分类: 嵌入式
2013-08-21 16:16:57
原文地址:Linux SPI框架(下) 作者:enzo26
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
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。