Linux Char Device Driver

Gary 2013/03/20

Outline

• Introduction

• Module

• Major Number and Minor Number

• Data Structure

• Registration

• Open and Release

• Read and Write

• Future Work

Introduction

• Device driver is a bridge between physical devices and programs, and it’s part of kernel. User program can manage physical devices via device driver.

• Driver can be roughly divided into Block device driver and Character device driver. The former transfer a fixed amount of data each time, and Character device driver transfer no-fixed amount of data.

Module

Module

Advantage:

• Reduce kernel image space

• Speed up the boot time

• Facilitate the development of the kernel function

Module

Linking a module to a kernel

Major Number and Minor Number

• Major number(0-255)

When the kernel receives open() system call,

it selects the driver based on major number.

• Minor number(0-255)

Identify individuals of similar devices.

Meaningless to kernel. Only the driver itself

knows the significance of the minor number.

Major Number and Minor Number

• “c” represents special file of char driver.

• “b” represents device file of block driver.

Major number Minor number

Major Number and Minor Number

• Use mknod command to create device node. Needs superuser priviledges and four arguments.

- <name> <device type> <major> <minor>

$mknod /dev/ant c 252 0

• Use rm command to delete device node.

$rm /dev/ant

Data Structure

• Struct file represents an opened-device.

• Struct file_operations is used for kernel to access the method in driver.

- defined in <linux/fs.h>

• Struct file has field f_op, which is the pointer point to struct file_operations

File_operations

File_operations

• struct module *owner;

Not a function pointer. For kernel to maintain

the usage count of module.

• loff_t (*llseek) (struct file *, loff_t, int);

Change the position of current file read write

point.

File

• The file mentioned here has no concerned with the file in normal application.

• For every file which is opened, there is a correspond struct file.

• The pointer point to file is named filp.

File

Old Registration Method

• Call register_chrdev()

- define in <linux/fs.h>

- /usr/src/<kernel version>/include/linux/fs.h

Old Registration Method

• $cat /proc/devices

New Registration Method

• Kernel uses struct cdev to represent char device driver. You need to include <linux/cdev.h>

• Use cdev_alloc() to configure struct cdev

• If you have your own designed struct, you need to use cdev_init()

Struct cdev *my_cdev = cdev_alloc();My_cdev->ops = &my_fops;

Void cdev_init(struct cdev *dev, struct file_operations *fops);

New Registration Method

• No matter how to initialize struct cdev, the owner field must be set

• The last step, use cdev_add() to add to kernel

struct cdev my_cdev;my_cdev.owner = THIS_MODULE;

int cdev_add(struct cdev *dev, dev_t num, unsigned int count);

Struct cdev you set

Major number

Total amount of device number

New Registration Method

• Destroy cdevvoid cdev_del(struct cdev *dev);

Open and Release

• Open operation offers driver initialization, and increase usage count.

• Release operation decrease usage count.

- Defined in <linux/module.h>

Open

• Most of the open operation of driver should do the following jobs.

- increase usage count

- check for device specific errors

(ex : no CD in CD-ROM)

- if the target device is opened the first time,

do initializtion

- Identify the minor number and update the f_op

pointer

- allocate and fill data structure in filp->private data

Open

• The first step of open operation is to check the target device’s minor number.

Release

• Release anything that open operation allocate to flip->private_data.

• Shut down the target device on the last close.

• Decrease usage count

Read and Write

• flip : file pointer

• buff : argument to the buffer in user-space

• count : transfer data amount

• offp : the file location

ssize_t read(struct file *filp, char *buff, size_t count, loff_t *offp);ssize_t write(struct file *filp, const char *buff, size_t count, loff_t *offp);

Read and Write

• Data transfer between kernel space and user space.

• Use the function defined in <asm/uaccess.h>

Read and Write

• The arguments to read

Future Work

• Read more

• Ioctl