linux spi驱动开发学习(三)-----spi_bitbang.c详解

经过了前面两节的学习,现在到了这个环节了,spi驱动的完整工作过程渐渐明朗起来
不多说废话了,直接进主题,大家共同学习,共同进步

首先,还是先唠叨以下,以方便接下来对bitbang机制的学习,那就是spi 的工作时序,这里直接转载自己看到的一篇文章
这里也同时感谢楼主的分享

SPI时序图详解

(2009-10-18 21:49)

SPI接口在模式0下输出第一位数据的时刻

SPI接口有四种不同的数据传输时序，取决于CPOL和CPHL这两位的组合。图1中表现了这四种时序，
时序与CPOL、CPHL的关系也可以从图中看出。

图1

CPOL是用来决定SCK时钟信号空闲时的电平，CPOL＝0，空闲电平为低电平，CPOL＝1时，

空闲电平为高电平。CPHA是用来决定采样时刻的，CPHA=0，在每个周期的第一个时钟沿采样，

CPHA＝1，在每个周期的第二个时钟沿采样。

由于我使用的器件工作在模式0这种时序（CPOL＝0，CPHA＝0），所以将图1简化为图2，
只关注模式0的时序。

图2

我们来关注SCK的第一个时钟周期，在时钟的前沿采样数据（上升沿，第一个时钟沿），
在时钟的后沿输出数据（下降沿，第二个时钟沿）。首先来看主器件，主器件的输出口（MOSI）输出的数据bit1，
在时钟的前沿被从器件采样，那主器件是在何时刻输出bit1的呢？bit1的输出时刻实际上在SCK信号有效以前，
比 SCK的上升沿还要早半个时钟周期。bit1的输出时刻与SSEL信号没有关系。再来看从器件，
主器件的输入口MISO同样是在时钟的前沿采样从器件输出的bit1的，那从器件又是在何时刻输出bit1的呢。
从器件是在SSEL信号有效后，立即输出bit1，尽管此时SCK信号还没有起效。关于上面的主器件
和从器件输出bit1位的时刻，可以从图3、4中得到验证。

图3

注意图3中，CS信号有效后（低电平有效，注意CS下降沿后发生的情况），故意用延时程序
延时了一段时间，之后再向数据寄存器写入了要发送的数据，来观察主器件输出bit1的情况（MOSI）。
可以看出，bit1（值为1）是在SCK信号有效之前的半个时钟周期的时刻开始输出的（与CS信号无关），
到了SCK的第一个时钟周期的上升沿正好被从器件采样。

图4

图4中，注意看CS和MISO信号。我们可以看出，CS信号有效后，从器件立刻输出了bit1（值为1）。

通常我们进行的spi操作都是16位的。图5记录了第一个字节和第二个字节间的相互衔接的过程。
第一个字节的最后一位在SCK的上升沿被采样，随后的SCK下降沿，从器件就输出了第二个字节的第一位。

图5

熟悉了spi 的工作时序,接着往下说

注意:

CPOL是用来决定SCK时钟信号空闲时的电平，CPOL＝0，空闲电平为低电平，CPOL＝1时，空闲电平为高电平。
CPHA是用来决定采样时刻的，
CPHA=0，在每个周期的第一个时钟沿采样，
CPHA＝1，在每个周期的第二个时钟沿采样。

linux的SPI模型中重要的有如下几个结构体,位置include/linux/spi/spi.h

struct spi_device {}  //Master side proxy for an SPI slave device

struct spi_driver {}  //Host side "protocol" driver

struct spi_master {}  //interface to SPI master controller

struct spi_transfer{}  //a read/write buffer pair

在这几个结构体中，我们只注意一下device结构体

struct spi_device
{
.....
#define SPI_CPHA 0x01 /* clock phase 同步*/
#define SPI_CPOL 0x02 /* clock polarity */
#define SPI_MODE_0 (0|0) /* (original MicroWire) */
#define SPI_MODE_1 (0|SPI_CPHA)
#define SPI_MODE_2 (SPI_CPOL|0)
#define SPI_MODE_3 (SPI_CPOL|SPI_CPHA)
.....
}

