linux网络流程分析（一）---网卡驱动

分析linux网络的书已经很多了，包括《追踪Linux TCP/IP代码运行》《Linux内核源码剖析——TCP/IP实现》，这里我只是从数据包在linux内核中的基本流程来分析，尽可能的展现一个主流程框架。

内核如何从网卡接收数据,传统的过程：

1.数据到达网卡；

2.网卡产生一个中断给内核；

3.内核使用I/O指令，从网卡I/O区域中去读取数据；

我们在许多网卡驱动中(很老那些)，都可以在网卡的中断函数中见到这一过程。   但是，这一种方法，有一种重要的问题，就是大流量的数据来到，网卡会产生大量的中断，内核在中断上下文 中，会浪费大量的资源来处理中断本身。所以，就有一个问题，“可不可以不使用中断”，这就是轮询技术，所谓NAPI技术，说来也不神秘，就是说，内核屏蔽 中断，然后隔一会儿就去问网卡，“你有没有数据啊？”……   从这个描述本身可以看到，如果数据量少，轮询同样占用大量的不必要的CPU资源，大家各有所长吧   OK，另一个问题，就是从网卡的I/O区域，包括I/O寄存器或I/O内存中去读取数据，这都要CPU 去读，也要占用CPU资源，“CPU从I/O区域读，然后把它放到内存（这个内存指的是系统本身的物理内存，跟外设的内存不相干，也叫主内存）中”。于是 自然地，就想到了DMA技术——让网卡直接从主内存之间读写它们的I/O数据，CPU，这儿不干你事，自己找乐子去：

1.首先，内核在主内存中为收发数据建立一个环形的缓冲队列（通常叫DMA环形缓冲区）。

2.内核将这个缓冲区通过DMA映射，把这个队列交给网卡；

3.网卡收到数据，就直接放进这个环形缓冲区了——也就是直接放进主内存了；然后，向系统产生一个中断；

4.内核收到这个中断，就取消DMA映射，这样，内核就直接从主内存中读取数据；   ——呵呵，这一个过程比传统的过程少了不少工作，因为设备直接把数据放进了主内存，不需要CPU的干预，效率是不是提高不少？   对应以上4步，来看它的具体实现：

1)分配环形DMA缓冲区

Linux内核中，用skb来描述一个缓存，所谓分配，就是建立一定数量的skb，然后用e1000_rx_ring 环形缓冲区队列描述符连接起来 2)建立DMA映射

内核通过调用

dma_map_single(struct device *dev,void *buffer,size_t size,enum dma_data_direction direction)

建立映射关系。

truct device *dev 描述一个设备；

uffer：把哪个地址映射给设备；也就是某一个skb——要映射全部，当然是做一个双向链表的循环即可；

ize：缓存大小；

direction：映射方向——谁传给谁：一般来说，是“双向”映射，数据在设备和内存之间双向流动；

对于PCI设备而言（网卡一般是PCI的），通过另一个包裹函数pci_map_single，这样，就把buffer交给设备了！设备可以直接从里边读/取数据。 3)这一步由硬件完成； 4)取消映射

dma_unmap_single，对PCI而言，大多调用它的包裹函数pci_unmap_single，不取消的话，缓存控制权还在设备手里，要调用 它，把主动权掌握在CPU手里——因为我们已经接收到数据了，应该由CPU把数据交给上层网络栈；当然，不取消之前，通常要读一些状态位信息，诸如此类， 一般是调用dma_sync_single_for_cpu()让CPU在取消映射前，就可以访问DMA缓冲区中的内容

首先，数据包从网卡光电信号来之后，先经过网卡驱动，转换成skb，进入链路层，那么我首先就先分析一下网卡驱动的流程。

源码位置：Driver/net/E1000e文件夹下面。

static int __init e1000_init_module(void)
{注册网卡驱动，按照PCI驱动开发方式来进行注册
int ret;
printk(KERN_INFO "%s: Intel(R) PRO/1000 Network Driver - %s\n",
       e1000e_driver_name, e1000e_driver_version);
printk(KERN_INFO "%s: Copyright (c) 1999-2008 Intel Corporation.\n",
       e1000e_driver_name);
ret = pci_register_driver(&e1000_driver);
pm_qos_add_requirement(PM_QOS_CPU_DMA_LATENCY, e1000e_driver_name,
       PM_QOS_DEFAULT_VALUE);

