Linux I2C 总线驱动

总线驱动和具体的平台CPU有关,以2440为例:drivers\i2c\busses\I2c-s3c2410.c 

I2C 适配器驱动加载与卸载

I2C 总线驱动模块的 加载函数要完成两个工作。
1.初始化 I2C 适配器所使用的硬件资源,如申请 I/O 地址、中断号等。

2.通过 i2c_add_adapter()添加 i2c_adapter 的数据结构,当然这个 i2c_adapter 数据结构的成员已经被 xxx 适配器的相应函数指针所初始化。

代码如下:

static int __init i2c_adap_s3c_init(void)
{
	int ret;
        //这个文件支持2440和2410的i2c驱动
	ret = platform_driver_register(&s3c2410_i2c_driver);
	if (ret == 0) {
		ret = platform_driver_register(&s3c2440_i2c_driver);
		if (ret)
			platform_driver_unregister(&s3c2410_i2c_driver);
	}

	return ret;
}
看下i2c_driver的定义:

static struct platform_driver s3c2440_i2c_driver = {
	.probe		= s3c24xx_i2c_probe,//看这个函数中如何初始化
	.remove		= s3c24xx_i2c_remove,
	.resume		= s3c24xx_i2c_resume,
	.driver		= {
		.owner	= THIS_MODULE,
		.name	= "s3c2440-i2c",
	},
};
s3c24xx_i2c_probe
static int s3c24xx_i2c_probe(struct platform_device *pdev)
{
	struct s3c24xx_i2c *i2c = &s3c24xx_i2c;
	struct resource *res;
	int ret;

	/* find the clock and enable it */

	i2c->dev = &pdev->dev;
	i2c->clk = clk_get(&pdev->dev, "i2c");
	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);
	if (res == NULL) {
		dev_err(&pdev->dev, "cannot find IO resource\n");
		ret = -ENOENT;
		goto err_clk;
	}

	i2c->ioarea = request_mem_region(res->start, (res->end-res->start)+1,
					 pdev->name);

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

	i2c->regs = ioremap(res->start, (res->end-res->start)+1);

	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);//对2440控制器 io的进行初始化设置,很重要,和具体的硬件寄存器相关
	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 
	 */

	res = platform_get_resource(pdev, IORESOURCE_IRQ, 0);
	if (res == NULL) {
		dev_err(&pdev->dev, "cannot find IRQ\n");
		ret = -ENOENT;
		goto err_iomap;
	}

	ret = request_irq(res->start, s3c24xx_i2c_irq, IRQF_DISABLED,
			  pdev->name, i2c);//申请中断

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

	i2c->irq = res;
		
	dev_dbg(&pdev->dev, "irq resource %p (%lu)\n", res,
		(unsigned long)res->start);

	ret = i2c_add_adapter(&i2c->adap);//添加adapter,很重要,和其他数据结构联系
	if (ret < 0) {
		dev_err(&pdev->dev, "failed to add bus to i2c core\n");
		goto err_irq;
	}

	platform_set_drvdata(pdev, i2c);

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

 err_irq:
	free_irq(i2c->irq->start, 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:
	return ret;
}

I2C 总线驱动模块的卸载函数要完成的工作与加载函数相反(卸载就不对2440分析了)。
1.释放 I2C 适配器所使用的硬件资源,如释放 I/O 地址、中断号等。
2.通过 i2c_del_adapter()删除 i2c_adapter 的数据结构。


下面 所示为 I2C 适配器驱动的模块加载和卸载函数的模板。

static int _ _init i2c_adapter_xxx_init(void)
{
 xxx_adpater_hw_init();
 i2c_add_adapter(&xxx_adapter);
}

static void _ _exit i2c_adapter_xxx_exit(void)
{
 xxx_adpater_hw_free();
 i2c_del_adapter(&xxx_adapter);
}
上述代码中 xxx_adpater_hw_init()和 xxx_adpater_hw_free()函数的实现都与具体的CPU 和 I2C 设备硬件直接相关和上面所列出的2440代码 s3c24xx_i2c_init非常类似

I2C 总线通信方法
我们需要为特定的 I2C 适配器实现其通信方法,主要实现 i2c_algorithm 的master_xfer()函数和 functionality()函数。

functionality()函数非常简单,用于返回algorithm所支持的通信协议,如I2C_FUNC_I2C、I2C_FUNC_10BIT_ADDRI2C_FUNC_SMBUS_READ_BYTEI2C_FUNC_SMBUS_WRITE_BYTE 等。

下面以2440为例看下实现:

定义通信方法:

