LAN9221網卡驅動之二 NAPI接收

轉載請註明出處：http://blog.csdn.net/qq405180763/article/details/8800391

LAN9221採用NAPI方式讀取網卡緩存內數據。當接主機收大量網絡數據時，網卡中斷觸發驅動程序接收數據，驅動程序被觸發後調用輪詢函數，在輪詢函數裏接收固定個數據包後，固定個數據爲budget，一般是輪詢註冊函數netif_napi_add的第三個參數weight，如果數據還沒接收完畢，則中斷再次被觸發（中斷標誌位沒有被清除），繼續輪詢接收數據，知道數據被完全接收完畢，則在輪詢函數裏將中斷標誌位清楚，關閉輪詢函數，重新使能中斷接收下一組數據。NAPI的好處就是既避免了在接收大量數據時頻繁的中斷，又不必因爲輪詢而耗費大量CPU資源，結合了中斷和輪詢的優點。

static int smsc911x_poll(struct napi_struct *napi, int budget)
{
 struct smsc911x_data *pdata =
  container_of(napi, struct smsc911x_data, napi);
 struct net_device *dev = pdata->dev;
 int npackets = 0;

 while (npackets < budget) { /*不管數據是否接收完畢，每個中斷最多輪詢budget=16次，
                                          因爲中斷被清零，如果還有數據則會再次觸發中斷*/
  unsigned int pktlength;
  unsigned int pktwords;
  struct sk_buff *skb;
  unsigned int rxstat = smsc911x_rx_get_rxstatus(pdata);

  if (!rxstat) {
   unsigned int temp;
   /* We processed all packets available.  Tell NAPI it can
    * stop polling then re-enable rx interrupts */
   smsc911x_reg_write(pdata, INT_STS, INT_STS_RSFL_); /*清數據接收中斷*/
   napi_complete(napi);
   temp = smsc911x_reg_read(pdata, INT_EN);
   temp |= INT_EN_RSFL_EN_;
   smsc911x_reg_write(pdata, INT_EN, temp); /*數據接收完，關閉輪詢，從新使能數據接收中斷*/
   break;
  }

  /* Count packet for NAPI scheduling, even if it has an error.
   * Error packets still require cycles to discard */
  npackets++;

  pktlength = ((rxstat & 0x3FFF0000) >> 16);
  pktwords = (pktlength + NET_IP_ALIGN + 3) >> 2; /*獲取數據包長度，並計算按字讀取的長度*/
  smsc911x_rx_counterrors(dev, rxstat);

  if (unlikely(rxstat & RX_STS_ES_)) {
   SMSC_WARNING(RX_ERR,
    "Discarding packet with error bit set");
   /* Packet has an error, discard it and continue with
    * the next */
   smsc911x_rx_fastforward(pdata, pktwords);
   dev->stats.rx_dropped++;
   continue;
  }

  skb = netdev_alloc_skb(dev, pktlength + NET_IP_ALIGN);  /*mac頭是14個字節，而緊接着的IP頭要求14字節對齊，所以多申請兩個字節，保證IP16字節對齊*/
  if (unlikely(!skb)) {
   SMSC_WARNING(RX_ERR,
    "Unable to allocate skb for rx packet");
   /* Drop the packet and stop this polling iteration */
   smsc911x_rx_fastforward(pdata, pktwords);
   dev->stats.rx_dropped++;
   break;
  }

  skb->data = skb->head; /*空頭*/ 
  skb_reset_tail_pointer(skb); /*空數據*/ 

  /* Align IP on 16B boundary */
  skb_reserve(skb, NET_IP_ALIGN); /*調節data的位置，使IP按16字節對齊*/
  skb_put(skb, pktlength - 4); /*將tail指針往後拉，接着就把數據拷貝到skb緩存中*/
  smsc911x_rx_readfifo(pdata, (unsigned int *)skb->head, pktwords);/*強制轉換成unsigned int，每加一次跳四個字節*/
  skb->protocol = eth_type_trans(skb, dev); /*使skb->mac_header指向mac協議頭，
                                                        skb->data後移14字節，剝離mac頭*/
  skb->ip_summed = CHECKSUM_NONE;
  netif_receive_skb(skb); /*將skb發往上一層*/

  /* Update counters */
  dev->stats.rx_packets++;
  dev->stats.rx_bytes += (pktlength - 4);
 }

 /* Return total received packets */
 return npackets;
}