linux内存管理之DMA

     说起DMA我们并不陌生，但是实际编程中去用的人不多吧，最多就是网卡驱动里的环形buffer，再有就是设备的dma，下面我们就分析分析.
   DMA用来在设备内存和内存之间直接数据交互。而无需cpu干预
  
内核为了方便驱动的开发，已经提供了几个dma 函数接口。
dma跟硬件架构相关，所以linux关于硬件部分已经给屏蔽了，有兴趣的可以深入跟踪学习.

按照linux内核对dma层的架构设计，各平台dma缓冲区映射之间的差异由内核定义的一个dma操作集

include/linux/dma-mapping.h:点击(此处)折叠或打开

struct dma_map_ops {
    void* (*alloc)(struct device *dev, size_t size,
                dma_addr_t *dma_handle, gfp_t gfp,
                struct dma_attrs *attrs);
    void (*free)(struct device *dev, size_t size,
                  void *vaddr, dma_addr_t dma_handle,
                  struct dma_attrs *attrs);
    int (*mmap)(struct device *, struct vm_area_struct *,
              void *, dma_addr_t, size_t, struct dma_attrs *attrs);

    int (*get_sgtable)(struct device *dev, struct sg_table *sgt, void *,
               dma_addr_t, size_t, struct dma_attrs *attrs);

    dma_addr_t (*map_page)(struct device *dev, struct page *page,
                   unsigned long offset, size_t size,
                   enum dma_data_direction dir,
                   struct dma_attrs *attrs);
    void (*unmap_page)(struct device *dev, dma_addr_t dma_handle,
               size_t size, enum dma_data_direction dir,
               struct dma_attrs *attrs);
    int (*map_sg)(struct device *dev, struct scatterlist *sg,
         int nents, enum dma_data_direction dir,
              struct dma_attrs *attrs);
    void (*unmap_sg)(struct device *dev,
             struct scatterlist *sg, int nents,
             enum dma_data_direction dir,
             struct dma_attrs *attrs);
    void (*sync_single_for_cpu)(struct device *dev,
                    dma_addr_t dma_handle, size_t size,
                    enum dma_data_direction dir);
    void (*sync_single_for_device)(struct device *dev,
                       dma_addr_t dma_handle, size_t size,
                       enum dma_data_direction dir);
    void (*sync_sg_for_cpu)(struct device *dev,
                struct scatterlist *sg, int nents,
                enum dma_data_direction dir);
    void (*sync_sg_for_device)(struct device *dev,
                   struct scatterlist *sg, int nents,
                   enum dma_data_direction dir);
    int (*mapping_error)(struct device *dev, dma_addr_t dma_addr);
    int (*dma_supported)(struct device *dev, u64 mask);
    int (*set_dma_mask)(struct device *dev, u64 mask);
#ifdef ARCH_HAS_DMA_GET_REQUIRED_MASK
    u64 (*get_required_mask)(struct device *dev);
#endif
    int is_phys;
}

来统一屏蔽实现的差异.
不同差异主要来来自cache的问题

Cache与dma同步问题，这里不深入讨论.

另外一个常用的函数是Dma_set_mask，  为了通知内核设备能够寻址的范围，很多时候设备能够寻址的范围有限。

Dma映射可以分为三类：

1.       一致性dma映射 dma_alloc_coherent （问题：驱动使用的buffer不是自身申请的，而是其他模块）

当驱动模块主动分配一个Dma缓冲区并且dma生存期和模块一样时

参数说明：

（1）这个函数的返回值是缓冲的一个内核虚拟地址, 它可被驱动使用

（2）第三个参数dma_handle：

其间相关的物理地址在 dma_handle 中返回

 

2.       流式dma映射  dma_map_single 
通常用于把内核一段buffer映射，返回物理地址.

如果驱动模块需要使用从别的模块传进来的虚拟地址空间作为dma缓冲区，保证地址的线性  cache一致性

一致性api接口：sync_single_for_cpu

3.分散/聚集映射（scatter/gather map）  Dma_map_sgs

有时候我们还需要

1. 回弹缓冲区 bounce  buffer：当cpu侧物理地址不适合设备的dma操作的时候

2.

