Writing device drivers in Linux(part 5)

Also, note the use of the kmalloc function. This function is used for memory allocation of the buffer in the device driver which resides in kernel space. Its use is very similar to the well known malloc function. Finally, if registering the major number or allocating the memory fails, the module acts accordingly.
The “memory” driver: removing the driver

In order to remove the module inside the memory_exit function, the function unregsiter_chrdev needs to be present. This will free the major number for the kernel.

=

void memory_exit(void) {
/* Freeing the major number */
unregister_chrdev(memory_major, "memory");

/* Freeing buffer memory */
if (memory_buffer) {
kfree(memory_buffer);
}

printk("<1>Removing memory module\n");

}

The buffer memory is also freed in this function, in order to leave a clean kernel when removing the device driver.
The “memory” driver: opening the device as a file

The kernel space function, which corresponds to opening a file in user space (fopen), is the member open: of the file_operations structure in the call to register_chrdev. In this case, it is the memory_open function. It takes as arguments: an inode structure, which sends information to the kernel regarding the major number and minor number; and a file structure with information relative to the different operations that can be performed on a file. Neither of these functions will be covered in depth within this article.

When a file is opened, it’s normally necessary to initialize driver variables or reset the device. In this simple example, though, these operations are not performed.

The memory_open function can be seen below:

=

int memory_open(struct inode *inode, struct file *filp) {

/* Success */
return 0;
}

This new function is now shown in Table 5.

Events User functions Kernel functions
Load module insmod module_init()
Open device fopen file_operations: open
Read device
Write device
Close device
Remove module rmmod module_exit()

Table 5. Device driver events and their associated interfacing functions between kernel space and user space.
The “memory” driver: closing the device as a file

The corresponding function for closing a file in user space (fclose) is the release: member of the file_operations structure in the call to register_chrdev. In this particular case, it is the function memory_release, which has as arguments an inode structure and a file structure, just like before.

When a file is closed, it’s usually necessary to free the used memory and any variables related to the opening of the device. But, once again, due to the simplicity of this example, none of these operations are performed.

The memory_release function is shown below:

=

int memory_release(struct inode *inode, struct file *filp) {
 
  /* Success */
  return 0;
}

This new function is shown in Table 6.

Events	User functions	Kernel functions
Load module	insmod	module_init()
Open device	fopen	file_operations: open
Read device
Write device
Close device	fclose	file_operations: release
Remove module	rmmod	module_exit()

Table 6. Device driver events and their associated interfacing functions between kernel space and user space.

The “memory” driver: reading the device

To read a device with the user function fread or similar, the member read: of the file_operations structure is used in the call to register_chrdev. This time, it is the function memory_read. Its arguments are: a type file structure; a buffer (buf), from which the user space function (fread) will read; a counter with the number of bytes to transfer (count), which has the same value as the usual counter in the user space function (fread); and finally, the position of where to start reading the file (f_pos).
In this simple case, the memory_read function transfers a single byte from the driver buffer (memory_buffer) to user space with the function copy_to_user:
=

ssize_t memory_read(struct file *filp, char *buf, 
                    size_t count, loff_t *f_pos) { 
 
  /* Transfering data to user space */ 
  copy_to_user(buf,memory_buffer,1);

  /* Changing reading position as best suits */ 
  if (*f_pos == 0) { 
    *f_pos+=1; 
    return 1; 
  } else { 
    return 0; 
  }
}

The reading position in the file (f_pos) is also changed. If the position is at the beginning of the file, it is increased by one and the number of bytes that have been properly read is given as a return value, 1. If not at the beginning of the file, an end of file (0) is returned since the file only stores one byte.
In Table 7 this new function has been added.

Events	User functions	Kernel functions
Load module	insmod	module_init()
Open device	fopen	file_operations: open
Read device	fread	file_operations: read
Write device
Close device	fclose	file_operations: release
Remove modules	rmmod	module_exit()

Table 7. Device driver events and their associated interfacing functions between kernel space and user space.

see you next time  :)
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