Linux Kernel Architecture

Chapter 6: Device Drivers

6.4 Character Device Operations

The hardware of character devices is usually very simple, and, not surprisingly, the associated drivers

are not very difficult to implement.

6.4.1 Representing Character Devices

Recall that character devices are represented bystruct cdev. We have seen how the kernel keeps a

database of all activecdevinstances, but have not yet looked into the content of the structure. It is defined

as follows:

<cdev.h>

struct cdev {

struct kobject kobj;

struct module *owner;

const struct file_operations *ops;

struct list_head list;

dev_t dev;

unsigned int count;

};

kobjis a kernel object embedded in the structure. As usual, it is used for general management of the data

structure.ownerpoints to the module (if any) that provides the driver, andopsis a list of file operations

that implement specific operations to communicate with the hardware.devspecifies the device number,

andcountdenotes how many minors are associated with the device.listallows for implementing a list

of all inodes that represent device special files for the device.

Initially, the file operations for character devices are limited to just a single method for opening the

associated device file (always the first action when using a driver). Logically, let’s examine this method

first.

6.4.2 Opening Device Files

chrdev_openfromfs/devices.cis the generic function for opening character devices. Figure 6-6 shows

the associated code flow diagram.

chrdev_open

Find device specific file_operations

f_op->open

Figure 6-6: Code flow diagram forchrdev_open.

Assume that the inode that represents the device file has not been opened before. Given the device

number,kobject_lookupqueries the character device database introduced in Section 6.2.5 and returns

thekobjectinstance associated with the driver. This, in turn, allows for obtaining thecdevinstance.