Linux SPI 数据收发框架

水平有限，描述不当之处还请之处，转载请注明出处 http://blog.csdn.net/vanbreaker/article/details/7737833   

     本节以spidev设备驱动为例，来阐述SPI数据传输的过程。spidev是内核中一个通用的设备驱动，我们注册的从设备都可以使用该驱动，只需在注册时将从设备的modalias字段设置为"spidev",这样才能和spidev驱动匹配成功。我们要传输的数据有时需要分为一段一段的(比如先发送，后读取，就需要两个字段)，每个字段都被封装成一个transfer,N个transfer可以被添加到message中，作为一个消息包进行传输。当用户发出传输数据的请求时，message并不会立刻传输到从设备，而是由之前定义的transfer()函数将message放入一个等待队列中，这些message会以FIFO的方式有workqueue调度进行传输，这样能够避免SPI从设备同一时间对主SPI控制器的竞争。和之前一样，还是习惯先画一张图来描述数据传输的主要过程。

 

         在使用spidev设备驱动时，需要先初始化spidev. spidev是以字符设备的形式注册进内核的。

 static int __init spidev_init(void)
 {
     int status;

     /* Claim our 256 reserved device numbers.  Then register a class
      * that will key udev/mdev to add/remove /dev nodes.  Last, register
      * the driver which manages those device numbers.
      */
     BUILD_BUG_ON(N_SPI_MINORS > 256);
     /*将spidev作为字符设备注册*/
     status = register_chrdev(SPIDEV_MAJOR, "spi", &spidev_fops);
     if (status < 0)
         return status;

     /*创建spidev类*/
     spidev_class = class_create(THIS_MODULE, "spidev");
     if (IS_ERR(spidev_class)) {
         unregister_chrdev(SPIDEV_MAJOR, spidev_spi.driver.name);
         return PTR_ERR(spidev_class);
     }

     /*注册spidev的driver,可与modalias字段为"spidev"的spi_device匹配*/
     status = spi_register_driver(&spidev_spi);
     if (status < 0) {
         class_destroy(spidev_class);
         unregister_chrdev(SPIDEV_MAJOR, spidev_spi.driver.name);
     }
     return status;
 }

与相应的从设备匹配成功后，则调用spidev中的probe函数

 static int spidev_probe(struct spi_device *spi)
 {
     struct spidev_data  *spidev;
     int         status;
     unsigned long       minor;

     /* Allocate driver data */
     spidev = kzalloc(sizeof(*spidev), GFP_KERNEL);
     if (!spidev)
         return -ENOMEM;

     /* Initialize the driver data */
     spidev->spi = spi;//设定spi
     spin_lock_init(&spidev->spi_lock);
     mutex_init(&spidev->buf_lock);

     INIT_LIST_HEAD(&spidev->device_entry);

     /* If we can allocate a minor number, hook up this device.
      * Reusing minors is fine so long as udev or mdev is working.
      */
     mutex_lock(&device_list_lock);
     minor = find_first_zero_bit(minors, N_SPI_MINORS);//寻找没被占用的次设备号
     if (minor < N_SPI_MINORS) {
         struct device *dev;
         /*计算设备号*/
         spidev->devt = MKDEV(SPIDEV_MAJOR, minor);
         /*在spidev_class下创建设备*/
         dev = device_create(spidev_class, &spi->dev, spidev->devt,
                     spidev, "spidev%d.%d",
                     spi->master->bus_num, spi->chip_select);
         status = IS_ERR(dev) ? PTR_ERR(dev) : 0;
     } else {
         dev_dbg(&spi->dev, "no minor number available!\n");
         status = -ENODEV;
     }
     if (status == 0) {
         set_bit(minor, minors);//将minors的相应位置位，表示该位对应的次设备号已被占用
         list_add(&spidev->device_entry, &device_list);//将创建的spidev添加到device_list
     }
     mutex_unlock(&device_list_lock);

     if (status == 0)
         spi_set_drvdata(spi, spidev);
     else
         kfree(spidev);

     return status;
 }

然后就可以利用spidev模块提供的接口来实现主从设备之间的数据传输了。我们以spidev_write()函数为例来分析数据传输的过程，实际上spidev_read()和其是差不多的，只是前面的一些步骤不一样，可以参照上图。

 static ssize_t
 spidev_write(struct file *filp, const char __user *buf,
         size_t count, loff_t *f_pos)
 {
     struct spidev_data  *spidev;
     ssize_t         status = 0;
     unsigned long       missing;

