Linux驱动----网络驱动开发实例

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Linux驱动----网络驱动开发实例 详细原理不介绍, 参照《Linux 设备驱动 Edition 3》中第17章网络驱动. 本文把Linux设备驱动示例代码snull移植到2.6.32版本,对应于发行版redhat6,centos6。 为了提高高端 Linux 系统性能的方法, 目前网络子系统开发一般采用基于查询的接口,所以把snull代码中接收中断去掉了,只使用NAPI。由于对snull比较复杂,便于理解,进行相应裁剪,去掉对功能没有影响代码,接下来对改动地方进行说明。对于修改原因,下面做了简单说明,具体原因测试过程,可以不修改(你觉得无关紧要或者无法确定代码),看看会出现什么问题,这样会理解更深。 注意: 1.本人内核版本: # uname -r 2.6.32-71.el6 2.发行版: # cat /etc/redhat-release CentOS Linux release 6.0 (Final) 一.修改说明: 1.snull_open函数  添加napi_enable(&vi->napi);语句,表示启动napi接受 2.snull_release函数 添加  napi_disable(&vi->napi);表示关闭napi接受  netif_carrier_off(dev);表示关闭载波 3.snull_napi_interrupt函数 由于内核接口,发生变化,以前代码改为如下:     if (napi_schedule_prep(&priv->napi)) {                         snull_rx_ints(dev, 0);  /* Disable further interrupts */                          printk("napi_schedule_prep\n");                         __napi_schedule(&priv->napi);      } 4.内核接口t struct net_device_ops发生改变,所以下面进行修改: static const struct net_device_ops virtnet_netdev = {         .ndo_open            = snull_open,         .ndo_stop            = snull_release,         .ndo_start_xmit      = snull_tx,         .ndo_get_stats       = snull_stats,         .ndo_set_rx_mode     = snull_set_rx_mode, }; static const struct header_ops      header_op =  {         .create              = snull_header,         .rebuild             = snull_rebuild_header, }; 5.snull_init_module函数改动较大 为了简单,直接用了以太网接口,所以网络接口不是sn0,sn1.  snull_devs[0] =  alloc_etherdev(sizeof(struct snull_priv));  snull_devs[1] =  alloc_etherdev(sizeof(struct snull_priv));   for (i = 0; i < 2;  i++) {                 snull_devs[i]->flags           |= IFF_NOARP;                 snull_devs[i]->features        |= NETIF_F_NO_CSUM;                 snull_devs[i]->header_ops = &header_op;                 snull_rx_ints(snull_devs[i], 1);                 snull_setup_pool(snull_devs[i]);                 snull_devs[i]->netdev_ops = &virtnet_netdev;                 vi = netdev_priv(snull_devs[i]);                 vi->dev = snull_devs[i];                 netif_napi_add(snull_devs[i], &vi->napi, snull_poll,2);         } 二.测试 1.编译,运行,加载内核模块,查看接口是否创建 #insmod snull.ko #ifconfig -a eth1      Link encap:Ethernet  HWaddr 00:00:00:00:00:00             BROADCAST NOARP MULTICAST  MTU:1500  Metric:1           RX packets:0 errors:0 dropped:0 overruns:0 frame:0           TX packets:0 errors:0 dropped:0 overruns:0 carrier:0           collisions:0 txqueuelen:1000           RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b) eth2      Link encap:Ethernet  HWaddr 00:00:00:00:00:00             BROADCAST NOARP MULTICAST  MTU:1500  Metric:1           RX packets:0 errors:0 dropped:0 overruns:0 frame:0           TX packets:0 errors:0 dropped:0 overruns:0 carrier:0           collisions:0 txqueuelen:1000           RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b) 2.编辑相应文件 #vi /etc/networks //添加下面 snullnet0 192.168.0.0 snullnet1 192.168.1.0 /etc/hosts 里面: #vi /etc/hosts 192.168.0.1  local0   192.168.0.2  remote0   192.168.1.2  local1   192.168.1.1  remote1   3.