十九、IIC驱动框架介绍及驱动代码解析

一、IIC驱动框架简介

1、IIC物理总线

• SCL:时钟线，数据收发同步。
• SDL:数据线，具体数据

支持一主多从，各设备地址独立，标准模式传输速率为100kbit/s,快速模式为400kbit/s

2、常见IIC设备

• EEPROM
• 触摸芯片
• 温湿度传感器
• mpu6050

3、框架图

• I2C核心

　　　　提供I2C总线驱动和设备驱动的注册方法、注销方法、I2C通信硬件无关代码。

• I2C总线驱动

　　　　主要包含I2C硬件体系结构中适配器（IIC控制器）的控制，用于I2C读写时序。

　　　　主要数据结构：I2C_adapter、 I2C_algorithm.

• I2C设备驱动

　　　　通过I2C适配器与CPU交换数据。

　　　　主要数据结构：i2c_driver和i2c_client.

• I2C适配器

　　　　i2c adapter是软件上抽象出来的i2c总线控制器接口,物理上一条i2c总线可以挂接多个硬件设备(slave)，一个CPU可以挂接多条i2c总线(想象一下PCI总线),i2c总线控制器就是CPU访问I2C总线的硬件接口，也就是你说的那几个寄存器  

　　　　简单来说，你的开发板上有几个I2C接口，就有几个adapter , 也就是有几条I2C bus ， I2C CLIENT 对应的就是你的外围I2C 设备，有几个就有几个CLIENT , 把这些设备插入开发板， 对应其中的一条BUS, 那么相应的就对应了其中的一个ADAPTER , 接下来的就是 　　　　　　　　        CLIENT 与 ADAPTER 勾搭成对了， 后面就是做该做的事了

二、I2C驱动文件分析

1、i2c-dev.c

(1)初始化

static int __init i2c_dev_init(void)
{
	int res;

	printk(KERN_INFO "i2c /dev entries driver\n");
　　　　//1、注册字符设备
	res = register_chrdev_region(MKDEV(I2C_MAJOR, 0), I2C_MINORS, "i2c");
	if (res)
		goto out;
　　　　//2、创建类
	i2c_dev_class = class_create(THIS_MODULE, "i2c-dev");
	if (IS_ERR(i2c_dev_class)) {
		res = PTR_ERR(i2c_dev_class);
		goto out_unreg_chrdev;
	}
	i2c_dev_class->dev_groups = i2c_groups;
　　　//3、追踪哪个i2c适配器被添加或者移除　　　
	/* Keep track of adapters which will be added or removed later */
	res = bus_register_notifier(&i2c_bus_type, &i2cdev_notifier);
	if (res)
		goto out_unreg_class;
　　　//4、立即绑定已经存在的适配器
	/* Bind to already existing adapters right away */
	i2c_for_each_dev(NULL, i2cdev_attach_adapter);

	return 0;

out_unreg_class:
	class_destroy(i2c_dev_class);
out_unreg_chrdev:
	unregister_chrdev_region(MKDEV(I2C_MAJOR, 0), I2C_MINORS);
out:
	printk(KERN_ERR "%s: Driver Initialisation failed\n", __FILE__);
	return res;
}

（2）文件操作集合　　

i2c设备驱动的文件操作集合中有两个ioctl:

static const struct file_operations i2cdev_fops = {
	.owner		= THIS_MODULE,
	.llseek		= no_llseek,
	.read		= i2cdev_read,   //读
	.write		= i2cdev_write,  //写
	.unlocked_ioctl	= i2cdev_ioctl,  
	.compat_ioctl	= compat_i2cdev_ioctl,
	.open		= i2cdev_open,
	.release	= i2cdev_release,
};　

两者的区别在于：

• 64位的用户程序运行在64位的kernel上，调用的是compat_ioctl，
• 32位的APP运行在32位的kernel上，调用的也是unlocked_ioctl。 