return ret;
}

然后看一下驱动结构体内容，这里不对PCI类型驱动开发做介绍了。

/* PCI Device API Driver */
static struct pci_driver e1000_driver = {
.name     = e1000e_driver_name,
.id_table = e1000_pci_tbl,
.probe    = e1000_probe,
.remove   = __devexit_p(e1000_remove),
#ifdef CONFIG_PM
/* Power Management Hooks */
.suspend  = e1000_suspend,
.resume   = e1000_resume,
#endif
.shutdown = e1000_shutdown,
.err_handler = &e1000_err_handler
};

这里面最重要的函数是e1000_probe，先看一下这个函数的作用是什么：“Device Initialization Routine”，这个应该不难理解。

static int __devinit e1000_probe(struct pci_dev *pdev,
 const struct pci_device_id *ent)
{
struct net_device *netdev;
struct e1000_adapter *adapter;
struct e1000_hw *hw;
const struct e1000_info *ei = e1000_info_tbl[ent->driver_data];
resource_size_t mmio_start, mmio_len;
resource_size_t flash_start, flash_len;

static int cards_found;
int i, err, pci_using_dac;
u16 eeprom_data = 0;
u16 eeprom_apme_mask = E1000_EEPROM_APME;

e1000e_disable_l1aspm(pdev);
从这里开始对设备驱动进行初始化，包括名称、内存之类的。
err = pci_enable_device_mem(pdev);
if (err)
return err;

pci_using_dac = 0;
err = pci_set_dma_mask(pdev, DMA_BIT_MASK(64));
if (!err) {
err = pci_set_consistent_dma_mask(pdev, DMA_BIT_MASK(64));
if (!err)
pci_using_dac = 1;
} else {
err = pci_set_dma_mask(pdev, DMA_BIT_MASK(32));
if (err) {
err = pci_set_consistent_dma_mask(pdev,
  DMA_BIT_MASK(32));
if (err) {
dev_err(&pdev->dev, "No usable DMA "
"configuration, aborting\n");
goto err_dma;
}
}
}

err = pci_request_selected_regions_exclusive(pdev,
                                  pci_select_bars(pdev, IORESOURCE_MEM),
                                  e1000e_driver_name);
if (err)
goto err_pci_reg;

/* AER (Advanced Error Reporting) hooks */
err = pci_enable_pcie_error_reporting(pdev);
if (err) {
dev_err(&pdev->dev, "pci_enable_pcie_error_reporting failed "
        "0x%x\n", err);
/* non-fatal, continue */
}

pci_set_master(pdev);
/* PCI config space info */
err = pci_save_state(pdev);
if (err)
goto err_alloc_etherdev;

err = -ENOMEM;
这里要为驱动分配一个容器之类的，因为驱动后面的一切操作都是在它的基础之上。
netdev = alloc_etherdev(sizeof(struct e1000_adapter));
if (!netdev)
goto err_alloc_etherdev;

SET_NETDEV_DEV(netdev, &pdev->dev);

pci_set_drvdata(pdev, netdev);
adapter = netdev_priv(netdev);
hw = &adapter->hw;
adapter->netdev = netdev;
adapter->pdev = pdev;
adapter->ei = ei;
adapter->pba = ei->pba;
adapter->flags = ei->flags;
adapter->flags2 = ei->flags2;
adapter->hw.adapter = adapter;
adapter->hw.mac.type = ei->mac;
adapter->max_hw_frame_size = ei->max_hw_frame_size;
adapter->msg_enable = (1 << NETIF_MSG_DRV | NETIF_MSG_PROBE) - 1;
0表示设备映射的内存的的bar
mmio_start = pci_resource_start(pdev, 0);
mmio_len = pci_resource_len(pdev, 0);

err = -EIO
这里我的理解是容器的硬件地址与bar进行映射，hw_addr代表的是网卡的硬件地址
adapter->hw.hw_addr = ioremap(mmio_start, mmio_len);
if (!adapter->hw.hw_addr)
goto err_ioremap;

if ((adapter->flags & FLAG_HAS_FLASH) &&
    (pci_resource_flags(pdev, 1) & IORESOURCE_MEM)) {
flash_start = pci_resource_start(pdev, 1);
flash_len = pci_resource_len(pdev, 1);
adapter->hw.flash_address = ioremap(flash_start, flash_len);
if (!adapter->hw.flash_address)
goto err_flashmap;
}