注意如上定义的 SPI_CPHA和SPI_CPOL

这个和一中的CPHA,以及CPOL是对应的，

然后在次基础上定义了MODE?,到此，你是否能相像出SPI接口的数据传输过程？

下面才是刚刚进入真正的主题
以下是spi_bitbang结构体的定义

struct spi_bitbang {
    struct workqueue_struct    *workqueue;
    struct work_struct    work;

    spinlock_t        lock;
    struct list_head    queue;
    u8            busy;
    u8            use_dma;
    u8            flags;        /* extra spi->mode support */

    struct spi_master    *master;

    /* setup_transfer() changes clock and/or wordsize to match settings
     * for this transfer; zeroes restore defaults from spi_device.
     */
    int    (*setup_transfer)(struct spi_device *spi,
            struct spi_transfer *t);

    void    (*chipselect)(struct spi_device *spi, int is_on);
#define    BITBANG_CS_ACTIVE    1    /* normally nCS, active low */
#define    BITBANG_CS_INACTIVE    0

    /* txrx_bufs() may handle dma mapping for transfers that don't
     * already have one (transfer.{tx,rx}_dma is zero), or use PIO
     */
    int    (*txrx_bufs)(struct spi_device *spi, struct spi_transfer *t);

    /* txrx_word[SPI_MODE_*]() just looks like a shift register */
    u32    (*txrx_word[4])(struct spi_device *spi,
            unsigned nsecs,
            u32 word, u8 bits);
};

这里我们只对加底纹部分做以下说明:
struct work_struct work;
这个让我们想入菲菲--bit bang是按照workqueue队列的形式来工作的吗？

worqueue这个是什么？简单地说，他就是一个队列，里面的每一个work节点代表着一个需要调度的工作。
具体可以百度，这个和tasklet有点相似的。以前自己有用过,经常用于中断的顶半部,底半部机制
既然是worqueue，那么我们可以猜想，spi是在每一个work中实现bit bang的。
事实上确实如此，在/driver/spi/spi_bitbang.c#L267中，我们可以看到如下函数
static void bitbang_work(struct work_struct *work){},
在/driver/spi/spi_bitbang.c中，我们可以发现
int spi_bitbang_start(struct spi_bitbang *bitbang);
这里还是有必要看一下这个方法的实现过程,看他都干了些什么:

int spi_bitbang_start(struct spi_bitbang *bitbang)
{
    int    status;

    if (!bitbang->master || !bitbang->chipselect)
        return -EINVAL;

    INIT_WORK(&bitbang->work, bitbang_work);
    spin_lock_init(&bitbang->lock);
    INIT_LIST_HEAD(&bitbang->queue);

    if (!bitbang->master->mode_bits)//此处在s3c24xx_spi_probe方法中已定义,采用s3c24xx_spi_probe的设置
        bitbang->master->mode_bits = SPI_CPOL | SPI_CPHA | bitbang->flags;

    if (!bitbang->master->transfer)//此处在s3c24xx_spi_probe方法中没有定义,所以使用这里设置的方法
        bitbang->master->transfer = spi_bitbang_transfer;//之后发送数据的时候会用到这个接口

    if (!bitbang->txrx_bufs) {//此处在s3c24xx_spi_probe方法中已定义,采用s3c24xx_spi_probe的设置
        bitbang->use_dma = 0;
        bitbang->txrx_bufs = spi_bitbang_bufs;
        if (!bitbang->master->setup) {
            if (!bitbang->setup_transfer)
                bitbang->setup_transfer =
                     spi_bitbang_setup_transfer;
            bitbang->master->setup = spi_bitbang_setup;
            bitbang->master->cleanup = spi_bitbang_cleanup;
        }
    } else if (!bitbang->master->setup)//此处在s3c24xx_spi_probe方法中已定义,从这里看来bitbang->master->setup必须定义,不可遗漏
        return -EINVAL;

    /* this task is the only thing to touch the SPI bits */
    bitbang->busy = 0;
    bitbang->workqueue = create_singlethread_workqueue(
            dev_name(bitbang->master->dev.parent));
    if (bitbang->workqueue == NULL) {
        status = -EBUSY;
        goto err1;
    }

    /* driver may get busy before register() returns, especially
     * if someone registered boardinfo for devices
     */
    status = spi_register_master(bitbang->master);
    if (status < 0)
        goto err2;

    return status;

err2:
    destroy_workqueue(bitbang->workqueue);
err1:
    return status;
}
EXPORT_SYMBOL_GPL(spi_bitbang_start);

