曾经的足迹——对CAN驱动中的NAPI机制的理解

本文主要是介绍曾经的足迹——对CAN驱动中的NAPI机制的理解，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

NAPI 是 Linux 上采用的一种提高网络处理效率的技术，它的核心概念就是不采用中断的方式读取数据，而代之以首先采用中断唤醒数据接收的服务程序，然后通过poll的方法来轮询数据。采用NAPI技术可以大大改善短长度数据包接收的效率，减少中断触发的时间。

可以这样理解，在NAPI机制没有出现时，由于网络设备接收数据是采用中断方式的，假设每次数据包很小，小到只有几个字节。而刚好在1秒内有5000个这样的数据包。那么系统就是在1秒内产生5000个中断，这无疑给会占据CPU的大部分资源。NAPI的出现就是要解决这样的问题。

概念的东西就不多提了。下面开始源码之旅啦！

首先在分配CAN设备时，采用netif_napi_add函数注册轮询函数d_can_poll()。

struct net_device *alloc_d_can_dev(int num_objs)

{

struct net_device *dev;

struct d_can_priv *priv;

dev = alloc_candev(sizeof(struct d_can_priv), num_objs/2);

if (!dev)

return NULL;

priv = netdev_priv(dev);

netif_napi_add(dev, &priv->napi, d_can_poll, num_objs/2);

priv->dev = dev;

priv->can.bittiming_const = &d_can_bittiming_const;

priv->can.do_set_mode = d_can_set_mode;

priv->can.do_get_berr_counter = d_can_get_berr_counter;

priv->can.ctrlmode_supported = (CAN_CTRLMODE_LOOPBACK |

CAN_CTRLMODE_LISTENONLY |

CAN_CTRLMODE_BERR_REPORTING |

CAN_CTRLMODE_3_SAMPLES);

return dev;

}

netif_napi_add()函数原型如下：

void netif_napi_add(struct net_device *dev, struct napi_struct *napi,

int (*poll)(struct napi_struct *, int), int weight)

{

INIT_LIST_HEAD(&napi->poll_list);

napi->gro_count = 0;

napi->gro_list = NULL;

napi->skb = NULL;

napi->poll = poll;

napi->weight = weight;

list_add(&napi->dev_list, &dev->napi_list);

napi->dev = dev;

#ifdef CONFIG_NETPOLL

spin_lock_init(&napi->poll_lock);

napi->poll_owner = -1;

#endif

set_bit(NAPI_STATE_SCHED, &napi->state);

}

轮询函数d_can_poll()如下：

static int d_can_poll(struct napi_struct *napi, int quota)

{

int lec_type = 0;

int work_done = 0;

struct net_device *dev = napi->dev;

struct d_can_priv *priv = netdev_priv(dev);

if (!priv->irqstatus)

goto end;

/* status events have the highest priority */

if (priv->irqstatus == STATUS_INTERRUPT) {

priv->current_status = d_can_read(priv, D_CAN_ES);

/* handle Tx/Rx events */

if (priv->current_status & D_CAN_ES_TXOK)

d_can_write(priv, D_CAN_ES,

priv->current_status & ~D_CAN_ES_TXOK);

if (priv->current_status & D_CAN_ES_RXOK)

d_can_write(priv, D_CAN_ES,

priv->current_status & ~D_CAN_ES_RXOK);

/* handle state changes */

if ((priv->current_status & D_CAN_ES_EWARN) &&

(!(priv->last_status & D_CAN_ES_EWARN))) {

netdev_dbg(dev, "entered error warning state\n");

work_done += d_can_handle_state_change(dev,

D_CAN_ERROR_WARNING);

}

if ((priv->current_status & D_CAN_ES_EPASS) &&

(!(priv->last_status & D_CAN_ES_EPASS))) {

netdev_dbg(dev, "entered error passive state\n");

work_done += d_can_handle_state_change(dev,

D_CAN_ERROR_PASSIVE);

}

if ((priv->current_status & D_CAN_ES_BOFF) &&

(!(priv->last_status & D_CAN_ES_BOFF))) {

netdev_dbg(dev, "entered bus off state\n");

work_done += d_can_handle_state_change(dev,

D_CAN_BUS_OFF);

}

/* handle bus recovery events */

if ((!(priv->current_status & D_CAN_ES_BOFF)) &&

(priv->last_status & D_CAN_ES_BOFF)) {

netdev_dbg(dev, "left bus off state\n");

priv->can.state = CAN_STATE_ERROR_ACTIVE;

}

if ((!(priv->current_status & D_CAN_ES_EPASS)) &&

(priv->last_status & D_CAN_ES_EPASS)) {

netdev_dbg(dev, "left error passive state\n");

priv->can.state = CAN_STATE_ERROR_ACTIVE;

}

priv->last_status = priv->current_status;

/* handle lec errors on the bus */

lec_type = d_can_has_handle_berr(priv);

if (lec_type)

work_done += d_can_handle_bus_err(dev, lec_type);

} else if ((priv->irqstatus >= D_CAN_MSG_OBJ_RX_FIRST) &&

(priv->irqstatus <= D_CAN_MSG_OBJ_RX_LAST)) {

/* handle events corresponding to receive message objects */

work_done += d_can_do_rx_poll(dev, (quota - work_done));

} else if ((priv->irqstatus >= D_CAN_MSG_OBJ_TX_FIRST) &&

(priv->irqstatus <= D_CAN_MSG_OBJ_TX_LAST)) {

/* handle events corresponding to transmit message objects */

d_can_do_tx(dev);

}

end:

if (work_done < quota) {

napi_complete(napi);

/* enable all IRQs */

d_can_interrupts(priv, ENABLE_ALL_INTERRUPTS);

}

return work_done;

}

在中断处理函数中，先禁止接收中断，且告诉网络子系统，将以轮询方式快速收包，其中禁止接收中断完全由硬件功能决定，而告诉内核将以轮询方式处理包则是使用函数napi_schedule()。

static irqreturn_t d_can_isr(int irq, void *dev_id)

{

struct net_device *dev = (struct net_device *)dev_id;

struct d_can_priv *priv = netdev_priv(dev);

priv->irqstatus = d_can_read(priv, D_CAN_INT);

if (!priv->irqstatus)

return IRQ_NONE;

/* disable all interrupts and schedule the NAPI */

d_can_interrupts(priv, DISABLE_ALL_INTERRUPTS);

napi_schedule(&priv->napi);

return IRQ_HANDLED;

}

其中的napi_schedule_prep()函数是为了判定现在是否已经进入了轮询模式。

static inline void napi_schedule(struct napi_struct *n)

{

if (napi_schedule_prep(n))

__napi_schedule(n);

}

轮询功能的关闭则需要使用：

if (work_done < quota) {

napi_complete(napi);

/* enable all IRQs */

d_can_interrupts(priv, ENABLE_ALL_INTERRUPTS);

}

因为可能存在多个napi_struct的实例，要求每个实例能够独立的使能或者禁止。因此，需要驱动开发人员保证在网卡接口关闭时，禁止所有的napi_struct的实例。在d_can_open()函数中使用napi_enable()，在d_can_close()函数中使用napi_disable()。

napi_enable(&priv->napi);

napi_disable(&priv->napi);