设置网络接口ip #ifconfig eth1 local0 #ifconfig eth2 local1 eth1      Link encap:Ethernet  HWaddr 00:53:4E:55:4C:30             inet addr:192.168.0.1  Bcast:192.168.0.255  Mask:255.255.255.0           inet6 addr: fe80::253:4eff:fe55:4c30/64 Scope:Link           UP BROADCAST RUNNING NOARP MULTICAST  MTU:1500  Metric:1           RX packets:3 errors:0 dropped:0 overruns:0 frame:0           TX packets:3 errors:0 dropped:0 overruns:0 carrier:0           collisions:0 txqueuelen:1000           RX bytes:210 (210.0 b)  TX bytes:210 (210.0 b) eth2      Link encap:Ethernet  HWaddr 00:53:4E:55:4C:31             inet addr:192.168.1.2  Bcast:192.168.1.255  Mask:255.255.255.0           inet6 addr: fe80::253:4eff:fe55:4c31/64 Scope:Link           UP BROADCAST RUNNING NOARP MULTICAST  MTU:1500  Metric:1           RX packets:3 errors:0 dropped:0 overruns:0 frame:0           TX packets:3 errors:0 dropped:0 overruns:0 carrier:0           collisions:0 txqueuelen:1000           RX bytes:210 (210.0 b)  TX bytes:210 (210.0 b) UP BROADCAST RUNNING NOARP MULTICAST  MTU:1500  Metric:1 说明接口正常运行了。 4.数据包发送与接受 查看包接受情况 #ifconfig eth1 eth1      Link encap:Ethernet  HWaddr 00:53:4E:55:4C:30             inet addr:192.168.0.1  Bcast:192.168.0.255  Mask:255.255.255.0           inet6 addr: fe80::253:4eff:fe55:4c30/64 Scope:Link           UP BROADCAST RUNNING NOARP MULTICAST  MTU:1500  Metric:1           RX packets:7 errors:0 dropped:0 overruns:0 frame:0           TX packets:9 errors:0 dropped:0 overruns:0 carrier:0           collisions:0 txqueuelen:1000           RX bytes:602 (602.0 b)  TX bytes:798 (798.0 b) [[email protected] ~]# ifconfig eth2 eth2      Link encap:Ethernet  HWaddr 00:53:4E:55:4C:31             inet addr:192.168.1.2  Bcast:192.168.1.255  Mask:255.255.255.0           inet6 addr: fe80::253:4eff:fe55:4c31/64 Scope:Link           UP BROADCAST RUNNING NOARP MULTICAST  MTU:1500  Metric:1           RX packets:9 errors:0 dropped:0 overruns:0 frame:0           TX packets:7 errors:0 dropped:0 overruns:0 carrier:0           collisions:0 txqueuelen:1000           RX bytes:798 (798.0 b)  TX bytes:602 (602.0 b) 发送数据包 # ping -c 2 remote0 PING remote0 (192.168.0.2) 56(84) bytes of data. 64 bytes from remote0 (192.168.0.2): icmp_seq=1 ttl=64 time=0.125 ms 64 bytes from remote0 (192.168.0.2): icmp_seq=2 ttl=64 time=0.211 ms --- remote0 ping statistics --- 2 packets transmitted, 2 received, 0% packet loss, time 999ms rtt min/avg/max/mdev = 0.125/0.168/0.211/0.043 ms # ping -c 2 remote1 PING remote1 (192.168.1.1) 56(84) bytes of data. 64 bytes from remote1 (192.168.1.1): icmp_seq=1 ttl=64 time=0.310 ms 64 bytes from remote1 (192.168.1.1): icmp_seq=2 ttl=64 time=0.216 ms --- remote1 ping statistics --- 2 packets transmitted, 2 received, 0% packet loss, time 1000ms rtt min/avg/max/mdev = 0.216/0.263/0.310/0.047 ms #再查看包接受情况 eth1      Link encap:Ethernet  HWaddr 00:53:4E:55:4C:30             inet addr:192.168.0.1  Bcast:192.168.0.255  Mask:255.255.255.0           inet6 addr: fe80::253:4eff:fe55:4c30/64 Scope:Link           UP BROADCAST RUNNING NOARP MULTICAST  MTU:1500  Metric:1           RX packets:11 errors:0 dropped:0 overruns:0 frame:0           TX packets:13 errors:0 dropped:0 overruns:0 carrier:0           collisions:0 txqueuelen:1000           RX bytes:994 (994.0 b)  TX bytes:1190 (1.1 KiB) eth2      Link encap:Ethernet  HWaddr 00:53:4E:55:4C:31             inet addr:192.168.1.2  Bcast:192.168.1.255  Mask:255.255.255.0           inet6 addr: fe80::253:4eff:fe55:4c31/64 Scope:Link           UP BROADCAST RUNNING NOARP MULTICAST  MTU:1500  Metric:1           RX packets:13 errors:0 dropped:0 overruns:0 frame:0           TX packets:11 errors:0 dropped:0 overruns:0 carrier:0           collisions:0 txqueuelen:1000           RX bytes:1190 (1.1 KiB)  TX bytes:994 (994.0 b) 5.卸载模块 # rmmod  snull # ifconfig -a 注意地方: 1.网卡接口名称发生变化 sn----eth 2.包发送情况统计 ndo_get_stats .ndo_get_stats       = snull_stats, 如果该函数不实现,包统计数据无法显示 RX packets:13 errors:0 dropped:0 overruns:0 frame:0 TX packets:11 errors:0 dropped:0 overruns:0 carrier:0 3.ARP地址解析与填充 static const struct header_ops      header_op =  {     .create             = snull_header,     .rebuild             = snull_rebuild_header, }; 如果该函数不实现,包无法进行通信的,即两个接口之间。