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公司网站建设宣传报道稿件,市场营销是做什么的,wordpress keywords插件,一键生成静态网页从前面的一篇文章应用层发送一个数据包的时候，是如何到达网卡的（上）可以知道，应用层发送一个数据包的时候首先经过tcp_write和ip_queue_xmit函数，然后调用mac层的dev_queue_xmit函数。该函数代码如下，主要功…

从前面的一篇文章应用层发送一个数据包的时候，是如何到达网卡的（上）可以知道，应用层发送一个数据包的时候首先经过tcp_write和ip_queue_xmit函数，然后调用mac层的dev_queue_xmit函数。该函数代码如下，主要功能是完成arp解析（如果还没解析的话）、把数据包复制一份和对所有数据包都感兴趣的协议、把数据包插入发送队列，然后发送发送队列中的数据包。如果发送失败则加到发送队列里等待重发。

// dev为路由项对应的设备，在数据包路由的时候赋值
void dev_queue_xmit(struct sk_buff *skb, struct device *dev, int pri)
{unsigned long flags;int nitcount;struct packet_type *ptype;int where = 0;		/* used to say if the packet should go	*//* at the front or the back of the	*//* queue - front is a retransmit try	*/if (dev == NULL) {printk("dev.c: dev_queue_xmit: dev = NULL\n");return;}if(pri>=0 && !skb_device_locked(skb))skb_device_lock(skb);	/* Shove a lock on the frame */
#ifdef CONFIG_SLAVE_BALANCINGsave_flags(flags);cli();if(dev->slave!=NULL && dev->slave->pkt_queue < dev->pkt_queue &&(dev->slave->flags & IFF_UP))dev=dev->slave;restore_flags(flags);
#endif		
#ifdef CONFIG_SKB_CHECK IS_SKB(skb);
#endif    skb->dev = dev;/**	This just eliminates some race conditions, but not all... */if (skb->next != NULL) {/**	Make sure we haven't missed an interrupt. */printk("dev_queue_xmit: worked around a missed interrupt\n");start_bh_atomic();dev->hard_start_xmit(NULL, dev);end_bh_atomic();return;}/**	Negative priority is used to flag a frame that is being pulled from the*	queue front as a retransmit attempt. It therefore goes back on the queue*	start on a failure.*/// 类似坐标轴，-1和1是对称的，如果传的是负数，则先取他的对边，因为数组是从0开始，需要减一，比如传2即数组下标是1if (pri < 0) {pri = -pri-1;// 是直接发送还是先缓存到发送队列，1说明直接发送,0说明当前的skb先入队尾，先发送队列的队头节点where = 1;}if (pri >= DEV_NUMBUFFS) {printk("bad priority in dev_queue_xmit.\n");pri = 1;}/**	If the address has not been resolved. Call the device header rebuilder.*	This can cover all protocols and technically not just ARP either.*//*还没有完成arp解析，重新构建mac头，如果当前arp解析还没成功则直接返回，等待解析完成重新执行该函数(arp_rcv->arp_send_q).*/if (!skb->arp && dev->rebuild_header(skb->data, dev, skb->raddr, skb)) {return;}save_flags(flags);cli();	/*1 where一般是0，即pri是正整数，这时候skb会先插入队尾，先发送队头的节点，并且把数据包复制一份给对数据包感兴趣的协议，然后发送。2 where等于1，即pri是负数代表这个skb是发送失败后重发的，这时候这个数据包时直接发送出去的，不再走1的那些流程*/if (!where) {
#ifdef CONFIG_SLAVE_BALANCING	skb->in_dev_queue=1;
#endif		// 插入队尾，取出队头节点发送skb_queue_tail(dev->buffs + pri,skb);skb_device_unlock(skb);		/* Buffer is on the device queue and can be freed safely */skb = skb_dequeue(dev->buffs + pri);skb_device_lock(skb);		/* New buffer needs locking down */
#ifdef CONFIG_SLAVE_BALANCING		skb->in_dev_queue=0;
#endif		}restore_flags(flags);/* copy outgoing packets to any sniffer packet handlers */if(!where){	// 把所有发出去的数据包传一份给其他协议for (nitcount= dev_nit, ptype = ptype_base; nitcount > 0 && ptype != NULL; ptype = ptype->next) {/* Never send packets back to the socket* they originated from - MvS (miquels@drinkel.ow.org)*/// 对所有包都感兴趣的、不是packet协议产生的packet_type节点if (ptype->type == htons(ETH_P_ALL) &&(ptype->dev == dev || !ptype->dev) &&((struct sock *)ptype->data != skb->sk)){struct sk_buff *skb2;if ((skb2 = skb_clone(skb, GFP_ATOMIC)) == NULL)break;/**	The protocol knows this has (for other paths) been taken off*	and adds it back.*/skb2->len-=skb->dev->hard_header_len;ptype->func(skb2, skb->dev, ptype);nitcount--;}}}start_bh_atomic();// 调用驱动层发送if (dev->hard_start_xmit(skb, dev) == 0) {end_bh_atomic();/**	Packet is now solely the responsibility of the driver*/return;}end_bh_atomic();/**	Transmission failed, put skb back into a list. Once on the list it's safe and*	no longer device locked (it can be freed safely from the device queue)*/cli();
#ifdef CONFIG_SLAVE_BALANCINGskb->in_dev_queue=1;dev->pkt_queue++;
#endif		skb_device_unlock(skb);// 发送失败则把数据包重新加入队列skb_queue_head(dev->buffs + pri,skb);restore_flags(flags);
}

