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從0學ARM-uboot中的網絡協議棧

uboot中網絡協議棧

網卡的驅動，對于上層協議來說，已經封裝好了發送和接收數據的接口，那么上層協議棧只需要按照順序調用對應的網卡驅動函數就可以進行網絡數據的收發。

uboot中的協議棧相對來說比較簡單，有以下幾個特點：

傳輸層只支持UDP協議;

目前只支持ICMP、TFTP、NFS、DNS、DHCP、CDP、SNTP等幾種協議;
網卡采用poll接收數據包，而不是中斷方式;
數據包的發送和接收操作是串行操作，不支持并發。

1. 網絡協議棧架構

下面是uboot網絡協議棧的函數調用流程：

從0學ARM-uboot中的網絡協議棧

2. 通過DNS命令來解析一個數據包的收發流程

uboot中，所有的命令都用宏U_BOOT_CMD來定義， dns命令的定義如下：

426 U_BOOT_CMD(
427 dns, 3, 1, do_dns,
428 "lookup the IP of a hostname",
429 "hostname [envvar]"
430 );

當我們在uboot的命令終端輸入命令dns后，命令解析函數就會調用dns執行函數do_dns()

389 int do_dns(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
390 {
……
406 if (strlen(argv[1]) >= 255) {
407 printf("dns error: hostname too long\n");
408 return 1;
409 }
410
411 NetDNSResolve = argv[1];
412
413 if (argc == 3)
414 NetDNSenvvar = argv[2];
415 else
416 NetDNSenvvar = NULL;
417
418 if (NetLoop(DNS) < 0) {
419 printf("dns lookup of %s failed, check setup\n", argv[1]);
420 return 1;
421 }
422
423 return 0;
424 }

406行判斷參數字符串長度，大于255非法 411行參數1必須是要解析的主機，存儲在NetDNSResolve 中 413~416行保存dns命令的環境參數，該參數可以沒有 418行進入網絡協議處理函數入口NetLoop()，并將對應的協議DNS傳遞給該函數

NetLoop()代碼比較長，我們只分析其中比較重要的幾段代碼

316 /**********************************************************************/
317 /*
318 * Main network processing loop.
319 */
320
321 int NetLoop(enum proto_t protocol)
322 {
323 bd_t *bd = gd->bd;
324 int ret = -1;
…………
352 NetInitLoop();
…………
367 switch (protocol) {
368 case TFTPGET:
369 #ifdef CONFIG_CMD_TFTPPUT
370 case TFTPPUT:
371 #endif
372 /* always use ARP to get server ethernet address */
373 TftpStart(protocol);
374 break;
…………
426 #if defined(CONFIG_CMD_DNS)
427 case DNS:
428 DnsStart();
429 break;
430 #endif
438 }
…………
461 for (;;) {
462 WATCHDOG_RESET();
463 #ifdef CONFIG_SHOW_ACTIVITY
464 show_activity(1);
465 #endif
466 /*
467 * Check the ethernet for a new packet. The ethernet
468 * receive routine will process it.
469 */
470 eth_rx();
471
472 /*
473 * Abort if ctrl-c was pressed.
474 */
475 if (ctrlc()) {
476 /* cancel any ARP that may not have completed */
477 NetArpWaitPacketIP = 0;
478
479 net_cleanup_loop();
480 eth_halt();
481 /* Invalidate the last protocol */
482 eth_set_last_protocol(BOOTP);
483
484 puts("\nAbort\n");
485 /* include a debug print as well incase the debug
486 messages are directed to stderr */
487 debug_cond(DEBUG_INT_STATE, "--- NetLoop Abort!\n");
488 goto done;
489 }
…………
522 switch (net_state) {
523
524 case NETLOOP_RESTART:
525 NetRestarted = 1;
526 goto restart;
527
528 case NETLOOP_SUCCESS:
529 net_cleanup_loop();
530 if (NetBootFileXferSize > 0) {
531 char buf[20];
532 printf("Bytes transferred = %ld (%lx hex)\n",
533 NetBootFileXferSize,
534 NetBootFileXferSize);
535 sprintf(buf, "%lX", NetBootFileXferSize);
536 setenv("filesize", buf);
537
538 sprintf(buf, "%lX", (unsigned long)load_addr);
539 setenv("fileaddr", buf);
540 }
541 if (protocol != NETCONS)
542 eth_halt();
543 else
544 eth_halt_state_only();
545
546 eth_set_last_protocol(protocol);
547
548 ret = NetBootFileXferSize;
549 debug_cond(DEBUG_INT_STATE, "--- NetLoop Success!\n");
550 goto done;
551
552 case NETLOOP_FAIL:
553 net_cleanup_loop();
554 /* Invalidate the last protocol */
555 eth_set_last_protocol(BOOTP);
556 debug_cond(DEBUG_INT_STATE, "--- NetLoop Fail!\n");
557 goto done;
558
559 case NETLOOP_CONTINUE:
560 continue;
561 }
562 }
563
564 done:
565 #ifdef CONFIG_CMD_TFTPPUT
566 /* Clear out the handlers */
567 net_set_udp_handler(NULL);
568 net_set_icmp_handler(NULL);
569 #endif
570 return ret;
571 }

