How to implement a writable proc file by using seq_file in a driver module

Question

In the book of Linux Device Driver 3rd ed, /proc file system is used as a output method to export the running state of a certain device driver.

However, in some circumstances, /proc file system is used as one interface to change the internal parameters of a driver module.

I googled a lot, and found some implementations on the Internet are too old that they used create_proc_entry() rather than proc_create().

What's more, I'm prefer to implement this by seq_file(actually, I'm not sure is it possible). I checked the seq_write() function, and obtained nothing.

Can anyone show me an example to finish this mission? seq_file implementation is more preferred.

Answer 1

seq_file provides helpers only for reading file. There is no similar helpers for write, but it is not difficult to implement .write callback for iterable data by hands:

Unlike to the reading, you can drop file's position handling in the .write callback, assuming that user always write to the beginning, or, optionally, to the end (using O_APPEND file's control flag). Second, again unlike to the reading, you can assume that user writes at once content of 1,2 or more elements, but not a half element.

The simplest way is allow to write by single element:

size_t write(struct file* file, const char __user* buf, size_t size, loff_t* pos)
{
    copy_from_user(str, buf, size); // Copy string from user space
    my_elem = my_parse(str, size); // Parse string
    if(file->f_flags & O_APPEND) {
         list_add_tail(my_elem, &my_list);//Append element to the end of list
    }
    else {
         clear_my_list(); // Clear old content of the list
         list_add_tail(my_elem, &my_list);// Add single element to it.
    }

    (void)pos; //Do not use file position at all
    return count; // Return number of bytes passed by the user
}

If user wants to write several elements, e.g., from a file on hard disk, any shell is able to split this file by, e.g., new lines, and feed lines to the your proc file one by one.