Difference between revisions of "Embedded-Linux-Device-Driver/C3/Registering-a-Character-Device-Number/English"

Welcome slide:

Learning objectives:

• Dynamically allocate the major and minor numbers to a device.
• Register a new character device inside the kernel.

System Requirements:

• VirtualBox version 5.2.
• Ubuntu Linux 18.04 LTS operating system.
• Linux kernel version 5.0.0-31 generic and
• gedit text editor

Prerequisites:

• C programming language and
• Basics of Linux kernel

If not, then go through the C/C++ and Linux spoken tutorials on this website.

Device file in Linux.

• Each device is represented as a file in Linux.
• The device files are located under the dev directory.
• Each device file in Linux has a unique number associated with it.

Press Enter key after every command.

Show the output

Type the command as shown here.

Highlight 4

The major number indicates the driver associated with the device.

It indicates that the tty0 and tty1 serial ports are managed by the driver 4.

The tty0 and tty1 have different minor numbers 0 and 1 as shown here.

The driver uses the minor number to distinguish individual physical or logical devices.

Using these minor numbers, the driver 4 is able to differentiate these devices.

Internal representation of device number

• The kernel uses the dev_t type variable to hold major and minor numbers.
• The size of the dev_t is 32-bit.
• 12 bits are used for major numbers and 20 bits are used for minor numbers.

Point to the files.

I have created a directory named RegisterDevice.

In this directory, I have saved simple_driver dot c and Makefile.

I will use these files for demonstration.

Code files:

• The files used in this tutorial are available in the Code Files link on this tutorial page.
• Please download and extract them
• Make a copy and then use them while practising

Go to the directory where RegisterDevice is saved in your system.

Type gedit space simple_driver dot c.

Let us see the code.

Highlight fs.h

The purpose of each header file is mentioned here.

fs.h contains the required functions to register a character device number.

It will hold the major and minor number of a device.

The kernel will find the unused major number and assign it to a device.

This function returns a value less than zero on failure.

Highlight firstminor

Highlight count

Highlight name

dev is the variable that holds the device number.

firstminor specifies the starting minor number which is 0.

count specifies the number of supported devices.

name is a device name used to identify the device.

Highlight alloc_chrdev_region()

my_device.

This function will register one device with the name my_device.

Highlight MINOR(dev_t dev)

Highlight macros

Similarly, MINOR macro is used to extract the minor number from the dev_t variable.

It will print the major and minor numbers of my_device using these macros.

You should free the device numbers when they are no longer in use.

Highlight count

count represents the number of devices to unregister.

Highlight printk()

When the driver is unloaded, these messages will be printed in the kernel log level.

Save the file and close the window.

Type >> clear

Type gedit space Makefile.

Type the code as shown or you can make use of the downloaded Makefile.

Save and close the file.

Type >> sudo su

Type sudo space su to be a superuser and type the system password.

Type >> clear

Clear the screen.

Type >> clear

Type insmod space simple_driver dot ko.

Clear the screen.

Highlight major and minor number

We can see the major and minor number of my_device

It indicates that the my_device is registered inside the kernel successfully.

Highlight devices

Type the command as shown here.

This file displays the various devices currently configured in the kernel.

It indicates that my_device is registered inside the kernel successfully.

Type rmmod space simple_driver dot ko

Highlight printk

Type >> clear

Type the command as shown here.

It indicates that the my_device is unregistered from the kernel successfully.

Clear the screen.

Type >> clear

It removes all objects files created after the compilation.

Clear the screen.

Summary

• Dynamically allocate the major and minor numbers to a device.
• Register a new character device inside the kernel.

Assignment :

1. Open the simple_driver.c file.
2. Allocate the device number using the kernel function.
3. Change the name of the device.
4. Compile and load the driver into the kernel.
5. See the registered device in the procfs
6. Unload the driver from the kernel.

About Spoken Tutorial Project:

• The video at the following link summarizes the Spoken Tutorial project.

• Please download and watch it.

Spoken Tutorial workshops:

For more details, please write to us.

Forum questions :

Forum for specific questions :

• Do you have any general or technical questions on Embedded Linux Device Driver?
• Please visit the FOSSEE forum and post your question.

Acknowledgment:

Thank you slide:

This is Usha from IIT Bombay signing off.

Thanks for watching.