Linux内核I2C子系统驱动(二)

原文地址：http://blog.csdn.net/liaozc/article/details/6655030

I2C子系统驱动（二）

      上一篇文章讲述了I2C子系统体系结构，总线驱动、设备驱动的知识点，下面就S3C2440 I2C总线驱动的实现详细讲解，它的源码位于drivers/i2c/busses/i2c-s3c2410.c

一、I2C平台设备资源

      IIC驱动中使用的平台设备与前面看门狗、rtc等方式原理相同，但定义路径有所不同，并且设置了额外一些参数。mach_smdk2440.c文件中smdk2440_machine_init函数初始化了平台设备，还对s3c_device_i2c0平台设备进行额外的设置(s3c_i2c0_set_platdata)，s3c_device_i2c0平台设备定义在arch/arm/plat-s3c/dev-i2c0.c。

static void __init smdk2440_machine_init(void)

{

       s3c24xx_fb_set_platdata(&smdk2440_fb_info);

       s3c_i2c0_set_platdata(NULL);

 

       platform_add_devices(smdk2440_devices, ARRAY_SIZE(smdk2440_devices));

       smdk_machine_init();

}

下面是s3c_device_i2c0平台设备相关部分，s3c_i2c0_set_platdata初始化s3c_device_i2c0.dev.platform_data为default_i2c_data0

static struct resource s3c_i2c_resource[] = {

       [0] = {

              .start = S3C_PA_IIC,

              .end   = S3C_PA_IIC + SZ_4K - 1,

              .flags = IORESOURCE_MEM,

       },

       [1] = {

              .start = IRQ_IIC,

              .end   = IRQ_IIC,

              .flags = IORESOURCE_IRQ,

       },

};

 

struct platform_device s3c_device_i2c0 = {

       .name               = "s3c2410-i2c",

#ifdef CONFIG_S3C_DEV_I2C1

       .id             = 0,

#else

       .id             = -1,

#endif

       .num_resources       = ARRAY_SIZE(s3c_i2c_resource),

       .resource   = s3c_i2c_resource,

};

 

static struct s3c2410_platform_i2c default_i2c_data0 __initdata = {

       .flags             = 0,

       .slave_addr     = 0x10,

       .frequency      = 100*1000,

       .sda_delay      = 100,

};

 

void __init s3c_i2c0_set_platdata(struct s3c2410_platform_i2c *pd)

{

       struct s3c2410_platform_i2c *npd;

 

       if (!pd)

              pd = &default_i2c_data0;

 

       npd = kmemdup(pd, sizeof(struct s3c2410_platform_i2c), GFP_KERNEL);

       if (!npd)

              printk(KERN_ERR "%s: no memory for platform data\n", __func__);

       else if (!npd->cfg_gpio)

              npd->cfg_gpio = s3c_i2c0_cfg_gpio;

       s3c_device_i2c0.dev.platform_data = npd;

}

void s3c_i2c0_cfg_gpio(struct platform_device *dev)

{         //  位于arch/arm/plat-s3c24xx/setup-i2c.c

       s3c2410_gpio_cfgpin(S3C2410_GPE(15), S3C2410_GPE15_IICSDA);

       s3c2410_gpio_cfgpin(S3C2410_GPE(14), S3C2410_GPE14_IICSCL);

}

二、总线驱动实现

1.I2C适配器驱动加载卸载

static int s3c24xx_i2c_calcdivisor(unsigned long clkin, unsigned int wanted,
                   unsigned int *div1, unsigned int *divs)
{
    unsigned int calc_divs = clkin / wanted;
    unsigned int calc_div1;

    if (calc_divs > (16*16))
        calc_div1 = 512;
    else
        calc_div1 = 16;

    calc_divs += calc_div1-1;
    calc_divs /= calc_div1;

    if (calc_divs == 0)
        calc_divs = 1;
    if (calc_divs > 17)
        calc_divs = 17;

    *divs = calc_divs;
    *div1 = calc_div1;

    return clkin / (calc_divs * calc_div1);
}

/* s3c24xx_i2c_clockrate
 *
 * work out a divisor for the user requested frequency setting,
 * either by the requested frequency, or scanning the acceptable
 * range of frequencies until something is found
*/

static int s3c24xx_i2c_clockrate(struct s3c24xx_i2c *i2c, unsigned int *got)
{
    struct s3c2410_platform_i2c *pdata = i2c->dev->platform_data;
    unsigned long clkin = clk_get_rate(i2c->clk);   /* PCLK */
    unsigned int divs, div1;
    unsigned long target_frequency;    /* 需要设置速率默认100khz */
    u32 iiccon;
    int freq;

    i2c->clkrate = clkin;
    clkin /= 1000;      /* clkin now in KHz */

    dev_dbg(i2c->dev, "pdata desired frequency %lu\n", pdata->frequency);

    target_frequency = pdata->frequency ? pdata->frequency : 100000;

    target_frequency /= 1000; /* Target frequency now in KHz */

    freq = s3c24xx_i2c_calcdivisor(clkin, target_frequency, &div1, &divs);  /* 计算出IICCON中Tx clock source selection和Transmit clock value */

    if (freq > target_frequency) {
        dev_err(i2c->dev,
            "Unable to achieve desired frequency %luKHz."   \
            " Lowest achievable %dKHz\n", target_frequency, freq);
        return -EINVAL;
    }

