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,
const struct usb_device_id *id)
{
struct snd_ftdi_audard_quirk *quirk =
(struct snd_ftdi_audard_quirk *)id->driver_info;
struct usb_interface *interface = serial->interface; // added
int ret;
...
usb_set_serial_data(serial, (void *)id->driver_info);
ret = my_extra_probe(interface, id);
dbg("%s: RET is %d ... (&interface->dev)->driver_data %p", __func__,
ret, (&interface->dev)->driver_data);
/*
* Here (&interface->dev)->driver_data is an OK pointer,
* same as the one traced from within my_extra_probe,
* where it is set.
*/
return 0;
}
static void ftdi_shutdown(struct usb_serial *serial)
{
struct usb_interface *interface = serial->interface; // added
dbg("%s - (&interface->dev)->driver_data %p", __func__,
(&interface->dev)->driver_data);
/*
* Here (&interface->dev)->driver_data is NULL pointer,
* which is a problem, because my_extra_disconnect needs it
*/
my_extra_disconnect(interface);
}
---
So these are the questions I have:
- Is there an easy/obvious reason, why this pointer is reset at
_shutdown, that I'm missing?
- How could I debug this? Ideally, I'd like to put a watch on
usb_serial->interface->dev->driver_data and see when it changes (and
what changes it) - but its not even a variable :)
I have just learned that I could recompile a kernel and use something
called kgdb, which needs two machines; I found
http://linux-hacks.blogspot.com/2008/05/setting-up-kgdb-using-kvmqemu.html
that uses a VM instead - but I'm afraid using a VM will mess up
everything USB related. Any links to some resources related to this kind
of debugging?
Thanks in advance for any answers,
Cheers!
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