（3）读操作

static ssize_t i2cdev_read(struct file *file, char __user *buf, size_t count,
		loff_t *offset)
{
	char *tmp;
	int ret;

	struct i2c_client *client = file->private_data;
        //最多读8192个字节
	if (count > 8192)
		count = 8192;

	tmp = kzalloc(count, GFP_KERNEL);
	if (tmp == NULL)
		return -ENOMEM;

	pr_debug("i2c-dev: i2c-%d reading %zu bytes.\n",
		iminor(file_inode(file)), count);
        //接收到i2c传过来的数据
	ret = i2c_master_recv(client, tmp, count);
	if (ret >= 0)
　　　　　　　　　　//将数据拷贝到用户空间
		if (copy_to_user(buf, tmp, ret))
			ret = -EFAULT;
	kfree(tmp);
	return ret;
}

（4）写操作

static ssize_t i2cdev_write(struct file *file, const char __user *buf,
		size_t count, loff_t *offset)
{
	int ret;
	char *tmp;
	struct i2c_client *client = file->private_data;

	if (count > 8192)
		count = 8192;
       //分配一块内存空间，将用户空间的数据拷贝进去
	tmp = memdup_user(buf, count);
	if (IS_ERR(tmp))
		return PTR_ERR(tmp);

	pr_debug("i2c-dev: i2c-%d writing %zu bytes.\n",
		iminor(file_inode(file)), count);
　　　　　//i2c发送　
	ret = i2c_master_send(client, tmp, count);
	kfree(tmp);
	return ret;
}　　

memdup_user函数

/**
 * memdup_user - duplicate memory region from user space
 *
 * @src: source address in user space
 * @len: number of bytes to copy
 *
 * Return: an ERR_PTR() on failure.  Result is physically
 * contiguous, to be freed by kfree().
 */
void *memdup_user(const void __user *src, size_t len)
{
	void *p;
        //分配内核态的空间
	p = kmalloc_track_caller(len, GFP_USER | __GFP_NOWARN);
	if (!p)
		return ERR_PTR(-ENOMEM);
　　　　　//从用户空间拷贝数据
	if (copy_from_user(p, src, len)) {
		kfree(p);
		return ERR_PTR(-EFAULT);
	}

	return p;
}

（5）i2cdev_ioctl

static long i2cdev_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
	struct i2c_client *client = file->private_data;
	unsigned long funcs;

	dev_dbg(&client->adapter->dev, "ioctl, cmd=0x%02x, arg=0x%02lx\n",
		cmd, arg);

	switch (cmd) {
　　　　　　//设置从机地址
　　　　　　//I2C_SLAVE和I2C_SLAVE_FORCE的区别在于I2C_SLACE会检查设备地址，如果地址已经使用，就不能使用重复的地址，否则会返回-EBUSY,而I2C_SLACE_FORCE会跳过检查
	case I2C_SLAVE:
	case I2C_SLAVE_FORCE:
		if ((arg > 0x3ff) ||
		    (((client->flags & I2C_M_TEN) == 0) && arg > 0x7f))
			return -EINVAL;
		if (cmd == I2C_SLAVE && i2cdev_check_addr(client->adapter, arg))
			return -EBUSY;
		/* REVISIT: address could become busy later */
		client->addr = arg;
		return 0;
　　　　//设置10bit地址模式
       //如果select不等于0选择10bit地址模式，如果等于0选择7bit模式，默认7bit。只有适配器支持I2C_FUNC_10BIT_ADDR，这个请求才是有效的
	case I2C_TENBIT:
		if (arg)
			client->flags |= I2C_M_TEN;
		else
			client->flags &= ~I2C_M_TEN;
		return 0;
        //设置传输后增加PEC标志(用于数据校验)
　　　　　//这个命令只对SMBus传输有效。这个请求只在适配器支持I2C_FUNC_SMBUS_PEC时有效；如果不支持这个命令也是安全的，它不做任何工作　
	case I2C_PEC:
		/*
		 * Setting the PEC flag here won't affect kernel drivers,
		 * which will be using the i2c_client node registered with
		 * the driver model core.  Likewise, when that client has
		 * the PEC flag already set, the i2c-dev driver won't see
		 * (or use) this setting.
		 */
		if (arg)
			client->flags |= I2C_CLIENT_PEC;
		else
			client->flags &= ~I2C_CLIENT_PEC;
		return 0;
　　　　//获取适配器支持的功能：SMbus或者普通的I2C
	case I2C_FUNCS:
		funcs = i2c_get_functionality(client->adapter);
		return put_user(funcs, (unsigned long __user *)arg);
       //i2c读写,和i2c_read、i2c_write的区别在于这两个函数一次只能处理一条消息，而i2c_RDWR一次可以处理多条消息
	case I2C_RDWR: {
		struct i2c_rdwr_ioctl_data rdwr_arg;
		struct i2c_msg *rdwr_pa;

