回环网卡驱动设计

一、loopback
协议栈需要发送数据时，将数据发送给网卡，网卡通过介质发送。而loopback回环网卡，是自发自收的。

二、代码分析

/*
 * INET        An implementation of the TCP/IP protocol suite for the LINUX
 *        operating system. INET is implemented using the  BSD Socket
 *        interface as the means of communication with the user level.
 *
 *        Pseudo-driver for the loopback interface.
 *
 * Version:    @(#)loopback.c    1.0.4b    08/16/93
 *
 * Authors:    Ross Biro
 *        Fred N. van Kempen, <[e-mail]>
 *        Donald Becker, <[e-mail]>
 *
 *        Alan Cox    :    Fixed oddments for NET3.014
 *        Alan Cox    :    Rejig for NET3.029 snap #3
 *        Alan Cox    :     Fixed NET3.029 bugs and sped up
 *        Larry McVoy    :    Tiny tweak to double performance
 *        Alan Cox    :    Backed out LMV's tweak - the linux mm
 *                    can't take it...
 * Michael Griffith: Don't bother computing the checksums
 * on packets received on the loopback
 * interface.
 *        Alexey Kuznetsov:    Potential hang under some extreme
 *                    cases removed.
 *
 *        This program is free software; you can redistribute it and/or
 *        modify it under the terms of the GNU General Public License
 *        as published by the Free Software Foundation; either version
 *        2 of the License, or (at your option) any later version.
 */
#include <linux/kernel.h>
#include <linux/jiffies.h>
#include <linux/module.h>
#include <linux/interrupt.h>
#include <linux/fs.h>
#include <linux/types.h>
#include <linux/string.h>
#include <linux/socket.h>
#include <linux/errno.h>
#include <linux/fcntl.h>
#include <linux/in.h>
#include <linux/init.h>

#include <asm/system.h>
#include <asm/uaccess.h>
#include <asm/io.h>

#include <linux/inet.h>
#include <linux/netdevice.h>
#include <linux/etherdevice.h>
#include <linux/skbuff.h>
#include <linux/ethtool.h>
#include <net/sock.h>
#include <net/checksum.h>
#include <linux/if_ether.h>    /* For the statistics structure. */
#include <linux/if_arp.h>    /* For ARPHRD_ETHER */
#include <linux/ip.h>
#include <linux/tcp.h>
#include <linux/percpu.h>
#include <net/net_namespace.h>

unsigned long bytes = 0;
unsigned long packets = 0;
struct net_device *dev;                                                                            //创建net_device结构

int loopback_xmit(struct sk_buff *skb, struct net_device *dev)                                     //发送函数
{
    skb->protocol = eth_type_trans(skb, dev);                                            //获取网络包协议

    bytes += skb->len;                                                                  //获取数据包的长度
    packets++;                                                                          //包的量

    netif_rx(skb);                                                                      //发送skb信息

    return 0;
}

struct net_device_stats *loopback_get_stats(struct net_device *dev)                     //获取状态
{
    struct net_device_stats *stats = &dev->stats;

    stats->rx_bytes = bytes;
    stats->tx_bytes = bytes;
    stats->rx_packets = packets;
    stats->tx_packets = packets;

    return &stats;
}

struct net_device_ops loopback_ops =                                                     //net_device_ops是回环网卡的函数结构指针
{
    .ndo_start_xmit = loopback_xmit,
    .ndo_get_stats = loopback_get_stats,
};

void loopback_setup(struct net_device *dev)
{
    dev->mtu = (16*1024)+20+20+12;                                                       //16kB+tcp+ip+net,设置最大的空间
    dev->flags = IFF_LOOPBACK;                                                           //表明是loopback函数
    dev->header_ops = &eth_header_ops;                                                   //使用默认的header_ops
    dev->netdev_ops = &loopback_ops;                                                     //使用自定义的netdev_ops
}

/* Setup and register the loopback device. */
static __net_init int loopback_net_init(struct net *net)
{
    int err = -ENOMEM;

    //1.
    dev = alloc_netdev(0, "lo", loopback_setup);                                       //给dev分配空间，0是自定义内容大小，lo是显示的名字，loopback_setup是初始化函数

    //4.
    err = register_netdev(dev);                                                        //注册dev

    net->loopback_dev = dev;

    return 0;
}

static __net_exit void loopback_net_exit(struct net *net)
{

    unregister_netdev(dev);                                                             //注销dev结构
}

/* Registered in net/core/dev.c */
struct pernet_operations __net_initdata loopback_net_ops = {
       .init = loopback_net_init,
       .exit = loopback_net_exit,
};