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

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++) {

		ret = s3c24xx_i2c_doxfer(i2c, msgs, num);//传输函数

		if (ret != -EAGAIN)
			return ret;

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

		udelay(100);
	}

	return -EREMOTEIO;
}
s3c24xx_i2c_doxfer的实现
static int s3c24xx_i2c_doxfer(struct s3c24xx_i2c *i2c, struct i2c_msg *msgs, int num)
{
	unsigned long timeout;
	int ret;

	ret = s3c24xx_i2c_set_master(i2c);//获取i2c总线,先判断i2c是否处于busy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;
	i2c->msg_num = num;
	i2c->msg_ptr = 0;
	i2c->msg_idx = 0;
	i2c->state   = STATE_START;
        //以下对中断,ack,读写就行相关的设置,是不是控制器设置好后就会自动的发送相关的信号?
	s3c24xx_i2c_enable_irq(i2c);//使能中断,
	s3c24xx_i2c_message_start(i2c, msgs);//相关的设置
	spin_unlock_irq(&i2c->lock);
	
	timeout = wait_event_timeout(i2c->wait, i2c->msg_num == 0, HZ * 5);

	ret = i2c->msg_idx;

	/* 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)
		dev_dbg(i2c->dev, "incomplete xfer (%d)\n", ret);

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

	msleep(1);

 out:
	return ret;
}
s3c24xx_i2c_message_start
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;//读写

	if (msg->flags & I2C_M_RD) {//判断是读还是写
		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);//发送地址
	
	/* 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;
	writel(stat, i2c->regs + S3C2410_IICSTAT);
}
 
 

从上面的分析可以看到master_xfer并没有完成i2c的整个传输,只是发送一个起始条件和从设备地址,那么传输数据是如何完成的??

先给出i2c传输的模板:

master_xfer()函数在 I2C 适配器上完成传递给它的 i2c_msg 数组中的每个 I2C 消息,下面所示为 xxx 设备的 master_xfer()函数模板。

static int i2c_adapter_xxx_xfer(struct i2c_adapter *adap, struct i2c_msg *msgs,int num)
{
...
for (i = 0; i < num; i++)
{
  i2c_adapter_xxx_start(); /*产生开始位*/
  /*是读消息*/
  if(msgs[i]->flags &I2C_M_RD)
  {
    i2c_adapter_xxx_setaddr((msg->addr << 1) | 1); /*发送从设备读地址*/
    i2c_adapter_xxx_wait_ack(); /*获得从设备的 ack*/
    i2c_adapter_xxx_readbytes(msgs[i]->buf, msgs[i]->len); /*读取msgs[i] ->len长的数据到 msgs[i]->buf*/
  }
  else/*是写消息*/
  {
    i2c_adapter_xxx_setaddr(msg->addr << 1); /*发送从设备写地址*/
    i2c_adapter_xxx_wait_ack(); /*获得从设备的 ack*/
    i2c_adapter_xxx_writebytes(msgs[i]->buf, msgs[i]->len); /*读取 msgs[i] ->len长的数据到 msgs[i]->buf*/
  }
}
i2c_adapter_xxx_stop(); /*产生停止位*/
}
上述代码实际上给出了一个 master_xfer()函数处理 I2C 消息数组的流程,对于数组中的每个消息,判断消息类型,若为 读消息,则赋从设备地址为 (msg->addr << 1)|1,否则为 msg->addr << 1。对每个消息产生一个开始位,紧接着传送从设备地址,然后开始数据的发送或接收,对最后的消息还需产生一个停止位。
如下图所示为整个 master_xfer()完成的时序。
Linux I2C 总线驱动_第1张图片
master_xfer()函数模板中的 i2c_adapter_xxx_start()、i2c_adapter_xxx_setaddr()、i2c_adapter_ xxx_wait_ack()、
i2c_adapter_xxx_readbytes()、i2c_adapter_xxx_writebytes()和 i2c_adapter_xxx_stop()函数用于完成适配器的底层硬件操作,与 I2C 适配器和 CPU的具体硬件直接相关,需要由工程师根据芯片的数据手册来实现
i2c_adapter_xxx_readbytes() 用 于 从 从 设 备 上 接 收 一 串 数 据 ,

i2c_adapter_xxx_writebytes()用于向从设备写入一串数据,这两个函数的内部也会涉及I2C 总线协议中的 ACK 应答。


下面看下S3C2410 I2C 总线驱动实例

S3C2410 I2C 控制器硬件描述
S3C2410 处理器内部集成了一个 I2C 控制器,通过 4 个寄存器就可方便地对其进行控制,这 4 个寄存器如下。
1.IICCON:I2C 控制寄存器。
2.IICSTAT:I2C 状态寄存器。
3.IICDS:I2C 收发数据移位寄存器。
4.IICADD:I2C 地址寄存器。
S3C2410 处理器内部集成的 I2C 控制器可支持主、从两种模式,我们主要使用其主模式。通过对 IICCON、IICDS 和 IICADD 寄存器的操作,可在 I2C 总线上产生开始位、停止位、数据和地址,而传输的状态则通过 IICSTAT 寄存器获取

