Linux lernel log level test

Makefile

obj-m += printk_device.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

printk_device.c

#define DEBUG
#define pr_fmt(fmt) "printk_device: " fmt

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/miscdevice.h>
#include <linux/fs.h>
#include <linux/printk.h>

#define print(level)	\
	printk(level "It is '" #level "' message generated from `printk`.")

static ssize_t printk_read(struct file *file, char __user *buf,
			   size_t count, loff_t *off) {
	pr_devel("This message is output because of macro 'DEBUG' defination.\n");
	pr_debug("This message is dynamic output.\n");
	pr_info_once("This message should only appear once.\n");
	print(KERN_EMERG);
	print(KERN_ALERT);
	print(KERN_CRIT);
	print(KERN_ERR);
	print(KERN_WARNING);
	print(KERN_NOTICE);
	print(KERN_INFO);
	print(KERN_DEBUG);
	printk(KERN_CONT " It is the first 'KERN_CONT' message generated"	\
	       " from printk, shouldn't start at newline.\n");
	printk(KERN_CONT "It is the second 'KERN_CONT' message generated"	\
	       " from printk, it should be at newline and appear immediately"	\
	       " because of the end character '\\n'.\n");
	printk(KERN_CONT "It is the third 'KERN_CONT' message generated"	\
	       " from printk, it starts at newline because of the '\\n'."	\
	       " in the end of the last 'KERN_CONT' line, but it appears"	\
	       " in the next action because of no '\\n' suffix.");
	return 0;
}

static ssize_t printk_write(struct file *file, const char __user *buf,
			    size_t count, loff_t *off) {
	WARN(true, "This message is generated from 'WARN'.\n");
	WARN_ONCE(true, "This message is generated from 'WARN_ONCE', and should"
		  " appear once.\n");
	return count;
}

static struct file_operations printk_fops = {
	.read = printk_read,
	.write = printk_write,
};

static struct miscdevice printk_device = {
	.minor = MISC_DYNAMIC_MINOR,
	.name = "printk",
	.fops = &printk_fops,
};

static int __init printk_init(void) {
	int retval = 0;

	retval = misc_register(&printk_device);
	if (retval != 0)
		pr_warn("Couldn't register device!\n");

//	BUILD_BUG_ON_MSG(true, "this line should be commented.");

	return retval;
}

static void __exit printk_exit(void) {
	misc_deregister(&printk_device);
}

module_init(printk_init);
module_exit(printk_exit);

MODULE_LICENSE("GPL");