函數參數為DNS 352行初始化網絡信息,讀取ipaddr、gatewayip、netmask、serverip、dnsip等環境變量的值并復制到對應的全局變量中

static void NetInitLoop(void)
{
static int env_changed_id;
int env_id = get_env_id();
/* update only when the environment has changed */
if (env_changed_id != env_id) {
NetOurIP = getenv_IPaddr("ipaddr");
NetOurGatewayIP = getenv_IPaddr("gatewayip");
NetOurSubnetMask = getenv_IPaddr("netmask");
NetServerIP = getenv_IPaddr("serverip");
NetOurNativeVLAN = getenv_VLAN("nvlan");
NetOurVLAN = getenv_VLAN("vlan");
#if defined(CONFIG_CMD_DNS)
NetOurDNSIP = getenv_IPaddr("dnsip");
#endif
env_changed_id = env_id;
}
memcpy(NetOurEther, eth_get_dev()->enetaddr, 6);
return;
}

367行對傳入的參數做switch操作，不同的協議進入到不同的處理流程 428行執行DnsStart(),

197 void
198 DnsStart(void)
199 {
200 debug("%s\n", __func__);
201
202 NetSetTimeout(DNS_TIMEOUT, DnsTimeout);
203 net_set_udp_handler(DnsHandler);
204
205 DnsSend();
206 }

203行函數net_set_udp_handler()主要將dns協議的回調函數DnsHandler()注冊到udp協議的回調指針udp_packet_handler，

void net_set_udp_handler(rxhand_f *f)
{
debug_cond(DEBUG_INT_STATE, "--- NetLoop UDP handler set (%p)\n", f);
if (f == NULL)
udp_packet_handler = dummy_handler;//注冊到udp協議回調函數指針
else
udp_packet_handler = f;
}

DnsStart()最終會調用函數DnsSend()發送dns協議數據包，該函數是根據dns協議填充udp數據包

37 static void
38 DnsSend(void)
39 {
40 struct header *header;
41 int n, name_len;
42 uchar *p, *pkt;
43 const char *s;
44 const char *name;
45 enum dns_query_type qtype = DNS_A_RECORD;
46
47 name = NetDNSResolve;
48 pkt = p = (uchar *)(NetTxPacket + NetEthHdrSize() + IP_UDP_HDR_SIZE);
49
50 /* Prepare DNS packet header */
51 header = (struct header *) pkt;
52 header->tid = 1;
53 header->flags = htons(0x100); /* standard query */
54 header->nqueries = htons(1); /* Just one query */
55 header->nanswers = 0;
56 header->nauth = 0;
57 header->nother = 0;
58
59 /* Encode DNS name */
60 name_len = strlen(name);
61 p = (uchar *) &header->data; /* For encoding host name into packet */
62
63 do {
64 s = strchr(name, '.');
65 if (!s)
66 s = name + name_len;
67
68 n = s - name; /* Chunk length */
69 *p++ = n; /* Copy length */
70 memcpy(p, name, n); /* Copy chunk */
71 p += n;
72
73 if (*s == '.')
74 n++;
75
76 name += n;
77 name_len -= n;
78 } while (*s != '\0');
79
80 *p++ = 0; /* Mark end of host name */
81 *p++ = 0; /* Some servers require double null */
82 *p++ = (unsigned char) qtype; /* Query Type */
83
84 *p++ = 0;
85 *p++ = 1; /* Class: inet, 0x0001 */
86
87 n = p - pkt; /* Total packet length */
88 debug("Packet size %d\n", n);
89
90 DnsOurPort = random_port();
91
92 NetSendUDPPacket(NetServerEther, NetOurDNSIP, DNS_SERVICE_PORT,
93 DnsOurPort, n);
94 debug("DNS packet sent\n");
95 }

