有了SPI總線設備對象，還需要實現總線的操作方法，操作方法的函數指針定義已經在SPI總線設備框架中給出了：

/**
 * SPI operators
 */
struct rt_spi_ops
{
    rt_err_t (*configure)(struct rt_spi_device *device, struct rt_spi_configuration *configuration);
    rt_uint32_t (*xfer)(struct rt_spi_device *device, struct rt_spi_message *message);
};

      configure：有兩個輸入參數，其作用就是根據configuration配置參數配置SPI總線設備的傳輸數據寬度、時鐘極性、時鐘相位和總線速率等參數，最后調用HAL庫初始化SPI總線。其stm32的實現代碼如下：

/**
 * SPI configuration structure
 */
struct rt_spi_configuration
{
    rt_uint8_t mode;
    rt_uint8_t data_width;
    rt_uint16_t reserved;

    rt_uint32_t max_hz;
};
static rt_err_t spi_configure(struct rt_spi_device *device,
                              struct rt_spi_configuration *configuration)
{
    RT_ASSERT(device != RT_NULL);
    RT_ASSERT(configuration != RT_NULL);

    struct stm32_spi *spi_drv =  rt_container_of(device->bus, struct stm32_spi, spi_bus);
    spi_drv->cfg = configuration;

    return stm32_spi_init(spi_drv, configuration);
}
static rt_err_t stm32_spi_init(struct stm32_spi *spi_drv, struct rt_spi_configuration *cfg)
{
    RT_ASSERT(spi_drv != RT_NULL);
    RT_ASSERT(cfg != RT_NULL);

    SPI_HandleTypeDef *spi_handle = &spi_drv->handle;

    if (cfg->mode & RT_SPI_SLAVE)
    {
        spi_handle->Init.Mode = SPI_MODE_SLAVE;
    }
    else
    {
        spi_handle->Init.Mode = SPI_MODE_MASTER;
    }

    if (cfg->mode & RT_SPI_3WIRE)
    {
        spi_handle->Init.Direction = SPI_DIRECTION_1LINE;
    }
    else
    {
        spi_handle->Init.Direction = SPI_DIRECTION_2LINES;
    }

    if (cfg->data_width == 8)
    {
        spi_handle->Init.DataSize = SPI_DATASIZE_8BIT;
        spi_handle->TxXferSize = 8;
        spi_handle->RxXferSize = 8;
    }
    else if (cfg->data_width == 16)
    {
        spi_handle->Init.DataSize = SPI_DATASIZE_16BIT;
    }
    else
    {
        return RT_EIO;
    }

    if (cfg->mode & RT_SPI_CPHA)
    {
        spi_handle->Init.CLKPhase = SPI_PHASE_2EDGE;
    }
    else
    {
        spi_handle->Init.CLKPhase = SPI_PHASE_1EDGE;
    }

    if (cfg->mode & RT_SPI_CPOL)
    {
        spi_handle->Init.CLKPolarity = SPI_POLARITY_HIGH;
    }
    else
    {
        spi_handle->Init.CLKPolarity = SPI_POLARITY_LOW;
    }

    if (cfg->mode & RT_SPI_NO_CS)
    {
        spi_handle->Init.NSS = SPI_NSS_HARD_OUTPUT;
    }
    else
    {
        spi_handle->Init.NSS = SPI_NSS_SOFT;
    }
    ...
    if (HAL_SPI_Init(spi_handle) != HAL_OK)
    {
        return RT_EIO;
    }
    ...
}

      當你需要更換MCU的時候，你就需要重寫上述的驅動部分代碼了。接下來看下xfer:用于傳輸數據，通過xger方法對SPI總線的控制來完成一條message的傳輸，這里的傳輸肯能是雙向的 也可能是單向的，也就是所謂的單雙工，最終都是通過stm32的hal庫來實現，直接看代碼：

static rt_uint32_t spixfer(struct rt_spi_device *device, struct rt_spi_message *message)
{
    HAL_StatusTypeDef state;
    rt_size_t message_length, already_send_length;
    rt_uint16_t send_length;
    rt_uint8_t *recv_buf;
    const rt_uint8_t *send_buf;