    *got = freq;

    iiccon = readl(i2c->regs + S3C2410_IICCON);
    iiccon &= ~(S3C2410_IICCON_SCALEMASK | S3C2410_IICCON_TXDIV_512);
    iiccon |= (divs-1);

    if (div1 == 512)
        iiccon |= S3C2410_IICCON_TXDIV_512;

    writel(iiccon, i2c->regs + S3C2410_IICCON);

    if (s3c24xx_i2c_is2440(i2c)) {    /* 2440设置IICLC */
        unsigned long sda_delay;

        if (pdata->sda_delay) {
            sda_delay = (freq / 1000) * pdata->sda_delay;
            sda_delay /= 1000000;
            sda_delay = DIV_ROUND_UP(sda_delay, 5);
            if (sda_delay > 3)
                sda_delay = 3;
            sda_delay |= S3C2410_IICLC_FILTER_ON;
        } else
            sda_delay = 0;

        dev_dbg(i2c->dev, "IICLC=%08lx\n", sda_delay);
        writel(sda_delay, i2c->regs + S3C2440_IICLC);
    }

    return 0;
}

#ifdef CONFIG_CPU_FREQ

#define freq_to_i2c(_n) container_of(_n, struct s3c24xx_i2c, freq_transition)

static int s3c24xx_i2c_cpufreq_transition(struct notifier_block *nb,
                      unsigned long val, void *data)
{
    struct s3c24xx_i2c *i2c = freq_to_i2c(nb);
    unsigned long flags;
    unsigned int got;
    int delta_f;
    int ret;

    delta_f = clk_get_rate(i2c->clk) - i2c->clkrate;

    /* if we're post-change and the input clock has slowed down
     * or at pre-change and the clock is about to speed up, then
     * adjust our clock rate. <0 is slow, >0 speedup.
     */

    if ((val == CPUFREQ_POSTCHANGE && delta_f < 0) ||
        (val == CPUFREQ_PRECHANGE && delta_f > 0)) {
        spin_lock_irqsave(&i2c->lock, flags);
        ret = s3c24xx_i2c_clockrate(i2c, &got);
        spin_unlock_irqrestore(&i2c->lock, flags);

        if (ret < 0)
            dev_err(i2c->dev, "cannot find frequency\n");
        else
            dev_info(i2c->dev, "setting freq %d\n", got);
    }

    return 0;
}

static inline int s3c24xx_i2c_register_cpufreq(struct s3c24xx_i2c *i2c)
{
    i2c->freq_transition.notifier_call = s3c24xx_i2c_cpufreq_transition;

    return cpufreq_register_notifier(&i2c->freq_transition,
                     CPUFREQ_TRANSITION_NOTIFIER);
}

static inline void s3c24xx_i2c_deregister_cpufreq(struct s3c24xx_i2c *i2c)
{
    cpufreq_unregister_notifier(&i2c->freq_transition,
                    CPUFREQ_TRANSITION_NOTIFIER);
}

#else
static inline int s3c24xx_i2c_register_cpufreq(struct s3c24xx_i2c *i2c)
{
    return 0;
}

static inline void s3c24xx_i2c_deregister_cpufreq(struct s3c24xx_i2c *i2c)
{
}
#endif

/* s3c24xx_i2c_init
 *
 * initialise the controller, set the IO lines and frequency
*/

static int s3c24xx_i2c_init(struct s3c24xx_i2c *i2c)
{
    unsigned long iicon = S3C2410_IICCON_IRQEN | S3C2410_IICCON_ACKEN;
    struct s3c2410_platform_i2c *pdata;
    unsigned int freq;

    /* get the plafrom data */

    pdata = i2c->dev->platform_data;    /* 获取platform_data */

    /* inititalise the gpio */

    if (pdata->cfg_gpio)
        pdata->cfg_gpio(to_platform_device(i2c->dev));

    /* write slave address */

    writeb(pdata->slave_addr, i2c->regs + S3C2410_IICADD);

    dev_info(i2c->dev, "slave address 0x%02x\n", pdata->slave_addr);

    writel(iicon, i2c->regs + S3C2410_IICCON);

    /* we need to work out the divisors for the clock... */

    if (s3c24xx_i2c_clockrate(i2c, &freq) != 0) {         /* 设置i2c速率 */
        writel(0, i2c->regs + S3C2410_IICCON);
        dev_err(i2c->dev, "cannot meet bus frequency required\n");
        return -EINVAL;
    }