/* construct the net_device struct */
netdev->netdev_ops= &e1000e_netdev_ops;
e1000e_set_ethtool_ops(netdev);
netdev->watchdog_timeo= 5 * HZ;
netif_napi_add(netdev, &adapter->napi, e1000_clean, 64);
strncpy(netdev->name, pci_name(pdev), sizeof(netdev->name) - 1);

netdev->mem_start = mmio_start;
netdev->mem_end = mmio_start + mmio_len;

adapter->bd_number = cards_found++;

e1000e_check_options(adapter);

/* setup adapter struct */
err = e1000_sw_init(adapter);
if (err)
goto err_sw_init;

err = -EIO;

memcpy(&hw->mac.ops, ei->mac_ops, sizeof(hw->mac.ops));
memcpy(&hw->nvm.ops, ei->nvm_ops, sizeof(hw->nvm.ops));
memcpy(&hw->phy.ops, ei->phy_ops, sizeof(hw->phy.ops));

err = ei->get_variants(adapter);
if (err)
goto err_hw_init;

if ((adapter->flags & FLAG_IS_ICH) &&
    (adapter->flags & FLAG_READ_ONLY_NVM))
e1000e_write_protect_nvm_ich8lan(&adapter->hw);

hw->mac.ops.get_bus_info(&adapter->hw);

adapter->hw.phy.autoneg_wait_to_complete = 0;

/* Copper options */
if (adapter->hw.phy.media_type == e1000_media_type_copper) {
adapter->hw.phy.mdix = AUTO_ALL_MODES;
adapter->hw.phy.disable_polarity_correction = 0;
adapter->hw.phy.ms_type = e1000_ms_hw_default;
}

if (e1000_check_reset_block(&adapter->hw))
e_info("PHY reset is blocked due to SOL/IDER session.\n");

netdev->features = NETIF_F_SG |
   NETIF_F_HW_CSUM |
   NETIF_F_HW_VLAN_TX |
   NETIF_F_HW_VLAN_RX;

if (adapter->flags & FLAG_HAS_HW_VLAN_FILTER)
netdev->features |= NETIF_F_HW_VLAN_FILTER;

netdev->features |= NETIF_F_TSO;
netdev->features |= NETIF_F_TSO6;

netdev->vlan_features |= NETIF_F_TSO;
netdev->vlan_features |= NETIF_F_TSO6;
netdev->vlan_features |= NETIF_F_HW_CSUM;
netdev->vlan_features |= NETIF_F_SG;

if (pci_using_dac)
netdev->features |= NETIF_F_HIGHDMA;

if (e1000e_enable_mng_pass_thru(&adapter->hw))
adapter->flags |= FLAG_MNG_PT_ENABLED;

/*
 * before reading the NVM, reset the controller to
 * put the device in a known good starting state
 */
adapter->hw.mac.ops.reset_hw(&adapter->hw);

/*
 * systems with ASPM and others may see the checksum fail on the first
 * attempt. Let\'s give it a few tries
 */
for (i = 0;; i++) {
if (e1000_validate_nvm_checksum(&adapter->hw) >= 0)
break;
if (i == 2) {
e_err("The NVM Checksum Is Not Valid\n");
err = -EIO;
goto err_eeprom;
}
}

e1000_eeprom_checks(adapter);

/* copy the MAC address out of the NVM */
if (e1000e_read_mac_addr(&adapter->hw))
e_err("NVM Read Error while reading MAC address\n");

memcpy(netdev->dev_addr, adapter->hw.mac.addr, netdev->addr_len);
memcpy(netdev->perm_addr, adapter->hw.mac.addr, netdev->addr_len);

if (!is_valid_ether_addr(netdev->perm_addr)) {
e_err("Invalid MAC Address: %pM\n", netdev->perm_addr);
err = -EIO;
goto err_eeprom;
}

init_timer(&adapter->watchdog_timer);
adapter->watchdog_timer.function = &e1000_watchdog;
adapter->watchdog_timer.data = (unsigned long) adapter;

init_timer(&adapter->phy_info_timer);
adapter->phy_info_timer.function = &e1000_update_phy_info;
adapter->phy_info_timer.data = (unsigned long) adapter;

