/*
 * procfs4.c -  create a "file" in /proc
 * This program uses the seq_file library to manage the /proc file.
 */

#include <linux/kernel.h>   /* We are doing kernel work */
#include <linux/module.h>   /* Specifically, a module */
#include <linux/proc_fs.h>  /* Necessary because we use proc fs */
#include <linux/seq_file.h> /* for seq_file */
#include <linux/version.h>

#if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 6, 0)
#define HAVE_PROC_OPS
#endif

#define PROC_NAME "iter"

/* This function is called at the beginning of a sequence.
 * ie, when:
 *   - the /proc file is read (first time)
 *   - after the function stop (end of sequence)
 */
static void *my_seq_start(struct seq_file *s, loff_t *pos)
{
    static unsigned long counter = 0;

    /* beginning a new sequence? */
    if (*pos == 0) {
        /* yes => return a non null value to begin the sequence */
        return &counter;
    }
    /* no => it is the end of the sequence, return end to stop reading */
    *pos = 0;
    return NULL;
}

/* This function is called after the beginning of a sequence.
 * It is called untill the return is NULL (this ends the sequence).
 */
static void *my_seq_next(struct seq_file *s, void *v, loff_t *pos)
{
    unsigned long *tmp_v = (unsigned long *) v;
    (*tmp_v)++;
    (*pos)++;
    return NULL;
}

/* This function is called at the end of a sequence. */
static void my_seq_stop(struct seq_file *s, void *v)
{
    /* nothing to do, we use a static value in start() */
}

/* This function is called for each "step" of a sequence. */
static int my_seq_show(struct seq_file *s, void *v)
{
    loff_t *spos = (loff_t *) v;

    seq_printf(s, "%Ld\n", *spos);
    return 0;
}

/* This structure gather "function" to manage the sequence */
static struct seq_operations my_seq_ops = {
    .start = my_seq_start,
    .next = my_seq_next,
    .stop = my_seq_stop,
    .show = my_seq_show,
};

/* This function is called when the /proc file is open. */
static int my_open(struct inode *inode, struct file *file)
{
    return seq_open(file, &my_seq_ops);
};

/* This structure gather "function" that manage the /proc file */
#ifdef HAVE_PROC_OPS
static const struct proc_ops my_file_ops = {
    .proc_open = my_open,
    .proc_read = seq_read,
    .proc_lseek = seq_lseek,
    .proc_release = seq_release,
};
#else
static const struct file_operations my_file_ops = {
    .open = my_open,
    .read = seq_read,
    .llseek = seq_lseek,
    .release = seq_release,
};
#endif

static int __init procfs4_init(void)
{
    struct proc_dir_entry *entry;

    entry = proc_create(PROC_NAME, 0, NULL, &my_file_ops);
    if (entry == NULL) {
        remove_proc_entry(PROC_NAME, NULL);
        pr_debug("Error: Could not initialize /proc/%s\n", PROC_NAME);
        return -ENOMEM;
    }

    return 0;
}

static void __exit procfs4_exit(void)
{
    remove_proc_entry(PROC_NAME, NULL);
    pr_debug("/proc/%s removed\n", PROC_NAME);
}

module_init(procfs4_init);
module_exit(procfs4_exit);

MODULE_LICENSE("GPL");