		if (copy_from_user(&rdwr_arg,
				   (struct i2c_rdwr_ioctl_data __user *)arg,
				   sizeof(rdwr_arg)))
			return -EFAULT;

		if (!rdwr_arg.msgs || rdwr_arg.nmsgs == 0)
			return -EINVAL;

		/*
		 * Put an arbitrary limit on the number of messages that can
		 * be sent at once
		 */
　　　　　　　　//最多处理42条message
		if (rdwr_arg.nmsgs > I2C_RDWR_IOCTL_MAX_MSGS)
			return -EINVAL;

		rdwr_pa = memdup_user(rdwr_arg.msgs,
				      rdwr_arg.nmsgs * sizeof(struct i2c_msg));
		if (IS_ERR(rdwr_pa))
			return PTR_ERR(rdwr_pa);

		return i2cdev_ioctl_rdwr(client, rdwr_arg.nmsgs, rdwr_pa);
	}
    //smbus协议
	case I2C_SMBUS: {
		struct i2c_smbus_ioctl_data data_arg;
		if (copy_from_user(&data_arg,
				   (struct i2c_smbus_ioctl_data __user *) arg,
				   sizeof(struct i2c_smbus_ioctl_data)))
			return -EFAULT;
		return i2cdev_ioctl_smbus(client, data_arg.read_write,
					  data_arg.command,
					  data_arg.size,
					  data_arg.data);
	}
　　　　//设置重试次数
　　　　//这句话设置适配器收不到ACK时重试的次数为m。默认的重试次数为1。

	case I2C_RETRIES:
		if (arg > INT_MAX)
			return -EINVAL;

		client->adapter->retries = arg;
		break;
　　　　//超时时间，时间单位为jiffes
	case I2C_TIMEOUT:
		if (arg > INT_MAX)
			return -EINVAL;

		/* For historical reasons, user-space sets the timeout
		 * value in units of 10 ms.
		 */
		client->adapter->timeout = msecs_to_jiffies(arg * 10);
		break;
	default:
		/* NOTE:  returning a fault code here could cause trouble
		 * in buggy userspace code.  Some old kernel bugs returned
		 * zero in this case, and userspace code might accidentally
		 * have depended on that bug.
		 */
		return -ENOTTY;
	}
	return 0;
}

I2C_RDWR应用实例：i2c驱动之调用ioctl函数进行读写at24c08

i2c数据传输主要通过i2c_transfer函数：

/**
 * i2c_transfer - execute a single or combined I2C message
 * @adap: Handle to I2C bus
 * @msgs: One or more messages to execute before STOP is issued to
 *	terminate the operation; each message begins with a START.
 * @num: Number of messages to be executed.
 *
 * Returns negative errno, else the number of messages executed.
 *
 * Note that there is no requirement that each message be sent to
 * the same slave address, although that is the most common model.
 */
int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msgs, int num)
{
	int ret;

	if (!adap->algo->master_xfer) {
		dev_dbg(&adap->dev, "I2C level transfers not supported\n");
		return -EOPNOTSUPP;
	}