INIT_WORK(&adapter->reset_task, e1000_reset_task);
INIT_WORK(&adapter->watchdog_task, e1000_watchdog_task);
INIT_WORK(&adapter->downshift_task, e1000e_downshift_workaround);
INIT_WORK(&adapter->update_phy_task, e1000e_update_phy_task);

/* Initialize link parameters. User can change them with ethtool */
adapter->hw.mac.autoneg = 1;
adapter->fc_autoneg = 1;
adapter->hw.fc.requested_mode = e1000_fc_default;
adapter->hw.fc.current_mode = e1000_fc_default;
adapter->hw.phy.autoneg_advertised = 0x2f;
这里是默认的接收环和发送环大小是256，其实一次中断，能接受的数据不会有太高，我做实验的时候也就是1个2个。这里的环不是一直存放skb_buff，而是DMA一次中断后能给内核的数据存放地，当中断结束后，skb_buff会被转移的。
/* ring size defaults */
adapter->rx_ring->count = 256;
adapter->tx_ring->count = 256;

/*
 * Initial Wake on LAN setting - If APM wake is enabled in
 * the EEPROM, enable the ACPI Magic Packet filter
 */
if (adapter->flags & FLAG_APME_IN_WUC) {
/* APME bit in EEPROM is mapped to WUC.APME */
eeprom_data = er32(WUC);
eeprom_apme_mask = E1000_WUC_APME;
if (eeprom_data & E1000_WUC_PHY_WAKE)
adapter->flags2 |= FLAG2_HAS_PHY_WAKEUP;
} else if (adapter->flags & FLAG_APME_IN_CTRL3) {
if (adapter->flags & FLAG_APME_CHECK_PORT_B &&
    (adapter->hw.bus.func == 1))
e1000_read_nvm(&adapter->hw,
NVM_INIT_CONTROL3_PORT_B, 1, &eeprom_data);
else
e1000_read_nvm(&adapter->hw,
NVM_INIT_CONTROL3_PORT_A, 1, &eeprom_data);
}

/* fetch WoL from EEPROM */
if (eeprom_data & eeprom_apme_mask)
adapter->eeprom_wol |= E1000_WUFC_MAG;

/*
 * now that we have the eeprom settings, apply the special cases
 * where the eeprom may be wrong or the board simply won\'t support
 * wake on lan on a particular port
 */
if (!(adapter->flags & FLAG_HAS_WOL))
adapter->eeprom_wol = 0;

/* initialize the wol settings based on the eeprom settings */
adapter->wol = adapter->eeprom_wol;
device_set_wakeup_enable(&adapter->pdev->dev, adapter->wol);

/* save off EEPROM version number */
e1000_read_nvm(&adapter->hw, 5, 1, &adapter->eeprom_vers);

/* reset the hardware with the new settings */
e1000e_reset(adapter);

/*
 * If the controller has AMT, do not set DRV_LOAD until the interface
 * is up.  For all other cases, let the f/w know that the h/w is now
 * under the control of the driver.
 */
if (!(adapter->flags & FLAG_HAS_AMT))
e1000_get_hw_control(adapter);

strcpy(netdev->name, "eth%d")
注册网卡驱动
err = register_netdev(netdev);
if (err)
goto err_register;

/* carrier off reporting is important to ethtool even BEFORE open */
netif_carrier_off(netdev);

e1000_print_device_info(adapter);

return 0;

err_register:
if (!(adapter->flags & FLAG_HAS_AMT))
e1000_release_hw_control(adapter);
err_eeprom:
if (!e1000_check_reset_block(&adapter->hw))
e1000_phy_hw_reset(&adapter->hw);
err_hw_init:

kfree(adapter->tx_ring);
kfree(adapter->rx_ring);
err_sw_init:
if (adapter->hw.flash_address)
iounmap(adapter->hw.flash_address);
e1000e_reset_interrupt_capability(adapter);
err_flashmap:
iounmap(adapter->hw.hw_addr);
err_ioremap:
free_netdev(netdev);
err_alloc_etherdev:
pci_release_selected_regions(pdev,
                             pci_select_bars(pdev, IORESOURCE_MEM));
err_pci_reg:
err_dma:
pci_disable_device(pdev);
return err;
}

