Linux SPI框架(下)

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;intstatus;unsigned longminor;/* Allocate driver data */spidev = kzalloc(sizeof(*spidev), GFP_KERNEL);if (!spidev)return -ENOMEM;/* Initialize the driver data */spidev->spi = spi;//设定spispin_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);elsekfree(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_tspidev_write(struct file *filp, const char __user *buf,size_t count, loff_t *f_pos){struct spidev_data*spidev;ssize_tstatus = 0;unsigned longmissing;/* 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->buffermissing = 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);} elsestatus = -EFAULT;mutex_unlock(&spidev->buf_lock);return status;}

static inline ssize_tspidev_sync_write(struct spidev_data *spidev, size_t len){struct spi_transfert = {//设置传输字段.tx_buf= spidev->buffer,.len= len,};struct spi_message m;//创建messagespi_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unsignedlen;    //传输数据的长度dma_addr_ttx_dma;dma_addr_trx_dma;unsignedcs_change:1; //该位如果为1，则表示当该transfer传输完后，改变片选信号u8bits_per_word;//字比特数u16delay_usecs;  //传输后的延时 u32speed_hz;  //指定的时钟struct list_head transfer_list;//用于将该transfer链入message};

struct spi_message {struct list_headtransfers;//用于链接spi_transferstruct spi_device*spi;      //指向目的从设备unsignedis_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unsignedactual_length;            //实际传输的长度intstatus;/* 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_headqueue; //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

static ssize_tspidev_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;elsestatus = 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 intspi_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 longflags;intstatus = 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bitbangunsigned longflags;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;unsignednsecs;struct spi_transfer*t = NULL;unsignedtmp;unsignedcs_change;intstatus;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。

1楼zi776424753天前 20:52

确实很勤快啊……希望能在毕业前 成为专家哦！