LwIP协议栈（２）：网络接口

　　在LwIP中，物理网络硬件接口结构保存在一个全局链表中，它们通过结构体中的 next 指针连接。

struct netif {
  /// pointer to next in linked list */
  struct netif *next;

  /// IP address configuration in network byte order */
  ip_addr_t ip_addr; //IP地址
  ip_addr_t netmask; //子网掩码
  ip_addr_t gw;      //网关地址

  netif_input_fn input; //设备驱动调用该函数传递一个包给TCP/IP协议栈

  //IP模块调用该函数传递一个包给网卡，output 函数的第三个參数 ipaddr 是应该接收实际的链路层帧的主机的 IP 地址。
  //它不必与 IP 信息包的目的地址同样。特别地，当要发送 IP 信息包到一个并不在本地网络里的主机上时，链路层帧会被发送到网络里的一个路由器上。
  //在这样的情况下给output函数的IP地址将是这个路由器的地址。

netif_output_fn output; 

  netif_linkoutput_fn linkoutput; //ARP模块调用该函数传递一个包给网卡

  void *state; //由设备驱动设置。指向设备状态信息
#if LWIP_DHCP
  /// the DHCP client state information for this netif */
  struct dhcp *dhcp;
#endif // LWIP_DHCP */
#if LWIP_AUTOIP
  /// the AutoIP client state information for this netif */
  struct autoip *autoip;
#endif
#if LWIP_NETIF_HOSTNAME
  // the hostname for this netif, NULL is a valid value */
  char*  hostname;
#endif // LWIP_NETIF_HOSTNAME */

  u16_t mtu; //一次传送的最大字节数，以太网一般为1500

  u8_t hwaddr_len; //物理地址长度，通常是以太网MAC地址长度。6字节

  u8_t hwaddr[NETIF_MAX_HWADDR_LEN]; //物理地址，通常是以太网MAC地址
  /// flags (see NETIF_FLAG_ above) */
  u8_t flags; //使能标志符。比方设置NETIF_FLAG_LINK_UP。接收到数据包才会向上传
  /// descriptive abbreviation */
  char name[2]; //设备驱动类型
  /// number of this interface */
  u8_t num;

    // List of packets to be queued for ourselves. */
  struct pbuf *loop_first; //环回。指向发给自己数据包的第一个pbuf
  struct pbuf *loop_last;  //环回，指向发给自己数据包的最后一个pbuf
};

/*
 * Add a network interface to the list of lwIP netifs.
 *
 * @param netif a pre-allocated netif structure
 * @param ipaddr IP address for the new netif
 * @param netmask network mask for the new netif
 * @param gw default gateway IP address for the new netif
 * @param state opaque data passed to the new netif
 * @param init callback function that initializes the interface
 * @param input callback function that is called to pass
 * ingress packets up in the protocol layer stack.
 *
 * @return netif, or NULL if failed.
 */
struct netif *
netif_add(struct netif *netif, ip_addr_t *ipaddr, ip_addr_t *netmask,
  ip_addr_t *gw, void *state, netif_init_fn init, netif_input_fn input)
{
  static u8_t netifnum = 0;

  // reset new interface configuration state */
  ip_addr_set_zero(&netif->ip_addr);
  ip_addr_set_zero(&netif->netmask);
  ip_addr_set_zero(&netif->gw);
  netif->flags = 0;

  // remember netif specific state information data */
  netif->state = state;
  netif->num = netifnum++;
  netif->input = input;

  netif_set_addr(netif, ipaddr, netmask, gw);

  // call user specified initialization function for netif */
  if (init(netif) != ERR_OK) { //用户自己定义初始化函数
    return NULL;
  }

  // add this netif to the list */
  netif->next = netif_list;
  netif_list = netif;
  snmp_inc_iflist();

  return netif;
}

//用户自己定义初始化函数
err_t ethernetif_init(struct netif *netif)
{
  netif->name[0] = IFNAME0;   //比方蓝牙设备（bluetooth）的网络接口能够是bt，随便啦
  netif->name[1] = IFNAME1;
  netif->output = etharp_output;  //IP模块发送数据包函数
  netif->linkoutput = low_level_output; // //ARP模块发送数据包函数
  low_level_init(netif); //底层硬件初始化函数
  return ERR_OK;
}

static void low_level_init(struct netif *netif)
{
  netif->hwaddr_len = ETHARP_HWADDR_LEN; //设置MAC地址长度

  netif->hwaddr[0] = 'F'; //设置网卡MAC地址
  netif->hwaddr[1] = 'O';
  netif->hwaddr[2] = 'R';
  netif->hwaddr[3] = 'E';
  netif->hwaddr[4] = 'S';
  netif->hwaddr[5] = 'T';

  netif->mtu = 1500; //最大同意传输单元

  //同意该网卡的广播和ARP功能。而且该网卡同意有硬件链路连接
  netif->flags = NETIF_FLAG_BROADCAST | \
  NETIF_FLAG_ETHARP | NETIF_FLAG_LINK_UP;