    /* todo - check that the i2c lines aren't being dragged anywhere */

    dev_info(i2c->dev, "bus frequency set to %d KHz\n", freq);
    dev_dbg(i2c->dev, "S3C2410_IICCON=0x%02lx\n", iicon);

    return 0;
}

/* s3c24xx_i2c_probe
 *
 * called by the bus driver when a suitable device is found
*/

static int s3c24xx_i2c_probe(struct platform_device *pdev)
{
    struct s3c24xx_i2c *i2c;
    struct s3c2410_platform_i2c *pdata;
    struct resource *res;
    int ret;

    pdata = pdev->dev.platform_data;    /* 在平台设备资源中初始化了platform_data，关于platform_data参考前一节 */
    if (!pdata) {
        dev_err(&pdev->dev, "no platform data\n");
        return -EINVAL;
    }

    i2c = kzalloc(sizeof(struct s3c24xx_i2c), GFP_KERNEL); /* s3c24xx_i2c分配空间 */
    if (!i2c) {
        dev_err(&pdev->dev, "no memory for state\n");
        return -ENOMEM;
    }

    strlcpy(i2c->adap.name, "s3c2410-i2c", sizeof(i2c->adap.name));   /* s3c24xx_i2c初始化 */
    i2c->adap.owner   = THIS_MODULE;
    i2c->adap.algo    = &s3c24xx_i2c_algorithm;   /* 通信方法，下一节重点介绍 */
    i2c->adap.retries = 2;
    i2c->adap.class   = I2C_CLASS_HWMON | I2C_CLASS_SPD;
    i2c->tx_setup     = 50;

    spin_lock_init(&i2c->lock);
    init_waitqueue_head(&i2c->wait);

    /* find the clock and enable it */

    i2c->dev = &pdev->dev;
    i2c->clk = clk_get(&pdev->dev, "i2c");   /* 时钟 */
    if (IS_ERR(i2c->clk)) {
        dev_err(&pdev->dev, "cannot get clock\n");
        ret = -ENOENT;
        goto err_noclk;
    }

    dev_dbg(&pdev->dev, "clock source %p\n", i2c->clk);

    clk_enable(i2c->clk);

    /* map the registers */

    res = platform_get_resource(pdev, IORESOURCE_MEM, 0);    /* 寄存器映射 */
    if (res == NULL) {
        dev_err(&pdev->dev, "cannot find IO resource\n");
        ret = -ENOENT;
        goto err_clk;
    }

    i2c->ioarea = request_mem_region(res->start, resource_size(res),
                     pdev->name);

    if (i2c->ioarea == NULL) {
        dev_err(&pdev->dev, "cannot request IO\n");
        ret = -ENXIO;
        goto err_clk;
    }

    i2c->regs = ioremap(res->start, resource_size(res));

    if (i2c->regs == NULL) {
        dev_err(&pdev->dev, "cannot map IO\n");
        ret = -ENXIO;
        goto err_ioarea;
    }

    dev_dbg(&pdev->dev, "registers %p (%p, %p)\n",
        i2c->regs, i2c->ioarea, res);

    /* setup info block for the i2c core */

    i2c->adap.algo_data = i2c;
    i2c->adap.dev.parent = &pdev->dev;

    /* initialise the i2c controller */

    ret = s3c24xx_i2c_init(i2c);     /* 初始化 */
    if (ret != 0)
        goto err_iomap;

    /* find the IRQ for this unit (note, this relies on the init call to
     * ensure no current IRQs pending
     */

    i2c->irq = ret = platform_get_irq(pdev, 0);     /* 中断申请 */
    if (ret <= 0) {
        dev_err(&pdev->dev, "cannot find IRQ\n");
        goto err_iomap;
    }

    ret = request_irq(i2c->irq, s3c24xx_i2c_irq, IRQF_DISABLED,
              dev_name(&pdev->dev), i2c);

    if (ret != 0) {
        dev_err(&pdev->dev, "cannot claim IRQ %d\n", i2c->irq);
        goto err_iomap;
    }

    ret = s3c24xx_i2c_register_cpufreq(i2c);  /* 关于cpufreq没明白，有关cpufreq可以暂时不管 */
    if (ret < 0) {
        dev_err(&pdev->dev, "failed to register cpufreq notifier\n");
        goto err_irq;
    }

    /* Note, previous versions of the driver used i2c_add_adapter()
     * to add the bus at any number. We now pass the bus number via
     * the platform data, so if unset it will now default to always
     * being bus 0.
     */

    i2c->adap.nr = pdata->bus_num;
    /*调用了i2c-core中的i2c_add_adapter函数来添加一个i2c控制器,i2c_add_numbered_adapter和i2c_add_adapter的
     区别在于前者用来添加一个在CPU内部集成的适配器，而后者用来添加一个CPU外部的适配器。显然这里应该用前者 */
    ret = i2c_add_numbered_adapter(&i2c->adap);
    if (ret < 0) {
        dev_err(&pdev->dev, "failed to add bus to i2c core\n");
        goto err_cpufreq;
    }

    platform_set_drvdata(pdev, i2c);

    dev_info(&pdev->dev, "%s: S3C I2C adapter\n", dev_name(&i2c->adap.dev));
    return 0;

 err_cpufreq:
    s3c24xx_i2c_deregister_cpufreq(i2c);

 err_irq:
    free_irq(i2c->irq, i2c);

