Using devfs to address device driver

Hi All
I am writing a device driver that should use the existing serial port +
custom hardware.
Problem is that I want to use loadable modules and need to talk with the
serial port driver via the devfs (is this the best way ?).
I can get a file handle , no problem,  but when I want to call the
tty_open() procedure it requires parameters (inode, file *).
How do I get hold of them ?

Thanks in advance
reyems