由以上代码可知mac层最后调用驱动层的函数去发送数据包，这里以3c501网卡为例。下面是发送函数的代码。


static int
el_start_xmit(struct sk_buff *skb, struct device *dev)
{struct net_local *lp = (struct net_local *)dev->priv;int ioaddr = dev->base_addr;unsigned long flags;if (dev->tbusy) {if (jiffies - dev->trans_start < 20) {if (el_debug > 2)printk(" transmitter busy, deferred.\n");return 1;}if (el_debug)printk ("%s: transmit timed out, txsr %#2x axsr=%02x rxsr=%02x.\n",dev->name, inb(TX_STATUS), inb(AX_STATUS), inb(RX_STATUS));lp->stats.tx_errors++;outb(TX_NORM, TX_CMD);outb(RX_NORM, RX_CMD);outb(AX_OFF, AX_CMD);	/* Just trigger a false interrupt. */outb(AX_RX, AX_CMD);	/* Aux control, irq and receive enabled */dev->tbusy = 0;dev->trans_start = jiffies;}if (skb == NULL) {dev_tint(dev);return 0;}save_flags(flags);/* Avoid incoming interrupts between us flipping tbusy and flippingmode as the driver assumes tbusy is a faithful indicator of cardstate */cli();/* Avoid timer-based retransmission conflicts. */if (set_bit(0, (void*)&dev->tbusy) != 0){restore_flags(flags);printk("%s: Transmitter access conflict.\n", dev->name);}else {int gp_start = 0x800 - (ETH_ZLEN < skb->len ? skb->len : ETH_ZLEN);unsigned char *buf = skb->data;lp->tx_pkt_start = gp_start;lp->collisions = 0;/**	Command mode with status cleared should [in theory]*	mean no more interrupts can be pending on the card.*/outb(AX_SYS, AX_CMD);inb(RX_STATUS);inb(TX_STATUS);/* *	Turn interrupts back on while we spend a pleasant afternoon*	loading bytes into the board */restore_flags(flags);outw(0x00, RX_BUF_CLR);		/* Set rx packet area to 0. */outw(gp_start, GP_LOW);		/* aim - packet will be loaded into buffer start */// 传输数据到网卡outsb(DATAPORT,buf,skb->len);	/* load buffer (usual thing each byte increments the pointer) */outw(gp_start, GP_LOW);		/* the board reuses the same register */outb(AX_XMIT, AX_CMD);		/* fire ... Trigger xmit.  */dev->trans_start = jiffies;}if (el_debug > 2)printk(" queued xmit.\n");dev_kfree_skb (skb, FREE_WRITE);return 0;
}