U-boot两个修改：ARP支持和UDP校验支持

正如上一篇文章所说，需要对U-boot的UDP做一些小修改。
本文地址：https://segmentfault.com/a/1190000005182243

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ARP支持

原文件：文件net.c的NetReceive()函数

代码位置：搜索“case ARPOP_REPLY:”行并且找到其return;语句。

修改：
#ifdef CONFIG_NETCONSOLE以下修改为：

#ifdef CONFIG_NETCONSOLE
    (*packetHandler)(0, 0, 0, 0);
#endif
    /** if Arp response by TFTP,
     ** send "TFTP Read Request"
     ** packet immediately */
    extern int TftpStarted;
    if (1 == TftpStarted) 
    {
        NetArpWaitPacketIP = 0;
        NetArpWaitTxPacketSize = 0;
        NetArpWaitPacketMAC = NULL;
        TftpSend();
    }
    else if (NetArpWaitTxPacketSize)
    {
        NetSendUDPPacket(NetArpWaitPacketMAC,
                         NetArpWaitPacketIP,
                         NetArpWaitDPort,        // 注
                         NetArpWaitSPort,
                         NetArpWaitTxPacketSize);
        NetArpWaitPacketIP = 0;
        NetArpWaitTxPacketSize = 0;
        NetArpWaitPacketMAC = NULL;
    }
    else
    {
        /* no arp request pending now */
    }

注：NetArpWaitDPort和NetArpWaitSPort都是新定义的全局变量。当然，也需要在NetSendUDPPacket将这两个值赋值。

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UDP校验和支持

U-boot默认不用UDP校验和（置零）。但是在OS X中，UDP校验和不正确的话，UDP包将会被系统丢弃。所以需要添加如下：

位置：文件net.c中的NetSetIp()函数

修改：
首先，在NetSetIp()中的“ip->udp_xsum = 0;”后面加上：

unsigned int tmpSum = 0;
tmpSum = NetUdpCksum(ip, len);
ip->udp_xsum = htons((ushort)(~tmpSum));

然后，在前面添加Cksum函数：

#define DB_UDP_XSUM(x)
unsigned NetUdpCksum(IP_t *ip, int len)
{
    ulong xsum = 0;
    uchar *data;
    len += 8;
    
    ip->udp_xsum = 0;
    
    /* sum IP data */
    data = &(ip->src);
    xsum += ((ulong)(*(data + 0))) << 8;
    xsum += ((ulong)(*(data + 1))) << 0;
    xsum += ((ulong)(*(data + 2))) << 8;
    xsum += ((ulong)(*(data + 3))) << 0;
    xsum += ((ulong)(*(data + 4))) << 8;
    xsum += ((ulong)(*(data + 5))) << 0;
    xsum += ((ulong)(*(data + 6))) << 8;
    xsum += ((ulong)(*(data + 7))) << 0;
    DB_UDP_XSUM(printf("sum: 0x%04X IP\n", xsum));
    
    /* sum IP protocol */
    xsum += (ushort)(ip->ip_p);
    
    /* sum UDP length */
    data = &(ip->udp_len);
    xsum += ((ulong)(*(data + 0))) << 8;
    xsum += ((ulong)(*(data + 1))) << 0;
    
    /* sum UDP content */
    data = &(ip->udp_src);
    while(len > 1)
    {
        xsum += ((ulong)(*(data + 0))) << 8;
        xsum += ((ulong)(*(data + 1))) << 0;
        data += 2;
        len -= 2;
    }
    
    if (len)    /* 1 == len */
    {
        xsum += ((ulong)(*(data + 0))) << 8;
    }
    
    xsum = (xsum & 0x0000FFFF) + (xsum >> 16);
    xsum = (xsum & 0x0000FFFF) + (xsum >> 16);
    
    return (xsum & 0x0000FFFF);
}