Device-Driver

Device drivers in Linux serve as intermediary software components, facilitating communication between the operating system and hardware devices. They abstract hardware complexities, providing a standardized interface for user activities while remaining flexible to accommodate diverse usage scenarios. By separating mechanism from policy, drivers enable efficient hardware access and simplify maintenance. Additionally, many drivers are accompanied by user programs for configuration and access, enriching the ecosystem with utilities and graphical interfaces.

first driver

steps

1. write main.c (which will be written in the kernel space)

Things written in kernel space don't see glibc library ---> so kernel provides equivelent functions in kernel space.

ex: printf in glibc == printk in kernel space

main.c

#include <linux/module.h>
#include <linux/init.h>

//Meta information
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("Second Driver");
MODULE_AUTHOR("HEMA");

static int __init hellodriver_init(void)
{
    printk("Hello, world\n");
    return 0;
}
static void __exit hellodriver_exit(void)
{
	printk("Goodbye, world\n");
}
module_init(hellodriver_init);
module_exit(hellodriver_exit);

module_init serves as the entry point for kernel modules, initializing resources and registering device drivers upon module loading. Unlike in GCC where the startup code searches for the main function, in kernel builds, it looks for module_init as the entry point, executing the specified function at the start of the program.

module_exit serves as the exit point for kernel modules, releasing resources and unregistering device drivers before unloading the module. Similar to module_init, it defines a function that is executed at the end of the program's execution.

static int __init to put the function hellodriver_init ---> .init section in memory , after executing that function it will be removed from Memory to save resources.

static void __exit to put the function hellodriver_exit ---> .exit section in memory , after executing that function it will be removed from Memory to save resources.

2. write Makefile

obj-m += hello.o
all:
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean:
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

obj-m build as a dyanmic module (can load and unload) , obj-y build as a static module (comiled as a part with zImage)

make -C /lib/modules/$(shell uname -r)/build ---> i want this make to use Makefile of the linux kernel.

uname -r refers to ---> 6.5.0-27-generic the version of linux in my device

modules just do this task in the Makefile of the kernel

clean just do this task in the Makefile of the kernel

3. Build

make

• After that will appear the kernel module hello.ko that will be inserted to the kernel

4. use insmod command to insert the module to the kernel

sudo insmod hello.ko

5. using (dmesg) command to see the messages from kernel

dmesg

it will print hello, world

6. To remove the module from the kernel use rmmod

sudo rmmod hello

7. Using dmesg

dmesg

it will print Goodbye, world so it's the function executed when the driver removed from the kernel.

Note:

you have to put \n in printk() function

Using \n in printk() ensures each log message appears on a new line in the kernel log file.

Omitting \n may cause subsequent log messages to concatenate, making it hard to read the log file with dmesg.

Including \n maintains clear and organized log output, aiding in effective debugging and system monitoring.