     /* chipselect only toggles at start or end of operation */
     if (count > bufsiz)
         return -EMSGSIZE;

     spidev = filp->private_data;

     mutex_lock(&spidev->buf_lock);
     //将用户要发送的数据拷贝到spidev->buffer
     missing = copy_from_user(spidev->buffer, buf, count);
     if (missing == 0) {//全部拷贝成功，则调用spidev_sysn_write()
         status = spidev_sync_write(spidev, count);
     } else
         status = -EFAULT;
     mutex_unlock(&spidev->buf_lock);

     return status;
 }

 

 static inline ssize_t
 spidev_sync_write(struct spidev_data *spidev, size_t len)
 {
     struct spi_transfer t = {//设置传输字段
             .tx_buf     = spidev->buffer,
             .len        = len,
         };
     struct spi_message   m;//创建message

     spi_message_init(&m);
     spi_message_add_tail(&t, &m);//将transfer添加到message中
     return spidev_sync(spidev, &m);
 }

我们来看看struct spi_transfer和struct spi_message是如何定义的

 struct spi_transfer {
     /* it's ok if tx_buf == rx_buf (right?)
      * for MicroWire, one buffer must be null
      * buffers must work with dma_*map_single() calls, unless
      *   spi_message.is_dma_mapped reports a pre-existing mapping
      */
     const void  *tx_buf;//发送缓冲区
     void        *rx_buf;//接收缓冲区
     unsigned    len;    //传输数据的长度

     dma_addr_t  tx_dma;
     dma_addr_t  rx_dma;

     unsigned    cs_change:1; //该位如果为1，则表示当该transfer传输完后，改变片选信号
     u8      bits_per_word;//字比特数
     u16     delay_usecs;  //传输后的延时
     u32     speed_hz;  //指定的时钟

     struct list_head transfer_list;//用于将该transfer链入message
 };

 

 struct spi_message {
     struct list_head    transfers;//用于链接spi_transfer

     struct spi_device   *spi;      //指向目的从设备

     unsigned        is_dma_mapped:1;

     /* REVISIT:  we might want a flag affecting the behavior of the
      * last transfer ... allowing things like "read 16 bit length L"
      * immediately followed by "read L bytes".  Basically imposing
      * a specific message scheduling algorithm.
      *
      * Some controller drivers (message-at-a-time queue processing)
      * could provide that as their default scheduling algorithm.  But
      * others (with multi-message pipelines) could need a flag to
      * tell them about such special cases.
      */

     /* completion is reported through a callback */
     void            (*complete)(void *context);//用于异步传输完成时调用的回调函数
     void            *context;                  //回调函数的参数
     unsigned        actual_length;            //实际传输的长度
     int         status;

     /* for optional use by whatever driver currently owns the
      * spi_message ...  between calls to spi_async and then later
      * complete(), that's the spi_master controller driver.
      */
     struct list_head    queue; //用于将该message链入bitbang等待队列
     void            *state;
 };

继续跟踪源码，进入spidev_sync(),从这一步开始，read和write就完全一样了

 "font-size:12px;">static ssize_t
 spidev_sync(struct spidev_data *spidev, struct spi_message *message)
 {
     DECLARE_COMPLETION_ONSTACK(done);
     int status;

     message->complete = spidev_complete;//设置回调函数
     message->context = &done;

     spin_lock_irq(&spidev->spi_lock);
     if (spidev->spi == NULL)
         status = -ESHUTDOWN;
     else
         status = spi_async(spidev->spi, message);//调用spi核心层的函数spi_async()
     spin_unlock_irq(&spidev->spi_lock);

     if (status == 0) {
         wait_for_completion(&done);
         status = message->status;
         if (status == 0)
             status = message->actual_length;
     }
     return status;
 }

 

 static inline int
 spi_async(struct spi_device *spi, struct spi_message *message)
 {
     message->spi = spi;
     /*调用master的transfer函数将message放入等待队列*/
     return spi->master->transfer(spi, message);
 }

