分类： 嵌入式
2017-07-20 15:38:41
原文地址：OpenWRT数据接收过程 作者：lwchsz
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:
...
}
}
}
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）。
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);
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);
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）。
/*
* 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）。
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);
…
}
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);
}
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;
}
/*
* 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）。
/*
* 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。