S3C2410 I2C 总线驱动总体分析
S3C2410 的 I2C 总线驱动设计主要要完成以下工作。
1.设计对应于 i2c_adapter_xxx_init()模板的 S3C2410 的模块加载函数和对应于i2c_ adapter_xxx_exit()函数模板的模块卸载函数。
2.设计对应于 i2c_adapter_xxx_xfer()模板的 S3C2410 适配器的通信方法函数。针 对 S3C2410 , functionality() 函 数 只 需 简 单 地 返 回 I2C_FUNC_I2C | I2C_FUNC_SMBUS_ EMUL | I2C_FUNC_PROTOCOL_MANGLING 表明其支持的功能。

这里分析内核中自带的 Ben Dooks 版本驱动,它同时支持 S3C2410 和 S3C2440。
下图给出了 S3C2410 驱动中的主要函数与 上面模板函数的对应关系,由于实现通信方法的方式不一样,模板的一个函数可能对应于 S3C2410 I2C 总线驱动的多个函数。


S3C2410 I2C 适配器驱动的模块加载与卸载
I2C 适配器驱动被作为一个单独的模块加载进内核,在模块的加载和卸载函数中,只需注册和注销一个 platform_driver 结构体,如代码清单如下所示:

static int __init i2c_adap_s3c_init(void)
{
	int ret;

	ret = platform_driver_register(&s3c2410_i2c_driver);
	if (ret == 0) {
		ret = platform_driver_register(&s3c2440_i2c_driver);
		if (ret)
			platform_driver_unregister(&s3c2410_i2c_driver);
	}

	return ret;
}

static void __exit i2c_adap_s3c_exit(void)
{
	platform_driver_unregister(&s3c2410_i2c_driver);
	platform_driver_unregister(&s3c2440_i2c_driver);
}
platform_driver 结构体包含了具体适配器的 probe()函数、remove()函数、resume()函数指针等信息,它需要被定义和赋值,如代码清单如下所示:

static struct platform_driver s3c2410_i2c_driver = {
	.probe		= s3c24xx_i2c_probe,
	.remove		= s3c24xx_i2c_remove,
	.resume		= s3c24xx_i2c_resume,
	.driver		= {
		.owner	= THIS_MODULE,
		.name	= "s3c2410-i2c",
	},
};
当 通 过 Linux 内 核 源 代 码 /drivers/base/platform.c 文 件 中 定 义platform_driver_unregister()函数注册 platform_driver 结构体时,其中 probe 指针指向的s3c24xx_i2c_probe()函数将被调用, 以初始化适配器硬件,如代码清单如下所示。
static int s3c24xx_i2c_probe(struct platform_device *pdev)
{
	struct s3c24xx_i2c *i2c = &s3c24xx_i2c;
	struct resource *res;
	int ret;

	/* find the clock and enable it */

	i2c->dev = &pdev->dev;
	i2c->clk = clk_get(&pdev->dev, "i2c");
	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);
	if (res == NULL) {
		dev_err(&pdev->dev, "cannot find IO resource\n");
		ret = -ENOENT;
		goto err_clk;
	}

	i2c->ioarea = request_mem_region(res->start, (res->end-res->start)+1,
					 pdev->name);

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

	i2c->regs = ioremap(res->start, (res->end-res->start)+1);

	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);
	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 
	 */

	res = platform_get_resource(pdev, IORESOURCE_IRQ, 0);
	if (res == NULL) {
		dev_err(&pdev->dev, "cannot find IRQ\n");
		ret = -ENOENT;
		goto err_iomap;
	}

	ret = request_irq(res->start, s3c24xx_i2c_irq, IRQF_DISABLED,
			  pdev->name, i2c);

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

	i2c->irq = res;
		
	dev_dbg(&pdev->dev, "irq resource %p (%lu)\n", res,
		(unsigned long)res->start);

	ret = i2c_add_adapter(&i2c->adap);
	if (ret < 0) {
		dev_err(&pdev->dev, "failed to add bus to i2c core\n");
		goto err_irq;
	}

	platform_set_drvdata(pdev, i2c);