 err_iomap:
    iounmap(i2c->regs);

 err_ioarea:
    release_resource(i2c->ioarea);
    kfree(i2c->ioarea);

 err_clk:
    clk_disable(i2c->clk);
    clk_put(i2c->clk);

 err_noclk:
    kfree(i2c);
    return ret;
}

/* s3c24xx_i2c_remove
 *
 * called when device is removed from the bus
*/

static int s3c24xx_i2c_remove(struct platform_device *pdev)
{
    struct s3c24xx_i2c *i2c = platform_get_drvdata(pdev);

    s3c24xx_i2c_deregister_cpufreq(i2c);

    i2c_del_adapter(&i2c->adap);     /* 删除adapter*/
    free_irq(i2c->irq, i2c);

    clk_disable(i2c->clk);
    clk_put(i2c->clk);

    iounmap(i2c->regs);

    release_resource(i2c->ioarea);
    kfree(i2c->ioarea);
    kfree(i2c);

    return 0;
}

#ifdef CONFIG_PM
static int s3c24xx_i2c_suspend_noirq(struct device *dev)
{
    struct platform_device *pdev = to_platform_device(dev);
    struct s3c24xx_i2c *i2c = platform_get_drvdata(pdev);

    i2c->suspended = 1;

    return 0;
}

static int s3c24xx_i2c_resume(struct device *dev)
{
    struct platform_device *pdev = to_platform_device(dev);
    struct s3c24xx_i2c *i2c = platform_get_drvdata(pdev);

    i2c->suspended = 0;
    s3c24xx_i2c_init(i2c);

    return 0;
}

static struct dev_pm_ops s3c24xx_i2c_dev_pm_ops = {
    .suspend_noirq = s3c24xx_i2c_suspend_noirq,
    .resume = s3c24xx_i2c_resume,
};

#define S3C24XX_DEV_PM_OPS (&s3c24xx_i2c_dev_pm_ops)
#else
#define S3C24XX_DEV_PM_OPS NULL
#endif

/* device driver for platform bus bits */

static struct platform_device_id s3c24xx_driver_ids[] = {
    {
        .name       = "s3c2410-i2c",
        .driver_data    = TYPE_S3C2410,
    }, {
        .name       = "s3c2440-i2c",
        .driver_data    = TYPE_S3C2440,
    }, { },
};
MODULE_DEVICE_TABLE(platform, s3c24xx_driver_ids);

static struct platform_driver s3c24xx_i2c_driver = {
    .probe      = s3c24xx_i2c_probe,
    .remove     = s3c24xx_i2c_remove,
    .id_table   = s3c24xx_driver_ids,   //解释参见代码后面说明
    .driver     = {
        .owner  = THIS_MODULE,
        .name   = "s3c-i2c",
        .pm = S3C24XX_DEV_PM_OPS,
    },
};

static int __init i2c_adap_s3c_init(void)
{
    return platform_driver_register(&s3c24xx_i2c_driver);
}
subsys_initcall(i2c_adap_s3c_init);

static void __exit i2c_adap_s3c_exit(void)
{
    platform_driver_unregister(&s3c24xx_i2c_driver);
}
module_exit(i2c_adap_s3c_exit);

MODULE_DESCRIPTION("S3C24XX I2C Bus driver");
MODULE_AUTHOR("Ben Dooks, ");
MODULE_LICENSE("GPL");

      i2c_adap_s3c_init注册i2c平台驱动s3c24xx_i2c_driver，它是platform_driver结构体，不要误解为i2c_driver。同时还实现了probe、remove、id_table、driver，其中suspend、resume在driver中实现。i2c_adap_s3c_exit注销s3c24xx_i2c_driver。

      关于平台驱动s3c24xx_i2c_driver中名字为s3c_i2c与平台设备中名字s3c2410-i2c不一样，怎么匹配？这里主要在于id_table，s3c24xx_driver_ids包含了驱动所支持的设备ID表，判断这个表中的名字与平台设备中名字一致，则匹配成功。

2.I2C通信方法

#include
#include

#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include

#include
#include

#include
#include

/* i2c controller state */

enum s3c24xx_i2c_state {
    STATE_IDLE,
    STATE_START,
    STATE_READ,
    STATE_WRITE,
    STATE_STOP
};

enum s3c24xx_i2c_type {
    TYPE_S3C2410,
    TYPE_S3C2440,
};

struct s3c24xx_i2c {
    spinlock_t      lock;
    wait_queue_head_t   wait;
    unsigned int        suspended:1;

    struct i2c_msg      *msg;
    unsigned int        msg_num;
    unsigned int        msg_idx;
    unsigned int        msg_ptr;

    unsigned int        tx_setup;
    unsigned int        irq;

    enum s3c24xx_i2c_state  state;
    unsigned long       clkrate;

    void __iomem        *regs;
    struct clk      *clk;
    struct device       *dev;
    struct resource     *ioarea;
    struct i2c_adapter  adap;

#ifdef CONFIG_CPU_FREQ
    struct notifier_block   freq_transition;
#endif
};