DmA内存池：一般dma映射都是单个page的整数倍，如果驱动程序需要更小的一致性映射的dma缓冲区，可以使用。类似于slab机制，

Dma_pool_create

下面我们就那网卡驱动的例子说说dma的具体应用，参考linux kernel e1000网卡
drivers/net/ethernet/intel/e1000/*

Ring buffer

Dma不能为高端内存，一般为32，默认低端内存，由于设备能够访问的地址范围有限。

设备使用物理地址，而代码使用虚拟地址。

就看看如何发送数据包：e1000_main.c:
e1000_xmit_frame: 关于帧的发送流程这里不多说.

static netdev_tx_t e1000_xmit_frame(struct sk_buff *skb,
                    struct net_device *netdev)
{
    struct e1000_adapter *adapter = netdev_priv(netdev);
    struct e1000_hw *hw = &adapter->hw;
    struct e1000_tx_ring *tx_ring;
    unsigned int first, max_per_txd = E1000_MAX_DATA_PER_TXD;
    unsigned int max_txd_pwr = E1000_MAX_TXD_PWR;
    unsigned int tx_flags = 0;
    unsigned int len = skb_headlen(skb);
    unsigned int nr_frags;
    unsigned int mss;
    int count = 0;
    int tso;
    unsigned int f;

    /* This goes back to the question of how to logically map a tx queue
     * to a flow. Right now, performance is impacted slightly negatively
     * if using multiple tx queues. If the stack breaks away from a
     * single qdisc implementation, we can look at this again. */
    tx_ring = adapter->tx_ring;

    if (unlikely(skb->len <= 0)) {
        dev_kfree_skb_any(skb);
        return NETDEV_TX_OK;
    }

    /* On PCI/PCI-X HW, if packet size is less than ETH_ZLEN,
     * packets may get corrupted during padding by HW.
     * To WA this issue, pad all small packets manually.
     */
    if (skb->len < ETH_ZLEN) {
        if (skb_pad(skb, ETH_ZLEN - skb->len))
            return NETDEV_TX_OK;
        skb->len = ETH_ZLEN;
        skb_set_tail_pointer(skb, ETH_ZLEN);
    }

    mss = skb_shinfo(skb)->gso_size;
    /* The controller does a simple calculation to
     * make sure there is enough room in the FIFO before
     * initiating the DMA for each buffer. The calc is:
     * 4 = ceil(buffer len/mss). To make sure we don't
     * overrun the FIFO, adjust the max buffer len if mss
     * drops. */
    if (mss) {
        u8 hdr_len;
        max_per_txd = min(mss << 2, max_per_txd);
        max_txd_pwr = fls(max_per_txd) - 1;

        hdr_len = skb_transport_offset(skb) + tcp_hdrlen(skb);
        if (skb->data_len && hdr_len == len) {
            switch (hw->mac_type) {
                unsigned int pull_size;
            case e1000_82544:
                /* Make sure we have room to chop off 4 bytes,
                 * and that the end alignment will work out to
                 * this hardware's requirements
                 * NOTE: this is a TSO only workaround
                 * if end byte alignment not correct move us
                 * into the next dword */
                if ((unsigned long)(skb_tail_pointer(skb) - 1) & 4)
                    break;
                /* fall through */
                pull_size = min((unsigned int)4, skb->data_len);
                if (!__pskb_pull_tail(skb, pull_size)) {
                    e_err(drv, "__pskb_pull_tail "
                     "failed.\n");
                    dev_kfree_skb_any(skb);
                    return NETDEV_TX_OK;
                }
                len = skb_headlen(skb);
                break;
            default:
                /* do nothing */
                break;
            }
        }
    }

    /* reserve a descriptor for the offload context */
    if ((mss) || (skb->ip_summed == CHECKSUM_PARTIAL))
        count++;
    count++;

    /* Controller Erratum workaround */
    if (!skb->data_len && tx_ring->last_tx_tso && !skb_is_gso(skb))
        count++;

    count += TXD_USE_COUNT(len, max_txd_pwr);

    if (adapter->pcix_82544)
        count++;

    /* work-around for errata 10 and it applies to all controllers
     * in PCI-X mode, so add one more descriptor to the count
     */
    if (unlikely((hw->bus_type == e1000_bus_type_pcix) &&
            (len > 2015)))
        count++;

    nr_frags = skb_shinfo(skb)->nr_frags;
    for (f = 0; f < nr_frags; f++)
        count += TXD_USE_COUNT(skb_frag_size(&skb_shinfo(skb)->frags[f]),
                       max_txd_pwr);
    if (adapter->pcix_82544)
        count += nr_frags;