通过上面的函数，我们完成了驱动的初始化和设备注册工作。下面是网卡设备注册的操作函数

static const struct net_device_ops e1000e_netdev_ops = {
.ndo_open= e1000_open,
.ndo_stop= e1000_close,
.ndo_start_xmit= e1000_xmit_frame,
.ndo_get_stats= e1000_get_stats,
.ndo_set_multicast_list= e1000_set_multi,
.ndo_set_mac_address= e1000_set_mac,
.ndo_change_mtu= e1000_change_mtu,
.ndo_do_ioctl= e1000_ioctl,
.ndo_tx_timeout= e1000_tx_timeout,
.ndo_validate_addr= eth_validate_addr,

.ndo_vlan_rx_register= e1000_vlan_rx_register,
.ndo_vlan_rx_add_vid= e1000_vlan_rx_add_vid,
.ndo_vlan_rx_kill_vid= e1000_vlan_rx_kill_vid,
#ifdef CONFIG_NET_POLL_CONTROLLER
.ndo_poll_controller= e1000_netpoll,
#endif
};

这里关注一下最后一个函数

static void e1000_netpoll(struct net_device *netdev)
{
struct e1000_adapter *adapter = netdev_priv(netdev);

disable_irq(adapter->pdev->irq);这里关闭容器设备中断
e1000_intr(adapter->pdev->irq, netdev); 初始化设备中断

enable_irq(adapter->pdev->irq);
}

这是网卡驱动的中断处理函数，也就是后半段的处理

static irqreturn_t e1000_intr(int irq, void *data)
{
struct net_device *netdev = data;
struct e1000_adapter *adapter = netdev_priv(netdev);
struct e1000_hw *hw = &adapter->hw;
u32 rctl, icr = er32(ICR);

if (!icr)
return IRQ_NONE;  /* Not our interrupt */

/*
 * IMS will not auto-mask if INT_ASSERTED is not set, and if it is
 * not set, then the adapter didn\'t send an interrupt
 */
if (!(icr & E1000_ICR_INT_ASSERTED))
return IRQ_NONE;

/*
 * Interrupt Auto-Mask...upon reading ICR,
 * interrupts are masked.  No need for the
 * IMC write
 */

if (icr & E1000_ICR_LSC) {
hw->mac.get_link_status = 1;
/*
 * ICH8 workaround-- Call gig speed drop workaround on cable
 * disconnect (LSC) before accessing any PHY registers
 */
if ((adapter->flags & FLAG_LSC_GIG_SPEED_DROP) &&
    (!(er32(STATUS) & E1000_STATUS_LU)))
schedule_work(&adapter->downshift_task);

/*
 * 80003ES2LAN workaround--
 * For packet buffer work-around on link down event;
 * disable receives here in the ISR and
 * reset adapter in watchdog
 */
if (netif_carrier_ok(netdev) &&
    (adapter->flags & FLAG_RX_NEEDS_RESTART)) {
/* disable receives */
rctl = er32(RCTL);
ew32(RCTL, rctl & ~E1000_RCTL_EN);
adapter->flags |= FLAG_RX_RESTART_NOW;
}
/* guard against interrupt when we\'re going down */
if (!test_bit(__E1000_DOWN, &adapter->state))
mod_timer(&adapter->watchdog_timer, jiffies + 1);
}
这里调用了_napi_schedule完成将设备的napi队列挂到CPU
if (napi_schedule_prep(&adapter->napi)) {
adapter->total_tx_bytes = 0;
adapter->total_tx_packets = 0;
adapter->total_rx_bytes = 0;
adapter->total_rx_packets = 0;
__napi_schedule(&adapter->napi);
}

return IRQ_HANDLED;
}

void __napi_schedule(struct napi_struct *n)
{
unsigned long flags;

local_irq_save(flags);
list_add_tail(&n->poll_list, &__get_cpu_var(softnet_data).poll_list);//adapter里面的队列地址挂到poll.list中
//设置软中断NET_RX_SOFTIRQ，等待调度其中断处理程序
__raise_softirq_irqoff(NET_RX_SOFTIRQ);
local_irq_restore(flags);
}

