数据结构

CAN数据接收链表dev_rcv_lists

CAN核心为每个CAN类型网络设备都关联一个该链表对象（表头为can_rx_dev_list），用于保存所有对该CAN网络设备数据感兴趣的接收者。

enum { RX_ERR, RX_ALL, RX_FIL, RX_INV, RX_EFF, RX_MAX };struct dev_rcv_lists {struct hlist_node list; // 将本对象放入can_rx_dev_list中struct rcu_head rcu;struct net_device *dev; // 关联网络设备struct hlist_head rx[RX_MAX]; // 两个接收者链表struct hlist_head rx_sff[0x800];int remove_on_zero_entries; // 链表清理标记int entries;
};

CAN数据接收者receiver

struct receiver {struct hlist_node list;struct rcu_head rcu;canid_t can_id; // filter信息canid_t mask;unsigned long matches; // 统计有多少数据包匹配了该接收者// skb接收函数&参数void (*func)(struct sk_buff *, void *);void *data;char *ident; // 标识接收者的字符串
};

CAN帧结构

/* special address description flags for the CAN_ID */
#define CAN_EFF_FLAG 0x80000000U/* EFF/SFF is set in the MSB */
#define CAN_RTR_FLAG 0x40000000U/* remote transmission request */
#define CAN_ERR_FLAG 0x20000000U/* error frame *//* valid bits in CAN ID for frame formats */
#define CAN_SFF_MASK 0x000007FFU/* standard frame format (SFF) */
#define CAN_EFF_MASK 0x1FFFFFFFU/* extended frame format (EFF) */
#define CAN_ERR_MASK 0x1FFFFFFFU/* omit EFF, RTR, ERR flags *//** Controller Area Network Identifier structure** bit 0-28    : CAN identifier (11/29 bit)* bit 29    : error frame flag (0 = data frame, 1 = error frame)* bit 30    : remote transmission request flag (1 = rtr frame)* bit 31  : frame format flag (0 = standard 11 bit, 1 = extended 29 bit)*/
// 低29bit对应CAN帧中的标识符字段，linux利用后3bit表示了其它帧信息
typedef __u32 canid_t;/*** struct can_frame - basic CAN frame structure* @can_id:  the CAN ID of the frame and CAN_*_FLAG flags, see above.* @can_dlc: the data length field of the CAN frame* @data:    the CAN frame payload.*/
struct can_frame {canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */__u8    can_dlc; /* data length code: 0 .. 8 */__u8    data[8] __attribute__((aligned(8)));
};

对于canid_t的使用主要是在接收过滤器设置和数据接收两个流程中，这里linux做的是比较混乱的，bit0~bit28没有疑问，对应的就是CAN帧中的标识符字段，但是高3bit的使用需要分情况梳理。

接收过滤器设置（can_id和mask两个参数）：

mask参数中这是bit29（CAN_ERR_FLAG）代表要监听错误帧；
can_id参数中设置bit29（CAN_INV_FILTER）代表匹配结果要取反；
mask的bit0~bit28都为0，表示不做过滤全匹配；

接收流程

接收者列表维护

CAN核心仅仅对接收到的CAN帧进行分发，分发过程就是围绕上面提到的can_rx_dev_list链表进行的。

在模块初始化时，CAN核心向网络设备管理层注册了notifier，对应的接收函数为can_notifier()。

/** af_can notifier to create/remove CAN netdevice specific structs*/
static int can_notifier(struct notifier_block *nb, unsigned long msg, void *data)
{struct net_device *dev = (struct net_device *)data;struct dev_rcv_lists *d;// 只处理CAN类型的网络设备事件if (!net_eq(dev_net(dev), &init_net))return NOTIFY_DONE;if (dev->type != ARPHRD_CAN)return NOTIFY_DONE;// 只处理网络设备注册和去注册事件switch (msg) {case NETDEV_REGISTER:// 为网络设备分配一个dev_rcv_lists对象，并将该链表加入接收列表can_rx_dev_list中/** create new dev_rcv_lists for this device** N.B. zeroing the struct is the correct initialization* for the embedded hlist_head structs.* Another list type, e.g. list_head, would require* explicit initialization.*/d = kzalloc(sizeof(*d), GFP_KERNEL);if (!d) {printk(KERN_ERR"can: allocation of receive list failed\n");return NOTIFY_DONE;}d->dev = dev; // 链表和网络设备进行了关联spin_lock(&can_rcvlists_lock);hlist_add_head_rcu(&d->list, &can_rx_dev_list);spin_unlock(&can_rcvlists_lock);break;case NETDEV_UNREGISTER:// 网络设备去注册时将其对应的dev_rcv_lists对象从can_rx_dev_list链表中移除spin_lock(&can_rcvlists_lock);d = find_dev_rcv_lists(dev);if (d) {if (d->entries) {d->remove_on_zero_entries = 1;d = NULL;} elsehlist_del_rcu(&d->list);} elseprintk(KERN_ERR "can: notifier: receive list not ""found for dev %s\n", dev->name);spin_unlock(&can_rcvlists_lock);if (d)call_rcu(&d->rcu, can_rx_delete_device);break;}return NOTIFY_DONE;
}

