Linux内核--网络栈实现分析（十一）--驱动程序层（下）

本文分析基于Linux Kernel 1.2.13

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作者：闫明

注：标题中的”（上）“，”（下）“表示分析过程基于数据包的传递方向：”（上）“表示分析是从底层向上分析、”（下）“表示分析是从上向下分析。

在博文Linux内核--网络栈实现分析（三）--驱动程序层（链路层）（上）中对网络设备结构，网络设备初始化等函数有了初步认识，并列出了设备的发送和接收函数。

设备接口层会调用函数设备驱动层ei_start_xmit()函数发送数据，这里没有详细分析。

static int ei_start_xmit(struct sk_buff *skb, struct device *dev)
{
    int e8390_base = dev->base_addr;
    struct ei_device *ei_local = (struct ei_device *) dev->priv;//取出网卡设备的私有数据，和具体的网卡型号有关，在ethdev_init()函数中已经分配空间
    int length, send_length;
    unsigned long flags;
    
/*
 *  We normally shouldn't be called if dev->tbusy is set, but the
 *  existing code does anyway. If it has been too long since the
 *  last Tx, we assume the board has died and kick it.
 */
 
    if (dev->tbusy) {	/* Do timeouts, just like the 8003 driver. */
		........................................
		........................................
    }
    
    /* Sending a NULL skb means some higher layer thinks we've missed an
       tx-done interrupt. Caution: dev_tint() handles the cli()/sti()
       itself. */
    if (skb == NULL) {//该条件似乎不会发生，这用于处理内核中的BUG
		dev_tint(dev);//发送设备中的所有缓存的数据包
		return 0;
    }
    
    length = skb->len;
    if (skb->len <= 0)
		return 0;

    save_flags(flags);
    cli();

    /* Block a timer-based transmit from overlapping. */
    if ((set_bit(0, (void*)&dev->tbusy) != 0) || ei_local->irqlock) {
	printk("%s: Tx access conflict. irq=%d lock=%d tx1=%d tx2=%d last=%d\n",
		dev->name, dev->interrupt, ei_local->irqlock, ei_local->tx1,
		ei_local->tx2, ei_local->lasttx);
	restore_flags(flags);
	return 1;
    }

    /* Mask interrupts from the ethercard. */
    outb(0x00, e8390_base + EN0_IMR);
    ei_local->irqlock = 1;
    restore_flags(flags);

    send_length = ETH_ZLEN < length ? length : ETH_ZLEN;

    if (ei_local->pingpong) {
		int output_page;
		if (ei_local->tx1 == 0) {
			output_page = ei_local->tx_start_page;
			ei_local->tx1 = send_length;
			if (ei_debug  &&  ei_local->tx2 > 0)
				printk("%s: idle transmitter tx2=%d, lasttx=%d, txing=%d.\n",
					   dev->name, ei_local->tx2, ei_local->lasttx,
					   ei_local->txing);
		} else if (ei_local->tx2 == 0) {
			output_page = ei_local->tx_start_page + 6;
			ei_local->tx2 = send_length;
			if (ei_debug  &&  ei_local->tx1 > 0)
				printk("%s: idle transmitter, tx1=%d, lasttx=%d, txing=%d.\n",
					   dev->name, ei_local->tx1, ei_local->lasttx,
					   ei_local->txing);
		} else {	/* We should never get here. */
			if (ei_debug)
				printk("%s: No Tx buffers free. irq=%d tx1=%d tx2=%d last=%d\n",
					dev->name, dev->interrupt, ei_local->tx1, 
					ei_local->tx2, ei_local->lasttx);
			ei_local->irqlock = 0;
			dev->tbusy = 1;
			outb_p(ENISR_ALL, e8390_base + EN0_IMR);
			return 1;
		}
		ei_block_output(dev, length, skb->data, output_page);
		if (! ei_local->txing) {
			ei_local->txing = 1;
			NS8390_trigger_send(dev, send_length, output_page);
			dev->trans_start = jiffies;
			if (output_page == ei_local->tx_start_page)
				ei_local->tx1 = -1, ei_local->lasttx = -1;
			else
				ei_local->tx2 = -1, ei_local->lasttx = -2;
		} else
			ei_local->txqueue++;

		dev->tbusy = (ei_local->tx1  &&  ei_local->tx2);
    } else {  /* No pingpong, just a single Tx buffer. */
		ei_block_output(dev, length, skb->data, ei_local->tx_start_page);
		ei_local->txing = 1;
		NS8390_trigger_send(dev, send_length, ei_local->tx_start_page);
		dev->trans_start = jiffies;
		dev->tbusy = 1;
    }
    
    /* Turn 8390 interrupts back on. */
    ei_local->irqlock = 0;
    outb_p(ENISR_ALL, e8390_base + EN0_IMR);

    dev_kfree_skb (skb, FREE_WRITE);
    
    return 0;

其中的dev_tint()函数是将设备的所有缓存队列中的数据全部调用dev_queue_xmit()发送全部数据包。

/*
 *	This routine is called when an device driver (i.e. an
 *	interface) is ready to transmit a packet.
 */
//该函数功能:遍历设备的缓冲队列，对所有的数据包调用dev_queue_xmit()函数发送数据
void dev_tint(struct device *dev)
{
	int i;
	struct sk_buff *skb;
	unsigned long flags;
	
	save_flags(flags);	
	/*
	 *	Work the queues in priority order
	 */
	 
	for(i = 0;i < DEV_NUMBUFFS; i++) 
	{
		/*
		 *	Pull packets from the queue
		 */
		 

		cli();
		while((skb=skb_dequeue(&dev->buffs[i]))!=NULL)
		{
			/*
			 *	Stop anyone freeing the buffer while we retransmit it
			 */
			skb_device_lock(skb);
			restore_flags(flags);
			/*
			 *	Feed them to the output stage and if it fails
			 *	indicate they re-queue at the front.
			 */
			dev_queue_xmit(skb,dev,-i - 1);//注意优先级的计算方式，在函数dev_queue_xmit()中优先级若<0则计算pri=-pri-1=-(-i-1)-1=i，
						       //这样做的目的就是为了得到正确的where值,函数(dev_queue_xmit())中
			/*
			 *	If we can take no more then stop here.
			 */
			if (dev->tbusy)
				return;
			cli();
		}
	}
	restore_flags(flags);
}

驱动层严格的说不属于内核网络栈的内容，和硬件关系密切，何况这种网卡硬件设备可能已经不用了，这里就没有详细分析，如果对网卡驱动有兴趣可以看一下之前的分析的ARM-Linux下的DM9000网卡驱动的分析，链接如下：

1. ARM-Linux驱动--DM9000网卡驱动分析（一）
   
2. ARM-Linux驱动--DM9000网卡驱动分析（二）
   
3. ARM-Linux驱动--DM9000网卡驱动分析（三）
   
4. ARM-Linux驱动--DM9000网卡驱动分析（四）