因为包的以太网包头MAC地址不存在或者不正确。 4.网络测试时,要保证 192.168.0.0 ,192.168.1.0两个网段没有使用。 三.代码 //////////////////////////////////////////////////////////////// /*  * snull.c --  the Simple Network Utility  *  * Copyright (C) 2001 Alessandro Rubini and Jonathan Corbet  * Copyright (C) 2001 O'Reilly & Associates  *  * The source code in this file can be freely used, adapted,  * and redistributed in source or binary form, so long as an  * acknowledgment appears in derived source files.  The citation  * should list that the code comes from the book "Linux Device  * Drivers" by Alessandro Rubini and Jonathan Corbet, published  * by O'Reilly & Associates.   No warranty is attached;  * we cannot take responsibility for errors or fitness for use.  *  * $Id: snull.c,v 1.21 2004/11/05 02:36:03 rubini Exp $  */ #include <linux/module.h> #include <linux/init.h> #include <linux/moduleparam.h> #include <linux/sched.h> #include <linux/kernel.h> /* printk() */ #include <linux/slab.h> /* kmalloc() */ #include <linux/errno.h>  /* error codes */ #include <linux/types.h>  /* size_t */ #include <linux/interrupt.h> /* mark_bh */ #include <linux/in.h> #include <linux/netdevice.h>   /* struct device, and other headers */ #include <linux/etherdevice.h> /* eth_type_trans */ #include <linux/ip.h>          /* struct iphdr */ #include <linux/tcp.h>         /* struct tcphdr */ #include <linux/skbuff.h> #include "snull.h" #include <linux/in6.h> #include <asm/checksum.h> #include <linux/netdevice.h> #include <linux/etherdevice.h> #include <linux/ethtool.h> MODULE_AUTHOR("Alessandro Rubini, Jonathan Corbet"); MODULE_LICENSE("Dual BSD/GPL"); /*  * Do we run in NAPI mode?  */ static int use_napi = 1; module_param(use_napi, int, 0); /*  * A structure representing an in-flight packet.  */ struct snull_packet {     struct snull_packet *next;     struct net_device *dev;     int    datalen;     u8 data[ETH_DATA_LEN]; }; int pool_size = 8; module_param(pool_size, int, 0); /*  * This structure is private to each device. It is used to pass  * packets in and out, so there is place for a packet  */ struct snull_priv {     struct net_device_stats stats;     struct net_device *dev;     struct napi_struct napi;     int status;     struct snull_packet *ppool;     struct snull_packet *rx_queue;  /* List of incoming packets */         int rx_int_enabled;         int tx_packetlen;     u8 *tx_packetdata;     struct sk_buff *skb;     spinlock_t lock; }; static void (*snull_interrupt)(int, void *, struct pt_regs *); /*  * Set up a device's packet pool.  */ void snull_setup_pool(struct net_device *dev) {     struct snull_priv *priv = netdev_priv(dev);     int i;     struct snull_packet *pkt;     priv->ppool = NULL;     for (i = 0; i < pool_size; i++) {         printk("%s\n",__func__);         pkt = kmalloc (sizeof (struct snull_packet), GFP_KERNEL);         if (pkt == NULL) {             printk (KERN_NOTICE "Ran out of memory allocating packet pool\n");             return;         }         pkt->dev = dev;         pkt->next = priv->ppool;         priv->ppool = pkt;     }     printk("%p,%p\n",dev,priv->ppool); } void snull_teardown_pool(struct net_device *dev) {     struct snull_priv *priv = netdev_priv(dev);     struct snull_packet *pkt;          while ((pkt = priv->ppool)) {         priv->ppool = pkt->next;         kfree (pkt);         /* FIXME - in-flight packets ? */     } }     /*  * Buffer/pool management.  */ struct snull_packet *snull_get_tx_buffer(struct net_device *dev) {     struct snull_priv *priv = netdev_priv(dev);     unsigned long flags;     struct snull_packet *pkt;             int i=0;     spin_lock_irqsave(&priv->lock, flags);     pkt = priv->ppool;     priv->ppool = pkt->next;     if (priv->ppool == NULL) {         printk (KERN_INFO "Pool empty %p\n",dev);         for(i=0;i<ETH_ALEN;i++)             printk("netif_stop_queue:%x\n",dev->dev_addr[i]);         netif_stop_queue(dev);     }     spin_unlock_irqrestore(&priv->lock, flags);     return pkt; } void snull_release_buffer(struct snull_packet *pkt) {     unsigned long flags;     struct snull_priv *priv = netdev_priv(pkt->dev);          spin_lock_irqsave(&priv->lock, flags);     pkt->next = priv->ppool;     priv->ppool = pkt;     spin_unlock_irqrestore(&priv->lock, flags);     if (netif_queue_stopped(pkt->dev) && pkt->next == NULL) {         printk("netif_wake_queue\n");         netif_wake_queue(pkt->dev);     } } void snull_enqueue_buf(struct net_device *dev, struct snull_packet *pkt) {     unsigned long flags;     struct snull_priv *priv = netdev_priv(dev);     spin_lock_irqsave(&priv->lock, flags);     pkt->next = priv->rx_queue;  /* FIXME - misorders packets */     priv->rx_queue = pkt;     spin_unlock_irqrestore(&priv->lock, flags); } struct snull_packet *snull_dequeue_buf(struct net_device *dev) {     struct snull_priv *priv = netdev_priv(dev);     struct snull_packet *pkt;     unsigned long flags;     spin_lock_irqsave(&priv->lock, flags);     pkt = priv->rx_queue;     if (pkt != NULL)         priv->rx_queue = pkt->next;     spin_unlock_irqrestore(&priv->lock, flags);     return pkt; } /*  *  * Enable and disable receive interrupts.  *   */ static void snull_rx_ints(struct net_device *dev, int enable) {         struct snull_priv *priv = netdev_priv(dev);         priv->rx_int_enabled = enable; }      /*  * Open and close  */ int snull_open(struct net_device *dev) {     /* request_region(), request_irq(), ....  (like fops->open) */     /*      * Assign the hardware address of the board: use "\0SNULx", where      * x is 0 or 1. The first byte is '\0' to avoid being a multicast      * address (the first byte of multicast addrs is odd).      */     struct snull_priv *vi = netdev_priv(dev);     netif_carrier_on(dev);          memcpy(dev->dev_addr, "\0SNUL0", ETH_ALEN);     if (dev == snull_devs[1])         dev->dev_addr[ETH_ALEN-1]++;           /* \0SNUL1 */     printk("########################\n");         napi_enable(&vi->napi);     //if (napi_schedule_prep(&vi->napi)) {     //    __napi_schedule(&vi->napi);     //}     netif_start_queue(dev);     return 0; } int snull_release(struct net_device *dev) {     /* release ports, irq and such -- like fops->close */     struct snull_priv *vi = netdev_priv(dev);         napi_disable(&vi->napi);         netif_stop_queue(dev);/* can't transmit any more */         netif_carrier_off(dev);     return 0; } /*  * The poll implementation.  */ static int snull_poll(struct napi_struct *napi, int budget) {     struct snull_priv *priv = container_of(napi, struct snull_priv, napi);     struct sk_buff *skb;         unsigned int received = 0;     struct snull_packet *pkt;         //while (received < budget && priv->rx_queue ) {         while ( priv->rx_queue ) {         printk("snull_dequeue_buf:%p@@@@@@@\n", priv->dev);         pkt = snull_dequeue_buf(priv->dev);                 skb = dev_alloc_skb(pkt->datalen + 2);                 if (! skb) {                         if (printk_ratelimit())                                 printk(KERN_NOTICE "snull: packet dropped\n");                         priv->stats.rx_dropped++;                         snull_release_buffer(pkt);                         continue;                 }                 skb_reserve(skb, 2); /* align IP on 16B boundary */                   memcpy(skb_put(skb, pkt->datalen), pkt->data, pkt->datalen);                 skb->dev = priv->dev;                 skb->protocol = eth_type_trans(skb, priv->dev);                 skb->ip_summed = CHECKSUM_UNNECESSARY; /* don't check it */                 netif_receive_skb(skb);                                  /* Maintain stats */                 priv->stats.rx_packets++;                 priv->stats.rx_bytes += pkt->datalen;                 snull_release_buffer(pkt);                      received++;         }     //if (received < budget || ! priv->rx_queue) {     if ( ! priv->rx_queue) {          printk("napi_complete\n");         napi_complete(napi);         snull_rx_ints(priv->dev, 1);         } //       netif_start_queue(priv->dev);         return received; } /*  * A NAPI interrupt handler.  */ static void snull_napi_interrupt(int irq, void *dev_id, struct pt_regs *regs) {     int statusword;     struct snull_priv *priv;     /*      * As usual, check the "device" pointer for shared handlers.      * Then assign "struct device *dev"      */     struct net_device *dev = (struct net_device *)dev_id;     /* ... and check with hw if it's really ours */     /* paranoid */     if (!dev)         return;     /* Lock the device */     priv = netdev_priv(dev);     spin_lock(&priv->lock);     /* retrieve statusword: real netdevices use I/O instructions */     statusword = priv->status;     priv->status = 0;     if (statusword & SNULL_RX_INTR) {         printk("napi_schedule\n");         if (napi_schedule_prep(&priv->napi)) {                     snull_rx_ints(dev, 0);  /* Disable further interrupts */              printk("napi_schedule_prep\n");                     __napi_schedule(&priv->napi);             }         }     if (statusword & SNULL_TX_INTR) {             /* a transmission is over: free the skb */         priv->stats.tx_packets++;         priv->stats.tx_bytes += priv->tx_packetlen;         dev_kfree_skb(priv->skb);     }     /* Unlock the device and we are done */     spin_unlock(&priv->lock);     return; } /*  * Transmit a packet (low level interface)  */ static void snull_hw_tx(char *buf, int len, struct net_device *dev) {     /*      * This function deals with hw details. This interface loops      * back the packet to the other snull interface (if any).      * In other words, this function implements the snull behaviour,      * while all other procedures are rather device-independent      */     struct iphdr *ih;     struct net_device *dest;     struct snull_priv *priv;     u32 *saddr, *daddr;     struct snull_packet *tx_buffer;          /* I am paranoid. Ain't I? */     if (len < sizeof(struct ethhdr) + sizeof(struct iphdr)) {         printk("snull: Hmm... packet too short (%i octets)\n",                 len);         return;     }     if (1) { /* enable this conditional to look at the data */         int i;         //PDEBUG("len is %i\n" KERN_DEBUG "data:",len);         printk("len is %i\n" KERN_DEBUG "data:",len);         //for (i=14 ; i<len; i++)         for (i=0 ; i<len; i++)             printk(" %02x",buf[i]&0xff);         printk("\n");     }     /*      * Ethhdr is 14 bytes, but the kernel arranges for iphdr      * to be aligned (i.e., ethhdr is unaligned)      */     ih = (struct