Someone

static int my_ioctl(struct block_device *bdev, fmode_t mode,
                    unsigned int cmd, unsigned long arg)
{
        long size;
        struct hd_geometry geo;

        printk(KERN_INFO "cmd=%d\n", cmd);

        switch (cmd) {
        case HDIO_GETGEO:
                printk(KERN_INFO "HIT HDIO_GETGEO\n");
                /*
                 * get geometry: we have to fake one...  trim the size to a
                 * multiple of 64 (32k): tell we have 16 sectors, 4 heads,
                 * whatever cylinders. Tell also that data starts at sector. 4.
                 */

                size = disk_size;
                size &= ~0x3f;  /* multiple of 64 */
                geo.cylinders = size >> 6;
                geo.heads = 4;
                geo.sectors = 16;
                geo.start = 4;

                if (copy_to_user((void __user *)arg, &geo, sizeof(geo)))
                        return -EFAULT;

                return 0;
        }
        printk(KERN_WARNING "return -ENOTTY\n");

        return -ENOTTY;         /* unknown command */
}