intn

1 What is it for ?

This driver is an further development from the one driver. It implements additionally 3 features:

• store (write) data (integer) provided from user space
• implements basic concurrency

In other words, the user can set a desired value (integer) and more than one user space programm can read/write in a coherent way. In order to test this driver a user space programm was written additionally, where it can start n threads in parallel (default: 4) that read the driver value, increment it and write it back.

2 Looking at the code

struct intn2_dev {
	struct cdev    intn_cdev;
	struct class  *intn_class;
	struct device *intn_device;
	struct mutex   intn_mutex;
};

2.1 Common code

The necessary header are the same as in the intndriver.

2.2 User-space communication

We take care now the concurrency operations in the open and release functions, so we must deal with multiple user-space accesses. They are executed when the user-space invoke the openand close system calls respectively.

int intn2_release(struct inode *inode, struct file *filp)
{
	mutex_unlock(&intn2->intn2_mutex);
	return 0;
}

int intn2_open(struct inode *inode, struct file *filp)
{
	mutex_lock(&intn2->intn2_mutex);
	return 0;
}

Write: write data from the device.

ssize_t (*read)(struct file *f, char __user *u, size_t size, loff_t *l);

Parameters	type	Description
*f	input	pointer to the structure that represents the file to be open
*u	input	pointer to the buffer that stores data to be read from the device
size	input	number of bytes to be read
*l	input	pointer to the current reading position/offset
ssize_t	return	a nonnegative return value represents the number of bytes successfully read, on failure a negative error code

The one_read implements this operation, returning only a "1" to user-space. In order to do so, the copy_to_userfunction is used. If an error occurs, an -EFAULT is returned to user space.

unsigned long copy_from_user(void __user *to, const void *from, unsigned long count);

Parameters	type	Description
*to	output	pointer to the buffer in kernel space
*from	input	pointer to the buffer holding the data in user space
count	input	number of bytes to be copied from user-space
unsigned long	return	returns zero on success or non-zero to indicate the number of bytes that weren't transferred

3 Executing the code

Run make to compile it. Run sudo make ins or sudo insmod one.ko to insert the driver in the Linux Kernel memory space. Then take a look at the kernel log and you should see the following line:

Jun 16 21:09:19 localhost kernel: [41443.956311] one<Major, Minor>: <248, 0>