	/* REVISIT the fault reporting model here is weak:
	 *
	 *  - When we get an error after receiving N bytes from a slave,
	 *    there is no way to report "N".
	 *
	 *  - When we get a NAK after transmitting N bytes to a slave,
	 *    there is no way to report "N" ... or to let the master
	 *    continue executing the rest of this combined message, if
	 *    that's the appropriate response.
	 *
	 *  - When for example "num" is two and we successfully complete
	 *    the first message but get an error part way through the
	 *    second, it's unclear whether that should be reported as
	 *    one (discarding status on the second message) or errno
	 *    (discarding status on the first one).
	 */
	ret = __i2c_lock_bus_helper(adap);
	if (ret)
		return ret;

	ret = __i2c_transfer(adap, msgs, num);
	i2c_unlock_bus(adap, I2C_LOCK_SEGMENT);

	return ret;
}

(6)退出函数

static void __exit i2c_dev_exit(void)
{
	bus_unregister_notifier(&i2c_bus_type, &i2cdev_notifier);
	i2c_for_each_dev(NULL, i2cdev_detach_adapter);
	class_destroy(i2c_dev_class);
	unregister_chrdev_region(MKDEV(I2C_MAJOR, 0), I2C_MINORS);
}

2、i2c_algorithm

两种i2c算法，一种是普通的I2c数据通讯协议，一种是SMbus协议，是两种不同的通信方法

/**
 * struct i2c_algorithm - represent I2C transfer method
 * @master_xfer: Issue a set of i2c transactions to the given I2C adapter
 *   defined by the msgs array, with num messages available to transfer via
 *   the adapter specified by adap.
 * @master_xfer_atomic: same as @master_xfer. Yet, only using atomic context
 *   so e.g. PMICs can be accessed very late before shutdown. Optional.
 * @smbus_xfer: Issue smbus transactions to the given I2C adapter. If this
 *   is not present, then the bus layer will try and convert the SMBus calls
 *   into I2C transfers instead.
 * @smbus_xfer_atomic: same as @smbus_xfer. Yet, only using atomic context
 *   so e.g. PMICs can be accessed very late before shutdown. Optional.
 * @functionality: Return the flags that this algorithm/adapter pair supports
 *   from the I2C_FUNC_* flags.
 * @reg_slave: Register given client to I2C slave mode of this adapter
 * @unreg_slave: Unregister given client from I2C slave mode of this adapter
 *
 * The following structs are for those who like to implement new bus drivers:
 * i2c_algorithm is the interface to a class of hardware solutions which can
 * be addressed using the same bus algorithms - i.e. bit-banging or the PCF8584
 * to name two of the most common.
 *
 * The return codes from the @master_xfer{_atomic} fields should indicate the
 * type of error code that occurred during the transfer, as documented in the
 * Kernel Documentation file Documentation/i2c/fault-codes.rst.
 */
struct i2c_algorithm {
	/*
	 * If an adapter algorithm can't do I2C-level access, set master_xfer
	 * to NULL. If an adapter algorithm can do SMBus access, set
	 * smbus_xfer. If set to NULL, the SMBus protocol is simulated
	 * using common I2C messages.
	 *
	 * master_xfer should return the number of messages successfully
	 * processed, or a negative value on error
	 */
	int (*master_xfer)(struct i2c_adapter *adap, struct i2c_msg *msgs,
			   int num);
	int (*master_xfer_atomic)(struct i2c_adapter *adap,
				   struct i2c_msg *msgs, int num);
	int (*smbus_xfer)(struct i2c_adapter *adap, u16 addr,
			  unsigned short flags, char read_write,
			  u8 command, int size, union i2c_smbus_data *data);
	int (*smbus_xfer_atomic)(struct i2c_adapter *adap, u16 addr,
				 unsigned short flags, char read_write,
				 u8 command, int size, union i2c_smbus_data *data);

	/* To determine what the adapter supports */
	u32 (*functionality)(struct i2c_adapter *adap);

#if IS_ENABLED(CONFIG_I2C_SLAVE)
	int (*reg_slave)(struct i2c_client *client);
	int (*unreg_slave)(struct i2c_client *client);
#endif
};

3、i2c总线驱动分析（i2c-core-base.c）

(1)i2c总线定义

struct bus_type i2c_bus_type = {
	.name		= "i2c",
	.match		= i2c_device_match,
	.probe		= i2c_device_probe,
	.remove		= i2c_device_remove,
	.shutdown	= i2c_device_shutdown,
};