/* default platform data removed, dev should always carry data. */

/* s3c24xx_i2c_is2440()
 *
 * return true is this is an s3c2440
*/

static inline int s3c24xx_i2c_is2440(struct s3c24xx_i2c *i2c)
{
    struct platform_device *pdev = to_platform_device(i2c->dev);
    enum s3c24xx_i2c_type type;

    type = platform_get_device_id(pdev)->driver_data;
    return type == TYPE_S3C2440;
}

/* s3c24xx_i2c_master_complete
 *
 * complete the message and wake up the caller, using the given return code,
 * or zero to mean ok.
*/

static inline void s3c24xx_i2c_master_complete(struct s3c24xx_i2c *i2c, int ret)
{
    dev_dbg(i2c->dev, "master_complete %d\n", ret);

    i2c->msg_ptr = 0;
    i2c->msg = NULL;
    i2c->msg_idx++;
    i2c->msg_num = 0;
    if (ret)
        i2c->msg_idx = ret;

    wake_up(&i2c->wait);
}

static inline void s3c24xx_i2c_disable_ack(struct s3c24xx_i2c *i2c)
{
    unsigned long tmp;

    tmp = readl(i2c->regs + S3C2410_IICCON);
    writel(tmp & ~S3C2410_IICCON_ACKEN, i2c->regs + S3C2410_IICCON);
}

static inline void s3c24xx_i2c_enable_ack(struct s3c24xx_i2c *i2c)
{
    unsigned long tmp;

    tmp = readl(i2c->regs + S3C2410_IICCON);
    writel(tmp | S3C2410_IICCON_ACKEN, i2c->regs + S3C2410_IICCON);
}

/* irq enable/disable functions */

static inline void s3c24xx_i2c_disable_irq(struct s3c24xx_i2c *i2c)
{
    unsigned long tmp;

    tmp = readl(i2c->regs + S3C2410_IICCON);
    writel(tmp & ~S3C2410_IICCON_IRQEN, i2c->regs + S3C2410_IICCON);
}

static inline void s3c24xx_i2c_enable_irq(struct s3c24xx_i2c *i2c)
{
    unsigned long tmp;

    tmp = readl(i2c->regs + S3C2410_IICCON);
    writel(tmp | S3C2410_IICCON_IRQEN, i2c->regs + S3C2410_IICCON);
}


/* s3c24xx_i2c_message_start
 *
 * put the start of a message onto the bus
*/

static void s3c24xx_i2c_message_start(struct s3c24xx_i2c *i2c,
                      struct i2c_msg *msg)
{
    unsigned int addr = (msg->addr & 0x7f) << 1;
    unsigned long stat;
    unsigned long iiccon;

    stat = 0;
    stat |=  S3C2410_IICSTAT_TXRXEN;      /* 使能TXRX */

    if (msg->flags & I2C_M_RD) {          /* 设备地址 */
        stat |= S3C2410_IICSTAT_MASTER_RX;
        addr |= 1;
    } else
        stat |= S3C2410_IICSTAT_MASTER_TX;

    if (msg->flags & I2C_M_REV_DIR_ADDR)
        addr ^= 1;

    /* todo - check for wether ack wanted or not */
    s3c24xx_i2c_enable_ack(i2c);          /* 使能ack */

    iiccon = readl(i2c->regs + S3C2410_IICCON);
    writel(stat, i2c->regs + S3C2410_IICSTAT);

    dev_dbg(i2c->dev, "START: %08lx to IICSTAT, %02x to DS\n", stat, addr);
    writeb(addr, i2c->regs + S3C2410_IICDS);    /* 写设备地址 */

    /* delay here to ensure the data byte has gotten onto the bus
     * before the transaction is started */

    ndelay(i2c->tx_setup);

    dev_dbg(i2c->dev, "iiccon, %08lx\n", iiccon);
    writel(iiccon, i2c->regs + S3C2410_IICCON);

    stat |= S3C2410_IICSTAT_START;             /* 启动i2c，当设备地址发送后就会进入中断，在中断中根据不同状态进行读写操作 */
    writel(stat, i2c->regs + S3C2410_IICSTAT);
}

static inline void s3c24xx_i2c_stop(struct s3c24xx_i2c *i2c, int ret)
{
    unsigned long iicstat = readl(i2c->regs + S3C2410_IICSTAT);

    dev_dbg(i2c->dev, "STOP\n");

    /* stop the transfer */
    iicstat &= ~S3C2410_IICSTAT_START;
    writel(iicstat, i2c->regs + S3C2410_IICSTAT);

    i2c->state = STATE_STOP;

    s3c24xx_i2c_master_complete(i2c, ret);
    s3c24xx_i2c_disable_irq(i2c);
}

/* helper functions to determine the current state in the set of
 * messages we are sending */

/* 关于以下三个函数功能区别，后文详解 */
/* is_lastmsg()
 *
 * returns TRUE if the current message is the last in the set
*/

static inline int is_lastmsg(struct s3c24xx_i2c *i2c)
{
    return i2c->msg_idx >= (i2c->msg_num - 1);
}

