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Linux Wi-Fi open source drivers -mac80211, ath9k/ath5k- Daniel Camps Mur

1. General Driver Overview hostapd wpa_supplicant

1. Transmission Path: kernelmac80211ath9k Kernel hands the packet to the virtual interface ieee80211_subif_start_xmit (tx.c): -Adds the 802.11 header. -Initializes transmission time in devtrans_start ieee80211_xmit (tx.c): -Makes headroom for encryption struct ieee80211_local: -This struct contains information about the real hardware, and is created when the interface is first added (ieee80211_register_hw() in main.c) - This data structure is created from the structure ieee80211_hw in mac80211.h -This data structure probably allows the hardware driver to register with mac80211 the proper function to be called by drv_tx(). - The driver and mac80211 communicate through this structure. ieee80211_set_qos_header (wme.c): -Sets TID in Wi-Fi header according to skbpriority ieee80211_tx (tx.c): -Obtains the proper transmission queue. -Prepares transmission -If the packet can not be transmitted, it is queued  timestamp queue entry. -Queues are maintained per sub-interface, in the structure qdata[4] in ieee80211_sub_if_data -Calls drv_tx() to pass the frame to the actual driver for transmission drv_tx() (driver-ops.h): -This is the entry point to the actual driver! -It is called for each packet to be transmitted. -Receives struct ieee80211_local -Receives the sk_buff (the data) skb_get_queue_mapping (?.c): ieee80211_tx_prepare (tx.c): invoke_tx_handlers(tx.c): -In an AP checks if the data has to be to be buffered -Many other stuff … struct qdata(ieee80211_i.h): -enqueued -max_enqueued -ewma tserv_ns_avg (can this be useful?)

1a. Transmission Path: ath9khardware drv_tx() in mac80211 ath9k_tx() (main.c): -Loads details of the hardware in struct ath_softc. -Updates the PM bit (not set by mac80211). -Wakes up hardware if it was sleeping. -Uses data struct ath_tx_control to track the status of the transmission -Starts transmission in ath_tx_start() ath_tx_start() (xmit.c): -Receives a pointer to an struct ath_tx_control containing a pointer to the queue that contains the frame? -If the destination is HT capable tries aggregation. ath_tx_send_ampdu() (xmit.c): -The ath9k driver registers the functions to interface with mac80211 in struct ieee80211_ops ath9k_ops. The input of this struct is then loaded in mac80211 using the function ieee80211_alloc_hw() in main.c of mac80211. - The function interfacing with drv_tx() is ath9k_tx() -The common data also contains other info like the number of hw queues ath9k_ps_wakeup() (x.c): struct ath_softc (ath9k.h): struct ieee80211_hw *hw; struct device *dev; bool ps_enabled; struct ath_config config; struct ath_rx rx; struct ath_tx tx; struct ath_beacon beacon; struct ath_tx (ath9k.h): struct ath_txq txq[ATH9K_NUM_TX_QUEUES]; struct ath_txq *txq_map[WME_NUM_AC]; -Contains the hw transmission queues. Ath9k has 10 hw tx queues. -Points to the mac80211 tx queues struct ath_txq(ath9k.h): -Contains the equivalent mac80211 queue -Contains the hw queue number -Actual queue. ath_tx_send_normal() (xmit.c): -Receives as argument a txq -Selects the data rate -Adds the data to be transmitted in the txq ath_tx_txqaddbuf ath_tx_txqaddbuf() (xmit.c): -Receives as parameter a txq, then inserts the frame into the outbound list, and sends it out to the hardware. The hardware physical queues seem to support only 8 frames! -If the packet can not be inserted in the physical queue then it is held here. ath9k_hw_txstart() (mac.c): -Writes the proper register in the hardware to trigger the transmission in a given queue. -The hardware is the one performing the CSMA-CA according to the configured parameters on that queue.

