Linux kernel module: Add entry to debugfs and a read/write file.

Goal

This module will create Ray_DBG directory in debugfs and create a file REG for read/write. it's similar to previous module(create /proc/Ray), but easier to use and need less line of code.

 

Code

	#include <linux/module.h>
	#include <linux/init.h>
	#include <linux/kernel.h>
	#include <linux/debugfs.h>
	MODULE_LICENSE("GPL");
	MODULE_AUTHOR("Ray Tsai");
	static u32 reg_data = 0xaa;
	static struct dentry *root;
	static int __init test_module_init(void)
	{
	pr_info("Init Ray test module!\n");
	pr_info("Create Ray_DBG !\n");
	root = debugfs_create_dir("Ray_DBG", NULL);
	if(!root)
	return -EFAULT;
	pr_info("Create REG debug !\n");
	if(!debugfs_create_x32("REG", 0777, root, (u32 *)&reg_data))
	goto err;
	return 0;
	err:
	debugfs_remove_recursive(root);
	pr_info("Failed to initialize debugfs\n");
	return -EFAULT;
	}
	static void __exit test_module_exit(void)
	{
	debugfs_remove_recursive(root);
	pr_info("Exit Ray test module!\n");
	}
	module_init(test_module_init);
	module_exit(test_module_exit);

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Makefile

	obj-m := test_module.o
	KERNELDIR ?= /lib/modules/$(shell uname -r)/build
	all default: modules
	install: modules_install
	modules modules_install help clean:
	$(MAKE) -C $(KERNELDIR) M=$(shell pwd) $@

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Usage

1. Insert module 

$>insmod test_module.ko
$>dmesg | tail
[7501255.251763] Create Ray_DBG !
[7501255.251776] Create REG debug !

2. Mount debugfs 

$>mount -t debugfs none /sys/kernel/debug

3. Read data

$>cat /sys/kernel/debug/Ray_DBG/REG
$>0x000000aa

4. Write data 

$>echo 0xff>  /sys/kernel/debug/Ray_DBG/REG 
$>cat /sys/kernel/debug/Ray_DBG/REG 
$>0x000000ff

Reference

1. Kernel document: debugfs
2. Debugfs