That means the "if" statement is not doing any tricks here, returning 0 means IS_ERR is not creating any problems which is a nice thing, now the next statement in that function is a call to vfs_read function, its declaration is shown in fs.h as:
extern ssize_t vfs_read(struct file *, char __user *, size_t, loff_t *);
You need to verify whether the parameters you are throwing in it have exactly the same types as above, do they need a typecast. I haven't used vfs_read before, so you also need to see whether what these parameters mean is same as your understanding of them!

You mean receiving the data from the device in user space? If yes, then you can read it in the kernel space and then "copy" it to the user space, there are predefined kernel functions for handling the same.

One thing I would like to clarify, I am no expert in these kernel matters. One year back I wrote the interrupt handlers for the keyboard and the serial port rs232, that's it. And secondly posting in the wrong forum is also not going to help you, "kernel section" post there.
Also: http://kernelnewbies.org/MailingList

About what you did on the kernel.... that's so much more than what I've done. :-)

ANd yes, I think taking this whole thing to userspace will be the right thing to try. I'll think it over and look around for samples of code.

I was referring to: copy_to_user.

I don't mean taking the whole driver to user space... just reading from the files so that I can skip this mess. I'll take a look at your link. Thank you very much.

Reading about call_usermodehelper_setup I'm wondering if it's possible to call an application and and communicate with it through its stdin/stdout? I'd like to keep implementation as simple as possible.

Other mechanism I found around is communicating through UDP. I was able to see messages sent from an application to the kernel, but I haven't been able to send information form the kernel to the outside world. Do you know of a simple example? Being able to send "hello world" through the net would be enough. Don't have to have a listening application on the other side to know that it's working.

I haven't used/read about the function you mentioned.
Do you mean to ask whether it is possible to pass arguments to a kernel module?
Which application are you asking about?

Sending information from kernel to outside world means what? I don't understand sending "hello world through net".

Is all this related to the driver errors? If no, then there should be a new thread for a new problem.
P.S. Did you verify vfs_read as I mentioned in post 31?

I've been following the inner workings of the kernel when I call vfs_read (by cloning function, adding my debug messages, recompiling and so on). The error (one of the possible errors, that is...) is being generated on file->f_ops->read() from vfs_read(). How can I know where that function is going? By the fs? I mean, if the file is being read from an ext3 partition, I'd check the function used to read files on ext3 and so on?