 

s3c24xx平台下的transfer函数是在bitbang_start()函数中定义的，为bitbang_transfer()

 int spi_bitbang_transfer(struct spi_device *spi, struct spi_message *m)
 {
     struct spi_bitbang  *bitbang;
     unsigned long       flags;
     int         status = 0;

     m->actual_length = 0;
     m->status = -EINPROGRESS;

     bitbang = spi_master_get_devdata(spi->master);

     spin_lock_irqsave(&bitbang->lock, flags);
     if (!spi->max_speed_hz)
         status = -ENETDOWN;
     else {
         list_add_tail(&m->queue, &bitbang->queue);//将message添加到bitbang的等待队列
         queue_work(bitbang->workqueue, &bitbang->work);//调度运行work
     }
     spin_unlock_irqrestore(&bitbang->lock, flags);

     return status;
 }

这里可以看到transfer函数不负责实际的数据传输，而是将message添加到等待队列中。同样在spi_bitbang_start()中，有这样一个定义INIT_WORK(&bitbang->work, bitbang_work);因此bitbang_work()函数会被调度运行，类似于底半部机制

 static void bitbang_work(struct work_struct *work)
 {
     struct spi_bitbang  *bitbang =
         container_of(work, struct spi_bitbang, work);//获取bitbang
     unsigned long       flags;

     spin_lock_irqsave(&bitbang->lock, flags);
     bitbang->busy = 1;
     while (!list_empty(&bitbang->queue)) {//等待队列不为空
         struct spi_message  *m;
         struct spi_device   *spi;
         unsigned        nsecs;
         struct spi_transfer *t = NULL;
         unsigned        tmp;
         unsigned        cs_change;
         int         status;
         int         (*setup_transfer)(struct spi_device *,
                         struct spi_transfer *);
         /*取出等待队列中的的第一个message*/
         m = container_of(bitbang->queue.next, struct spi_message,
                 queue);
         list_del_init(&m->queue);//将message从队列中删除
         spin_unlock_irqrestore(&bitbang->lock, flags);

         /* FIXME this is made-up ... the correct value is known to
          * word-at-a-time bitbang code, and presumably chipselect()
          * should enforce these requirements too?
          */
         nsecs = 100;

         spi = m->spi;
         tmp = 0;
         cs_change = 1;
         status = 0;
         setup_transfer = NULL;

         /*遍历message中的所有传输字段,逐一进行传输*/
         list_for_each_entry (t, &m->transfers, transfer_list) {

             /* override or restore speed and wordsize */
             if (t->speed_hz || t->bits_per_word) {
                 setup_transfer = bitbang->setup_transfer;
                 if (!setup_transfer) {
                     status = -ENOPROTOOPT;
                     break;
                 }
             }
             /*调用setup_transfer根据transfer中的信息进行时钟、字比特数的设定*/
             if (setup_transfer) {
                 status = setup_transfer(spi, t);
                 if (status < 0)
                     break;
             }

             /* set up default clock polarity, and activate chip;
              * this implicitly updates clock and spi modes as
              * previously recorded for this device via setup().
              * (and also deselects any other chip that might be
              * selected ...)
              */
             if (cs_change) {//使能外设的片选
                 bitbang->chipselect(spi, BITBANG_CS_ACTIVE);
                 ndelay(nsecs);
             }
             cs_change = t->cs_change;//这里确定进行了这个字段的传输后是否要改变片选状态
             if (!t->tx_buf && !t->rx_buf && t->len) {
                 status = -EINVAL;
                 break;
             }

             /* transfer data.  the lower level code handles any
              * new dma mappings it needs. our caller always gave
              * us dma-safe buffers.
              */
             if (t->len) {
                 /* REVISIT dma API still needs a designated
                  * DMA_ADDR_INVALID; ~0 might be better.
                  */
                 if (!m->is_dma_mapped)
                     t->rx_dma = t->tx_dma = 0;
                 /*调用针对于平台的传输函数txrx_bufs*/
                 status = bitbang->txrx_bufs(spi, t);
             }
             if (status > 0)
                 m->actual_length += status;
             if (status != t->len) {
                 /* always report some kind of error */
                 if (status >= 0)
                     status = -EREMOTEIO;
                 break;
             }
             status = 0;

             /* protocol tweaks before next transfer */
             /*如果要求在传输完一个字段后进行delay,则进行delay*/
             if (t->delay_usecs)
                 udelay(t->delay_usecs);

             if (!cs_change)
                 continue;