iphdr *)(buf+sizeof(struct ethhdr));     saddr = &ih->saddr;     daddr = &ih->daddr;     //printk("###%08x --> %08x\n", ntohl(ih->saddr), ntohl(ih->daddr));     ((u8 *)saddr)[2] ^= 1; /* change the third octet (class C) */     ((u8 *)daddr)[2] ^= 1;     ih->check = 0;         /* and rebuild the checksum (ip needs it) */     ih->check = ip_fast_csum((unsigned char *)ih,ih->ihl);     if (dev == snull_devs[0]) {             printk("dev0:%08x --> %08x\n", ntohl(ih->saddr), ntohl(ih->daddr));         }         //PDEBUGG("%08x:%05i --> %08x:%05i\n",         //printk("%08x:%05i --> %08x:%05i\n",         //        ntohl(ih->saddr),ntohs(((struct tcphdr *)(ih+1))->source),         //        ntohl(ih->daddr),ntohs(((struct tcphdr *)(ih+1))->dest));     else {         printk("dev1:%08x <-- %08x\n", ntohl(ih->daddr), ntohl(ih->saddr));         //printk("%08x:%05i <-- %08x:%05i\n",         //        ntohl(ih->daddr),ntohs(((struct tcphdr *)(ih+1))->dest),         //        ntohl(ih->saddr),ntohs(((struct tcphdr *)(ih+1))->source));     }     /*      * Ok, now the packet is ready for transmission: first simulate a      * receive interrupt on the twin device, then  a      * transmission-done on the transmitting device      */     tx_buffer = snull_get_tx_buffer(dev);     tx_buffer->datalen = len;     memcpy(tx_buffer->data, buf, len);     dest = snull_devs[dev == snull_devs[0] ? 1 : 0];     priv = netdev_priv(dest);     snull_enqueue_buf(dest, tx_buffer);     printk("snull_enqueue_buf\n");     if (priv->rx_int_enabled) {         priv->status |= SNULL_RX_INTR;         snull_interrupt(0, dest, NULL);     }     priv = netdev_priv(dev);     priv->tx_packetlen = len;     priv->tx_packetdata = buf;     priv->status |= SNULL_TX_INTR;     snull_interrupt(0, dev, NULL); } /*  * Transmit a packet (called by the kernel)  */ int snull_tx(struct sk_buff *skb, struct net_device *dev) {     int len;     char *data, shortpkt[ETH_ZLEN];     struct snull_priv *priv = netdev_priv(dev);          data = skb->data;     len = skb->len;     if (len < ETH_ZLEN) {         memset(shortpkt, 0, ETH_ZLEN);         memcpy(shortpkt, skb->data, skb->len);         len = ETH_ZLEN;         data = shortpkt;     }     dev->trans_start = jiffies; /* save the timestamp */     /* Remember the skb, so we can free it at interrupt time */     priv->skb = skb;     /* actual deliver of data is device-specific, and not shown here */     snull_hw_tx(data, len, dev);     return 0; /* Our simple device can not fail */ } /*  *  * Return statistics to the caller  *   */ struct net_device_stats *snull_stats(struct net_device *dev) {         struct snull_priv *priv = netdev_priv(dev);         return &priv->stats; } /*  *  * This function is called to fill up an eth header, since arp is not  *   * available on the interface  *    */ int snull_rebuild_header(struct sk_buff *skb) {         struct ethhdr *eth = (struct ethhdr *) skb->data;         struct net_device *dev = skb->dev;         memcpy(eth->h_source, dev->dev_addr, dev->addr_len);         memcpy(eth->h_dest, dev->dev_addr, dev->addr_len);         eth->h_dest[ETH_ALEN-1]   ^= 0x01;   /* dest is us xor 1 */         return 0; } int snull_header(struct sk_buff *skb, struct net_device *dev,                 unsigned short type, void *daddr, void *saddr,                 unsigned int len) {         struct ethhdr *eth = (struct ethhdr *)skb_push(skb,ETH_HLEN);         eth->h_proto = htons(type);         memcpy(eth->h_source, saddr ? saddr : dev->dev_addr, dev->addr_len);         memcpy(eth->h_dest,   daddr ? daddr : dev->dev_addr, dev->addr_len);         eth->h_dest[ETH_ALEN-1]   ^= 0x01;   /* dest is us xor 1 */         return (dev->hard_header_len); } static void snull_set_rx_mode(struct net_device *dev) { } static const struct net_device_ops virtnet_netdev = {         .ndo_open            = snull_open,         .ndo_stop            = snull_release,         .ndo_start_xmit      = snull_tx,     .ndo_get_stats       = snull_stats,     .ndo_set_rx_mode     = snull_set_rx_mode, }; static const struct header_ops      header_op =  {     .create             = snull_header,     .rebuild             = snull_rebuild_header, }; /*  * The devices  */ struct net_device *snull_devs[2]; /*  * Finally, the module stuff  */ void snull_cleanup(void) {     int i;          for (i = 0; i < 2;  i++) {         if (snull_devs[i]) {             unregister_netdev(snull_devs[i]);             snull_teardown_pool(snull_devs[i]);             free_netdev(snull_devs[i]);         }     }     return; } int snull_init_module(void) {     int result, i, ret = -ENOMEM;     struct snull_priv *vi;     snull_interrupt = snull_napi_interrupt ;     /* Allocate the devices */     snull_devs[0] =  alloc_etherdev(sizeof(struct snull_priv));     snull_devs[1] =  alloc_etherdev(sizeof(struct snull_priv));     if (!snull_devs[0] || !snull_devs[1])         return  -ENOMEM;     for (i = 0; i < 2;  i++) {         snull_devs[i]->flags           |= IFF_NOARP;         snull_devs[i]->features        |= NETIF_F_NO_CSUM;         snull_devs[i]->header_ops = &header_op;         snull_rx_ints(snull_devs[i], 1);         snull_setup_pool(snull_devs[i]);         snull_devs[i]->netdev_ops = &virtnet_netdev;         vi = netdev_priv(snull_devs[i]);         vi->dev = snull_devs[i];         netif_napi_add(snull_devs[i], &vi->napi, snull_poll,2);     }     for (i = 0; i < 2;  i++)         if ((result = register_netdev(snull_devs[i])))             printk("snull: error %i registering device \"%s\"\n",                     result, snull_devs[i]->name);         else {             ret = 0;         }     if (ret)         snull_cleanup();     return ret; } /*  * snull.h -- definitions for the network module  *  * Copyright (C) 2001 Alessandro Rubini and Jonathan Corbet  * Copyright (C) 2001 O'Reilly & Associates  *  * The source code in this file can be freely used, adapted,  * and redistributed in source or binary form, so long as an  * acknowledgment appears in derived source files.  The citation  * should list that the code comes from the book "Linux Device  * Drivers" by Alessandro Rubini and Jonathan Corbet, published  * by O'Reilly & Associates.   No warranty is attached;  * we cannot take responsibility for errors or fitness for use.  */ /*  * Macros to help debugging  */ #undef PDEBUG             /* undef it, just in case */ #ifdef SNULL_DEBUG #  ifdef __KERNEL__      /* This one if debugging is on, and kernel space */ #    define PDEBUG(fmt, args...) printk( KERN_DEBUG "snull: " fmt, ## args) #  else      /* This one for user space */ #    define PDEBUG(fmt, args...) fprintf(stderr, fmt, ## args) #  endif #else #  define PDEBUG(fmt, args...) /* not debugging: nothing */ #endif #undef PDEBUGG #define PDEBUGG(fmt, args...) /* nothing: it's a placeholder */ /* These are the flags in the statusword */ #define SNULL_RX_INTR 0x0001 #define SNULL_TX_INTR 0x0002 /* Default timeout period */ #define SNULL_TIMEOUT 5   /* In jiffies */ extern struct net_device *snull_devs[]; module_init(snull_init_module); module_exit(snull_cleanup);

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