/* is_msglast
 *
 * returns TRUE if we this is the last byte in the current message
*/

static inline int is_msglast(struct s3c24xx_i2c *i2c)
{
    return i2c->msg_ptr == i2c->msg->len-1;
}

/* is_msgend
 *
 * returns TRUE if we reached the end of the current message
*/

static inline int is_msgend(struct s3c24xx_i2c *i2c)
{
    return i2c->msg_ptr >= i2c->msg->len;
}

/* i2s_s3c_irq_nextbyte
 *
 * process an interrupt and work out what to do
 */

static int i2s_s3c_irq_nextbyte(struct s3c24xx_i2c *i2c, unsigned long iicstat)
{
       /* 此函数比较复杂，在后文中将举一个具体例子来说明整个过程 */
    unsigned long tmp;
    unsigned char byte;
    int ret = 0;

    switch (i2c->state) {

    case STATE_IDLE:
        dev_err(i2c->dev, "%s: called in STATE_IDLE\n", __func__);
        goto out;
        break;

    case STATE_STOP:
        dev_err(i2c->dev, "%s: called in STATE_STOP\n", __func__);
        s3c24xx_i2c_disable_irq(i2c);
        goto out_ack;

    case STATE_START:
        /* last thing we did was send a start condition on the
         * bus, or started a new i2c message
         */

        if (iicstat & S3C2410_IICSTAT_LASTBIT &&
            !(i2c->msg->flags & I2C_M_IGNORE_NAK)) {
            /* ack was not received... */

            dev_dbg(i2c->dev, "ack was not received\n");
            s3c24xx_i2c_stop(i2c, -ENXIO);
            goto out_ack;
        }

        if (i2c->msg->flags & I2C_M_RD)
            i2c->state = STATE_READ;
        else
            i2c->state = STATE_WRITE;

        /* terminate the transfer if there is nothing to do
         * as this is used by the i2c probe to find devices. */

        if (is_lastmsg(i2c) && i2c->msg->len == 0) {
            s3c24xx_i2c_stop(i2c, 0);
            goto out_ack;
        }

        if (i2c->state == STATE_READ)
            goto prepare_read;

        /* fall through to the write state, as we will need to
         * send a byte as well */

    case STATE_WRITE:
        /* we are writing data to the device... check for the
         * end of the message, and if so, work out what to do
         */

        if (!(i2c->msg->flags & I2C_M_IGNORE_NAK)) {
            if (iicstat & S3C2410_IICSTAT_LASTBIT) {
                dev_dbg(i2c->dev, "WRITE: No Ack\n");

                s3c24xx_i2c_stop(i2c, -ECONNREFUSED);
                goto out_ack;
            }
        }

 retry_write:

        if (!is_msgend(i2c)) {
            byte = i2c->msg->buf[i2c->msg_ptr++];
            writeb(byte, i2c->regs + S3C2410_IICDS);

            /* delay after writing the byte to allow the
             * data setup time on the bus, as writing the
             * data to the register causes the first bit
             * to appear on SDA, and SCL will change as
             * soon as the interrupt is acknowledged */

            ndelay(i2c->tx_setup);

        } else if (!is_lastmsg(i2c)) {
            /* we need to go to the next i2c message */

            dev_dbg(i2c->dev, "WRITE: Next Message\n");

            i2c->msg_ptr = 0;
            i2c->msg_idx++;
            i2c->msg++;

            /* check to see if we need to do another message */
            if (i2c->msg->flags & I2C_M_NOSTART) {

                if (i2c->msg->flags & I2C_M_RD) {
                    /* cannot do this, the controller
                     * forces us to send a new START
                     * when we change direction */

                    s3c24xx_i2c_stop(i2c, -EINVAL);
                }

                goto retry_write;
            } else {
                /* send the new start */
                s3c24xx_i2c_message_start(i2c, i2c->msg);
                i2c->state = STATE_START;
            }

        } else {
            /* send stop */

            s3c24xx_i2c_stop(i2c, 0);
        }
        break;

    case STATE_READ:
        /* we have a byte of data in the data register, do
         * something with it, and then work out wether we are
         * going to do any more read/write
         */

        byte = readb(i2c->regs + S3C2410_IICDS);
        i2c->msg->buf[i2c->msg_ptr++] = byte;

 prepare_read:
        if (is_msglast(i2c)) {
            /* last byte of buffer */

            if (is_lastmsg(i2c))
                s3c24xx_i2c_disable_ack(i2c);

        } else if (is_msgend(i2c)) {
            /* ok, we've read the entire buffer, see if there
             * is anything else we need to do */

            if (is_lastmsg(i2c)) {
                /* last message, send stop and complete */
                dev_dbg(i2c->dev, "READ: Send Stop\n");

                s3c24xx_i2c_stop(i2c, 0);
            } else {
                /* go to the next transfer */
                dev_dbg(i2c->dev, "READ: Next Transfer\n");

                i2c->msg_ptr = 0;
                i2c->msg_idx++;
                i2c->msg++;
            }
        }

        break;
    }

    /* acknowlegde the IRQ and get back on with the work */

 out_ack:              /* 退出时请pend flag */
    tmp = readl(i2c->regs + S3C2410_IICCON);
    tmp &= ~S3C2410_IICCON_IRQPEND;
    writel(tmp, i2c->regs + S3C2410_IICCON);
 out:
    return ret;
}

