ftdi_sio, and usb_serial->interface->dev->driver_data being null

Hi all,

I am trying to learn more about interfacing with FT232, and I started 
with ftdi_sio.c as a base, and my final goal is being able to interface 
with snd_pcm and ALSA (and I'm trying to use usbaudio functions as 
examples).

For that purpose, I am trying to build a module, where a modification of 
ftdi_sio.c is one object, and another .c file with corresponding ALSA 
functions is another object built into this module. Needless to say, I'm 
quite struggling to keep all the structs involved in mind :)

For now, I just want to come to the point where probe and disconnect 
work. I have tried to add additional commands to ftdi_sio_probe and 
ftdi_shutdown, so functions (my_extra_probe and my_extra_disconnect) in 
the other .c file get called.

Probe seems to work fine (i.e. a new card can be seen in cat 
/dev/sndstat), however, it is the disconnect I have a problem with. The 
problem is that in ftdi_sio_probe, after my_extra_probe, what 
corresponds to usb_serial->interface->dev->driver_data is a valid 
pointer -- however, whenever ftdi_shutdown is called, this 
...->driver_data is always a null pointer (even if ftdi_open was never 
called). Unfortunately, the functions from usbaudio are using precisely 
this "property" to proceed with deregistering. Below is a snippet from 
these two functions:

---
/* Probe function to check for special devices */
static int ftdi_sio_probe(struct usb_serial *serial,