关于RT-Thread+LWIP在STM32F4平台上的驱动

发布于 2017-06-27 11:31:51 浏览：3460 订阅该版

现在RT-Thread的BSP里包含了STM32F7的移植，但还没有网络功能，所以想参照之前RealTouch例程，将RT-Thread+LWIP移植到STM32F7平台上。
看了一下与以太网硬件驱动有关的代码，现在有个疑问，
```/* reception packet. */
struct pbuf *rt_stm32_eth_rx(rt_device_t dev)
{
    struct pbuf* p;
    rt_uint32_t offset = 0, framelength = 0;

/* init p pointer */
    p = RT_NULL;

/* Check if the descriptor is owned by the ETHERNET DMA (when set) or CPU (when reset) */
    if(((DMARxDescToGet->Status & ETH_DMARxDesc_OWN) != (uint32_t)RESET))
        return p;

if (((DMARxDescToGet->Status & ETH_DMARxDesc_ES) == (uint32_t)RESET) &&
            ((DMARxDescToGet->Status & ETH_DMARxDesc_LS) != (uint32_t)RESET) &&
            ((DMARxDescToGet->Status & ETH_DMARxDesc_FS) != (uint32_t)RESET))
    {
        /* Get the Frame Length of the received packet: substruct 4 bytes of the CRC */
        framelength = ((DMARxDescToGet->Status & ETH_DMARxDesc_FL) >> ETH_DMARXDESC_FRAME_LENGTHSHIFT) - 4;

/* allocate buffer */
        p = pbuf_alloc(PBUF_LINK, framelength, PBUF_RAM);
......```

该函数的功能将硬件接收到的Packet放到LWIP的pbuf中，其中
```if (((DMARxDescToGet->Status & ETH_DMARxDesc_ES) == (uint32_t)RESET) &&
            ((DMARxDescToGet->Status & ETH_DMARxDesc_LS) != (uint32_t)RESET) &&
            ((DMARxDescToGet->Status & ETH_DMARxDesc_FS) != (uint32_t)RESET))```

if语句的功能好像是针对了一次接收到的数据只使用了一个以太网DMA传输的描述符（链表形式）。
有没有可能一次接收到的数据使用了一个以上的DMA描述符（就是一次接收到的数据超过一帧的长度）。

我看ST官方的LWIP例程里面，似乎有针对一次接收数据使用多个DMA描述符的处理。
```HAL_StatusTypeDef HAL_ETH_GetReceivedFrame_IT(ETH_HandleTypeDef *heth)
{
  uint32_t descriptorscancounter = 0U;
  
  /* Process Locked */
  __HAL_LOCK(heth);
  
  /* Set ETH HAL State to BUSY */
  heth->State = HAL_ETH_STATE_BUSY;
  
  /* Scan descriptors owned by CPU */
  while (((heth->RxDesc->Status & ETH_DMARXDESC_OWN) == (uint32_t)RESET) && (descriptorscancounter < ETH_RXBUFNB))
  {
    /* Just for security */
    descriptorscancounter++;
    
    /* Check if first segment in frame */
    /* ((heth->RxDesc->Status & ETH_DMARXDESC_FS) != (uint32_t)RESET) && ((heth->RxDesc->Status & ETH_DMARXDESC_LS) == (uint32_t)RESET)) */  
    if((heth->RxDesc->Status & (ETH_DMARXDESC_FS | ETH_DMARXDESC_LS)) == (uint32_t)ETH_DMARXDESC_FS)
    { 
      heth->RxFrameInfos.FSRxDesc = heth->RxDesc;
      heth->RxFrameInfos.SegCount = 1U;   
      /* Point to next descriptor */
      heth->RxDesc = (ETH_DMADescTypeDef*) (heth->RxDesc->Buffer2NextDescAddr);
    }
    /* Check if intermediate segment */
    /* ((heth->RxDesc->Status & ETH_DMARXDESC_LS) == (uint32_t)RESET)&& ((heth->RxDesc->Status & ETH_DMARXDESC_FS) == (uint32_t)RESET)) */
    else if ((heth->RxDesc->Status & (ETH_DMARXDESC_LS | ETH_DMARXDESC_FS)) == (uint32_t)RESET)
    {
      /* Increment segment count */
      (heth->RxFrameInfos.SegCount)++;
      /* Point to next descriptor */
      heth->RxDesc = (ETH_DMADescTypeDef*)(heth->RxDesc->Buffer2NextDescAddr);
    }
    /* Should be last segment */
    else
    { 
      /* Last segment */
      heth->RxFrameInfos.LSRxDesc = heth->RxDesc;
      
      /* Increment segment count */
      (heth->RxFrameInfos.SegCount)++;
      
      /* Check if last segment is first segment: one segment contains the frame */
      if ((heth->RxFrameInfos.SegCount) == 1U)
      {
        heth->RxFrameInfos.FSRxDesc = heth->RxDesc;
      }
      
      /* Get the Frame Length of the received packet: substruct 4 bytes of the CRC */
      heth->RxFrameInfos.length = (((heth->RxDesc)->Status & ETH_DMARXDESC_FL) >> ETH_DMARXDESC_FRAMELENGTHSHIFT) - 4U;
      
      /* Get the address of the buffer start address */ 
      heth->RxFrameInfos.buffer =((heth->RxFrameInfos).FSRxDesc)->Buffer1Addr;
      
      /* Point to next descriptor */      
      heth->RxDesc = (ETH_DMADescTypeDef*) (heth->RxDesc->Buffer2NextDescAddr);
      
      /* Set HAL State to Ready */
      heth->State = HAL_ETH_STATE_READY;
      
      /* Process Unlocked */
      __HAL_UNLOCK(heth);
  
      /* Return function status */
      return HAL_OK;
    }
  }

/* Set HAL State to Ready */
  heth->State = HAL_ETH_STATE_READY;
  
  /* Process Unlocked */
  __HAL_UNLOCK(heth);
  
  /* Return function status */
  return HAL_ERROR;
}```
以上是ST官方的代码，针对一次接收到的数据可能使用了多个DMA描述符的情况作了处理。

现在想问一下，
1）是因为RT-Thread针对LWIP做了优化处理后，不会出现一次接收到的数据超过一个Frame(最长1524字节)的情况？还是说需要考虑这种情况。
2）p = pbuf_alloc(PBUF_LINK, framelength, PBUF_RAM)，type是PBUF_RAM，
    RT-Thread用的是自己的内存管理，从系统HEAP上分配内存给pbuf，没有使用LWIP自己的mem.c？
3）从最底层接收到的数据，针对pbuf_alloc，RT-Thread用的是PBUF_LINK，ST官方例程用的是PBUF_RAW，有什么区别吗？