    /* need: count + 2 desc gap to keep tail from touching
     * head, otherwise try next time */
    if (unlikely(e1000_maybe_stop_tx(netdev, tx_ring, count + 2)))
        return NETDEV_TX_BUSY;

    if (unlikely((hw->mac_type == e1000_82547) &&
         (e1000_82547_fifo_workaround(adapter, skb)))) {
        netif_stop_queue(netdev);
        if (!test_bit(__E1000_DOWN, &adapter->flags))
            schedule_delayed_work(&adapter->fifo_stall_task, 1);
        return NETDEV_TX_BUSY;
    }

    if (vlan_tx_tag_present(skb)) {
        tx_flags |= E1000_TX_FLAGS_VLAN;
        tx_flags |= (vlan_tx_tag_get(skb) << E1000_TX_FLAGS_VLAN_SHIFT);
    }

    first = tx_ring->next_to_use;

    tso = e1000_tso(adapter, tx_ring, skb);
    if (tso < 0) {
        dev_kfree_skb_any(skb);
        return NETDEV_TX_OK;
    }

    if (likely(tso)) {
        if (likely(hw->mac_type != e1000_82544))
            tx_ring->last_tx_tso = true;
        tx_flags |= E1000_TX_FLAGS_TSO;
    } else if (likely(e1000_tx_csum(adapter, tx_ring, skb)))
        tx_flags |= E1000_TX_FLAGS_CSUM;

    if (likely(skb->protocol == htons(ETH_P_IP)))
        tx_flags |= E1000_TX_FLAGS_IPV4;

    if (unlikely(skb->no_fcs))
        tx_flags |= E1000_TX_FLAGS_NO_FCS;

    count = e1000_tx_map(adapter, tx_ring, skb, first, max_per_txd,
                         nr_frags, mss);

    if (count) {
        netdev_sent_queue(netdev, skb->len);
        skb_tx_timestamp(skb);

        e1000_tx_queue(adapter, tx_ring, tx_flags, count);
        /* Make sure there is space in the ring for the next send. */
        e1000_maybe_stop_tx(netdev, tx_ring, MAX_SKB_FRAGS + 2);

    } else {
        dev_kfree_skb_any(skb);
        tx_ring->buffer_info[first].time_stamp = 0;
        tx_ring->next_to_use = first;
    }

    return NETDEV_TX_OK;
}

经过上次，邻居子系统后，数据帧已经到达驱动，数据放在skb指定的内存里. 
看代码
tx_ring = adapter->tx_ring;  //  获取发送的ring buffer
接着我们看关键代码：
count = e1000_tx_map(adapter, tx_ring, skb, first, max_per_txd,    nr_frags, mss);
它做了什么呢？

static int e1000_tx_map(struct e1000_adapter *adapter,
            struct e1000_tx_ring *tx_ring,
            struct sk_buff *skb, unsigned int first,
            unsigned int max_per_txd, unsigned int nr_frags,
            unsigned int mss)
{
    struct e1000_hw *hw = &adapter->hw;
    struct pci_dev *pdev = adapter->pdev;
    struct e1000_buffer *buffer_info;
    unsigned int len = skb_headlen(skb);
    unsigned int offset = 0, size, count = 0, i;
    unsigned int f, bytecount, segs;

    i = tx_ring->next_to_use;

    while (len) {
        buffer_info = &tx_ring->buffer_info[i];
        size = min(len, max_per_txd);
        /* Workaround for Controller erratum --
         * descriptor for non-tso packet in a linear SKB that follows a
         * tso gets written back prematurely before the data is fully
         * DMA'd to the controller */
        if (!skb->data_len && tx_ring->last_tx_tso &&
         !skb_is_gso(skb)) {
            tx_ring->last_tx_tso = false;
            size -= 4;
        }

        /* Workaround for premature desc write-backs
         * in TSO mode. Append 4-byte sentinel desc */
        if (unlikely(mss && !nr_frags && size == len && size > 8))
            size -= 4;
        /* work-around for errata 10 and it applies
         * to all controllers in PCI-X mode
         * The fix is to make sure that the first descriptor of a
         * packet is smaller than 2048 - 16 - 16 (or 2016) bytes
         */
        if (unlikely((hw->bus_type == e1000_bus_type_pcix) &&
         (size > 2015) && count == 0))
                size = 2015;

