[Kernel driver] Compile and load the kernel driver module

00. Table of Contents

Article directory

01. Overview

For the linux kernel, there are two ways to add kernel modules to the kernel:

1. Compile the kernel module into a kernel module file, which is dynamically loaded manually by the user after the kernel is started.
2. Compile the module directly into the kernel and load it automatically when the kernel starts.

02. Test code

test.c

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

static int __init hello_init(void)
{
    
    
    printk(KERN_EMERG "Hello  Module Init\n");
    return 0;
}

static void __exit hello_exit(void)
{
    
    
    printk(KERN_EMERG "Hello  Module Exit\n");
}

module_init(hello_init);
module_exit(hello_exit);

MODULE_LICENSE("GPL v2");
MODULE_AUTHOR("uplooking");
MODULE_DESCRIPTION("hello module");
MODULE_ALIAS("test_module");

03. Kernel module compilation

The building steps and compilation required for the kernel driver module object are very complicated. It takes advantage of the powerful functions of the Linux kernel build system. Of course, we do not need to understand this part of knowledge in depth. We can use a simple Make tool to compile the kernel driver we want. module.

Makefile reference is as follows

KERNEL_DIR=/home/deng/code/x3399/kernel
CROSS_COMPILE=aarch64-linux-gnu-gcc

obj-m := test.o

all:
	$(MAKE) -C $(KERNEL_DIR) M=`pwd` modules

.PHONE:clean
clean:
	$(MAKE) -C $(KERNEL_DIR) M=$(PWD) clean

Line 1: Specify the storage location of the compiled kernel
Line 2: Specify the cross-compilation tool chain
Line 3: Indicates compilation as module
Line 4: all is just a label, which can be defined by yourself. It is the default execution target of make.
Line 5: $(MAKE):MAKE is a macro variable in the Makefile. To reference the macro variable, you must use symbols. This actually points to the make program, so you can also replace $(MAKE) with make. -C: is an option of the make command. -C is used to change directory. -C dir is to go to the dir directory. M= pwd: Return to the current directory. What this sentence means is: when make executes the default target all, -C (KERNEL_DIR) specifies to jump to the kernel source code directory to execute the Makefile there, and -C $ (KERNEL_DIR) specifies to jump to the kernel source code directory to execute there. Makefile, M= pwdmeans returning to the current directory to execute the current Makefile.
Line 6: clean is to delete the files generated by make.

Check the current folder and add test.ko. This is the kernel driver module we wrote and compiled ourselves.

Insert image description here

deng@local:~/code/test/1module$ make 
make -C /home/deng/code/x3399/kernel M=`pwd` modules
make[1]: 进入目录“/home/deng/code/x3399/kernel”
  CC [M]  /home/deng/code/test/1module/test.o
  Building modules, stage 2.
  MODPOST 1 modules
  CC      /home/deng/code/test/1module/test.mod.o
  LD [M]  /home/deng/code/test/1module/test.ko
make[1]: 离开目录“/home/deng/code/x3399/kernel”
deng@local:~/code/test/1module$ 


deng@local:~/code/test/1module$ ls
Makefile       Module.symvers  test.ko     test.mod.o
modules.order  test.c          test.mod.c  test.o
deng@local:~/code/test/1module$

04. Kernel module loading and unloading

After compiling the kernel driver module, test.ko can be copied to the development board in a variety of ways. Generally, the NFS network file system or SCP command is used.

Load module

[root@rk3399:/mnt/code/test/1module]# ls
Makefile	modules.order  test.ko	   test.mod.o
Module.symvers	test.c	       test.mod.c  test.o
[root@rk3399:/mnt/code/test/1module]# insmod test.ko
[  652.669514] Hello  Module Init
[root@rk3399:/mnt/code/test/1module]#

Uninstall module

[root@rk3399:/mnt/code/test/1module]# rmmod test
[  668.206247] Hello  Module Exit
[root@rk3399:/mnt/code/test/1module]#