看看spi_register_master的实现过程:

int spi_register_master(struct spi_master *master)
{
    static atomic_t        dyn_bus_id = ATOMIC_INIT((1<<15) - 1);
    struct device        *dev = master->dev.parent;
    int            status = -ENODEV;
    int            dynamic = 0;

    if (!dev)
        return -ENODEV;

    /* even if it's just one always-selected device, there must
     * be at least one chipselect
     */
    if (master->num_chipselect == 0)
        return -EINVAL;

    /* convention: dynamically assigned bus IDs count down from the max */
    if (master->bus_num < 0) {
        /* FIXME switch to an IDR based scheme, something like
         * I2C now uses, so we can't run out of "dynamic" IDs
         */
        master->bus_num = atomic_dec_return(&dyn_bus_id);
        dynamic = 1;
    }

    /* register the device, then userspace will see it.
     * registration fails if the bus ID is in use.
     */
    dev_set_name(&master->dev, "spi%u", master->bus_num);
    status = device_add(&master->dev);//添加设备
    if (status < 0)
        goto done;
    dev_dbg(dev, "registered master %s%sn", dev_name(&master->dev),
            dynamic ? " (dynamic)" : "");

    /* populate children from any spi device tables */
    scan_boardinfo(master);
    status = 0;
done:
    return status;
}
EXPORT_SYMBOL_GPL(spi_register_master);

看看scan_boardinfo原型:

static void scan_boardinfo(struct spi_master *master)
{
    struct boardinfo    *bi;

    mutex_lock(&board_lock);
    list_for_each_entry(bi, &board_list, list) {
        struct spi_board_info    *chip = bi->board_info;
        unsigned        n;

        for (n = bi->n_board_info; n > 0; n--, chip++) {
            if (chip->bus_num != master->bus_num)
                continue;
            /* NOTE: this relies on spi_new_device to
             * issue diagnostics when given bogus inputs
             */
            (void) spi_new_device(master, chip);
        }
    }
    mutex_unlock(&board_lock);
}

struct spi_device *spi_new_device(struct spi_master *master,
                  struct spi_board_info *chip)
{
    struct spi_device    *proxy;
    int            status;

    /* NOTE: caller did any chip->bus_num checks necessary.
     *
     * Also, unless we change the return value convention to use
     * error-or-pointer (not NULL-or-pointer), troubleshootability
     * suggests syslogged diagnostics are best here (ugh).
     */

    proxy = spi_alloc_device(master);
    if (!proxy)
        return NULL;

    WARN_ON(strlen(chip->modalias) >= sizeof(proxy->modalias));

    proxy->chip_select = chip->chip_select;
    proxy->max_speed_hz = chip->max_speed_hz;
    proxy->mode = chip->mode;
    proxy->irq = chip->irq;
    strlcpy(proxy->modalias, chip->modalias, sizeof(proxy->modalias));
    proxy->dev.platform_data = (void *) chip->platform_data;
    proxy->controller_data = chip->controller_data;
    proxy->controller_state = NULL;

    status = spi_add_device(proxy);
    if (status < 0) {
        spi_dev_put(proxy);
        return NULL;
    }

    return proxy;
}
EXPORT_SYMBOL_GPL(spi_new_device);

struct spi_device *spi_alloc_device(struct spi_master *master)
{
    struct spi_device    *spi;
    struct device        *dev = master->dev.parent;

    if (!spi_master_get(master))
        return NULL;

    spi = kzalloc(sizeof *spi, GFP_KERNEL);
    if (!spi) {
        dev_err(dev, "cannot alloc spi_devicen");
        spi_master_put(master);
        return NULL;
    }

    spi->master = master;
    spi->dev.parent = dev;
    spi->dev.bus = &spi_bus_type;
    spi->dev.release = spidev_release;
    device_initialize(&spi->dev);
    return spi;
}
EXPORT_SYMBOL_GPL(spi_alloc_device);

int spi_add_device(struct spi_device *spi)
{
    static DEFINE_MUTEX(spi_add_lock);
    struct device *dev = spi->master->dev.parent;
    int status;