CAN核心为每个CAN类型的网络设备都维护了一个dev_rcv_lists链表对象。有了链表对象，需要向链表对象中注册接收者才能完成数据包的分发，注册接口为can_rx_register()，当然对应的也有can_rx_unregister()接口。

/*** can_rx_register - subscribe CAN frames from a specific interface* @dev: pointer to netdevice (NULL => subcribe from 'all' CAN devices list)* @can_id: CAN identifier (see description)* @mask: CAN mask (see description)* @func: callback function on filter match* @data: returned parameter for callback function* @ident: string for calling module indentification** Description:*  Invokes the callback function with the received sk_buff and the given*  parameter 'data' on a matching receive filter. A filter matches, when**          <received_can_id> & mask == can_id & mask**  The filter can be inverted (CAN_INV_FILTER bit set in can_id) or it can*  filter for error frames (CAN_ERR_FLAG bit set in mask).**  The provided pointer to the sk_buff is guaranteed to be valid as long as*  the callback function is running. The callback function must *not* free*  the given sk_buff while processing it's task. When the given sk_buff is*  needed after the end of the callback function it must be cloned inside*  the callback function with skb_clone().** Return:*  0 on success*  -ENOMEM on missing cache mem to create subscription entry*  -ENODEV unknown device*/
int can_rx_register(struct net_device *dev, canid_t can_id, canid_t mask,void (*func)(struct sk_buff *, void *), void *data, char *ident)
{struct receiver *r;struct hlist_head *rl;struct dev_rcv_lists *d;int err = 0;/* insert new receiver  (dev,canid,mask) -> (func,data) */// 分配receiver对象r = kmem_cache_alloc(rcv_cache, GFP_KERNEL);if (!r)return -ENOMEM;spin_lock(&can_rcvlists_lock);// 找到网络设备对应的dev_rcv_lists对象d = find_dev_rcv_lists(dev);if (d) {// 根据参数决定将接收者放入哪个接收者链表rl = find_rcv_list(&can_id, &mask, d);r->can_id  = can_id;r->mask    = mask;r->matches = 0;r->func    = func;r->data    = data;r->ident   = ident;// 将接收者加入链表hlist_add_head_rcu(&r->list, rl);d->entries++;can_pstats.rcv_entries++;if (can_pstats.rcv_entries_max < can_pstats.rcv_entries)can_pstats.rcv_entries_max = can_pstats.rcv_entries;} else {kmem_cache_free(rcv_cache, r);err = -ENODEV;}spin_unlock(&can_rcvlists_lock);return err;
}

find_rcv_list()

