一、Linux的I2C体系结构
主要由三部分组成：
(1) I2C核心
提供I2C控制器和设备驱动的注册和注销方法，I2C通信方法，与适配器无关的代码以及探测设备等。
(2) I2C控制器驱动(适配器)
(3) I2C设备驱动
二、重要的结构体
- i2c_adapter
//i2c控制器(适配器)
struct i2c_adapter {
struct module *owner;
unsigned int class; /* classes to allow probing for */
const struct i2c_algorithm *algo; /* 总线通信结构体指针 */
void *algo_data; //algorithm数据
/* data fields that are valid for all devices */
//并发同步，互斥锁
const struct i2c_lock_operations *lock_ops;
struct rt_mutex bus_lock;
struct rt_mutex mux_lock;
int timeout; /* in jiffies */
int retries; //重试次数
struct device dev; /* the adapter device */
int nr;
char name[48]; //适配器名称
struct completion dev_released;
struct mutex userspace_clients_lock;
struct list_head userspace_clients; //client链表
struct i2c_bus_recovery_info *bus_recovery_info;
const struct i2c_adapter_quirks *quirks;
struct irq_domain *host_notify_domain;
};- i2c_algorithm
//I2C传输方法
struct i2c_algorithm {
//i2c传输函数指针
int (*master_xfer)(struct i2c_adapter *adap, struct i2c_msg *msgs,
int num);
//smbus传输函数指针
int (*smbus_xfer) (struct i2c_adapter *adap, u16 addr,
unsigned short flags, char read_write,
u8 command, int size, union i2c_smbus_data *data);
//返回适配器支持的功能
u32 (*functionality) (struct i2c_adapter *);
//作为从机时使用
#if IS_ENABLED(CONFIG_I2C_SLAVE)
int (*reg_slave)(struct i2c_client *client);
int (*unreg_slave)(struct i2c_client *client);
#endif
};SMBus是基于I2C总线规范的，所以上面的传输函数要根据自己的总线来选择，选择其一就可以。
- i2c_driver
//I2C驱动，和platform_driver,spi_driver类似
struct i2c_driver {
unsigned int class;
/* Standard driver model interfaces */
int (*probe)(struct i2c_client *, const struct i2c_device_id *);
int (*remove)(struct i2c_client *);
int (*probe_new)(struct i2c_client *);
void (*shutdown)(struct i2c_client *);
void (*alert)(struct i2c_client *, enum i2c_alert_protocol protocol,
unsigned int data);
int (*command)(struct i2c_client *client, unsigned int cmd, void *arg);
struct device_driver driver;
const struct i2c_device_id *id_table; //该设备所支持的设备ID表
/* Device detection callback for automatic device creation */
int (*detect)(struct i2c_client *, struct i2c_board_info *);
const unsigned short *address_list;
struct list_head clients; //client链表
bool disable_i2c_core_irq_mapping;
};- i2c_client
//I2C设备
struct i2c_client {
unsigned short flags; //标志
unsigned short addr; //芯片地址，保存在addr低7位
char name[I2C_NAME_SIZE]; //设备名称
struct i2c_adapter *adapter; //依附的i2c_adapter
struct device dev; //设备结构体
int irq; //设备使用的中断号
struct list_head detected; //client链表，和i2c_driver中clients的一样
#if IS_ENABLED(CONFIG_I2C_SLAVE)
i2c_slave_cb_t slave_cb; //从机模式回调
#endif
};- i2c_msg
//I2C传输数据结构体，代表一个消息数据
struct i2c_msg {
__u16 addr; //设备地址
__u16 flags; //标志
#define I2C_M_RD 0x0001 /* read data, from slave to master */
/* I2C_M_RD is guaranteed to be 0x0001! */
#define I2C_M_TEN 0x0010 /* this is a ten bit chip address */
#define I2C_M_DMA_SAFE 0x0200 /* the buffer of this message is DMA safe */
/* makes only sense in kernelspace */
/* userspace buffers are copied anyway */
#define I2C_M_RECV_LEN 0x0400 /* length will be first received byte */
#define I2C_M_NO_RD_ACK 0x0800 /* if I2C_FUNC_PROTOCOL_MANGLING */
#define I2C_M_IGNORE_NAK 0x1000 /* if I2C_FUNC_PROTOCOL_MANGLING */
#define I2C_M_REV_DIR_ADDR 0x2000 /* if I2C_FUNC_PROTOCOL_MANGLING */
#define I2C_M_NOSTART 0x4000 /* if I2C_FUNC_NOSTART */
#define I2C_M_STOP 0x8000 /* if I2C_FUNC_PROTOCOL_MANGLING */
__u16 len; //消息长度
__u8 *buf; //消息数据
};总结上面结构体关系：
1. i2c_adapter和i2c_algorithm
i2c_adapter对应物理上的一个适配器，而i2c_algorithm对应一套通信方法，适配器需要通过i2c_algorithm提供的通信函数来产生对应的访问时序。所以i2c_adapter中包含i2c_algorithm的指针。
i2c_algorithm使用master_xfer()来产生I2C时序，以i2c_msg为单位，i2c_msg代表一次传输的数据。
2. i2c_driver和i2c_client
i2c_driver对应一套驱动方法，包含probe,remove等方法。i2c_clent对应真实的物理设备，每个i2c设备都需要一个i2c_client来描述。i2c_driver与i2c_client是一对多的关系，一个i2c_driver上可以支持多个同类型的i2c_client。
3. i2c_adapter和i2c_client
