Linux内核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, <[email protected]>");
 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 <linux/kernel.h>
 #include <linux/module.h>

 #include <linux/i2c.h>
 #include <linux/i2c-id.h>
 #include <linux/init.h>
 #include <linux/time.h>
 #include <linux/interrupt.h>
 #include <linux/delay.h>
 #include <linux/errno.h>
 #include <linux/err.h>
 #include <linux/platform_device.h>
 #include <linux/clk.h>
 #include <linux/cpufreq.h>

 #include <asm/irq.h>
 #include <asm/io.h>

 #include <plat/regs-iic.h>
 #include <plat/iic.h>

 /* 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。