    /* Chipselects are numbered 0..max; validate. */
    if (spi->chip_select >= spi->master->num_chipselect) {
        dev_err(dev, "cs%d >= max %dn",
            spi->chip_select,
            spi->master->num_chipselect);
        return -EINVAL;
    }

    /* Set the bus ID string */
    dev_set_name(&spi->dev, "%s.%u", dev_name(&spi->master->dev),
            spi->chip_select);


    /* We need to make sure there's no other device with this
     * chipselect **BEFORE** we call setup(), else we'll trash
     * its configuration. Lock against concurrent add() calls.
     */
    mutex_lock(&spi_add_lock);

    if (bus_find_device_by_name(&spi_bus_type, NULL, dev_name(&spi->dev))
            != NULL) {
        dev_err(dev, "chipselect %d already in usen",
                spi->chip_select);
        status = -EBUSY;
        goto done;
    }

    /* Drivers may modify this initial i/o setup, but will
     * normally rely on the device being setup. Devices
     * using SPI_CS_HIGH can't coexist well otherwise...
     */
    status = spi_setup(spi);
    if (status < 0) {
        dev_err(dev, "can't %s %s, status %dn",
                "setup", dev_name(&spi->dev), status);
        goto done;
    }

    /* Device may be bound to an active driver when this returns */
    status = device_add(&spi->dev);
    if (status < 0)
        dev_err(dev, "can't %s %s, status %dn",
                "add", dev_name(&spi->dev), status);
    else
        dev_dbg(dev, "registered child %sn", dev_name(&spi->dev));

done:
    mutex_unlock(&spi_add_lock);
    return status;
}
EXPORT_SYMBOL_GPL(spi_add_device);

int spi_setup(struct spi_device *spi)
{
    unsigned    bad_bits;
    int        status;

    /* help drivers fail *cleanly* when they need options
     * that aren't supported with their current master
     */
    bad_bits = spi->mode & ~spi->master->mode_bits;
    if (bad_bits) {
        dev_dbg(&spi->dev, "setup: unsupported mode bits %xn",
            bad_bits);
        return -EINVAL;
    }

    if (!spi->bits_per_word)
        spi->bits_per_word = 8;

    status = spi->master->setup(spi);//这里其实调用的就是s3c24xx_spi_probe里填充的spi->master->setup

    dev_dbg(&spi->dev, "setup mode %d, %s%s%s%s"
                "%u bits/w, %u Hz max --> %dn",
            (int) (spi->mode & (SPI_CPOL | SPI_CPHA)),
            (spi->mode & SPI_CS_HIGH) ? "cs_high, " : "",
            (spi->mode & SPI_LSB_FIRST) ? "lsb, " : "",
            (spi->mode & SPI_3WIRE) ? "3wire, " : "",
            (spi->mode & SPI_LOOP) ? "loopback, " : "",
            spi->bits_per_word, spi->max_speed_hz,
            status);

    return status;
}
EXPORT_SYMBOL_GPL(spi_setup);

static int s3c24xx_spi_setup(struct spi_device *spi)
{
    struct s3c24xx_spi_devstate *cs = spi->controller_state;
    struct s3c24xx_spi *hw = to_hw(spi);
    int ret;

    /* allocate settings on the first call */
    if (!cs) {
        cs = kzalloc(sizeof(struct s3c24xx_spi_devstate), GFP_KERNEL);
        if (!cs) {
            dev_err(&spi->dev, "no memory for controller staten");
            return -ENOMEM;
        }

        cs->spcon = SPCON_DEFAULT;
        cs->hz = -1;
        spi->controller_state = cs;
    }

    /* initialise the state from the device */
    ret = s3c24xx_spi_update_state(spi, NULL);
    if (ret)
        return ret;

    spin_lock(&hw->bitbang.lock);
    if (!hw->bitbang.busy) {
        hw->bitbang.chipselect(spi, BITBANG_CS_INACTIVE);
        /* need to ndelay for 0.5 clocktick ? */
    }
    spin_unlock(&hw->bitbang.lock);

    return 0;
}

好了，至此spi主控制器(驱动)和板上spi设备注册完毕，以后要使用spi来传输数据的话，只要先获得spi设备结构，然后就可以利用它来和spi驱动打交道了(就好像你要操作一个文件，先要获取文件句柄一样，明白吧^_^)

具体的数据传输过程将在下一节进行,待续.....