這裏分析一下RT-Thread中串口DMA方式的實現,以供做新處理器串口支持時的參考。
背景
在如今的芯片性能和外設強大功能的情況下,串口不實現DMA/中斷方式操作,我認爲在實際項目中基本是不可接受的,但遺憾的是,rt-thread現有支持的實現中,基本上沒有支持串口的DMA,文檔也沒有關於串口DMA支持相關的說明,這裏以STM32實現爲背景,梳理一下串口DMA的實現流程,以供新處理器實現時以作參考。
DMA接收準備
啓用DMA接收,需要在打開設備的時候做一些處理,入口函數爲rt_device_open()。主體實現是:
rt_err_t rt_device_open(rt_device_t dev, rt_uint16_t oflag)
{
......
result = device_init(dev);
......
result = device_open(dev, oflag);
......
}
device_init()就是rt_serial_init()函數,其主要是調用configure()函數,
static rt_err_t rt_serial_init(struct rt_device *dev)
{
......
if (serial->ops->configure)
result = serial->ops->configure(serial, &serial->config);
......
}
在stm32下,其configure()函數是stm32_configure(),其根據設備打開參數,配置STM32外設的寄存器。包括波特率、校驗等串口工作參數。
device_open()函數就是rt_serial_open()函數,其主要實現是:
static rt_err_t rt_serial_open(struct rt_device *dev, rt_uint16_t oflag)
{
......
#ifdef RT_SERIAL_USING_DMA
else if (oflag & RT_DEVICE_FLAG_DMA_RX)
{
if (serial->config.bufsz == 0) {
struct rt_serial_rx_dma* rx_dma;
rx_dma = (struct rt_serial_rx_dma*) rt_malloc (sizeof(struct rt_serial_rx_dma));
RT_ASSERT(rx_dma != RT_NULL);
rx_dma->activated = RT_FALSE;
serial->serial_rx = rx_dma;
} else {
struct rt_serial_rx_fifo* rx_fifo;
rx_fifo = (struct rt_serial_rx_fifo*) rt_malloc (sizeof(struct rt_serial_rx_fifo) +
serial->config.bufsz);
RT_ASSERT(rx_fifo != RT_NULL);
rx_fifo->buffer = (rt_uint8_t*) (rx_fifo + 1);
rt_memset(rx_fifo->buffer, 0, serial->config.bufsz);
rx_fifo->put_index = 0;
rx_fifo->get_index = 0;
rx_fifo->is_full = RT_FALSE;
serial->serial_rx = rx_fifo;
/* configure fifo address and length to low level device */
serial->ops->control(serial, RT_DEVICE_CTRL_CONFIG, (void *) RT_DEVICE_FLAG_DMA_RX);
}
dev->open_flag |= RT_DEVICE_FLAG_DMA_RX;
}
#endif /* RT_SERIAL_USING_DMA */
......
#ifdef RT_SERIAL_USING_DMA
else if (oflag & RT_DEVICE_FLAG_DMA_TX)
{
struct rt_serial_tx_dma* tx_dma;
tx_dma = (struct rt_serial_tx_dma*) rt_malloc (sizeof(struct rt_serial_tx_dma));
RT_ASSERT(tx_dma != RT_NULL);
tx_dma->activated = RT_FALSE;
rt_data_queue_init(&(tx_dma->data_queue), 8, 4, RT_NULL);
serial->serial_tx = tx_dma;
dev->open_flag |= RT_DEVICE_FLAG_DMA_TX;
/* configure low level device */
serial->ops->control(serial, RT_DEVICE_CTRL_CONFIG, (void *)RT_DEVICE_FLAG_DMA_TX);
}
#endif /* RT_SERIAL_USING_DMA */
......
}
可見,其主要工作是爲DMA接收準備FIFO緩衝區;爲DMA發送準備發送數據緩衝隊列,但是好像STM32中斷並沒有用到發送數據緩衝。
DMA配置數據來源是rt_hw_usart_init()函數,缺省的配置參數由宏RT_SERIAL_CONFIG_DEFAULT決定, 這裏決定了缺省的接收緩衝區參數是64字節,通訊缺省參數是:115200,8N1。
#define RT_SERIAL_RB_BUFSZ 64
DMA接收
DMA接收我們從DMA中斷開始分析,DMA接收中斷服務函數爲UARTn_DMA_RX_IRQHandler(),其調用HAL庫的DMA處理函數HAL_DMA_IRQHandler(),該函數調用回調函數HAL_UART_RxCpltCallback()或HAL_UART_RxHalfCpltCallback(),這兩個函數進入真正的中斷服務處理函數dma_isr(struct rt_serial_device *),主體代碼如下:
static void dma_isr(struct rt_serial_device *serial)
{
......
