RT-Thread中的串口DMA分析

這裏分析一下RT-Thread中串口DMA方式的實現，以供作新處理器串口支持時的參考。

背景

在現在的芯片性能和外設強大功能的狀況下，串口不實現DMA/中斷方式操做，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做參考。

DMA接收準備

啓用DMA接收，須要在打開設備的時候作一些處理，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、校驗等串口工做參數。

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，其主要工做是爲DMA接收準備FIFO緩衝區；爲DMA發送準備發送數據緩衝隊列，可是好像STM32中斷並無用到發送數據緩衝。

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，通信缺省參數是：115200,8N1。

#define RT_SERIAL_RB_BUFSZ 64 

DMA接收

DMA接收咱們從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 *)，主體代碼以下：

static void dma_isr(struct rt_serial_device *serial) { ...... /* 若是是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(); /* 獲得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 { /* 減去之前接收到的數據量，獲得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) { /* 若是有新數據，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實現並無處理這個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()再調用操做的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) ...... } 

實現很是簡單。