             /*最后一个字段传输完毕了，则跳出循环*/
             if (t->transfer_list.next == &m->transfers)
                 break;

             /* sometimes a short mid-message deselect of the chip
              * may be needed to terminate a mode or command
              */
             ndelay(nsecs);
             bitbang->chipselect(spi, BITBANG_CS_INACTIVE);
             ndelay(nsecs);
         }

         m->status = status;
         m->complete(m->context);

         /* restore speed and wordsize */
         if (setup_transfer)
             setup_transfer(spi, NULL);

         /* normally deactivate chipselect ... unless no error and
          * cs_change has hinted that the next message will probably
          * be for this chip too.
          */
         if (!(status == 0 && cs_change)) {
             ndelay(nsecs);
             bitbang->chipselect(spi, BITBANG_CS_INACTIVE);
             ndelay(nsecs);
         }

         spin_lock_irqsave(&bitbang->lock, flags);
     }
     bitbang->busy = 0;
     spin_unlock_irqrestore(&bitbang->lock, flags);
 }

只要bitbang->queue等待队列不为空，就表示相应的SPI主控制器上还有传输任务没有完成，因此bitbang_work()会被不断地调度执行。 bitbang_work()中的工作主要是两个循环，外循环遍历等待队列中的message,内循环遍历message中的transfer,在bitbang_work()中，传输总是以transfer为单位的。当选定了一个transfer后，便会调用transfer_txrx()函数，进行实际的数据传输，显然这个函数是针对于平台的SPI控制器而实现的，在s3c24xx平台中，该函数为s3c24xx_spi_txrx();

 static int s3c24xx_spi_txrx(struct spi_device *spi, struct spi_transfer *t)
 {
     struct s3c24xx_spi *hw = to_hw(spi);

     dev_dbg(&spi->dev, "txrx: tx %p, rx %p, len %d\n",
         t->tx_buf, t->rx_buf, t->len);

     hw->tx = t->tx_buf;//获取发送缓冲区
     hw->rx = t->rx_buf;//获取读取缓存区
     hw->len = t->len;  //获取数据长度
     hw->count = 0;

     init_completion(&hw->done);//初始化完成量

     /* send the first byte */
     /*只发送第一个字节，其他的在中断中发送(读取)*/
     writeb(hw_txbyte(hw, 0), hw->regs + S3C2410_SPTDAT);

     wait_for_completion(&hw->done);

     return hw->count;
 }

 

 static inline unsigned int hw_txbyte(struct s3c24xx_spi *hw, int count)
 {
     /*如果tx不为空，也就是说当前是从主机向从机发送数据，则直接将tx[count]发送过去，
       如果tx为空，也就是说当前是从从机向主机发送数据，则向从机写入0*/
     return hw->tx ? hw->tx[count] : 0;
 }

负责SPI数据传输的中断函数：

 static irqreturn_t s3c24xx_spi_irq(int irq, void *dev)
 {
     struct s3c24xx_spi *hw = dev;
     unsigned int spsta = readb(hw->regs + S3C2410_SPSTA);
     unsigned int count = hw->count;

     /*冲突检测*/
     if (spsta & S3C2410_SPSTA_DCOL) {
         dev_dbg(hw->dev, "data-collision\n");
         complete(&hw->done);
         goto irq_done;
     }

     /*设备忙检测*/
     if (!(spsta & S3C2410_SPSTA_READY)) {
         dev_dbg(hw->dev, "spi not ready for tx?\n");
         complete(&hw->done);
         goto irq_done;
     }

     hw->count++;

     if (hw->rx)//读取数据到缓冲区
         hw->rx[count] = readb(hw->regs + S3C2410_SPRDAT);

     count++;

     if (count < hw->len)//向从机写入数据
         writeb(hw_txbyte(hw, count), hw->regs + S3C2410_SPTDAT);
     else//count == len，一个字段发送完成，唤醒完成量
         complete(&hw->done);

  irq_done:
     return IRQ_HANDLED;
 }

这里可以看到一点，即使tx为空，也就是说用户申请的是从从设备读取数据，也要不断地向从设备写入数据，只不过写入从设备的是无效数据(0)，这样做得目的是为了维持SPI总线上的时钟。至此，SPI框架已分析完毕。