/*
 * Ring buffer character device - example for Operating System course
 *
 * Copyright (C) 2016, 2017  Krzysztof Mazur <krzysiek@podlesie.net>
 *
 * Add your copyright here and change MODULE_AUTHOR.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 */

#define pr_fmt(fmt) "ringdev: " fmt
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/miscdevice.h>
#include <linux/sched.h>
#include <linux/uaccess.h>

/*
 * pożądany numer minor urządzenia znakowego ringdev,
 * MISC_DYNAMIC_MINOR dla automatycznego wyboru
 */
#define RINGDEV_MINOR	MISC_DYNAMIC_MINOR

/*
 * mutex używany do synchronizacji dostępu do wszystkich zmiennych modułu
 */
static struct mutex ringdev_lock;

/* bufor i liczba bajtów w buforze */
static char ringdev_buf[4096];
static size_t ringdev_len;

static int ringdev_open(struct inode *inode, struct file *filp)
{
	return 0;
}

static ssize_t ringdev_read(struct file *filp, char __user *buf, size_t count,
		loff_t *off)
{
	ssize_t ret = 0;

	/*
	 * bufor wraz z długością mogą być zmieniane przez inny proces,
	 * by się przed tym zabezpieczyć dane muszą być czytane
	 * trzymając blokadę
	 */
	mutex_lock(&ringdev_lock);
	if (*off > ringdev_len)
		count = 0;
	else if (count >= ringdev_len - *off)
		count = ringdev_len - *off;

	/*
	 * kopiowanie danych do przestrzeni użytkownika, trzeba użyć
	 * copy_*_user(), na niektórych architekturach pisanie/czytanie
	 * pamięci użytkownika może być niemożliwe, na innych
	 * wprowadza luki bezpieczeństwa
	 */
	ret = -EFAULT;
	if (copy_to_user(buf, ringdev_buf + *off, count))
		goto out_unlock;
	ret = count;
	*off += ret;

out_unlock:
	mutex_unlock(&ringdev_lock);
	return ret;
}

static ssize_t ringdev_write(struct file *filp, const char __user *buf,
		size_t count, loff_t *off)
{
	ssize_t ret;

	if (count > sizeof(ringdev_buf))
		count = sizeof(ringdev_buf);
	mutex_lock(&ringdev_lock);
	ret = -EFAULT;
	if (copy_from_user(ringdev_buf, buf, count))
		goto out_unlock;
	ringdev_len = count;
	ret = count;
out_unlock:
	mutex_unlock(&ringdev_lock);
	return ret;
}

static int ringdev_release(struct inode *inode, struct file *filp)
{
	return 0;
}

static const struct file_operations ringdev_fops = {
	.owner = THIS_MODULE,
	.open = ringdev_open,
	.read = ringdev_read,
	.write = ringdev_write,
	.release = ringdev_release,
};

static struct miscdevice ringdev_miscdevice = {
	.minor = RINGDEV_MINOR,
	.name = "ringdev",
	.fops = &ringdev_fops
};

static int __init ringdev_init(void)
{
	int ret;

	mutex_init(&ringdev_lock);
	ringdev_len = snprintf(ringdev_buf, sizeof(ringdev_buf),
			"Hello world!\n");
	ret = misc_register(&ringdev_miscdevice);
	if (ret < 0) {
		pr_err("can't register miscdevice.\n");
		return ret;
	}

	pr_info("minor %d\n", ringdev_miscdevice.minor);

	return 0;
}

static void __exit ringdev_exit(void)
{
	misc_deregister(&ringdev_miscdevice);
	mutex_destroy(&ringdev_lock);
}

module_init(ringdev_init);
module_exit(ringdev_exit);

MODULE_DESCRIPTION("Ring buffer device");
MODULE_AUTHOR("Real Name <email@example.com>");
MODULE_LICENSE("GPL");