/*** find_rcv_list - determine optimal filterlist inside device filter struct* @can_id: pointer to CAN identifier of a given can_filter* @mask: pointer to CAN mask of a given can_filter* @d: pointer to the device filter struct** Description:*  Returns the optimal filterlist to reduce the filter handling in the*  receive path. This function is called by service functions that need*  to register or unregister a can_filter in the filter lists.**  A filter matches in general, when**          <received_can_id> & mask == can_id & mask**  so every bit set in the mask (even CAN_EFF_FLAG, CAN_RTR_FLAG) describe*  relevant bits for the filter.**  The filter can be inverted (CAN_INV_FILTER bit set in can_id) or it can*  filter for error frames (CAN_ERR_FLAG bit set in mask). For error frames*  there is a special filterlist and a special rx path filter handling.** Return:*  Pointer to optimal filterlist for the given can_id/mask pair.*  Constistency checked mask.*  Reduced can_id to have a preprocessed filter compare value.*/
static struct hlist_head *find_rcv_list(canid_t *can_id, canid_t *mask,struct dev_rcv_lists *d)
{canid_t inv = *can_id & CAN_INV_FILTER; /* save flag before masking */// 调用者通过在mask中设置CAN_ERR_FLAG可以实现对错误帧的监听if (*mask & CAN_ERR_FLAG) {/* clear CAN_ERR_FLAG in filter entry */*mask &= CAN_ERR_MASK; // 最终在receiver->mask中保存的又去掉了CAN_ERR_FLAGreturn &d->rx[RX_ERR];}/* with cleared CAN_ERR_FLAG we have a simple mask/value filterpair */#define CAN_EFF_RTR_FLAGS (CAN_EFF_FLAG | CAN_RTR_FLAG)// mask指定要接收扩展帧，但是can_id却没有指定要接收扩展帧，那么mask非法，将其修正为接收标准帧/* ensure valid values in can_mask for 'SFF only' frame filtering */if ((*mask & CAN_EFF_FLAG) && !(*can_id & CAN_EFF_FLAG))*mask &= (CAN_SFF_MASK | CAN_EFF_RTR_FLAGS);// can_id保存的已经是和mask相与的结果了，这样在接收路径匹配时可以减少计算*can_id &= *mask;// 将监听者放入取反匹配列表if (inv)return &d->rx[RX_INV];// mask的bit0~bit28都为0，那么放入全匹配列表if (!(*mask))return &d->rx[RX_ALL];/* extra filterlists for the subscription of a single non-RTR can_id */if (((*mask & CAN_EFF_RTR_FLAGS) == CAN_EFF_RTR_FLAGS) && !(*can_id & CAN_RTR_FLAG)) {if (*can_id & CAN_EFF_FLAG) {if (*mask == (CAN_EFF_MASK | CAN_EFF_RTR_FLAGS)) {/* RFC: a future use-case for hash-tables? */return &d->rx[RX_EFF];}} else {if (*mask == (CAN_SFF_MASK | CAN_EFF_RTR_FLAGS))return &d->rx_sff[*can_id];}}/* default: filter via can_id/can_mask */return &d->rx[RX_FIL];
}

CAN核心收到数据can_rcv()

在模块初始化时，CAN核心向网络设备接口层注册了数据包接收接口can_rcv()。can_rcv()就是根据网络设备找到其对应的dev_rcv_lists链表，然后根据匹配规则，将skb分发给接收者。

static struct packet_type can_packet __read_mostly = {.type = __constant_htons(ETH_P_CAN),.dev  = NULL,.func = can_rcv,
};static int can_rcv(struct sk_buff *skb, struct net_device *dev,struct packet_type *pt, struct net_device *orig_dev)
{struct dev_rcv_lists *d;struct can_frame *cf = (struct can_frame *)skb->data;int matches;if (dev->type != ARPHRD_CAN || !net_eq(dev_net(dev), &init_net)) {kfree_skb(skb);return 0;}BUG_ON(skb->len != sizeof(struct can_frame) || cf->can_dlc > 8);/* update statistics */can_stats.rx_frames++;can_stats.rx_frames_delta++;rcu_read_lock();/* deliver the packet to sockets listening on all devices */matches = can_rcv_filter(&can_rx_alldev_list, skb);// 找到网络设备对象的dev_rcv_lists链表d = find_dev_rcv_lists(dev);if (d)matches += can_rcv_filter(d, skb);rcu_read_unlock();/* free the skbuff allocated by the netdevice driver */kfree_skb(skb);if (matches > 0) {can_stats.matches++;can_stats.matches_delta++;}return 0;
}

CAN核心分发接收数据: can_rcv_filter()

