精簡的介面
在lwIP架構下，網路卡驅動程式撰寫者需實作三個介面：初始化、傳送封包、接收封包。亦即，err_t (*init)(struct netif *netif)、err_t (*input)(struct pbuf *p, struct netif *netif)、err_t linkoutput(struct netif *netif, struct pbuf *p)。 初始化
網路卡驅動程式的初始化需透過函式err_t (* init)(struct netif *netif)。此函式將在註冊驅動程式之時，經由netif_add()傳遞至lwIP。
接收封包
網路卡接收到封包後， 可透過下列四個lwIP提供函式其中之一：ethernet_input()、ip_input()、tcpip_ethinput()、tcpip_input()將封包分派至通謝協定堆疊進行處理。上述四個函式各有不同的使用情境，於單執行緒並提供ARP功能之平台，如uboot，接收到的封包可直接搭配lwIP內建之ethernet_input()運作。為了確保對未來版本之lwIP的相容性，接收函式建議於netif_add()中指定，並透過netif內名為input之函式指標呼叫。
傳送封包
要能傳送封包，驅動程式需實作兩個介面linkoutput()與output()，這兩個介面以函式指標的方式存在於資料結構netif中。output()函式作用於幫IP封包添加link layer的header；在大多數情況下，output()可直接選用lwIP內建之etharp_output()。最終，output()函式結束時，網路封包將透過linkoutput()經由網路卡傳輸。總結來說，lwIP中網路封包之傳輸順序如下：IP堆疊->output()->linkoutput()->網路卡硬體。
output函式之原型：
err_t output(struct netif *netif, struct pbuf *p);linkoutput函式之宣告：
err_t linkoutput(struct netif *netif, struct pbuf *p);
驅動程式範例
驅動程式的範例可參考lwip原始碼提供之ethernetif.c。ethernetif.c是一個範例，也是一個lwip網路驅動程式之框架(skeleton)。任何想要撰寫lwip網路卡驅動程式者，都應當詳細閱讀該原始檔。在lwip官網上也有其它的驅動程式實作可供下載。
/* Define those to better describe your network interface. */
#define IFNAME0 'e'
#define IFNAME1 'n'
/**
* Helper struct to hold private data used to operate your ethernet interface.
* Keeping the ethernet address of the MAC in this struct is not necessary
* as it is already kept in the struct netif.
* But this is only an example, anyway...
*/
struct ethernetif {
struct eth_addr *ethaddr;
/* Add whatever per-interface state that is needed here. */
};
/* Forward declarations. */
static void ethernetif_input(struct netif *netif);
/**
* In this function, the hardware should be initialized.
* Called from ethernetif_init().
*
* @param netif the already initialized lwip network interface structure
* for this ethernetif
*/
static void
low_level_init(struct netif *netif)
{
struct ethernetif *ethernetif = netif->state;
/* set MAC hardware address length */
netif->hwaddr_len = ETHARP_HWADDR_LEN;
/* set MAC hardware address */
netif->hwaddr[0] = ;
...
netif->hwaddr[5] = ;
/* maximum transfer unit */
netif->mtu = 1500;
/* device capabilities */
/* don't set NETIF_FLAG_ETHARP if this device is not an ethernet one */
netif->flags = NETIF_FLAG_BROADCAST | NETIF_FLAG_ETHARP | NETIF_FLAG_LINK_UP;
/* Do whatever else is needed to initialize interface. */
}
/**
* This function should do the actual transmission of the packet. The packet is
* contained in the pbuf that is passed to the function. This pbuf
* might be chained.
*
* @param netif the lwip network interface structure for this ethernetif
* @param p the MAC packet to send (e.g. IP packet including MAC addresses and type)
* @return ERR_OK if the packet could be sent
* an err_t value if the packet couldn't be sent
*
* @note Returning ERR_MEM here if a DMA queue of your MAC is full can lead to
* strange results. You might consider waiting for space in the DMA queue
* to become availale since the stack doesn't retry to send a packet
* dropped because of memory failure (except for the TCP timers).
*/
static err_t
low_level_output(struct netif *netif, struct pbuf *p)
{
struct ethernetif *ethernetif = netif->state;
struct pbuf *q;
initiate transfer();
#if ETH_PAD_SIZE
pbuf_header(p, -ETH_PAD_SIZE); /* drop the padding word */
#endif
for(q = p; q != NULL; q = q->next) {
/* Send the data from the pbuf to the interface, one pbuf at a
time. The size of the data in each pbuf is kept in the ->len
variable. */
send data from(q->payload, q->len);
}
signal that packet should be sent();
#if ETH_PAD_SIZE
pbuf_header(p, ETH_PAD_SIZE); /* reclaim the padding word */
#endif
LINK_STATS_INC(link.xmit);
return ERR_OK;
}
/**
* Should allocate a pbuf and transfer the bytes of the incoming
* packet from the interface into the pbuf.
