Drivers en linux (II)

Ahora crearemos un dispositivo de caracteres en /dev/algo, para esto debemos definir cuatro nuevas funciones: para abrir y cerrar el dispositivo, y para leer y escribirlo. Hay mas operaciones, pero con esas es suficiente por ahora (http://www.linuxtopia.org/online_books/linux_kernel/linux_kernel_module_programming_2.6/x569.html).

El archivo test.h queda asi:

#ifndef _TEST_SPI_H_
#define _TEST_SPI_H_

#define DEVICE_NAME "test-spi"

MODULE_AUTHOR("Jorge Eduardo Cardona <jorgeecardona@gmail.com>");
MODULE_DESCRIPTION("SPI test driver.");
MODULE_LICENSE("GPL");

// Character device functions.
static int this_open(struct inode *inode, struct file *file);
static int this_release(struct inode *inode, struct file *file);
static ssize_t this_read(struct file *file, char __user *buffer, size_t len, loff_t *offset);
static ssize_t this_write(struct file *file, const char __user *buffer, size_t len, loff_t *offset);

// File Operations structure.
struct file_operations this_fops = {
    .open       = this_open,
    .release    = this_release,
    .read       = this_read,
    .write      = this_write
};

// Linux module functions.
static int __init this_init(void);
static void __exit this_exit(void);
module_init(this_init);
module_exit(this_exit);

#endif

Para crear el dispositivo de caracteres debemos llamar a la funcion: register_chrdev (http://kerneltrap.org/man/linux/man9/register_chrdev.9)(http://www.silicontao.com/ProgrammingGuide/GNU_function_list/register_chrdev.html)

#include <linux/module.h>
#include <linux/fs.h>
#include <linux/err.h>


#include "test.h"

// Global data
static int this_major;


// Function that is called every time we try to open a device.
static int this_open(struct inode *inode, struct file *file)
{
    printk(DEVICE_NAME": Open device.\n");
    return 0;
}

// Function that is called when we close a device.
static int this_release(struct inode *inode, struct file *file)
{
    printk(DEVICE_NAME": Close device.\n");
    return 0;
}

// Function that is called when we read from a device.
static ssize_t this_read(struct file *file, char __user *buffer, size_t len, loff_t *offset)
{
    printk(DEVICE_NAME": Read device.\n");
    return 0;
}

// Function that is called when we write to a device.
static ssize_t this_write(struct file *file, const char __user *buffer, size_t len, loff_t *offset)
{
    printk(DEVICE_NAME": Write device.\n");
    return 0;
}

// Function that is called when we  load the module.
static int __init this_init(void)
{
    int ret;

    printk(DEVICE_NAME ": Init.\n");

    // Register char device: 0 for dynamically allocation of the major number.
    ret = register_chrdev(0, DEVICE_NAME, &this_fops);
    if (~IS_ERR_VALUE(ret))
    {
        printk(DEVICE_NAME ": Character device registered, with major=%d.\n",ret);
        this_major = ret;

        return 0;
    }

    unregister_chrdev(this_major, DEVICE_NAME);
    printk(DEVICE_NAME ": Character device unregistered.\n");

    return ret;
}

// Function that is called when we remove the module.
static void __exit this_exit(void)
{
    unregister_chrdev(this_major, DEVICE_NAME);
    printk(DEVICE_NAME ": Character device unregistered.\n");
    printk(DEVICE_NAME ": Exit.\n");
}

(http://tomoyo.sourceforge.jp/cgi-bin/lxr/ident?i=IS_ERR_VALUE)

Bueno, igual que antes, configuramos, compilamos y tenemos:

root@gumstix-custom-connex:~$ modprobe test
root@gumstix-custom-connex:~$ dmesg | grep "test-spi"
<4>test-spi: Init.
<4>test-spi: Character device registered, with major=254.
root@gumstix-custom-connex:~$ mknod /dev/test c 254 0
root@gumstix-custom-connex:~$ cat /dev/test 
root@gumstix-custom-connex:~$ dd bs=1 count=0 if=/dev/zero of=/dev/test
0+0 records in
0+0 records out
root@gumstix-custom-connex:~$ echo "ja" > /dev/test

root@gumstix-custom-connex:~$ rmmod test
root@gumstix-custom-connex:~$ dmesg | grep "test-spi"
<4>test-spi: Init.
<4>test-spi: Character device registered, with major=254.
<4>test-spi: Open device.
<4>test-spi: Read device.
<4>test-spi: Close device.
<4>test-spi: Open device.
<4>test-spi: Close device.
<4>test-spi: Open device.
<4>test-spi: Write device.
<4>test-spi: Write device.
<4>test-spi: Write device.
<4>test-spi: Write device.
<4>test-spi: Write device.
<4>test-spi: Write device.
<4>test-spi: Write device.
<4>test-spi: Write device.
<4>test-spi: Write device.
<4>test-spi: Write device.
<4>test-spi: Close device.
<4>test-spi: Character device unregistered.
<4>test-spi: Exit.
root@gumstix-custom-connex:~$ 

Los varios mensajes de Write se deben a que retornamos 0 esto quiere decir que no escribimos ningun caracter y la funcion es llamada hasta que vacie el buffer, pero aqui toca matarla con un Ctrl-C.

Bueno, un paso mas.