    RT_ASSERT(device != RT_NULL);
    RT_ASSERT(device->bus != RT_NULL);
    RT_ASSERT(device->bus->parent.user_data != RT_NULL);
    RT_ASSERT(message != RT_NULL);

    struct stm32_spi *spi_drv =  rt_container_of(device->bus, struct stm32_spi, spi_bus);
    SPI_HandleTypeDef *spi_handle = &spi_drv->handle;
    struct stm32_hw_spi_cs *cs = device->parent.user_data;

    if (message->cs_take && !(device->config.mode & RT_SPI_NO_CS))
    {
        HAL_GPIO_WritePin(cs->GPIOx, cs->GPIO_Pin, GPIO_PIN_RESET);
    }
	...
    while (message_length)
    {


        /* calculate the start address */
        already_send_length = message->length - send_length - message_length;
        send_buf = (rt_uint8_t *)message->send_buf + already_send_length;
        recv_buf = (rt_uint8_t *)message->recv_buf + already_send_length;

        /* start once data exchange in DMA mode */
        if (message->send_buf && message->recv_buf)
        {
            if ((spi_drv->spi_dma_flag & SPI_USING_TX_DMA_FLAG) && (spi_drv->spi_dma_flag & SPI_USING_RX_DMA_FLAG))
            {
                state = HAL_SPI_TransmitReceive_DMA(spi_handle, (uint8_t *)send_buf, (uint8_t *)recv_buf, send_length);
            }
            else
            {
                state = HAL_SPI_TransmitReceive(spi_handle, (uint8_t *)send_buf, (uint8_t *)recv_buf, send_length, 1000);
            }
        }
        else if (message->send_buf)
        {
            if (spi_drv->spi_dma_flag & SPI_USING_TX_DMA_FLAG)
            {
                state = HAL_SPI_Transmit_DMA(spi_handle, (uint8_t *)send_buf, send_length);
            }
            else
            {
                state = HAL_SPI_Transmit(spi_handle, (uint8_t *)send_buf, send_length, 1000);
            }

            if (message->cs_release && (device->config.mode & RT_SPI_3WIRE))
            {
                /* release the CS by disable SPI when using 3 wires SPI */
                __HAL_SPI_DISABLE(spi_handle);
            }
        }
        else
        {
            memset((uint8_t *)recv_buf, 0xff, send_length);
            if (spi_drv->spi_dma_flag & SPI_USING_RX_DMA_FLAG)
            {
                state = HAL_SPI_Receive_DMA(spi_handle, (uint8_t *)recv_buf, send_length);
            }
            else
            {
                /* clear the old error flag */
                __HAL_SPI_CLEAR_OVRFLAG(spi_handle);
                state = HAL_SPI_Receive(spi_handle, (uint8_t *)recv_buf, send_length, 1000);
            }
        }

        if (state != HAL_OK)
        {
            LOG_I("spi transfer error : %d", state);
            message->length = 0;
            spi_handle->State = HAL_SPI_STATE_READY;
        }
        else
        {
            LOG_D("%s transfer done", spi_drv->config->bus_name);
        }
        while (HAL_SPI_GetState(spi_handle) != HAL_SPI_STATE_READY);
    }

    if (message->cs_release && !(device->config.mode & RT_SPI_NO_CS))
    {
        HAL_GPIO_WritePin(cs->GPIOx, cs->GPIO_Pin, GPIO_PIN_SET);
    }

    return message->length;
}

      這里刪減了一些不影響函數主要功能的代碼，主要體現函數的功能，根據message中recv_buf和send_buf判斷是全雙工還是半雙工發送接收數據，調用hal庫函數完成數據的傳輸，最后釋放cs引腳。 最后就是完成SPI總線設備注冊到操作系統中，需要定義rt_spi_ops來完成初始化時注冊借口中的ops參數：

static const struct rt_spi_ops stm_spi_ops =
{
    .configure = spi_configure,
    .xfer = spixfer,
};

static int rt_hw_spi_bus_init(void)
{
    rt_err_t result;
    for (int i = 0; i < sizeof(spi_config) / sizeof(spi_config[0]); i++)
    {
        spi_bus_obj[i].config = &spi_config[i];
        spi_bus_obj[i].spi_bus.parent.user_data = &spi_config[i];
        spi_bus_obj[i].handle.Instance = spi_config[i].Instance;
        result = rt_spi_bus_register(&spi_bus_obj[i].spi_bus, spi_config[i].bus_name, &stm_spi_ops);
        RT_ASSERT(result == RT_EOK);

        LOG_D("%s bus init done", spi_config[i].bus_name);
    }

    return result;
}

          到這里就是關于SPI驅動部分的核心代碼講解完畢了，當然還有attach、DMA、SPI_IRQHandler部分源碼沒有詳細的羅列，這一部分就交給對驅動感興趣的小伙伴去源碼里探索吧。