再看一下如何打开网络设备

static int e1000_open(struct net_device *netdev)
{
struct e1000_adapter *adapter = netdev_priv(netdev);
struct e1000_hw *hw = &adapter->hw;
int err;

/* disallow open during test */
if (test_bit(__E1000_TESTING, &adapter->state))
return -EBUSY;

netif_carrier_off(netdev);
初始化传输和接收描述符，这里主要是对接收环和发送环进行初始化，他们需要256个单元空间
/* allocate transmit descriptors */
err = e1000e_setup_tx_resources(adapter);
if (err)
goto err_setup_tx;

/* allocate receive descriptors */
err = e1000e_setup_rx_resources(adapter);
if (err)
goto err_setup_rx;

e1000e_power_up_phy(adapter);

adapter->mng_vlan_id = E1000_MNG_VLAN_NONE;
if ((adapter->hw.mng_cookie.status &
     E1000_MNG_DHCP_COOKIE_STATUS_VLAN))
e1000_update_mng_vlan(adapter);

/*
 * If AMT is enabled, let the firmware know that the network
 * interface is now open
 */
if (adapter->flags & FLAG_HAS_AMT)
e1000_get_hw_control(adapter);

/*
 * before we allocate an interrupt, we must be ready to handle it.
 * Setting DEBUG_SHIRQ in the kernel makes it fire an interrupt
 * as soon as we call pci_request_irq, so we have to setup our
 * clean_rx handler before we do so.
 */这个函数比较重要，在这里面完成对容器的配置，包括软中断设置
e1000_configure(adapter);
{

static void e1000_configure(struct e1000_adapter *adapter)
{
e1000_set_multi(adapter->netdev);

e1000_restore_vlan(adapter);
e1000_init_manageability(adapter);

e1000_configure_tx(adapter);配置发送
e1000_setup_rctl(adapter);
e1000_configure_rx(adapter);配置接收
adapter->alloc_rx_buf(adapter, e1000_desc_unused(adapter->rx_ring));
}

}
err = e1000_request_irq(adapter);
if (err)
goto err_req_irq;

/*
 * Work around PCIe errata with MSI interrupts causing some chipsets to
 * ignore e1000e MSI messages, which means we need to test our MSI
 * interrupt now
 */
if (adapter->int_mode != E1000E_INT_MODE_LEGACY) {
err = e1000_test_msi(adapter);
if (err) {
e_err("Interrupt allocation failed\n");
goto err_req_irq;
}
}

/* From here on the code is the same as e1000e_up() */
clear_bit(__E1000_DOWN, &adapter->state);

napi_enable(&adapter->napi);

e1000_irq_enable(adapter);

netif_start_queue(netdev);

/* fire a link status change interrupt to start the watchdog */
ew32(ICS, E1000_ICS_LSC);

return 0;

err_req_irq:
e1000_release_hw_control(adapter);
e1000_power_down_phy(adapter);
e1000e_free_rx_resources(adapter);
err_setup_rx:
e1000e_free_tx_resources(adapter);
err_setup_tx:
e1000e_reset(adapter);

return err;

这里看一下接收容器中断设置

static void e1000_configure_rx(struct e1000_adapter *adapter)
{
struct e1000_hw *hw = &adapter->hw;
struct e1000_ring *rx_ring = adapter->rx_ring;
u64 rdba;
u32 rdlen, rctl, rxcsum, ctrl_ext;

if (adapter->rx_ps_pages) {
/* this is a 32 byte descriptor */
rdlen = rx_ring->count *
sizeof(union e1000_rx_desc_packet_split);
adapter->clean_rx = e1000_clean_rx_irq_ps;
adapter->alloc_rx_buf = e1000_alloc_rx_buffers_ps;
} else if (adapter->netdev->mtu > ETH_FRAME_LEN + ETH_FCS_LEN) {
rdlen = rx_ring->count * sizeof(struct e1000_rx_desc);
adapter->clean_rx = e1000_clean_jumbo_rx_irq;
adapter->alloc_rx_buf = e1000_alloc_jumbo_rx_buffers;
} else {
rdlen = rx_ring->count * sizeof(struct e1000_rx_desc);
adapter->clean_rx = e1000_clean_rx_irq; 这里的函数是对前半段的一个处理流程，主要是将数据从DMA中获取然后放到队列中，供后半段进行处理。
adapter->alloc_rx_buf = e1000_alloc_rx_buffers;
}