i2c_adapter与i2c_client的关系和硬件上适配器与设备的关系一致，即i2c_client依附于i2c_adapter。一个适配器可以连接多个设备，所以i2c_adapter中包含i2c_client的链表。
三、API函数
//增加/删除i2c_adapter
int i2c_add_adapter(struct i2c_adapter *adapter)
void i2c_del_adapter(struct i2c_adapter *adap)
//增加/删除i2c_driver
int i2c_register_driver(struct module *owner, struct i2c_driver *driver)
void i2c_del_driver(struct i2c_driver *driver)
#define i2c_add_driver(driver) \
i2c_register_driver(THIS_MODULE, driver)
//i2c传输、发送和接收
//完成I2C总线和I2C设备之间的一定数目的I2C message交互
int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msgs, int num)
//通过封装i2c_transfer()完成一次I2c发送操作
int i2c_master_send(const struct i2c_client *client,
const char *buf, int count)
//通过封装i2c_transfer()完成一次I2c接收操作
int i2c_master_recv(const struct i2c_client *client,
char *buf, int count) i2c_transfer()函数本身不具备驱动适配器物理硬件以完成消息交互的能力，它只是寻找到与i2c_adapter对应的i2c_algorithm, 并使用i2c_algorithm的master_xfer()函数真正驱动硬件流程。
追踪i2c_transfer()的源码会发现下面的代码
for (ret = 0, try = 0; try <= adap->retries; try++) {
ret = adap->algo->master_xfer(adap, msgs, num); //真正发送的函数
if (ret != -EAGAIN)
break;
if (time_after(jiffies, orig_jiffies + adap->timeout))
break;
}四、适配器(控制器)驱动
由于I2C控制器通常是在内存上的，所以它本身也连接在platform总线上的，通过platform_driver和platform_device的匹配还执行。
(1) probe()完成如下工作：
- 初始化I2C控制器所使用的硬件资源，如申请IO地址，中断号，时钟等。
- 为特定I2C控制器实现通信方法，主要是实现i2c_algorithm的master_xfer()和functionality()函数。
- 通过i2c_add_adapter()添加i2c_adapter的数据结构(i2c_adapter成员已被初始化)。
模板代码：
static const struct i2c_algorithm xxx_i2c_algo = {
.master_xfer = xxx_i2c_master_xfer,
.functionality = xxx_i2c_func,
};
static u32 xxx_i2c_func(struct i2c_adapter *adap)
{
return I2C_FUNC_I2C | I2C_FUNC_10BIT_ADDR |
(I2C_FUNC_SMBUS_EMUL & ~I2C_FUNC_SMBUS_QUICK) |
I2C_FUNC_SMBUS_BLOCK_DATA;
}
static int xxx_i2c_master_xfer(struct i2c_adapter *adap, struct i2c_msg *msgs,
int num)
{
int ret, ;
u32 reg;
struct xxx_i2c *id = adap->algo_data;
/* Process the msg one by one */
for (i = 0; i < num; i++, msgs++) {
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*/
return num;
}
static int xxx_i2c_probe(struct platform_device *pdev) // dts里的设备信息传递进来了
{
struct resource *r_mem;
struct xxx_i2c *id;
int ret;
id = devm_kzalloc(&pdev->dev, sizeof(*id), GFP_KERNEL);
if (!id)
return -ENOMEM;
platform_set_drvdata(pdev, id);
xxx_adapter_hw_init(); //通常初始化iic适配器使用的硬件资源，如申请IO地址、中断号、时钟等
id->adap.dev.of_node = pdev->dev.of_node;
id->adap.algo = &xxx_i2c_algo; // 把altorithm连进来
id->adap.timeout = XXX_I2C_TIMEOUT;
id->adap.retries = 3; /* Default retry value. */
id->adap.algo_data = id;
id->adap.dev.parent = &pdev->dev;
ret = i2c_add_adapter(&id->adap);
...
}
static int xxx_i2c_remove(struct platform_device *pdev)
{
struct xxx_i2c *id = platform_get_drvdata(pdev);
i2c_del_adapter(&id->adap);
xxx_adapter_hw_free(); // 硬件相关资源的free
return 0;
}
static const struct of_device_id xxx_i2c_of_match[] = {
{ .compatible = "cdns,i2c-r1p10", },
{ /* end of table */ }
};
MODULE_DEVICE_TABLE(of, xxx_i2c_of_match);
static struct platform_driver xxx_i2c_drv = {
.driver = {
.name = DRIVER_NAME,
.owner = THIS_MODULE,
.of_match_table = xxx_i2c_of_match, // dts匹配的依据
.pm = &xxx_i2c_dev_pm_ops,
},
.probe = xxx_i2c_probe,
.remove = xxx_i2c_remove,
};
module_platform_driver(xxx_i2c_drv);xxx_adapter_hw_init实现和具体的CPU和I2C控制器硬件相关的初始化。
functionality() 函数比较简单，返回支持的通信协议。
master_xfer() 函数在适配器上完成i2c_msg的数据传输。
五、设备(外设)驱动
i2c_dirver就是i2c标准总线设备驱动模型中的驱动部分，i2c_client可理解为i2c总线上挂的外设。
模板代码：
static struct i2c_driver xxx_driver = {
.driver = {
.name = "xxx",
.of_match_table = xxx_of_match,
.acpi_match_table = ACPI_PTR(xxx_acpi_ids),
},
.probe_new = xxx_probe,
.remove = xxx_remove,
.id_table = xxx_ids,
};
static int __init xxx_init(void)
{
.......
return i2c_add_driver(&xxx_driver); // 匹配后，driver中的probe就能执行
}
static void __exit xxx_exit(void)
{
i2c_del_driver(&xxx_driver);
}
module_exit(xxx_exit);
module_init(xxx_init);