51~57行根據dns協議填充dns協議頭，數據幀首地址為NetTxPacket，此處通過指針pkt和p來填充dns數據幀 60~85行根據協議格式要求填充要解析的host名字到數據包 87行計算數據包長度 90行產生一個隨機的端口號 92~93行調用udp協議的發送函數NetSendUDPPacket(),參數依次是：以太頭信息，DNS服務器 ip地址，DNS服務器端口號，我們的dns服務端口號，數據包長度

688 int NetSendUDPPacket(uchar *ether, IPaddr_t dest, int dport, int sport,
689 int payload_len)
690 {
691 uchar *pkt;
692 int eth_hdr_size;
693 int pkt_hdr_size;
694
695 /* make sure the NetTxPacket is initialized (NetInit() was called) */
696 assert(NetTxPacket != NULL);
697 if (NetTxPacket == NULL)
698 return -1;
699
700 /* convert to new style broadcast */
701 if (dest == 0)
702 dest = 0xFFFFFFFF;
703
704 /* if broadcast, make the ether address a broadcast and don't do ARP */
705 if (dest == 0xFFFFFFFF)
706 ether = NetBcastAddr;
707
708 pkt = (uchar *)NetTxPacket;
709
710 eth_hdr_size = NetSetEther(pkt, ether, PROT_IP);
711 pkt += eth_hdr_size;
712 net_set_udp_header(pkt, dest, dport, sport, payload_len);
713 pkt_hdr_size = eth_hdr_size + IP_UDP_HDR_SIZE;
714
715 /* if MAC address was not discovered yet, do an ARP request */
716 if (memcmp(ether, NetEtherNullAddr, 6) == 0) {
717 debug_cond(DEBUG_DEV_PKT, "sending ARP for %pI4\n", &dest);
718
719 /* save the ip and eth addr for the packet to send after arp */
720 NetArpWaitPacketIP = dest;
721 NetArpWaitPacketMAC = ether;
722
723 /* size of the waiting packet */
724 NetArpWaitTxPacketSize = pkt_hdr_size + payload_len;
725
726 /* and do the ARP request */
727 NetArpWaitTry = 1;
728 NetArpWaitTimerStart = get_timer(0);
729 ArpRequest();
730 return 1; /* waiting */
731 } else {
732 debug_cond(DEBUG_DEV_PKT, "sending UDP to %pI4/%pM\n",
733 &dest, ether);
734 NetSendPacket(NetTxPacket, pkt_hdr_size + payload_len);
735 return 0; /* transmitted */
736 }
737 }

696~706行參數檢查 710行設置以太頭 713行設置udp協議頭 716~730行如果沒有目的MAC地址，就要先發送ARP請求 734行調用函數NetSendPacket()，參數分別是：要發送數據幀的首地址，數據包長度

529 /* Transmit a packet */
530 static inline void NetSendPacket(uchar *pkt, int len)
531 {
532 (void) eth_send(pkt, len);
533 }

532行調用我們注冊的函數dm9000_send() 該函數已經分析過，根據流程圖，回到函數NetLoop()

461~562行循環接收網絡數據包 470行調用網卡驅動接收函數eth_rx()

int eth_rx(void)
{
if (!eth_current)
return -1;
return eth_current->recv(eth_current);
}

eth_current->recv(eth_current)函數就是我們注冊的網卡的接收函數dm9000_rx()，該函數我們上一章已經分析過，最終通過調用函數NetReceive()，將數據幀上傳到協議棧

