STM32F401用配置串口2的dma，并用串口dma发送数据给上位机，请只用c语言标准库详细编写相关代码

以下是使用标准库配置STM32F401串口2 DMA发送数据的代码：

#include "stm32f4xx.h"
#include <string.h>

#define USART_TX_BUFFER_SIZE 100

uint8_t usart_tx_buffer[USART_TX_BUFFER_SIZE]; // 串口发送缓冲区
volatile uint16_t usart_tx_head = 0;           // 缓冲区头指针
volatile uint16_t usart_tx_tail = 0;           // 缓冲区尾指针

void USART2_Init(void) {
    GPIO_InitTypeDef GPIO_InitStruct;
    USART_InitTypeDef USART_InitStruct;
    DMA_InitTypeDef DMA_InitStruct;

    // 使能GPIOA时钟
    RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOA, ENABLE);

    // 配置PA2为USART2_TX
    GPIO_InitStruct.GPIO_Pin = GPIO_Pin_2;
    GPIO_InitStruct.GPIO_Mode = GPIO_Mode_AF;
    GPIO_InitStruct.GPIO_Speed = GPIO_Speed_50MHz;
    GPIO_InitStruct.GPIO_OType = GPIO_OType_PP;
    GPIO_InitStruct.GPIO_PuPd = GPIO_PuPd_UP;
    GPIO_Init(GPIOA, &GPIO_InitStruct);

    // 配置USART2时钟
    RCC_APB1PeriphClockCmd(RCC_APB1Periph_USART2, ENABLE);

    // 初始化USART2参数
    USART_InitStruct.USART_BaudRate = 115200;
    USART_InitStruct.USART_WordLength = USART_WordLength_8b;
    USART_InitStruct.USART_StopBits = USART_StopBits_1;
    USART_InitStruct.USART_Parity = USART_Parity_No;
    USART_InitStruct.USART_HardwareFlowControl = USART_HardwareFlowControl_None;
    USART_InitStruct.USART_Mode = USART_Mode_Tx;
    USART_Init(USART2, &USART_InitStruct);

    // 使能USART2
    USART_Cmd(USART2, ENABLE);

    // 配置DMA1时钟
    RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_DMA1, ENABLE);

    // 配置DMA1_Stream6
    DMA_InitStruct.DMA_Channel = DMA_Channel_4;
    DMA_InitStruct.DMA_PeripheralBaseAddr = (uint32_t)&USART2->DR;
    DMA_InitStruct.DMA_Memory0BaseAddr = (uint32_t)usart_tx_buffer;
    DMA_InitStruct.DMA_DIR = DMA_DIR_MemoryToPeripheral;
    DMA_InitStruct.DMA_BufferSize = 0;
    DMA_InitStruct.DMA_PeripheralInc = DMA_PeripheralInc_Disable;
    DMA_InitStruct.DMA_MemoryInc = DMA_MemoryInc_Enable;
    DMA_InitStruct.DMA_PeripheralDataSize = DMA_PeripheralDataSize_Byte;
    DMA_InitStruct.DMA_MemoryDataSize = DMA_MemoryDataSize_Byte;
    DMA_InitStruct.DMA_Mode = DMA_Mode_Normal;
    DMA_InitStruct.DMA_Priority = DMA_Priority_High;
    DMA_InitStruct.DMA_FIFOMode = DMA_FIFOMode_Disable;
    DMA_InitStruct.DMA_FIFOThreshold = DMA_FIFOThreshold_HalfFull;
    DMA_InitStruct.DMA_MemoryBurst = DMA_MemoryBurst_Single;
    DMA_InitStruct.DMA_PeripheralBurst = DMA_PeripheralBurst_Single;
    DMA_Init(DMA1_Stream6, &DMA_InitStruct);

    // 使能DMA1_Stream6传输完成中断
    DMA_ITConfig(DMA1_Stream6, DMA_IT_TC, ENABLE);

    // 配置NVIC中断优先级
    NVIC_InitTypeDef NVIC_InitStruct;
    NVIC_InitStruct.NVIC_IRQChannel = DMA1_Stream6_IRQn;
    NVIC_InitStruct.NVIC_IRQChannelPreemptionPriority = 0;
    NVIC_InitStruct.NVIC_IRQChannelSubPriority = 0;
    NVIC_InitStruct.NVIC_IRQChannelCmd = ENABLE;
    NVIC_Init(&NVIC_InitStruct);
}

void USART2_DMA_Send(uint8_t *data, uint16_t len) {
    uint16_t i;

    for (i = 0; i < len; i++) {
        // 将数据写入缓冲区
        usart_tx_buffer[usart_tx_head] = data[i];
        usart_tx_head = (usart_tx_head + 1) % USART_TX_BUFFER_SIZE;
    }

    // 如果DMA未在传输，则启动DMA传输
    if (DMA_GetCmdStatus(DMA1_Stream6) == DISABLE) {
        DMA_Cmd(DMA1_Stream6, DISABLE);
        DMA_SetCurrDataCounter(DMA1_Stream6, usart_tx_head > usart_tx_tail ? usart_tx_head - usart_tx_tail : USART_TX_BUFFER_SIZE - usart_tx_tail);
        DMA_Cmd(DMA1_Stream6, ENABLE);
    }
}

void DMA1_Stream6_IRQHandler(void) {
    // 清除传输完成中断标志位
    DMA_ClearITPendingBit(DMA1_Stream6, DMA_IT_TCIF6);

    // 更新缓冲区尾指针
    usart_tx_tail = usart_tx_head;

    // 如果缓冲区不为空，则启动DMA传输
    if (usart_tx_head != usart_tx_tail) {
        DMA_Cmd(DMA1_Stream6, DISABLE);
        DMA_SetCurrDataCounter(DMA1_Stream6, usart_tx_head > usart_tx_tail ? usart_tx_head - usart_tx_tail : USART_TX_BUFFER_SIZE - usart_tx_tail);
        DMA_Cmd(DMA1_Stream6, ENABLE);
    }
}

int main(void) {
    USART2_Init();

    // 发送字符串 "Hello, world!\r\n"
    USART2_DMA_Send((uint8_t *)"Hello, world!\r\n", strlen("Hello, world!\r\n"));

    while (1);

    return 0;
}

在上面的代码中，首先定义了一个`usart_tx_buffer`数组作为串口发送缓冲区，以及`usart_tx_head`和`usart_tx_tail`两个指针来指示缓冲区的头和尾。然后，在`USART2_Init()`函数中，配置了USART2和DMA1，使其能够通过DMA发送数据。其中，`USART2_DMA_Send()`函数用于将数据写入缓冲区，并启动DMA传输。最后，在`main()`函数中，调用`USART2_DMA_Send()`发送字符串"Hello, world!\r\n"。当DMA传输完成时，会触发`DMA1_Stream6_IRQHandler()`中断处理函数，更新缓冲区的尾指针，并在缓冲区不为空时启动DMA传输。