char driver node is not opening - c

Question

I wrote a simple char driver for my beaglebone kernel
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <asm/uaccess.h>
#include <linux/device.h>
#include <linux/slab.h>
#include <linux/fs.h>
MODULE_LICENSE("GPL");
MODULE_AUTHOR("mrigendra.chaubey#gmail.com");
#define DEVICE_NAME "experm"
#define CLASS_NAME "exp"
static struct class* myclass = NULL; ///< The device-driver class struct pointer
static struct device* mychardevice = NULL; ///< The device-driver device struct pointer
static int myopen(struct inode *, struct file *);
static int release(struct inode *, struct file *);
static int myioctl(struct inode *, struct file *, unsigned int cmd, unsigned long arg);
static size_t myread(struct file *,char * , size_t, loff_t *);
static size_t mywrite(struct file *,char * , size_t, loff_t *);
static dev_t mydev;
static int myopen(struct inode *nd, struct file *fp)
{
printk(KERN_INFO "myopen\n");
return 0;
}
static int myrelease(struct inode *nd, struct file *fp)
{
printk(KERN_INFO "myrelease\n");
return 0;
}
static int myioctl(struct inode *nd, struct file *fp, unsigned int cmd, unsigned long arg)
{
printk(KERN_INFO "myioctl\n");
return 0;
}
static size_t myread(struct file *fp, char *buf, size_t len, loff_t *ofs)
{
printk(KERN_INFO "myread\n");
return 0;
}
static size_t mywrite(struct file *fp, char *buf, size_t len, loff_t *ofs)
{
printk(KERN_INFO "mywrite\n");
return 0;
}
static struct file_operations fops = {
.open = myopen,
.release = myrelease,
.read = myread,
.write = mywrite,
.unlocked_ioctl = myioctl,
};
static int __init myinit(void)
{
int err;
//extern int alloc_chrdev_region(dev_t *, unsigned minor number, unsigned total, const char *);
err = alloc_chrdev_region(&mydev, 0, 1, "expermdev");
if(err<0)
{
printk(KERN_INFO "major and minor can't be created, err = %d\n", err);
return err;
}
//struct class * class_create ( struct module *owner, const char *name);
myclass = class_create(THIS_MODULE, CLASS_NAME);
if(IS_ERR(myclass))
{
unregister_chrdev(MAJOR(mydev), "expermdev");
printk(KERN_ALERT "Failed to register device class\n");
return PTR_ERR(myclass);
}
//struct device *device_create(struct class *cls, struct device *parent, dev_t devt, void *drvdata, const char *fmt, ...);
//This function can be used by char device classes. A struct device will be created in sysfs, registered to the specified class.
mychardevice = device_create(myclass, NULL, mydev, NULL, "expermdev");
if(IS_ERR(mychardevice))
{
class_destroy(myclass);
unregister_chrdev(MAJOR(mydev), "expermdev");
printk(KERN_ALERT "Failed to create the device\n");
return PTR_ERR(mychardevice);
}
printk(KERN_INFO "my device created correctly\n");
return 0;
}
static void __exit myexit(void)
{
device_destroy(mychardevice, mydev);
class_unregister(myclass);
class_destroy(myclass);
unregister_chrdev(MAJOR(mydev), "expermdev");
printk(KERN_INFO "exited\n");
}
module_init(myinit);
module_exit(myexit);
and app.c file
#include <stdio.h>
#include<fcntl.h>
int main()
{
int fp;
fp = open ("/dev/expermdev", O_RDWR);
if(fp < 0)
printf("file can't be opened\n");
else
printf("file opened\n");
return 0;
}
I am compiling the driver as a module and insmod it, also compiled app.c with the same cross compiler, and put this binary in bin directory. I ran this bin file, but it says
file can't be opened
What am I doing wrong?

Answer 1

Got it. I did not used cdev struture and added fops to it. After that I have to tell kernel about it.
cdev = cdev_alloc();//allocate memory to Char Device structure
cdev->ops = &fops;//link our file operations to the char device
result=cdev_add(cdev,mydev,1);//Notify the kernel abt the new device
also delete this cdev in exit function before unregistering the driver.
In register_chrdev_region we do both things at the same time, but in dynamic allocation of major and minor numbers we have to use cdev to do the rest.