  E1280_init(netif->hwaddr); //详细网卡初始化函数
}

/*
 * Send an IP packet to be received on the same netif (loopif-like).
 * The pbuf is simply copied and handed back to netif->input.
 * In multithreaded mode, this is done directly since netif->input must put
 * the packet on a queue.
 * In callback mode, the packet is put on an internal queue and is fed to
 * netif->input by netif_poll().
 *
 * @param netif the lwip network interface structure
 * @param p the (IP) packet to 'send'
 * @param ipaddr the ip address to send the packet to (not used)
 * @return ERR_OK if the packet has been sent
 *         ERR_MEM if the pbuf used to copy the packet couldn't be allocated
 */
err_t
netif_loop_output(struct netif *netif, struct pbuf *p,
       ip_addr_t *ipaddr)
{
  struct pbuf *r;
  err_t err;
  struct pbuf *last;

  SYS_ARCH_DECL_PROTECT(lev);
  LWIP_UNUSED_ARG(ipaddr);

  // Allocate a new pbuf */
  r = pbuf_alloc(PBUF_LINK, p->tot_len, PBUF_RAM);
  if (r == NULL) {
    return ERR_MEM;
  }

  // Copy the whole pbuf queue p into the single pbuf r */
  if ((err = pbuf_copy(r, p)) != ERR_OK) { //拷贝要发送的数据到r中
    pbuf_free(r);
    return err;
  }

  // Put the packet on a linked list which gets emptied through calling
     netif_poll(). */
  //能够调用netif_poll函数将当前loop链表上全部数据提交给IP层

  // let last point to the last pbuf in chain r */
  for (last = r; last->next != NULL; last = last->next); 

  SYS_ARCH_PROTECT(lev);
  if(netif->loop_first != NULL) { //原来的loop中还有数据
    netif->loop_last->next = r;   //接到原来数据的后面
    netif->loop_last = last;
  } else {
    netif->loop_first = r;
    netif->loop_last = last;
  }
  SYS_ARCH_UNPROTECT(lev);

  return ERR_OK;
}

/*
 * This function should be called when a packet is ready to be read
 * from the interface. It uses the function low_level_input() that
 * should handle the actual reception of bytes from the network
 * interface. Then the type of the received packet is determined and
 * the appropriate input function is called.
 *
 * @param netif the lwip network interface structure for this ethernetif
 */
static void
ethernetif_input(struct netif *netif)
{
  struct ethernetif *ethernetif;
  struct eth_hdr *ethhdr;
  struct pbuf *p;

  ethernetif = netif->state;

  // move received packet into a new pbuf */
  p = low_level_input(netif);
  // no packet could be read, silently ignore this */
  if (p == NULL) return;
  // points to packet payload, which starts with an Ethernet header */
  ethhdr = p->payload;

  switch (htons(ethhdr->type)) {
  // IP or ARP packet?

*/
  case ETHTYPE_IP:
  case ETHTYPE_ARP:
#if PPPOE_SUPPORT
  // PPPoE packet? */
  case ETHTYPE_PPPOEDISC:
  case ETHTYPE_PPPOE:
#endif // PPPOE_SUPPORT */
    // full packet send to tcpip_thread to process */
    if (netif->input(p, netif)!=ERR_OK)
     { LWIP_DEBUGF(NETIF_DEBUG, ("ethernetif_input: IP input error\n"));
       pbuf_free(p);
       p = NULL;
     }
    break;

  default:
    pbuf_free(p);
    p = NULL;
    break;
  }
}

/*
 * This function should be called when a packet is ready to be read
 * from the interface. It uses the function low_level_input() that
 * should handle the actual reception of bytes from the network
 * interface. Then the type of the received packet is determined and
 * the appropriate input function is called.
 *
 * @param netif the lwip network interface structure for this ethernetif
 */
static void
ethernetif_input(struct netif *netif)
{
  struct ethernetif *ethernetif;
  struct eth_hdr *ethhdr;
  struct pbuf *p;

  ethernetif = netif->state;

  // move received packet into a new pbuf */
  p = low_level_input(netif); //拷贝接收到的数据到一个新的pbuf
  // no packet could be read, silently ignore this */
  if (p == NULL) return;
  // points to packet payload, which starts with an Ethernet header */
  ethhdr = p->payload;

  switch (htons(ethhdr->type)) {
  // IP or ARP packet?

*/
  case ETHTYPE_IP:
  case ETHTYPE_ARP:
    // full packet send to tcpip_thread to process */
    if (netif->input(p, netif)!=ERR_OK) //调用上层（IP/ARP）函数进行处理
     {
       pbuf_free(p);
       p = NULL;
     }
    break;

  default:
    pbuf_free(p);
    p = NULL;
    break;
  }
}