/* disable receives while setting up the descriptors */ //写接收控制寄存器 暂时停止接收
rctl = er32(RCTL);
ew32(RCTL, rctl & ~E1000_RCTL_EN);
e1e_flush();
msleep(10);

/* set the Receive Delay Timer Register *///设置RDTR寄存器 有关
ew32(RDTR, adapter->rx_int_delay);

/* irq moderation */ //设置RADV寄存器 有关RADV具体详见开发者手册  
ew32(RADV, adapter->rx_abs_int_delay);
if (adapter->itr_setting != 0)
ew32(ITR, 1000000000 / (adapter->itr * 256));

ctrl_ext = er32(CTRL_EXT);
/* Reset delay timers after every interrupt */
ctrl_ext |= E1000_CTRL_EXT_INT_TIMER_CLR;
/* Auto-Mask interrupts upon ICR access */
ctrl_ext |= E1000_CTRL_EXT_IAME;
ew32(IAM, 0xffffffff);
ew32(CTRL_EXT, ctrl_ext);
e1e_flush();

/*
 * Setup the HW Rx Head and Tail Descriptor Pointers and
 * the Base and Length of the Rx Descriptor Ring
 */
 //与接收描述符环有关的有4个寄存器：RDBA存放描述符缓冲的首地址 做为基地址 供64位 包括各32位的高低地址  
//RDLEN：为缓冲区分配的总空间的大小 RDH和RDT是头尾指针 存放相对基址的偏移量 RDH的值由硬件增加 表示指向下一次DMA将用的描述符 
//RDT由软件增加 表示下一次要处理并送交协议栈的有关描述符 
rdba = rx_ring->dma;
ew32(RDBAL, (rdba & DMA_BIT_MASK(32)));
ew32(RDBAH, (rdba >> 32));
ew32(RDLEN, rdlen);
ew32(RDH, 0);
ew32(RDT, 0);
rx_ring->head = E1000_RDH;
rx_ring->tail = E1000_RDT;

/* Enable Receive Checksum Offload for TCP and UDP */
rxcsum = er32(RXCSUM);
if (adapter->flags & FLAG_RX_CSUM_ENABLED) {
rxcsum |= E1000_RXCSUM_TUOFL;

/*
 * IPv4 payload checksum for UDP fragments must be
 * used in conjunction with packet-split.
 */
if (adapter->rx_ps_pages)
rxcsum |= E1000_RXCSUM_IPPCSE;
} else {
rxcsum &= ~E1000_RXCSUM_TUOFL;
/* no need to clear IPPCSE as it defaults to 0 */
}
ew32(RXCSUM, rxcsum);

/*
 * Enable early receives on supported devices, only takes effect when
 * packet size is equal or larger than the specified value (in 8 byte
 * units), e.g. using jumbo frames when setting to E1000_ERT_2048
 */
if ((adapter->flags & FLAG_HAS_ERT) &&
    (adapter->netdev->mtu > ETH_DATA_LEN)) {
u32 rxdctl = er32(RXDCTL(0));
ew32(RXDCTL(0), rxdctl | 0x3);
ew32(ERT, E1000_ERT_2048 | (1 << 13));
/*
 * With jumbo frames and early-receive enabled, excessive
 * C4->C2 latencies result in dropped transactions.
 */
pm_qos_update_requirement(PM_QOS_CPU_DMA_LATENCY,
  e1000e_driver_name, 55);
} else {
pm_qos_update_requirement(PM_QOS_CPU_DMA_LATENCY,
  e1000e_driver_name,
  PM_QOS_DEFAULT_VALUE);
}

/* Enable Receives */
ew32(RCTL, rctl);
}