        /* Workaround for potential 82544 hang in PCI-X. Avoid
         * terminating buffers within evenly-aligned dwords. */
        if (unlikely(adapter->pcix_82544 &&
         !((unsigned long)(skb->data + offset + size - 1) & 4) &&
           size > 4))
            size -= 4;

        buffer_info->length = size;
        /* set time_stamp *before* dma to help avoid a possible race */
        buffer_info->time_stamp = jiffies;
        buffer_info->mapped_as_page = false;
        buffer_info->dma = dma_map_single(&pdev->dev,
                          skb->data + offset,
                          size,    DMA_TO_DEVICE);
        if (dma_mapping_error(&pdev->dev, buffer_info->dma))
            goto dma_error;
        buffer_info->next_to_watch = i;

        len -= size;
        offset += size;
        count++;
        if (len) {
            i++;
            if (unlikely(i == tx_ring->count))
                i = 0;
        }
    }

    for (f = 0; f < nr_frags; f++) {
        const struct skb_frag_struct *frag;

        frag = &skb_shinfo(skb)->frags[f];
        len = skb_frag_size(frag);
        offset = 0;

        while (len) {
            unsigned long bufend;
            i++;
            if (unlikely(i == tx_ring->count))
                i = 0;

            buffer_info = &tx_ring->buffer_info[i];
            size = min(len, max_per_txd);
            /* Workaround for premature desc write-backs
             * in TSO mode. Append 4-byte sentinel desc */
            if (unlikely(mss && f == (nr_frags-1) && size == len && size > 8))
                size -= 4;
            /* Workaround for potential 82544 hang in PCI-X.
             * Avoid terminating buffers within evenly-aligned
             * dwords. */
            bufend = (unsigned long)
                page_to_phys(skb_frag_page(frag));
            bufend += offset + size - 1;
            if (unlikely(adapter->pcix_82544 &&
                 !(bufend & 4) &&
                     size > 4))
                size -= 4;

            buffer_info->length = size;
            buffer_info->time_stamp = jiffies;
            buffer_info->mapped_as_page = true;
            buffer_info->dma = skb_frag_dma_map(&pdev->dev, frag,
                        offset, size, DMA_TO_DEVICE);
            if (dma_mapping_error(&pdev->dev, buffer_info->dma))
                goto dma_error;
            buffer_info->next_to_watch = i;

            len -= size;
            offset += size;
            count++;
        }
    }

    segs = skb_shinfo(skb)->gso_segs ?: 1;
    /* multiply data chunks by size of headers */
    bytecount = ((segs - 1) * skb_headlen(skb)) + skb->len;

    tx_ring->buffer_info[i].skb = skb;
    tx_ring->buffer_info[i].segs = segs;
    tx_ring->buffer_info[i].bytecount = bytecount;
    tx_ring->buffer_info[first].next_to_watch = i;

    return count;

dma_error:
    dev_err(&pdev->dev, "TX DMA map failed\n");
    buffer_info->dma = 0;
    if (count)
        count--;

    while (count--) {
        if (i==0)
            i += tx_ring->count;
        i--;
        buffer_info = &tx_ring->buffer_info[i];
        e1000_unmap_and_free_tx_resource(adapter, buffer_info);
    }

    return 0;
}

默认数据报文没有分片或者碎片什么的。
那么进入第一个while(len)
获取buffer_info = &tx_ring->buffer_info[i];
然后：调用dma_map_single进行流式映射. 即把skb->data(虚拟地址) 和buffer_info->dma（物理地址）对应起来.操作两个地址等于操作同一片区域。

buffer_info->length = size;
        /* set time_stamp *before* dma to help avoid a possible race */
        buffer_info->time_stamp = jiffies;
        buffer_info->mapped_as_page = false;
        buffer_info->dma = dma_map_single(&pdev->dev,
                          skb->data + offset,
                          size,    DMA_TO_DEVICE);

回到主发送函数：

if (count) {
        netdev_sent_queue(netdev, skb->len);
        skb_tx_timestamp(skb);

        e1000_tx_queue(adapter, tx_ring, tx_flags, count);
        /* Make sure there is space in the ring for the next send. */
        e1000_maybe_stop_tx(netdev, tx_ring, MAX_SKB_FRAGS + 2);

    }

调用e1000_tx_queue把数据发送出去：

static void e1000_tx_queue(struct e1000_adapter *adapter,
               struct e1000_tx_ring *tx_ring, int tx_flags,
             int count)
{
    struct e1000_hw *hw = &adapter->hw;
    struct e1000_tx_desc *tx_desc = NULL;
    struct e1000_buffer *buffer_info;
    u32 txd_upper = 0, txd_lower = E1000_TXD_CMD_IFCS;
    unsigned int i;