*
* @param netif the lwip network interface structure for this ethernetif
* @return a pbuf filled with the received packet (including MAC header)
* NULL on memory error
*/
static struct pbuf *
low_level_input(struct netif *netif)
{
struct ethernetif *ethernetif = netif->state;
struct pbuf *p, *q;
u16_t len;
/* Obtain the size of the packet and put it into the "len"
variable. */
len = ;
#if ETH_PAD_SIZE
len += ETH_PAD_SIZE; /* allow room for Ethernet padding */
#endif
/* We allocate a pbuf chain of pbufs from the pool. */
p = pbuf_alloc(PBUF_RAW, len, PBUF_POOL);
if (p != NULL) {
#if ETH_PAD_SIZE
pbuf_header(p, -ETH_PAD_SIZE); /* drop the padding word */
#endif
/* We iterate over the pbuf chain until we have read the entire
* packet into the pbuf. */
for(q = p; q != NULL; q = q->next) {
/* Read enough bytes to fill this pbuf in the chain. The
* available data in the pbuf is given by the q->len
* variable.
* This does not necessarily have to be a memcpy, you can also preallocate
* pbufs for a DMA-enabled MAC and after receiving truncate it to the
* actually received size. In this case, ensure the tot_len member of the
* pbuf is the sum of the chained pbuf len members.
*/
read data into(q->payload, q->len);
}
acknowledge that packet has been read();
#if ETH_PAD_SIZE
pbuf_header(p, ETH_PAD_SIZE); /* reclaim the padding word */
#endif
LINK_STATS_INC(link.recv);
} else {
drop packet();
LINK_STATS_INC(link.memerr);
LINK_STATS_INC(link.drop);
}
return p;
}
/**
* 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;
}
}
/**
* Should be called at the beginning of the program to set up the
* network interface. It calls the function low_level_init() to do the
* actual setup of the hardware.
*
* This function should be passed as a parameter to netif_add().
*
* @param netif the lwip network interface structure for this ethernetif
* @return ERR_OK if the loopif is initialized
* ERR_MEM if private data couldn't be allocated
* any other err_t on error
*/
err_t
ethernetif_init(struct netif *netif)
{
struct ethernetif *ethernetif;
LWIP_ASSERT("netif != NULL", (netif != NULL));
ethernetif = mem_malloc(sizeof(struct ethernetif));
if (ethernetif == NULL) {
LWIP_DEBUGF(NETIF_DEBUG, ("ethernetif_init: out of memory\n"));
return ERR_MEM;
}
#if LWIP_NETIF_HOSTNAME
/* Initialize interface hostname */
netif->hostname = "lwip";
#endif /* LWIP_NETIF_HOSTNAME */
/*
* Initialize the snmp variables and counters inside the struct netif.
* The last argument should be replaced with your link speed, in units
* of bits per second.
*/
NETIF_INIT_SNMP(netif, snmp_ifType_ethernet_csmacd, LINK_SPEED_OF_YOUR_NETIF_IN_BPS);
netif->state = ethernetif;
netif->name[0] = IFNAME0;
netif->name[1] = IFNAME1;
/* We directly use etharp_output() here to save a function call.
* You can instead declare your own function an call etharp_output()
* from it if you have to do some checks before sending (e.g. if link
* is available...) */
netif->output = etharp_output;
netif->linkoutput = low_level_output;
ethernetif->ethaddr = (struct eth_addr *)&(netif->hwaddr[0]);
/* initialize the hardware */
low_level_init(netif);
return ERR_OK;
}
使用範例
驅動程式完成後，應用程式撰寫方式很簡單，首先把驅動程式加入lwip，接著在主迴圈對網路卡不斷進行輪詢即可。
static void ethernet_timer_init(void)
{
... ... ...
}
int ethernet_init(void)
{
... ... ...
// Initialize LWIP
lwip_init();
// Add our netif to LWIP
if (netif_add(ðernetif, &myip_addr, &netmask, &gw_addr, NULL,
ethernetif_init, ethernet_input) == NULL)
{
return -1;
}
netif_set_default(ðernetif);
netif_set_up(ðernetif);
... ... ...
return 0;
}
void ethernet_poll(void)
{
... ... ...
// Invokes network interface driver to process incoming packets
ethernetif_input(ðernetif);
// Process lwip network-related timers.
... ... ...
}
int main(void)
{
... ... ...
ethernet_timer_init(); // Initialize timer for lwip and network interface
ethernet_init();
while (1) {
ethernet_poll(); // Poll network stack
... ... ...
}
}
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