ebbchar.c

/**
 * @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.
 */

#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/mutex.h>	  // Required for the mutex functionality
#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[256] = {0};           ///< 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
static DEFINE_MUTEX(ebbchar_mutex);	    /// A macro that is used to declare a new mutex that is visible in this file
					    /// results in a semaphore variable ebbchar_mutex with value 1 (unlocked)
					    /// DEFINE_MUTEX_LOCKED() results in a variable with value 0 (locked)

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

    mutex_init(&ebbchar_mutex);		/// Initialize the mutex lock dynamically at runtime
    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){
    mutex_destroy(&ebbchar_mutex);		/// destroy the dynamically-allocated mutex
    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
    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){
    if (!mutex_trylock(&ebbchar_mutex)){		/// Try acquire the mutez (i.e., put the lock on/down)
        						/// returns 1 if successful and 0 if there is contention
        printk(KERN_ALERT "EBBChar: Device in use by another process");
        return -EBUSY;
    }
    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
 *  @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){
    sprintf(message, "%s(%zu letters)", buffer, len);   // appending received string with its length
    size_of_message = strlen(message);                 // store the length of the stored message
    printk(KERN_INFO "EBBChar: Received %zu characters from the user\n", len);
    return len;
}

/** @brief 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){
    mutex_unlock(&ebbchar_mutex);
    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);