    ...

    i = tx_ring->next_to_use;

    while (count--) {
        buffer_info = &tx_ring->buffer_info[i];
        tx_desc = E1000_TX_DESC(*tx_ring, i);
        tx_desc->buffer_addr = cpu_to_le64(buffer_info->dma);
        tx_desc->lower.data =
            cpu_to_le32(txd_lower | buffer_info->length);
        tx_desc->upper.data = cpu_to_le32(txd_upper);
        if (unlikely(++i == tx_ring->count)) i = 0;
    }

    tx_desc->lower.data |= cpu_to_le32(adapter->txd_cmd);

    /* txd_cmd re-enables FCS, so we'll re-disable it here as desired. */
    if (unlikely(tx_flags & E1000_TX_FLAGS_NO_FCS))
        tx_desc->lower.data &= ~(cpu_to_le32(E1000_TXD_CMD_IFCS));

    /* Force memory writes to complete before letting h/w
     * know there are new descriptors to fetch. (Only
     * applicable for weak-ordered memory model archs,
     * such as IA-64). */
    wmb();

    tx_ring->next_to_use = i;
    writel(i, hw->hw_addr + tx_ring->tdt);
    /* we need this if more than one processor can write to our tail
     * at a time, it syncronizes IO on IA64/Altix systems */
    mmiowb();
}

我们看到它把刚才dma_map_singe里的映射赋值了：
tx_desc->buffer_addr = cpu_to_le64(buffer_info->dma);
说明发送的时候是根据发送描述符来发送的。
然后操作寄存器：
writel(i, hw->hw_addr + tx_ring->tdt);
那么网卡就会自动读取tx desc 然后把数据发送出去。
总结下流程：
1. linux os会调用网卡的start_xmit（）函数。在e1000里，对应的函数是 e1000_xmit_frame,
2.   e1000_xmit_frame又会调用e1000_tx_queue(adapter, tx_ring, tx_flags, count)。
这里的tx_queue指的是发送Descriptor的queue。
3. e1000_tx_queue 在检查了一些参数后，最终调用 writel(i, hw->hw_addr + tx_ring->tdt)。
这里的tx_ring->tdt中的tdt全写为 tx_descriptor_tail。从网卡的开发手册中可以查到，如果写了descriptor tail，那么网卡就会自动读取 descriptor,然后把包发送出去。
descroptor的主要内容是addr pointer和length。前者是要发送的包的起始物理地址。后者是包的长度。有了这些，硬件就可以通过dma来读取包并发出去了。其他网卡也基本会用descriptor的结构。

虽然流程明白了，但是还有几个点，
1. tx_ring在哪初始化？
2. 网卡到底是如何操作映射的dma地址的，把数据发送出去的？

tx ring 在e1000_open 的时候：
调用：

/**
 * e1000_setup_all_tx_resources - wrapper to allocate Tx resources
 *                  (Descriptors) for all queues
 * @adapter: board private structure
 *
 * Return 0 on success, negative on failure
 **/

int e1000_setup_all_tx_resources(struct e1000_adapter *adapter)
{
    int i, err = 0;

    for (i = 0; i < adapter->num_tx_queues; i++) {
        err = e1000_setup_tx_resources(adapter, &adapter->tx_ring[i]);
        if (err) {
            e_err(probe, "Allocation for Tx Queue %u failed\n", i);
            for (i-- ; i >= 0; i--)
                e1000_free_tx_resources(adapter,
                            &adapter->tx_ring[i]);
            break;
        }
    }

    return err;
}

/**
 * e1000_setup_tx_resources - allocate Tx resources (Descriptors)
 * @adapter: board private structure
 * @txdr: tx descriptor ring (for a specific queue) to setup
 *
 * Return 0 on success, negative on failure
 **/

static int e1000_setup_tx_resources(struct e1000_adapter *adapter,
                    struct e1000_tx_ring *txdr)
{
    struct pci_dev *pdev = adapter->pdev;
    int size;

    size = sizeof(struct e1000_buffer) * txdr->count;
    txdr->buffer_info = vzalloc(size);
    if (!txdr->buffer_info) {
        e_err(probe, "Unable to allocate memory for the Tx descriptor "
         "ring\n");
        return -ENOMEM;
    }

