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）