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

 err_irq:
	free_irq(i2c->irq->start, 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:
	return ret;
}
上述代码中的主体工作是使能硬件并申请 I2C 适配器使用的 I/O 地址、在这些工作都完成无误后,通过 I2C 核心提供的 i2c_add_adapter()函数添加这个适配器 。 因 为 S3C2410 内 部 集 成 I2C 控 制 器 , 可 以 确 定 I2C 适 配 器 一 定 存 在 ,
s3c24xx_i2c_probe()函数虽然命名“探测”,但实际没有也不必进行任何探测工作,之所以这样命名完全是一种设计习惯。
与 s3c24xx_i2c_probe()函数完成相反功能的函数是 s3c24xx_i2c_remove()函数,它在 适 配 器 模 块 卸 载 函 数 调 用 platform_driver_unregister() 函 数 时 所 示 通 过platform_driver 的 remove 指针方式被调用。xxx_i2c_remove()的设计模板如代码清单如下所示。
static int s3c24xx_i2c_remove(struct platform_device *pdev)
{
	struct s3c24xx_i2c *i2c = platform_get_drvdata(pdev);

	i2c_del_adapter(&i2c->adap);
	free_irq(i2c->irq->start, i2c);

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

	iounmap(i2c->regs);

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

	return 0;
}
上面代码用到的 s3c24xx_i2c 结构体进行适配器所有信息的封装,类似于私有信息结构体,下面所示的 s3c24xx_i2c 结构体的定义,以及驱动模块定义的一个s3c24xx_i2c 结构体全局实例。
struct s3c24xx_i2c {
	spinlock_t		lock;
	wait_queue_head_t	wait;//等待队列

	struct i2c_msg		*msg;//传输的信息结构体
	unsigned int		msg_num;
	unsigned int		msg_idx;
	unsigned int		msg_ptr;

	unsigned int		tx_setup;

	enum s3c24xx_i2c_state	state;

	void __iomem		*regs;
	struct clk		*clk;
	struct device		*dev;
	struct resource		*irq;
	struct resource		*ioarea;
	struct i2c_adapter	adap;//适配器
};
static struct s3c24xx_i2c s3c24xx_i2c = {
	.lock		= __SPIN_LOCK_UNLOCKED(s3c24xx_i2c.lock),
	.wait		= __WAIT_QUEUE_HEAD_INITIALIZER(s3c24xx_i2c.wait),
	.tx_setup	= 50,
	.adap		= {
		.name			= "s3c2410-i2c",
		.owner			= THIS_MODULE,
		.algo			= &s3c24xx_i2c_algorithm,
		.retries		= 2,
		.class			= I2C_CLASS_HWMON,
	},
};
S3C2410 I2C 总线通信方法
从上面代码可以看出,I2C 适配器对应的 i2c_algorithm 结构体实例为 s3c24xx_i2c_algorithm,下面代码清单 所示 s3c24xx_i2c_algorithm 的定义。
static const struct i2c_algorithm s3c24xx_i2c_algorithm = {
	.master_xfer		= s3c24xx_i2c_xfer,
	.functionality		= s3c24xx_i2c_func,
};
上述代码指定了 S3C2410 I2C 总线通信传输函数 s3c24xx_i2c_xfer(),这个函数非常关键,所有 I2C 总线上对设备的访问最终应该由它来完成,下面代码所 示 为 这 个 重 要 函 数 以 及 其 依 赖 的 s3c24xx_i2c_doxfer() 函 数 和s3c24xx_i2c_message_start()函数的源代码。
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++) {

		ret = s3c24xx_i2c_doxfer(i2c, msgs, num);//重要传输函数

		if (ret != -EAGAIN)
			return ret;

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

		udelay(100);
	}

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

	ret = s3c24xx_i2c_set_master(i2c);
	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;
	i2c->msg_num = num;
	i2c->msg_ptr = 0;
	i2c->msg_idx = 0;
	i2c->state   = STATE_START;

	s3c24xx_i2c_enable_irq(i2c);
	s3c24xx_i2c_message_start(i2c, msgs);//重要函数
	spin_unlock_irq(&i2c->lock);
	
	timeout = wait_event_timeout(i2c->wait, i2c->msg_num == 0, HZ * 5);

	ret = i2c->msg_idx;

	/* 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)
		dev_dbg(i2c->dev, "incomplete xfer (%d)\n", ret);

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

	msleep(1);//确保停止位被发送

 out:
	return ret;
}
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;

	if (msg->flags & I2C_M_RD) {
		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);/*如果要使能 ACK,则使能*/

	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);
	
