain conditions; see the file COPYING for details
^C
>