OpenWRT数据接收过程

1. ieee80211_tasklet_handler()

内核是通过中断来对接收到的数据进行响应的。当硬件检测到有接收数据的时候，产生一个中断，中断触发下半部的tasklet机制，在802.11协议栈这里会调用ieee80211_tasklet_handler()函数。我们来看一看函数体：（位于OpenWRT内核文件夹子目录/net/mac80211，文件main.c中）

static void ieee80211_tasklet_handler(unsigned long data)
{
       struct ieee80211_local *local = (struct ieee80211_local *) data;
       struct sk_buff *skb;
       while ((skb = skb_dequeue(&local->skb_queue)) ||
              (skb = skb_dequeue(&local->skb_queue_unreliable))) {
              switch (skb->pkt_type) {
              case IEEE80211_RX_MSG:
                     /* Clear skb->pkt_type in order to not confuse kernel
                      * netstack. */
                     skb->pkt_type = 0;
                     ieee80211_rx(&local->hw, skb);
                     break;
              case IEEE80211_TX_STATUS_MSG:
                     ...
              default:
                     ...
              }
       }
}

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2. ieee80211_rx()

系统收到数据时会开辟一个sk_buff缓存空间进行数据的存储，ieee80211_tasklet_handler()触发后对sk_buff中存储的数据帧进行判断，如果是接收来的数据（MPDU），则进入ieee80211_rx()函数：（位于OpenWRT内核文件夹子目录/net/mac80211，文件rx.c中）。

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void ieee80211_rx(struct ieee80211_hw *hw, struct sk_buff *skb)
{
       struct ieee80211_local *local = hw_to_local(hw);
       struct ieee80211_rate *rate = NULL;
       struct ieee80211_supported_band *sband;
       struct ieee80211_rx_status *status = IEEE80211_SKB_RXCB(skb);
       ...
       __ieee80211_rx_handle_packet(hw, skb);
       rcu_read_unlock();
       return;
 drop:
       kfree_skb(skb);
}
EXPORT_SYMBOL(ieee80211_rx);

3. __ieee80211_rx_handle_packet()

ieee80211_rx()函数再调用__ieee80211_rx_handle_packet()（位于OpenWRT内核文件夹子目录/net/mac80211，文件rx.c中），__ieee80211_rx_handle_packet()是接收帧的处理函数，会对帧类型进行判断，如果检测出该帧是beacon帧（或sta主动扫描后从AP端返回的响应帧），则进入ieee80211_scan_rx()函数对帧信息进行扫描，如果是数据帧，则调用ieee80211_prepare_and_rx_handle()对帧进行处理，下面分析接收帧为数据帧的情况。

void ieee80211_rx(struct ieee80211_hw *hw, struct sk_buff *skb)
{
       struct ieee80211_local *local = hw_to_local(hw);
       struct ieee80211_rate *rate = NULL;
       struct ieee80211_supported_band *sband;
       struct ieee80211_rx_status *status = IEEE80211_SKB_RXCB(skb);
       ...
       __ieee80211_rx_handle_packet(hw, skb);
       rcu_read_unlock();
       return;
 drop:
       kfree_skb(skb);
}
EXPORT_SYMBOL(ieee80211_rx);

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4. ieee80211_prepare_and_rx_handle()

调用ieee80211_prepare_and_rx_handle()（位于OpenWRT内核文件夹子目录/net/mac80211，文件rx.c中）。

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/*
 * This function returns whether or not the SKB
 * was destined for RX processing or not, which,
 * if consume is true, is equivalent to whether
 * or not the skb was consumed.
 */
static bool ieee80211_prepare_and_rx_handle(struct ieee80211_rx_data *rx, struct sk_buff *skb, bool consume)
{
…
       ieee80211_invoke_rx_handlers(rx);
       return true;
}

5. ieee80211_invoke_rx_handlers()

调用ieee80211_invoke_rx_handlers()（位于OpenWRT内核文件夹子目录/net/mac80211，文件rx.c中）。

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static void ieee80211_invoke_rx_handlers(struct ieee80211_rx_data *rx)
{
…
       ieee80211_rx_reorder_ampdu(rx, &reorder_release);
       ieee80211_rx_handlers(rx, &reorder_release);
       return;
 rxh_next:
       ieee80211_rx_handlers_result(rx, res);
…
}

6. ieee80211_rx_handlers()

