/*
 * A Simple Filesystem for the Linux Kernel.
 *
 * Initial author: Sankar P <sankar.curiosity@gmail.com>
 * License: Creative Commons Zero License - http://creativecommons.org/publicdomain/zero/1.0/
 */

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

#include "simple.h"

static int simplefs_readdir(struct file *filp, void *dirent, filldir_t filldir)
{
	/* ls will list nothing as of now.
	 * Basic skeleton code to make ls (readdir) work for simplefs */
	return 0;
}

const struct file_operations simplefs_dir_operations = {
	.owner = THIS_MODULE,
	.readdir = simplefs_readdir,
};

struct dentry *simplefs_lookup(struct inode *parent_inode,
			       struct dentry *child_dentry, unsigned int flags)
{
	/* The lookup function is used for dentry association.
	 * As of now, we don't deal with dentries in simplefs.
	 * So we will keep this simple for now and revisit later */
	return NULL;
}

static struct inode_operations simplefs_inode_ops = {
	.lookup = simplefs_lookup,
};

/* This function creates, configures and returns an inode,
 * for the asked file (or) directory (differentiated by the mode param),
 * under the directory specified by the dir param
 * on the device dev, managed by the superblock sb param) */
struct inode *simplefs_get_inode(struct super_block *sb,
				 const struct inode *dir, umode_t mode,
				 dev_t dev)
{
	struct inode *inode = new_inode(sb);

	if (inode) {
		inode->i_ino = get_next_ino();
		inode_init_owner(inode, dir, mode);

		inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;

		switch (mode & S_IFMT) {
		case S_IFDIR:
			/* i_nlink will be initialized to 1 in the inode_init_always function
			 * (that gets called inside the new_inode function),
			 * We change it to 2 for directories, for covering the "." entry */
			inc_nlink(inode);
			break;
		case S_IFREG:
		case S_IFLNK:
		default:
			printk(KERN_ERR
			       "simplefs can create meaningful inode for only root directory at the moment\n");
			return NULL;
			break;
		}
	}
	return inode;
}

/* This function, as the name implies, Makes the super_block valid and
 * fills filesystem specific information in the super block */
int simplefs_fill_super(struct super_block *sb, void *data, int silent)
{
	struct inode *inode;

	/* A magic number that uniquely identifies our filesystem type */
	sb->s_magic = SIMPLEFS_MAGIC;

	inode = simplefs_get_inode(sb, NULL, S_IFDIR, 0);
	inode->i_op = &simplefs_inode_ops;
	inode->i_fop = &simplefs_dir_operations;
	sb->s_root = d_make_root(inode);
	if (!sb->s_root)
		return -ENOMEM;

	return 0;
}

static struct dentry *simplefs_mount(struct file_system_type *fs_type,
				     int flags, const char *dev_name,
				     void *data)
{
	struct dentry *ret;

	ret = mount_bdev(fs_type, flags, dev_name, data, simplefs_fill_super);

	if (unlikely(IS_ERR(ret)))
		printk(KERN_ERR "Error mounting simplefs");
	else
		printk(KERN_INFO "simplefs is succesfully mounted on [%s]\n",
		       dev_name);

	return ret;
}

static void simplefs_kill_superblock(struct super_block *s)
{
	printk(KERN_INFO
	       "simplefs superblock is destroyed. Unmount succesful.\n");
	/* This is just a dummy function as of now. As our filesystem gets matured,
	 * we will do more meaningful operations here */
	return;
}

struct file_system_type simplefs_fs_type = {
	.owner = THIS_MODULE,
	.name = "simplefs",
	.mount = simplefs_mount,
	.kill_sb = simplefs_kill_superblock,
};

static int simplefs_init(void)
{
	int ret;

	ret = register_filesystem(&simplefs_fs_type);
	if (likely(ret == 0))
		printk(KERN_INFO "Sucessfully registered simplefs\n");
	else
		printk(KERN_ERR "Failed to register simplefs. Error:[%d]", ret);

	return ret;
}

static void simplefs_exit(void)
{
	int ret;

	ret = unregister_filesystem(&simplefs_fs_type);

	if (likely(ret == 0))
		printk(KERN_INFO "Sucessfully unregistered simplefs\n");
	else
		printk(KERN_ERR "Failed to unregister simplefs. Error:[%d]",
		       ret);
}

module_init(simplefs_init);
module_exit(simplefs_exit);

MODULE_LICENSE("CC0");
MODULE_AUTHOR("Sankar P");