	/* delay here to ensure the data byte has gotten onto the bus
	 * before the transaction is started */
        /*在发送新的开始位前延迟 i2c->tx_setup 位*/
	ndelay(i2c->tx_setup);

	dev_dbg(i2c->dev, "iiccon, %08lx\n", iiccon);
	writel(iiccon, i2c->regs + S3C2410_IICCON);
	
	stat |=  S3C2410_IICSTAT_START;
	writel(stat, i2c->regs + S3C2410_IICSTAT);//发送开始位
}
s3c24xx_i2c_xfer()函数调用 s3c24xx_i2c_doxfer()函数传输 I2C 消息,for循环意味着最多可以重试 adap->retries 次。
s3c24xx_i2c_doxfer()首先将 S3C2410 的 I2C 适配器设置为 I2C 主设备,其后初始化 s3c24xx_i2c 结构体,使能 I2C 中断,并调用 s3c24xx_i2c_message_start()函数启动I2C 消息的传输。s3c24xx_i2c_message_start()函数写 S3C2410 适配器对应的控制寄存器,向 I2C 从设备传递开始位和从设备地址。
上述代码只是启动了 I2C 消息数组的传输周期,并没有完整实现模板图中给出的algorithm master_xfer 流程。 这个流程的完整实现需要借助 I2C 适配器上的中断来步步推进。下面代码所示为 S3C2410 I2C 适配器中断处理函数以及其依赖的 i2c_s3c_irq_nextbyte()函数的源代码。
static irqreturn_t s3c24xx_i2c_irq(int irqno, void *dev_id)
{
	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);//重要的推进函数

 out:
	return IRQ_HANDLED;
}
i2s_s3c_irq_nextbyte

static int i2s_s3c_irq_nextbyte(struct s3c24xx_i2c *i2c, unsigned long iicstat)
{
	unsigned long tmp;
	unsigned char byte;
	int ret = 0;

	switch (i2c->state) {

	case STATE_IDLE://空闲状态
		dev_err(i2c->dev, "%s: called in STATE_IDLE\n", __FUNCTION__);
		goto out;
		break;

	case STATE_STOP://传输停止
		dev_err(i2c->dev, "%s: called in STATE_STOP\n", __FUNCTION__);
		s3c24xx_i2c_disable_irq(i2c);//关闭中断		
		goto out_ack;

	case STATE_START://开始传输
		/* 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... */
                        /* 没有收到 ACK */
			dev_dbg(i2c->dev, "ack was not received\n");
			s3c24xx_i2c_stop(i2c, -EREMOTEIO);//停止i2c传输
			goto out_ack;
		}

		if (i2c->msg->flags & I2C_M_RD) //检查是读还是写
			i2c->state = STATE_READ;
		else
			i2c->state = STATE_WRITE;
    
		/* terminate the transfer if there is nothing to do
		 * (used by the i2c probe to find devices */
                /* 仅一条消息,而且长度为 0(主要用于适配器探测),发送停止位*/
		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:/* 进入写状态 */
		/* we are writing data to the device... check for the
		 * end of the message, and if so, work out what to do
		 */

	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)) {/* 推进到下一条消息 */
			/* 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++;
			/* 检查是否要为该消息产生开始位 */
			/* 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 {
			        /* 发送新的开始位 */
				/* 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
		 */
                /* 有一个字节可读,看是否还有消息要处理*/
		if (!(i2c->msg->flags & I2C_M_IGNORE_NAK) &&
		    !(is_msglast(i2c) && is_lastmsg(i2c))) {

			if (iicstat & S3C2410_IICSTAT_LASTBIT) {
				dev_dbg(i2c->dev, "READ: No Ack\n");

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

		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)) {/* 还有消息要处理吗? */
			/* 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 *//* 推进到下一条消息 */
				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:/* irq 清除 */
	tmp = readl(i2c->regs + S3C2410_IICCON);	
	tmp &= ~S3C2410_IICCON_IRQPEND;
	writel(tmp, i2c->regs + S3C2410_IICCON);
 out:
	return ret;
}
中断处理函数 s3c24xx_i2c_irq()主要通过调用 i2s_s3c_irq_nextbyte()函数进行传输工 作 的 进 一 步 推 进 。 i2s_s3c_irq_nextbyte() 函 数 通 过 switch(i2c->state) 语 句 分 成i2c->state 的不同状态进行处理,在每种状态下,先检查 i2c->state 的状态与硬件寄存器应该处于的状态是否一致,如果不一致,则证明有误,直接返回。当 I2C 处于读状态 STATE_READ 或写状态 STATE_WRITE 时,通过 is_lastmsg()函数判断是否传输的是最后一条 I2C 消息,如果是,则产生停止位,否则通过 i2c->msg_idx++、
i2c->msg++推进到下一条消息。






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