调用ieee80211_rx_handlers()（位于OpenWRT内核文件夹子目录/net/mac80211，文件rx.c中）。

static void ieee80211_rx_handlers(struct ieee80211_rx_data *rx, struct sk_buff_head *frames)
{
       ieee80211_rx_result res = RX_DROP_MONITOR;
       struct sk_buff *skb;
#define CALL_RXH(rxh)                     \
       do {                            \
              res = rxh(rx);              \
              if (res != RX_CONTINUE)    \
                     goto rxh_next;  \
       } while (0);
       spin_lock_bh(&rx->local->rx_path_lock);
       while ((skb = __skb_dequeue(frames))) {
              /*
               * all the other fields are valid across frames
               * that belong to an aMPDU since they are on the
               * same TID from the same station
               */
              rx->skb = skb;
              CALL_RXH(ieee80211_rx_h_decrypt)
              CALL_RXH(ieee80211_rx_h_check_more_data)
              CALL_RXH(ieee80211_rx_h_uapsd_and_pspoll)
              CALL_RXH(ieee80211_rx_h_sta_process)
              CALL_RXH(ieee80211_rx_h_defragment)
              CALL_RXH(ieee80211_rx_h_michael_mic_verify)
              /* must be after MMIC verify so header is counted in MPDU mic */
#ifdef CPTCFG_MAC80211_MESH
              if (ieee80211_vif_is_mesh(&rx->sdata->vif))
                     CALL_RXH(ieee80211_rx_h_mesh_fwding);
#endif
              CALL_RXH(ieee80211_rx_h_amsdu)
              CALL_RXH(ieee80211_rx_h_data)
              /* special treatment -- needs the queue */
              res = ieee80211_rx_h_ctrl(rx, frames);
              if (res != RX_CONTINUE)
                     goto rxh_next;
              CALL_RXH(ieee80211_rx_h_mgmt_check)
              CALL_RXH(ieee80211_rx_h_action)
              CALL_RXH(ieee80211_rx_h_userspace_mgmt)
              CALL_RXH(ieee80211_rx_h_action_return)
              CALL_RXH(ieee80211_rx_h_mgmt)
 rxh_next:
              ieee80211_rx_handlers_result(rx, res);
#undef CALL_RXH
       }
       spin_unlock_bh(&rx->local->rx_path_lock);
}

7. ieee80211_rx_h_data()

只看是数据帧的情况，会继续调用ieee80211_rx_h_data()（位于OpenWRT内核文件夹子目录/net/mac80211，文件rx.c中）。

static ieee80211_rx_result debug_noinline ieee80211_rx_h_data(struct ieee80211_rx_data *rx)
{
…
       rx->skb->dev = dev;
       dev->stats.rx_packets++;
       dev->stats.rx_bytes += rx->skb->len;
       if (local->ps_sdata && local->hw.conf.dynamic_ps_timeout > 0 &&
           !is_multicast_ether_addr(
                  ((struct ethhdr *)rx->skb->data)->h_dest) &&
           (!local->scanning &&
            !test_bit(SDATA_STATE_OFFCHANNEL, &sdata->state))) {
                     mod_timer(&local->dynamic_ps_timer, jiffies +
                      msecs_to_jiffies(local->hw.conf.dynamic_ps_timeout));
       }
       ieee80211_deliver_skb(rx);
       return RX_QUEUED;
}

8. ieee80211_deliver_skb()

/*
 * requires that rx->skb is a frame with ethernet header
 */
static void
ieee80211_deliver_skb(struct ieee80211_rx_data *rx)
{
       …
       skb = rx->skb;
       …
       if (skb) {
              int align __maybe_unused;
#ifndef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
              /*
               * 'align' will only take the values 0 or 2 here
               * since all frames are required to be aligned
               * to 2-byte boundaries when being passed to
               * mac80211; the code here works just as well if
               * that isn't true, but mac80211 assumes it can
               * access fields as 2-byte aligned (e.g. for
               * compare_ether_addr)
               */
              align = ((unsigned long)(skb->data + sizeof(struct ethhdr))) & 3;
              if (align) {
                     if (WARN_ON(skb_headroom(skb) < 3)) {
                            dev_kfree_skb(skb);
                            skb = NULL;
                     } else {
                            u8 *data = skb->data;
                            size_t len = skb_headlen(skb);
                            skb->data -= align;
                            memmove(skb->data, data, len);
                            skb_set_tail_pointer(skb, len);
                     }
              }
#endif
              if (skb) {
                     /* deliver to local stack */
                     skb->protocol = eth_type_trans(skb, dev);
                     memset(skb->cb, 0, sizeof(skb->cb));
                     netif_receive_skb(skb);
              }
       }
       …
}

调用ieee80211_deliver_skb()（位于OpenWRT内核文件夹子目录/net/mac80211，文件rx.c中）。

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/*
 * requires that rx->skb is a frame with ethernet header
 */
static void
ieee80211_deliver_skb(struct ieee80211_rx_data *rx)
{
       …
       skb = rx->skb;
       …
       if (skb) {
              int align __maybe_unused;
#ifndef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
              /*
               * 'align' will only take the values 0 or 2 here
               * since all frames are required to be aligned
               * to 2-byte boundaries when being passed to
               * mac80211; the code here works just as well if
               * that isn't true, but mac80211 assumes it can
               * access fields as 2-byte aligned (e.g. for
               * compare_ether_addr)
               */
              align = ((unsigned long)(skb->data + sizeof(struct ethhdr))) & 3;
              if (align) {
                     if (WARN_ON(skb_headroom(skb) < 3)) {
                            dev_kfree_skb(skb);
                            skb = NULL;
                     } else {
                            u8 *data = skb->data;
                            size_t len = skb_headlen(skb);
                            skb->data -= align;
                            memmove(skb->data, data, len);
                            skb_set_tail_pointer(skb, len);
                     }
              }
#endif
              if (skb) {
                     /* deliver to local stack */
                     skb->protocol = eth_type_trans(skb, dev);
                     memset(skb->cb, 0, sizeof(skb->cb));
                     netif_receive_skb(skb);
              }
       }
       …
}

这里最核心的代码就是netif_receive_skb(skb)了，至此，数据已经接收到并发送至内核的网络子系统去处理。netif_receive_skb定义于内核文件夹linux-3.3.8的子目录/net/core的文件dev.c中。