Char Device Driver¶

obj-m+=ebbchar.o

all:
    make -C /lib/modules/$(shell uname -r)/build/ M=$(PWD)  modules
    $(CC) test.c -o test
clean:
    make -C /lib/modules/$(shell uname -r)/build/ M=$(PWD) clean
    rm test

/*
* @file   ebbchar.c
* @author Derek Molloy
* @date   7 April 2015
* @version 0.1
* @brief   An introductory character driver to support the second article of my series on
* Linux loadable kernel module (LKM) development. This module maps to /dev/ebbchar and
* comes with a helper C program that can be run in Linux user space to communicate with
* this the LKM.
* @see http://www.derekmolloy.ie/ for a full description and follow-up descriptions.
*
* Corsi 20b: Modifiquei para usar kmalloc no lugar de de alocacao estática.
*
*/

#include <linux/init.h>           // Macros used to mark up functions e.g. __init __exit
#include <linux/module.h>         // Core header for loading LKMs into the kernel
#include <linux/device.h>         // Header to support the kernel Driver Model
#include <linux/kernel.h>         // Contains types, macros, functions for the kernel
#include <linux/fs.h>             // Header for the Linux file system support
#include <linux/uaccess.h>          // Required for the copy to user function
#include <linux/slab.h>

#define  DEVICE_NAME "ebbchar"    ///< The device will appear at /dev/ebbchar using this value
#define  CLASS_NAME  "ebb"        ///< The device class -- this is a character device driver

MODULE_LICENSE("GPL");            ///< The license type -- this affects available functionality
MODULE_AUTHOR("Derek Molloy");    ///< The author -- visible when you use modinfo
MODULE_DESCRIPTION("A simple Linux char driver for the BBB");  ///< The description -- see modinfo
MODULE_VERSION("0.1");            ///< A version number to inform users

static int    majorNumber;                  ///< Stores the device number -- determined automatically
static char   *message;                     ///< Memory for the string that is passed from userspace
static short  size_of_message;              ///< Used to remember the size of the string stored
static int    numberOpens = 0;              ///< Counts the number of times the device is opened
static struct class*  ebbcharClass  = NULL; ///< The device-driver class struct pointer
static struct device* ebbcharDevice = NULL; ///< The device-driver device struct pointer

// The prototype functions for the character driver -- must come before the struct definition
static int     dev_open(struct inode *, struct file *);
static int     dev_release(struct inode *, struct file *);
static ssize_t dev_read(struct file *, char *, size_t, loff_t *);
static ssize_t dev_write(struct file *, const char *, size_t, loff_t *);

/** @brief Devices are represented as file structure in the kernel. The file_operations structure from
*  /linux/fs.h lists the callback functions that you wish to associated with your file operations
*  using a C99 syntax structure. char devices usually implement open, read, write and release calls
*/
static struct file_operations fops =
{
.open = dev_open,
.read = dev_read,
.write = dev_write,
.release = dev_release,
};

/** @brief The LKM initialization function
*  The static keyword restricts the visibility of the function to within this C file. The __init
*  macro means that for a built-in driver (not a LKM) the function is only used at initialization
*  time and that it can be discarded and its memory freed up after that point.
*  @return returns 0 if successful
*/
static int __init ebbchar_init(void){
printk(KERN_INFO "EBBChar: Initializing the EBBChar LKM\n");

// Try to dynamically allocate a major number for the device -- more difficult but worth it
majorNumber = register_chrdev(0, DEVICE_NAME, &fops);
if (majorNumber<0){
    printk(KERN_ALERT "EBBChar failed to register a major number\n");
    return majorNumber;
}
printk(KERN_INFO "EBBChar: registered correctly with major number %d\n", majorNumber);

// Register the device class
ebbcharClass = class_create(THIS_MODULE, CLASS_NAME);
if (IS_ERR(ebbcharClass)){                // Check for error and clean up if there is
    unregister_chrdev(majorNumber, DEVICE_NAME);
    printk(KERN_ALERT "Failed to register device class\n");
    return PTR_ERR(ebbcharClass);          // Correct way to return an error on a pointer
}
printk(KERN_INFO "EBBChar: device class registered correctly\n");

// Register the device driver
ebbcharDevice = device_create(ebbcharClass, NULL, MKDEV(majorNumber, 0), NULL, DEVICE_NAME);
if (IS_ERR(ebbcharDevice)){               // Clean up if there is an error
    class_destroy(ebbcharClass);           // Repeated code but the alternative is goto statements
    unregister_chrdev(majorNumber, DEVICE_NAME);
    printk(KERN_ALERT "Failed to create the device\n");
    return PTR_ERR(ebbcharDevice);
}

// alocate mem
message = kmalloc(32, GFP_KERNEL);
if (IS_ERR(message)){
    printk(KERN_INFO "Failed to allocate mem \n");
    return PTR_ERR(message);
}

printk(KERN_INFO "EBBChar: device class created correctly\n"); // Made it! device was initialized
return 0;
}

