# Kernel Development

The Debian system includes the kernel header files required for kernel module development and provides the toolchain needed to compile kernel modules. Users can directly develop and build new kernel modules on the device.

# Environment Preparation

```plaintext
sudo apt update
sudo apt install -y make
```

# Write the Source Code

## helloworld.c File

The contents of the helloworld.c file are as follows:

```cpp
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Your Name");
MODULE_DESCRIPTION("A simple Hello World kernel module");
MODULE_VERSION("0.1");

static int __init helloworld_init(void) {
        printk(KERN_INFO "Hello World!\n");
        return 0;
}

static void __exit helloworld_exit(void) {
        printk(KERN_INFO "Goodbye!\n");
}

module_init(helloworld_init);
module_exit(helloworld_exit);
```

## Makefile File

The contents of the Makefile are as follows:

```javascript
obj-m := helloworld.o

KERNELDIR ?= /lib/modules/$(shell uname -r)/build

PWD := $(shell pwd)

all:
        $(MAKE) -C $(KERNELDIR) M=$(PWD) modules

clean:
        $(MAKE) -C $(KERNELDIR) M=$(PWD) clean

install:
        $(MAKE) -C $(KERNELDIR) M=$(PWD) modules_install

help:
        $(MAKE) -C $(KERNELDIR) M=$(PWD) help

.PHONY: all clean install help
```

# Build

Build command:

```plaintext
LD_LIBRARY_PATH=/opt/qcom/lib:$LD_LIBRARY_PATH PATH=/opt/quectel/bin:$PATH make LLVM=1
```

# Run and Test

Load the module:

```plaintext
insmod helloworld.ko
```

Check the result:

```plaintext
lsmod | grep helloworld
```

Unload the module:

```plaintext
rmmod helloworld
```

Check the kernel log:

```plaintext
dmesg | grep "Hello World"
```