1b. Transmission Path: ath5khardware drv_tx() in mac80211 -The ath5k driver registers the functions to interface with mac80211 in struct ieee80211_ops ath5k_hw_ops in mac80211-ops.c. The input of this struct is then loaded in mac80211 using the function ieee80211_alloc_hw() in main.c of mac80211. - The function interfacing with drv_tx() is ath5k_tx() in mac80211-ops.c ath5k_tx() in mac80211-ops.c -Receives the skb to be transmitted. -Finds the appropriate hw queue for that skb ath5k_tx_queue() in base.c -If the queue is already at its maximum size (max is 50 pkts) it sends a signal back to mac80211 so that this stops his queues (ieee80211_stop_queue). It also may drop the packet -Copies the data to be transmitted in the txqueues. struct ath5k_buf represents a single queued frame (buffer). ath5k_tasklet_tx() in base.c -Tasklet that is called when the following interrupts happen: AR5K_INT_TXOK, AR5K_INT_TXDESC, AR5K_INT_TXERR, AR5K_INT_TXEOL ath5k_tx_processq() in base.c ieee80211_stop_queue() in ? ath5k_tx_frame_completed() in base.c -Check a timestamp here ? ath5k_txbuf_setup() in base.c -Obtains data rate for this transmission -Prepare for transmission ah_setup_tx_desc() in desc.c ath5k_hw_setup_mrr_tx_desc() in desc.c Ieee80211_tx_status() iin mac80211 ath5k_hw_start_tx_dma() in dma.c -Starts a transmission in the hw. -My understanding is that the hw may already be trying to transmit other frames that were previously submitted. AR5K_REG_WRITE_Q() in dma.c -Writes a certain register in the hw, so that the transmission on a certain queue starts

1a. Reception Path: hardwareath9k When receiving a packet, and also for other reasons, the hardware sends an interruption that ath9k has previously registered. The function in charge of handling the interruption seems to be irqreturn_t ath_isr in main.c. This function discovers the type of interruption and asks the kernel to schedule the execution of a tasklet, ath9k_tasklet in main.c. This function in turn calls the the receive tasklet ath_rx_tasklet() struct ieee80211_hdr (include/linux/ieee80211.h) struct ieee80211_rx_status (mac80211.h) u64 mactime : value in microseconds of the 64-bit Time Synchronization Function (TSF) timer when the first data symbol (MPDU) arrived at the hardware. enum ieee80211_band band; int rate_idx; unsigned int rx_flags; … ath_rx_tasklet (recv.c) -Obtains the frame header -Obtains the current tsf value -Records info about received packet in struct ieee80211_rx_status -Insert received data in the receive buffer -Creates a new skb to contain the received data -Passes the skb to ieee80211_rx() -Can we compute the airtime duration of the received frame in this function, as “airtime = tsf - rxs->mactime” ? ath9k_rx_skb_preprocess (recv.c) -Checks if the received data has CRC errors and in that case drops it. However, crypto errors are still passed up to mac80211 -Populates ath_rx_status ath9k_rx_skb_postprocess (recv.c) -Remove padding from the received header struct ath_rx_status (mac.h) u32 rs_tstamp; This is given by the hardware and eventually carried on to mactime in ieee80211_rx_status u16 rs_datalen; u8 rs_status; u8 rs_phyerr; int8_t rs_rssi; u8 rs_keyix; u8 rs_rate; u8 rs_antenna; … ieee80211_rx() (mac80211 rx.c) -This is the entry point to mac80211

1b. Reception Path: hardwareath5k ath5k_tasklet_rx in base.c -Receives the interruption that a new frame has been received ath5k_receive_frame_ok in base.c ath5k_receive_frame in base.c - rxs->mactime contains the time the first bit was received in the air ieee80211_rx() (mac80211 rx.c) -This is the entry point to mac80211

1. Reception Path: mac80211kernel ieee80211_rx() (mac80211 rx.c) -Removes the radiotap header -Parses QoS from header. -Decides if this packet is addressed to this interface or to another STA in the BSS -Calls ieee80211_prepare_and_rx_handle() ieee80211_prepare_and_rx_handle() -Receives as input an ieee80211_rx_data -Calls the rx handlers ieee80211_rx_handlers(rx.c) -Among other calls the following functions: ieee80211_rx_h_decrypt ieee80211_rx_h_check_more_data ieee80211_rx_h_sta_process ieee80211_rx_h_data ieee80211_rx_h_ctrl … ieee80211_parse_qos() (x.c): struct ieee80211_rx_data (ieee80211_i.h) struct sk_buff *skb; struct ieee80211_local *local; struct ieee80211_sub_if_data *sdata; struct sta_info *sta; struct ieee80211_key *key; int queue …. ieee80211_rx_h_sta_process() (rx.c): -Updates the sta_info struct that contains information about this station ieee80211_rx_h_data() (rx.c): -Removes the 802.11 header and passes up the 802.3 frame struct sta_info (sta_info.h): -Info about this station, like: -TX and RX statistics -PS buffers if we are an AP struct ieee80211_sta sta; ieee80211_deliver_skb() (rx.c): -Receives a frame with an Ethernet header -Decides if the frame has to go up to the stack, or must be reflected back to the wireless medium (if we are an AP). netif_receive_skb() (x.c): -Delivers the skb to the local stack (kernel) dev_queue_xmit() (x.c): -Resends the skb to the wireless medium

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