/* s3c24xx_i2c_irq
 *
 * top level IRQ servicing routine
*/

static irqreturn_t s3c24xx_i2c_irq(int irqno, void *dev_id)
{
    /* 中断第一次进入在调用s3c24xx_i2c_message_start写入设备地址后 */
    struct s3c24xx_i2c *i2c = dev_id;
    unsigned long status;
    unsigned long tmp;

    status = readl(i2c->regs + S3C2410_IICSTAT);

    if (status & S3C2410_IICSTAT_ARBITR) {
        /* deal with arbitration loss */
        dev_err(i2c->dev, "deal with arbitration loss\n");
    }

    if (i2c->state == STATE_IDLE) {
        dev_dbg(i2c->dev, "IRQ: error i2c->state == IDLE\n");

        tmp = readl(i2c->regs + S3C2410_IICCON);
        tmp &= ~S3C2410_IICCON_IRQPEND;
        writel(tmp, i2c->regs +  S3C2410_IICCON);
        goto out;
    }

    /* pretty much this leaves us with the fact that we've
     * transmitted or received whatever byte we last sent */

    i2s_s3c_irq_nextbyte(i2c, status);    /* 根据不同状态步步推进读写操作，i2s_s3c_irq_nextbyte难点 */

 out:
    return IRQ_HANDLED;
}


/* s3c24xx_i2c_set_master
 *
 * get the i2c bus for a master transaction
*/

static int s3c24xx_i2c_set_master(struct s3c24xx_i2c *i2c)
{
    unsigned long iicstat;
    int timeout = 400;

    while (timeout-- > 0) {
        iicstat = readl(i2c->regs + S3C2410_IICSTAT);

        if (!(iicstat & S3C2410_IICSTAT_BUSBUSY))
            return 0;

        msleep(1);
    }

    return -ETIMEDOUT;
}

/* s3c24xx_i2c_doxfer
 *
 * this starts an i2c transfer
*/

static int s3c24xx_i2c_doxfer(struct s3c24xx_i2c *i2c,
                  struct i2c_msg *msgs, int num)
{
    unsigned long timeout;
    int ret;

    if (i2c->suspended)
        return -EIO;

    ret = s3c24xx_i2c_set_master(i2c);       /* 检查i2c总线状态 */
    if (ret != 0) {
        dev_err(i2c->dev, "cannot get bus (error %d)\n", ret);
        ret = -EAGAIN;
        goto out;
    }

    spin_lock_irq(&i2c->lock);

    i2c->msg     = msgs;        /* 把消息写入i2c结构体 */
    i2c->msg_num = num;
    i2c->msg_ptr = 0;
    i2c->msg_idx = 0;
    i2c->state   = STATE_START;

    s3c24xx_i2c_enable_irq(i2c);      /* 使能中断*/
    s3c24xx_i2c_message_start(i2c, msgs);   /* 写设备地址，启动i2c */
    spin_unlock_irq(&i2c->lock);

    timeout = wait_event_timeout(i2c->wait, i2c->msg_num == 0, HZ * 5);  /* 等待消息传输完成，否则超时 */

    ret = i2c->msg_idx;     /* 成功传输消息条数 */

    /* having these next two as dev_err() makes life very
     * noisy when doing an i2cdetect */

    if (timeout == 0)
        dev_dbg(i2c->dev, "timeout\n");
    else if (ret != num)              /* 如果ret不等于原有消息条数，传输失败 */
        dev_dbg(i2c->dev, "incomplete xfer (%d)\n", ret);

    /* ensure the stop has been through the bus */

    msleep(1);

 out:
    return ret;
}

/* s3c24xx_i2c_xfer
 *
 * first port of call from the i2c bus code when an message needs
 * transferring across the i2c bus.
*/

static int s3c24xx_i2c_xfer(struct i2c_adapter *adap,
            struct i2c_msg *msgs, int num)
{
    struct s3c24xx_i2c *i2c = (struct s3c24xx_i2c *)adap->algo_data;
    int retry;
    int ret;

    for (retry = 0; retry < adap->retries; retry++) {   /* 传输不成功重复次数 */

        ret = s3c24xx_i2c_doxfer(i2c, msgs, num);   /* 重点在这里实现 */

        if (ret != -EAGAIN)
            return ret;

        dev_dbg(i2c->dev, "Retrying transmission (%d)\n", retry);

        udelay(100);
    }

    return -EREMOTEIO;
}

/* declare our i2c functionality */
static u32 s3c24xx_i2c_func(struct i2c_adapter *adap)
{      /* 支持的功能，定义在i2c.h */
    return I2C_FUNC_I2C | I2C_FUNC_SMBUS_EMUL | I2C_FUNC_PROTOCOL_MANGLING;
}

