Character Driver

First we will start with the basics of modules.

Module Information

• License
• Author
• Module Description
• Module Version

These pieces of information are present in the Linux/module.h as macros.

License

GPL, or the GNU General Public License, is an open-source license meant for software. If your software is licensed under the terms of the GPL, it is free. However, “free” here does not essentially mean freeware—it can also be a paid software. Instead, “free” as per the GPL means freedom. As proponents of GPL proudly proclaim, free as in freedom, not free beer.

Author

Using this Macro we can mention that who is writing this driver or module. So modinfo can show the author name for users wanting to know. We can give the Author name for our driver (module) like below. For this, you need to include the Linux/module.h header file.

Module Description

Using this Macro we can give a description of the module or driver. So modinfo can show a module description for users wanting to know. We can give the description for our driver (module) like below. For this, you need to include the Linux/module.h header file.

Module Version

Using this Macro we can give the version of the module or driver. So modinfo can show module version for users wanting to know.

Version of form [<epoch>:]<version>[-<extra-version>].

<epoch>: A (small) unsigned integer which allows you to start versions anew. If not mentioned, it’s zero. eg. “2:1.0” is after “1:2.0”.

<version>: The <version> may contain only alphanumerics and the character `.’. Ordered by numeric sort for numeric parts, ASCII sort for ASCII parts (as per RPM or DEB algorithm).

<extraversion>: Like <version>, but inserted for local customizations, eg “rh3” or “rusty1”.

Simple Kernel Module Programming

So as of now, we know the very basic things that needed for writing drivers. Now we will move into programming. In every programming language, how we will start to write the code? Any ideas? Well, in all programming there would be a starting point and ending point. If you take C Language, the starting point would be the main function, Isn’t it? It will start from the starting of the main function and run through the functions which are calling from the main function. Finally, it exits at the main function closing point. But Here two separate functions used for that starting and ending.

1. Init function
2. Exit function

Kernel modules require a different set of header files than user programs require. And keep in mind, Module code should not invoke user space Libraries or API’s or System calls.

Init function

This is the function that will execute first when the driver is loaded into the kernel. For example, when we load the driver using insmod, this function will execute. Please see below to know the syntax of this function.

This function should register itself by using module_init() macro.

Exit function

This is the function that will execute last when the driver is unloaded from the kernel. For example, when we unload the driver using rmmod, this function will execute. Please see below to know the syntax of this function.

This function should register itself by using module_exit() macro.

Printk()

In C programming how we will print the values or whatever? Correct. Using printf() function. printf() is a user-space function. So we cant use this here. So they created one another function for the kernel which is printk().

One of the differences is that printk lets you classify messages according to their severity by associating different log levels, or priorities, with the messages. You usually indicate the log level with a macro. I will explain about the macros now. There are several macros used for printk.

KERN_EMERG:

Used for emergency messages, usually those that precede a crash.

KERN_ALERT:

A situation requiring immediate action.

KERN_CRIT:

Critical conditions, often related to serious hardware or software failures.

KERN_ERR:

Used to report error conditions; device drivers often use KERN_ERR to report hardware difficulties.

KERN_WARNING:

Warnings about problematic situations that do not, in themselves, create serious problems with the system.

KERN_NOTICE:

Situations that are normal, but still worthy of note. A number of security-related conditions are reported at this level.

KERN_INFO:

Informational messages. Many drivers print information about the hardware they find at startup time at this level.

KERN_DEBUG:

Used for debugging messages.

Difference between printf and printk

• Printk() is a kernel-level function, which has the ability to print out to different log levels as defined in. We can see the prints using dmesg command.
• printf() will always print to a file descriptor – STD_OUT. We can see the prints in the STD_OUT console.

Simple Driver

This is the complete code for a simple module.

Compiling this Module / Driver code.

Now its time to compile the code by using a Makefile.

Sample Makefile.

Loading and Unloading the Module / Device driver

A Kernel Module is a small file that may be loaded into the running Kernel and unloaded.

Loading

To load a Kernel Module, use the insmod command with root privileges.

eg: root@egeeks:$ insmod <sampleProgram>.ko

sudo insmod <sampleProgram>.ko

lsmod used to see the modules were inserted. In the below image, I’ve shown the prints in init function. Use dmesg to see the kernel prints.

Listing the Modules

In order to see the list of currently loaded modules, use the lsmod command. In the above image, you can see that I have used lsmod command.

Unloading

To un-load, a Kernel module, use the rmmod command with root privileges.

sudo rmmod <sampleProgram>.koor sudo rmmod <sampleProgram>

Getting Module Details

In order to get information about a Module (author, supported options), we may use the modinfo command.

For example

modinfo hello_world_module.ko