（2）总线注册

static int __init i2c_init(void)
{
	int retval;

	retval = of_alias_get_highest_id("i2c");

	down_write(&__i2c_board_lock);
	if (retval >= __i2c_first_dynamic_bus_num)
		__i2c_first_dynamic_bus_num = retval + 1;
	up_write(&__i2c_board_lock);

	retval = bus_register(&i2c_bus_type);
	if (retval)
		return retval;

	is_registered = true;

#ifdef CONFIG_I2C_COMPAT
	i2c_adapter_compat_class = class_compat_register("i2c-adapter");
	if (!i2c_adapter_compat_class) {
		retval = -ENOMEM;
		goto bus_err;
	}
#endif
	retval = i2c_add_driver(&dummy_driver);
	if (retval)
		goto class_err;

	if (IS_ENABLED(CONFIG_OF_DYNAMIC))
		WARN_ON(of_reconfig_notifier_register(&i2c_of_notifier));
	if (IS_ENABLED(CONFIG_ACPI))
		WARN_ON(acpi_reconfig_notifier_register(&i2c_acpi_notifier));

	return 0;

class_err:
#ifdef CONFIG_I2C_COMPAT
	class_compat_unregister(i2c_adapter_compat_class);
bus_err:
#endif
	is_registered = false;
	bus_unregister(&i2c_bus_type);
	return retval;
}

（3）i2c设备和驱动匹配规则

static int i2c_device_match(struct device *dev, struct device_driver *drv)
{
	struct i2c_client	*client = i2c_verify_client(dev);
	struct i2c_driver	*driver;


	/* Attempt an OF style match */
	if (i2c_of_match_device(drv->of_match_table, client))
		return 1;

	/* Then ACPI style match */
	if (acpi_driver_match_device(dev, drv))
		return 1;

	driver = to_i2c_driver(drv);

	/* Finally an I2C match */
	if (i2c_match_id(driver->id_table, client))
		return 1;

	return 0;
}

• i2c_of_match_device：设备树匹配方式

　　　　比较I2C设备节点的compatible属性和of_device_id中的compatible属性

• acpi_driver_match_device: ACPI匹配方式
• i2c_match_id: i2c总线传统匹配方式

　　　　比较i2c设备名字和i2c驱动的id_table->name字段是否相等

三、SMbus介绍 

1、介绍

• 系统管理总线(SMBus)是一个两线接口。通过它，各设备之间以及设备与系统的其他部分之间可以互相通信。
• 它基于I2C操作原理。SMBus为系统和电源管理相关的任务提供一条控制总线。一个系统利用SMBus可以和多个设备互传信息，而不需使用独立的控制线路。
• 系统管理总线(SMBus)标准涉及三类设备。从设备-接收或响应命令的设备。主设备-用来发布命令，产生时钟和终止发送的设备。主机，是一种专用的主设备，它提供与系统CPU的主接口。主机必须具有主-从机功能，并且必须支持SMBus通报协议。
• 在一个系统里只允许有一个主机。

2、SMBus和I2C之间的相似点

• 2条线的总线协议(1个时钟，1个数据) + 可选的SMBus提醒线
• 主-从通信，主设备提供时钟
• 多主机功能
• SMBus数据格式类似于I2C的7位地址格式

3、SMBus和I2C之间的不同点

SMbus	i2c
传输速度 10khz~100khz	最小传输速度 10kHz 无最小传输速度
最小传输速度 35ms时钟低超时	无时钟超时
固定的逻辑电平	逻辑电平由VDD决定
不同的地址类型(保留、动态等)	7位、10位和广播呼叫从地址类型
不同的总线协议（快速命令、处理呼叫等)	无总线协议

4、SMBus应用用途

　　利用系统管理总线，设备可提供制造商信息，告诉系统它的型号/部件号，保存暂停事件的状态，报告不同类型的错误，接收控制参数，和返回它的状态。SMBus为系统和电源管理相关的任务提供控制总线。