// 将数据包分发给接收者
static inline void deliver(struct sk_buff *skb, struct receiver *r)
{r->func(skb, r->data);r->matches++;
}// 根据过滤规则分发数据
static int can_rcv_filter(struct dev_rcv_lists *d, struct sk_buff *skb)
{struct receiver *r;struct hlist_node *n;int matches = 0;struct can_frame *cf = (struct can_frame *)skb->data;canid_t can_id = cf->can_id;if (d->entries == 0)return 0;// 接收到1个错误帧，分发给对错误帧感兴趣的接收者if (can_id & CAN_ERR_FLAG) {/* check for error frame entries only */hlist_for_each_entry_rcu(r, n, &d->rx[RX_ERR], list) {if (can_id & r->mask) {deliver(skb, r);matches++;}}return matches;}// 将帧分发给未设置filter的接收者hlist_for_each_entry_rcu(r, n, &d->rx[RX_ALL], list) {deliver(skb, r);matches++;}// <received_can_id> & mask == can_id & maskhlist_for_each_entry_rcu(r, n, &d->rx[RX_FIL], list) {if ((can_id & r->mask) == r->can_id) {deliver(skb, r);matches++;}}// 取反过滤器: <received_can_id> & mask != can_id & maskhlist_for_each_entry_rcu(r, n, &d->rx[RX_INV], list) {if ((can_id & r->mask) != r->can_id) {deliver(skb, r);matches++;}}/* check filterlists for single non-RTR can_ids */if (can_id & CAN_RTR_FLAG)return matches;// 扩展帧和标准帧if (can_id & CAN_EFF_FLAG) {hlist_for_each_entry_rcu(r, n, &d->rx[RX_EFF], list) {if (r->can_id == can_id) {deliver(skb, r);matches++;}}} else {can_id &= CAN_SFF_MASK;hlist_for_each_entry_rcu(r, n, &d->rx_sff[can_id], list) {deliver(skb, r);matches++;}}return matches;
}

至此数据就交给了BCM等传输协议了。

发送流程

上层协议组织好CAN帧后，调用CAN核心的can_send()将数据包发送出去。

/*** can_send - transmit a CAN frame (optional with local loopback)* @skb: pointer to socket buffer with CAN frame in data section* @loop: loopback for listeners on local CAN sockets (recommended default!)** Return:*  0 on success*  -ENETDOWN when the selected interface is down*  -ENOBUFS on full driver queue (see net_xmit_errno())*  -ENOMEM when local loopback failed at calling skb_clone()*  -EPERM when trying to send on a non-CAN interface*  -EINVAL when the skb->data does not contain a valid CAN frame*/
int can_send(struct sk_buff *skb, int loop)
{struct sk_buff *newskb = NULL;struct can_frame *cf = (struct can_frame *)skb->data;int err;// 校验CAN帧长度if (skb->len != sizeof(struct can_frame) || cf->can_dlc > 8) {kfree_skb(skb);return -EINVAL;}// 校验出口网络设备必须是CAN类型网络设备if (skb->dev->type != ARPHRD_CAN) {kfree_skb(skb);return -EPERM;}// 网络设备已UPif (!(skb->dev->flags & IFF_UP)) {kfree_skb(skb);return -ENETDOWN;}// 设置数据包为CAN类型skb->protocol = htons(ETH_P_CAN);skb_reset_network_header(skb);skb_reset_transport_header(skb);if (loop) { // 发送者指示该数据包需要环回/* indication for the CAN driver: do loopback */skb->pkt_type = PACKET_LOOPBACK;/** The reference to the originating sock may be required* by the receiving socket to check whether the frame is* its own. Example: can_raw sockopt CAN_RAW_RECV_OWN_MSGS* Therefore we have to ensure that skb->sk remains the* reference to the originating sock by restoring skb->sk* after each skb_clone() or skb_orphan() usage.*/if (!(skb->dev->flags & IFF_ECHO)) {// 驱动不支持环回，这里软件来模拟：将skb复制一份，直接走netif_rx()接收流程newskb = skb_clone(skb, GFP_ATOMIC);if (!newskb) {kfree_skb(skb);return -ENOMEM;}newskb->sk = skb->sk;newskb->ip_summed = CHECKSUM_UNNECESSARY;newskb->pkt_type = PACKET_BROADCAST;}} else {/* indication for the CAN driver: no loopback required */skb->pkt_type = PACKET_HOST;}// 发送数据包给网络设备接口层err = dev_queue_xmit(skb);if (err > 0)err = net_xmit_errno(err);if (err) {if (newskb)kfree_skb(newskb);return err;}if (newskb)netif_rx(newskb);/* update statistics */can_stats.tx_frames++;can_stats.tx_frames_delta++;return 0;
}

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CAN核心帧收发流程

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