943 void
944 NetReceive(uchar *inpkt, int len)
945 {
946 struct ethernet_hdr *et;
947 struct ip_udp_hdr *ip;
948 IPaddr_t dst_ip;
949 IPaddr_t src_ip;
950 int eth_proto;
……
957
958 NetRxPacket = inpkt;
959 NetRxPacketLen = len;
960 et = (struct ethernet_hdr *)inpkt;
961
962 /* too small packet? */
963 if (len < ETHER_HDR_SIZE)
964 return;
965
……
984
985 eth_proto = ntohs(et->et_protlen);
986
987 if (eth_proto < 1514) {
988 struct e802_hdr *et802 = (struct e802_hdr *)et;
……
997
998 } else if (eth_proto != PROT_VLAN) { /* normal packet */
999 ip = (struct ip_udp_hdr *)(inpkt + ETHER_HDR_SIZE);
1000 len -= ETHER_HDR_SIZE;
1001
1002 } else { /* VLAN packet */
……
1026 }
1027
……
1045 switch (eth_proto) {
……
1056 case PROT_IP:
1057 debug_cond(DEBUG_NET_PKT, "Got IP\n");
1058 /* Before we start poking the header, make sure it is there */
1059 if (len < IP_UDP_HDR_SIZE) {
1060 debug("len bad %d < %lu\n", len,
1061 (ulong)IP_UDP_HDR_SIZE);
1062 return;
1063 }
1064 /* Check the packet length */
1065 if (len < ntohs(ip->ip_len)) {
1066 debug("len bad %d < %d\n", len, ntohs(ip->ip_len));
1067 return;
1068 }
1069 len = ntohs(ip->ip_len);
1070 debug_cond(DEBUG_NET_PKT, "len=%d, v=%02x\n",
1071 len, ip->ip_hl_v & 0xff);
1072
1073 /* Can't deal with anything except IPv4 */
1074 if ((ip->ip_hl_v & 0xf0) != 0x40)
1075 return;
1076 /* Can't deal with IP options (headers != 20 bytes) */
1077 if ((ip->ip_hl_v & 0x0f) > 0x05)
1078 return;
1079 /* Check the Checksum of the header */
1080 if (!NetCksumOk((uchar *)ip, IP_HDR_SIZE / 2)) {
1081 debug("checksum bad\n");
1082 return;
1083 }
1084 /* If it is not for us, ignore it */
1085 dst_ip = NetReadIP(&ip->ip_dst);
1092 /* Read source IP address for later use */
1093 src_ip = NetReadIP(&ip->ip_src);
1184 /*
1185 * IP header OK. Pass the packet to the current handler.
1186 */
1187 (*udp_packet_handler)((uchar *)ip + IP_UDP_HDR_SIZE,
1188 ntohs(ip->udp_dst),
1189 src_ip,
1190 ntohs(ip->udp_src),
1191 ntohs(ip->udp_len) - UDP_HDR_SIZE);
1192 break;
1193 }
1194 }

參數inpkt：指向接收到的以太數據包頭 len：接收到的數據包的長度 960行變量NetRxPacket指向接收的數據頭，以太數據包包頭比定位以太協議頭 985行從以太協議頭提取出協議字段，該字段表示后面是否是ip協議 999行解析出ip協議頭 1045行根據以太頭協議進行switch操作 1059~1083行對協議頭進行合法性檢查 1085行讀取出目的ip地址 1093行讀取出源ip地址， 1187行 ip協議頭解析成功，調用udp協議回調函數udp_packet_handler(),該函數在之前的DnStart()注冊了DnsHandler

104 static void
105 DnsHandler(uchar *pkt, unsigned dest, IPaddr_t sip, unsigned src, unsigned len)
106 {
193
194 net_set_state(NETLOOP_SUCCESS);
195 }

該函數用于解析DNS協議，在此不再詳解解析成功后194行，會設置當前執行狀態為NETLOOP_SUCCESS，代碼回到函數NetLoop()470行 475行判斷是否按下ctrl+c快捷鍵，并作出操作 522~562行對執行結果進行處理，計入統計信息 564行如果net_state為NETLOOP_SUCCESS、NETLOOP_FAIL最終都會進入done，從而置空udp回調函數如果net_state為NETLOOP_CONTINUE，表明仍然有后續數據包要接收，則回到461行，繼續下一個數據包的接收

至此DNS協議的處理流程分析完畢，大家可以根據這個流程自行分析其他幾個協議的處理流程。