Controller login.php

<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');

class Login extends CI_Controller
{
    function __construct()
    {
        parent::__construct();
    }

    function index()
    {
        $data_session = $this->session->all_userdata();
        if(!empty($data_session)){
            redirect('admin/main');
        }else{
            $this->load->view('login_view');
        }
    }

    function dologin()
    {
        $rules = array(
            array(
                'field' => 'username',
                'label' => 'Username',
                'rules' => 'required|callback_cek_username'
            ),
            array(
                'field' => 'password',
                'label' => 'Password',
                'rules' => 'required'
            )
        );

        $this->form_validation->set_rules($rules);
        $this->form_validation->set_error_delimiters('<div class="alert alert-error">','</div>');

        $data = array(
            'username' => $this->input->post('username'),
            'password' => $this->input->post('password')
        );

        if($this->form_validation->run() == FALSE){
            $this->load->view('login_view');
        }else{
            //set session
            $this->session->set_userdata($data['username']);
            redirect('admin/main');
        }
    }

    function cek_username($str){
        if($str != 'admin'){
            $this->form_validation->set_message('cek_username','%s Salah! Coba Lagi.');
            return FALSE;
        }else{
            return TRUE;
        }
    }
}