/** @brief The LKM cleanup function
*  Similar to the initialization function, it is static. The __exit macro notifies that if this
*  code is used for a built-in driver (not a LKM) that this function is not required.
*/
static void __exit ebbchar_exit(void){
device_destroy(ebbcharClass, MKDEV(majorNumber, 0));     // remove the device
class_unregister(ebbcharClass);                          // unregister the device class
class_destroy(ebbcharClass);                             // remove the device class
unregister_chrdev(majorNumber, DEVICE_NAME);             // unregister the major number
kfree(message);                                          // release memmory
printk(KERN_INFO "EBBChar: Goodbye from the LKM!\n");
}

/** @brief The device open function that is called each time the device is opened
*  This will only increment the numberOpens counter in this case.
*  @param inodep A pointer to an inode object (defined in linux/fs.h)
*  @param filep A pointer to a file object (defined in linux/fs.h)
*/
static int dev_open(struct inode *inodep, struct file *filep){
numberOpens++;
printk(KERN_INFO "EBBChar: Device has been opened %d time(s)\n", numberOpens);
return 0;
}

/** @brief This function is called whenever device is being read from user space i.e. data is
*  being sent from the device to the user. In this case is uses the copy_to_user() function to
*  send the buffer string to the user and captures any errors.
*  @param filep A pointer to a file object (defined in linux/fs.h)
*  @param buffer The pointer to the buffer to which this function writes the data
*  @param len The length of the b
*  @param offset The offset if required
*/
static ssize_t dev_read(struct file *filep, char *buffer, size_t len, loff_t *offset){
int error_count = 0;
// copy_to_user has the format ( * to, *from, size) and returns 0 on success
error_count = copy_to_user(buffer, message, size_of_message);

if (error_count==0){            // if true then have success
    printk(KERN_INFO "EBBChar: Sent %d characters to the user\n", size_of_message);
    return (size_of_message=0);  // clear the position to the start and return 0
}
else {
    printk(KERN_INFO "EBBChar: Failed to send %d characters to the user\n", error_count);
    return -EFAULT;              // Failed -- return a bad address message (i.e. -14)
}
}

/** @brief This function is called whenever the device is being written to from user space i.e.
*  data is sent to the device from the user. The data is copied to the message[] array in this
*  LKM using the sprintf() function along with the length of the string.
*  @param filep A pointer to a file object
*  @param buffer The buffer to that contains the string to write to the device
* aaaaaaa @param len The length of the array of data that is being passed in the const char buffer
*  @param offset The offset if required
*/
static ssize_t dev_write(struct file *filep, const char *buffer, size_t len, loff_t *offset){
copy_from_user(message, buffer, len);
size_of_message = len;                 // store the length of the stored message
printk(KERN_INFO "EBBChar: Received %zu characters from the user\n", len);
return len;
}

/** @brieaf The device release function that is called whenever the device is closed/released by
*  the userspace program
*  @param inodep A pointer to an inode object (defined in linux/fs.h)
*  @param filep A pointer to a file object (defined in linux/fs.h)
*/
static int dev_release(struct inode *inodep, struct file *filep){
printk(KERN_INFO "EBBChar: Device successfully closed\n");
return 0;
}

/** @brief A module must use the module_init() module_exit() macros from linux/init.h, which
*  identify the initialization function at insertion time and the cleanup function (as
*  listed above)
*/
module_init(ebbchar_init);
module_exit(ebbchar_exit);

/**
* @file   testebbchar.c
* @author Derek Molloy
* @date   7 April 2015
* @version 0.1
* @brief  A Linux user space program that communicates with the ebbchar.c LKM. It passes a
* string to the LKM and reads the response from the LKM. For this example to work the device
* must be called /dev/ebbchar.
* @see http://www.derekmolloy.ie/ for a full description and follow-up descriptions.
*/
#include<stdio.h>
#include<stdlib.h>
#include<errno.h>
#include<fcntl.h>
#include<string.h>
#include<unistd.h>

#define BUFFER_LENGTH 256               ///< The buffer length (crude but fine)
static char receive[BUFFER_LENGTH];     ///< The receive buffer from the LKM

int main(){
int ret, fd;
char stringToSend[BUFFER_LENGTH];
printf("Starting device test code example...\n");
fd = open("/dev/ebbchar", O_RDWR);             // Open the device with read/write access
if (fd < 0){
    perror("Failed to open the device...");
    return errno;
}

while(1){
    printf("Type in a short string to send to the kernel module:\n");
    scanf("%[^\n]%*c", stringToSend);                // Read in a string (with spaces)
    printf("Writing message to the device [%s].\n", stringToSend);
    ret = write(fd, stringToSend, strlen(stringToSend)); // Send the string to the LKM
    if (ret < 0){
        perror("Failed to write the message to the device.");
        return errno;
    }

    //printf("Press ENTER to read back from the device...\n");
    //getchar();

    printf("Reading from the device...\n");
    ret = read(fd, receive, BUFFER_LENGTH);        // Read the response from the LKM
    if (ret < 0){
        perror("Failed to read the message fm the device.");
        return errno;
    }
    printf("The received message is: [%s]\n", receive);
}

printf("End of the program\n");
close(fd);
return 0;
}