/* 如果是DMA-RX中斷 */
if ((__HAL_DMA_GET_IT_SOURCE(&(uart->dma_rx.handle), DMA_IT_TC) != RESET) ||
(__HAL_DMA_GET_IT_SOURCE(&(uart->dma_rx.handle), DMA_IT_HT) != RESET))
{
level = rt_hw_interrupt_disable();
/* 得到本次接收到的數據量 */
recv_total_index = serial->config.bufsz - __HAL_DMA_GET_COUNTER(&(uart->dma_rx.handle));
if (recv_total_index == 0)
{
/* 這一句代碼,是什麼意思? */
recv_len = serial->config.bufsz - uart->dma_rx.last_index;
}
else
{
/* 減去以前接收到的數據量,得到本次接收到的數據數量 */
recv_len = recv_total_index - uart->dma_rx.last_index;
}
/* 更新接收歷史數據量 */
uart->dma_rx.last_index = recv_total_index;
rt_hw_interrupt_enable(level);
if (recv_len)
{
/* 如果有新數據,調用serial設備模塊的通用處理 */
rt_hw_serial_isr(serial, RT_SERIAL_EVENT_RX_DMADONE | (recv_len << 8));
}
}
}
在serial模塊的函數rt_hw_serial_isr()中,主體代碼是:
void rt_hw_serial_isr(struct rt_serial_device *serial, int event)
{
......
case RT_SERIAL_EVENT_RX_DMADONE:
{
int length;
rt_base_t level;
/* get DMA rx length */
length = (event & (~0xff)) >> 8;
if (serial->config.bufsz == 0)
{
/* 這個case的處理邏輯不知道怎麼應用,看起來STM32實現並沒有處理這個case */
struct rt_serial_rx_dma* rx_dma;
rx_dma = (struct rt_serial_rx_dma*) serial->serial_rx;
RT_ASSERT(rx_dma != RT_NULL);
RT_ASSERT(serial->parent.rx_indicate != RT_NULL);
serial->parent.rx_indicate(&(serial->parent), length);
rx_dma->activated = RT_FALSE;
}
else
{
/* disable interrupt */
level = rt_hw_interrupt_disable();
/* update fifo put index, 將數據放入接收緩衝區 */
rt_dma_recv_update_put_index(serial, length);
/* calculate received total length, 更新緩衝區信息 */
length = rt_dma_calc_recved_len(serial);
/* enable interrupt */
rt_hw_interrupt_enable(level);
/* invoke callback, 通知上層,有新數據到達 */
if (serial->parent.rx_indicate != RT_NULL)
{
serial->parent.rx_indicate(&(serial->parent), length);
}
}
break;
}
......
}
上層接到通知後,讀取函數最終調用驅動讀函數rt_serial_read()函數,在DMA的條件下,調用_serial_dma_rx()從緩衝區讀取數據。其代碼爲:
static rt_size_t rt_serial_read(struct rt_device *dev, rt_off_t pos, void *buffer, rt_size_t size)
{
......
else if (dev->open_flag & RT_DEVICE_FLAG_DMA_RX)
{
return _serial_dma_rx(serial, (rt_uint8_t *)buffer, size);
}
......
}
DMA發送
DMA發送從驅動寫函數rt_serial_write()開始,在DMA的條件下,調用_serial_dma_tx(),_serial_dma_tx()再調用操作的DMA發送函數發送數據,代碼爲:
static rt_size_t rt_serial_write(struct rt_device *dev, rt_off_t pos, const void *buffer, rt_size_t size)
{
......
else if (dev->open_flag & RT_DEVICE_FLAG_DMA_TX)
{
return _serial_dma_tx(serial, (const rt_uint8_t *)buffer, size);
}
......
}
rt_inline int _serial_dma_tx(struct rt_serial_device *serial, const rt_uint8_t *data, int length)
{
......
/* make a DMA transfer */
serial->ops->dma_transmit(serial, (rt_uint8_t *)data, length, RT_SERIAL_DMA_TX);
......
}
STM32的dma_transmit()實現函數是stm32_dma_transmit(),其實現就是簡單調用HAL_UART_Transmit_DMA(),代碼爲:
static rt_size_t stm32_dma_transmit(struct rt_serial_device *serial, rt_uint8_t *buf, rt_size_t size, int direction)
{
......
if (HAL_UART_Transmit_DMA(&uart->handle, buf, size) == HAL_OK)
......
}
實現非常簡單。