/* i2c bus registration info */

static const struct i2c_algorithm s3c24xx_i2c_algorithm = {
    .master_xfer        = s3c24xx_i2c_xfer,
    .functionality      = s3c24xx_i2c_func,
};

      I2C适配器的通信方法整个驱动的重点，主要实现i2c_algorithm的master_xfer()和functionality()。s3c24xx_i2c_xfer中调用了s3c24xx_i2c_doxfer，然后启动i2c，并且通过中断s3c24xx_i2c_irq和i2s_s3c_irq_nextbyte来一步步推进传输工作。

      通信方法传输是以消息为单位的，所有先了解消息结构体。消息i2c_msg包括地址、标志、一条消息包含的数据及长度。

struct i2c_msg {

       __u16 addr;    /* slave address                    */

       __u16 flags;

#define I2C_M_TEN            0x0010    /* this is a ten bit chip address */

#define I2C_M_RD              0x0001    /* read data, from slave to master */

#define I2C_M_NOSTART           0x4000    /* if I2C_FUNC_PROTOCOL_MANGLING */

#define I2C_M_REV_DIR_ADDR 0x2000    /* if I2C_FUNC_PROTOCOL_MANGLING */

#define I2C_M_IGNORE_NAK    0x1000    /* if I2C_FUNC_PROTOCOL_MANGLING */

#define I2C_M_NO_RD_ACK             0x0800    /* if I2C_FUNC_PROTOCOL_MANGLING */

#define I2C_M_RECV_LEN         0x0400    /* length will be first received byte */

       __u16 len;             /* msg length                       */

       __u8 *buf;             /* pointer to msg data                  */

};

      代码中提到的is_lastmsg、is_msglast、is_msgend到底判断是什么？在s3c24xx_i2c中定义的struct i2c_msg       *msg看出可以包含多条消息，而一条消息有可能包含多个数据，比如对于AT24c02页写就包含多个数据。is_lastmsg判断是否是消息中最后一条，使用了变量msg_idx；is_msglast判断是否一条消息中最后一个数据，is_msgend判断是否是一条消息全部完成，所以这两个函数使用变量时msg_ptr。

      下面就根据AT24C02具体的讲解s3c24xx_i2c_irq和i2s_s3c_irq_nextbyte如何实现传输。

      任意地址字节写时序如上所示，只需要一条消息即可。其中flag位为写，写0即可，消息包括两个数据目标地址、数据，消息如下所示。

struct i2c_msg *msg;

msg=malloc(sizeof(struct i2c_msg));

msg.len=2;    // 1个目标地址和1个数据

msg.addr=0x50; // 设备地址

msg.flags=0;  // write

msg.buf=(unsigned char*)malloc(2);

msg.buf[0]=0x10;// 目标地址

msg.buf[1]=0x58;// the data to write

      s3c24xx_i2c_xfer中调用了s3c24xx_i2c_doxfer，在s3c24xx_i2c_doxfer中把消息传入i2c->msg，使能中断，置i2c->state 为STATE_START，调用s3c24xx_i2c_message_start启动i2c发送设备地址，就等待中断来做后续工作。当设备地址发送后就会进入中断，继续进入i2s_s3c_irq_nextbyte的STATE_START，判断消息为写，置i2c->state 为STATE_WRITE，跳入STATE_WRITE，在retry_write这一段中，if先判断是否一条消息所有数据发送完，没发送完，则每次发送一条等待下次中断进入，每发送一个数据都要清pend位；发送完else if判断是否最后一条消息，如果不是则要指针指向下一条消息继续if的步骤；最后else为发送完成，停止i2c。针对任意字节写只有一条消息，if中发送两次就完成本条消息传输。

      任意地址字节读时序如上所示，需两条消息。第一条，写目标地址，flag位为写；第二条，读取数据，flag位为读，第一条与第二条消息之间要发送START。

struct i2c_msg *msgs;

msgs=malloc(2*sizeof(struct i2c_msg));

msgs[0].len=1; // 目标地址

msgs[0].addr=0x50; // 设备地址

msgs[0].flags=0; // write

msgs[0].buf=(unsigned char*)malloc(1);

msgs[0].buf[0]=0x10; // 目标地址

msgs[1].len=1; // 读出的数据

msgs[1].addr=0x50; // 设备地址

msgs[1].flags=I2C_M_RD; // read

msgs[1].buf=(unsigned char*)malloc(1);

msgs[1].buf[0]=0;// 初始化读缓冲

      直接从中断开始讲，发送设备地址后，进入STATE_START，判断第一条消息为写，置i2c->state 为STATE_WRITE，跳入STATE_WRITE，第一条消息有一个数据，发送完成后，在else if中判断不是此条消息不是最后一条，就会执行else if中指向下一条消息，s3c24xx_i2c_message_start重新发送START，置i2c->state 为STATE_START。下次进入中跳到STATE_START，判断第二条消息为读，置i2c->state 为STATE_READ跳入STATE_READ，第二条消息只有一个数据，关闭ack，接收一个字节，停止i2c。看完这两个例子，再看i2s_s3c_irq_nextbyte就容易理解。