/*
 * A simple char device driver : globalmem without mutex
 * */





#include <linux/module.h>
#include <linux/fs.h>
#include <linux/init.h>
#include <linux/cdev.h>
#include <linux/slab.h>
#include <linux/uaccess.h>

#define GLOBALMEM_SIZE 0x1000
#define MEM_CLEAR 0x1
#define GLOBALMEM_MAJOR 230

static int globalmem_major = GLOBALMEM_MAJOR;
module_param(globalmem_major, int, S_IRUGO);

struct globalmem_dev {
    struct cdev cdev;
    unsigned char mem[GLOBALMEM_SIZE];
};

struct globalmem_dev *globalmem_devp;

static int globalmem_open(struct inode *inode, struct file *filp)
{
    filp->private_data = globalmem_devp;
    return 0;
}

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

static long globalmem_ioctl(struct file *filp, unsigned int cmd,
                unsigned long arg)
{
    struct globalmem_dev *dev = filp->private_data;

    switch (cmd) {
    case MEM_CLEAR:
        memset(dev->mem, 0, GLOBALMEM_SIZE);
        printk(KERN_INFO "globalmem is set to zero\n");
        break;
    default:
        return -EINVAL;
    }
    
    return 0;
}

static ssize_t globalmem_read(struct file *filp, char __user *buf, size_t size,
        loff_t * ppos)
{
    unsigned long p = *ppos;
    unsigned int count = size;
    int ret = 0;
    struct globalmem_dev *dev = filp->private_data;

    if (p >= GLOBALMEM_SIZE)
        return 0;
    if (count > GLOBALMEM_SIZE - p)
        count = GLOBALMEM_SIZE - p;

    if (copy_to_user(buf, dev->mem + p, count)) {
        ret = -EFAULT;
    } else {
        *ppos += count;
        ret = count;
        
        printk(KERN_INFO "read %u bytes(s) from %lu\n", count, p);
    }

    return ret;
}

static ssize_t globalmem_write(struct file *filp, const char __user * buf, 
                    size_t size, loff_t * ppos)
{
    unsigned long p = *ppos;
    unsigned int count = size;
    int ret = 0;
    struct globalmem_dev *dev = filp->private_data;

    if (p >= GLOBALMEM_SIZE)
        return 0;
    if (count > GLOBALMEM_SIZE - p)
        count = GLOBALMEM_SIZE - p;
    
    if (copy_from_user(dev->mem +p, buf, count))
        ret = -EFAULT;
    else {
        *ppos += count;
        ret = count;

        printk(KERN_INFO "written %u bytes(s) from %lu\n", count, p);
    }

    return ret;
}

static loff_t globalmem_llseek(struct file *filp, loff_t offset, int orig)
{
    loff_t ret = 0;
    switch (orig) {
    case 0:
        if (offset < 0) {
            ret = -EINVAL;
            break;
        }
        if ((unsigned int)offset > GLOBALMEM_SIZE) {
            ret = -EINVAL;
            break;
        }
        filp->f_pos = (unsigned int)offset;
        ret = filp->f_pos;
        break;
    case 1:
        if((filp->f_pos + offset) > GLOBALMEM_SIZE) {
            ret = -EINVAL;
            break;
        }
        if ((filp->f_pos + offset) < 0){
            ret = -EINVAL;
            break;
        }
        filp->f_pos += offset;
        ret = filp->f_pos;
        break;
    default:
        ret = -EINVAL;
        break;
    }
    return ret;
} 

static const struct file_operations globalmem_fops = {
    .owner = THIS_MODULE,
    .llseek = globalmem_llseek,
    .read = globalmem_read,
    .write = globalmem_write,
    .unlocked_ioctl = globalmem_ioctl,
    .open = globalmem_open,
    .release = globalmem_release,
};

static void globalmem_setup_cdev(struct globalmem_dev *dev, int index)
{
    int err, devno = MKDEV(globalmem_major, index);

    cdev_init(&dev->cdev, &globalmem_fops);
    dev->cdev.owner = THIS_MODULE;
    err = cdev_add(&dev->cdev, devno, 1);
    if(err)
        printk(KERN_NOTICE "Error %d adding globalmem %d", err, index);
}

static int __init globalmem_init(void)
{
    int ret;
    dev_t devno = MKDEV(globalmem_major, 0);
    
    if (globalmem_major)
        ret = register_chrdev_region(devno, 1, "globalmem");
    else{
        ret = alloc_chrdev_region(&devno, 0, 1, "globalmem");
        globalmem_major = MAJOR(devno);
    }
    if (ret <0)
        return ret;
    
    globalmem_devp = kzalloc(sizeof(struct globalmem_dev), GFP_KERNEL);
    if (!globalmem_devp) {
        ret = -ENOMEM;
        goto fail_malloc;
    }
    
    globalmem_setup_cdev(globalmem_devp, 0);
    return 0;
fail_malloc:
    unregister_chrdev_region(devno, 1);
    return ret;
}
module_init(globalmem_init);

static void __exit globalmem_exit(void)
{
    cdev_del(&globalmem_devp->cdev);
    kfree(globalmem_devp);
    unregister_chrdev_region(MKDEV(globalmem_major, 0), 1);
}
module_exit(globalmem_exit);

MODULE_AUTHOR("Barry Song <baohua@kernel.org>");
MODULE_LICENSE("GPL v2");