    /* round up to nearest 4K */

    txdr->size = txdr->count * sizeof(struct e1000_tx_desc);
    txdr->size = ALIGN(txdr->size, 4096);

    txdr->desc = dma_alloc_coherent(&pdev->dev, txdr->size, &txdr->dma,
                    GFP_KERNEL);
    if (!txdr->desc) {
setup_tx_desc_die:
        vfree(txdr->buffer_info);
        e_err(probe, "Unable to allocate memory for the Tx descriptor "
         "ring\n");
        return -ENOMEM;
    }

    /* Fix for errata 23, can't cross 64kB boundary */
    if (!e1000_check_64k_bound(adapter, txdr->desc, txdr->size)) {
        void *olddesc = txdr->desc;
        dma_addr_t olddma = txdr->dma;
        e_err(tx_err, "txdr align check failed: %u bytes at %p\n",
              txdr->size, txdr->desc);
        /* Try again, without freeing the previous */
        txdr->desc = dma_alloc_coherent(&pdev->dev, txdr->size,
                        &txdr->dma, GFP_KERNEL);
        /* Failed allocation, critical failure */
        if (!txdr->desc) {
            dma_free_coherent(&pdev->dev, txdr->size, olddesc,
                      olddma);
            goto setup_tx_desc_die;
        }

        if (!e1000_check_64k_bound(adapter, txdr->desc, txdr->size)) {
            /* give up */
            dma_free_coherent(&pdev->dev, txdr->size, txdr->desc,
                      txdr->dma);
            dma_free_coherent(&pdev->dev, txdr->size, olddesc,
                      olddma);
            e_err(probe, "Unable to allocate aligned memory "
             "for the transmit descriptor ring\n");
            vfree(txdr->buffer_info);
            return -ENOMEM;
        } else {
            /* Free old allocation, new allocation was successful */
            dma_free_coherent(&pdev->dev, txdr->size, olddesc,
                      olddma);
        }
    }
    memset(txdr->desc, 0, txdr->size);

    txdr->next_to_use = 0;
    txdr->next_to_clean = 0;

    return 0;
}

我们看：它建立了一致性dma映射.

        txdr->desc = dma_alloc_coherent(&pdev->dev, txdr->size,
                        &txdr->dma, GFP_KERNEL);

desc是结构指针：它的结构跟网卡寄存器结构有关，e1000_hw.h

/* Transmit Descriptor */
struct e1000_tx_desc {
    __le64 buffer_addr;    /* Address of the descriptor's data buffer */
    union {
        __le32 data;
        struct {
            __le16 length;    /* Data buffer length */
            u8 cso;    /* Checksum offset */
            u8 cmd;    /* Descriptor control */
        } flags;
    } lower;
    union {
        __le32 data;
        struct {
            u8 status;    /* Descriptor status */
            u8 css;    /* Checksum start */
            __le16 special;
        } fields;
    } upper;
}

我们稍微屡一下，
1. skb->data  --- ring->buffer_info->dma
2.ring->dma  ---  ring->desc
3. ring->desc->buffer_addr ---ring->buffer_info->dma
那么网卡又是如何和dma地址关联的呢?

/**
 * e1000_configure_tx - Configure 8254x Transmit Unit after Reset
 * @adapter: board private structure
 *
 * Configure the Tx unit of the MAC after a reset.
 **/

static void e1000_configure_tx(struct e1000_adapter *adapter)
{
    u64 tdba;
    struct e1000_hw *hw = &adapter->hw;
    u32 tdlen, tctl, tipg;
    u32 ipgr1, ipgr2;

    /* Setup the HW Tx Head and Tail descriptor pointers */

    switch (adapter->num_tx_queues) {
    case 1:
    default:
        tdba = adapter->tx_ring[0].dma;
        tdlen = adapter->tx_ring[0].count *
            sizeof(struct e1000_tx_desc);
        ew32(TDLEN, tdlen);
        ew32(TDBAH, (tdba >> 32));
        ew32(TDBAL, (tdba & 0x00000000ffffffffULL));
        ew32(TDT, 0);
        ew32(TDH, 0);
        adapter->tx_ring[0].tdh = ((hw->mac_type >= e1000_82543) ? E1000_TDH : E1000_82542_TDH);
        adapter->tx_ring[0].tdt = ((hw->mac_type >= e1000_82543) ? E1000_TDT : E1000_82542_TDT);
        break;
    }

很明显它把dma地址写入了网卡dma寄存器。所以dma还需要网卡硬件的支持才行.

当然e1000这个网卡驱动还是相当的复杂,不过它把一致性映射和流式映射都用上了。