驱动模块使用I2C总线范例

在嵌入式中，I2C接口的意义非常重大，许多外围芯片控制接口都采用I2C。因此，了解在驱动模块中如何进行I2C总线通信是很有必要的。我们先看看I2C驱动代码的树形结构：

.|-- Kconfig|-- Makefile|-- algos|   |-- Kconfig|   |-- Makefile|   |-- i2c-algo-bit.c|   |-- i2c-algo-pca.c|   |-- i2c-algo-pcf.c|   `-- i2c-algo-pcf.h|-- busses|   |-- Kconfig|   |-- Makefile|   |-- i2c-acorn.c|   |-- ...|   |-- i2c-gpio.c|   |-- ...|   |-- i2c-pxa.c|   |-- i2c-s3c2410.c|   `-- ...|-- chips|   |-- Kconfig|   |-- Makefile|   |-- ds1682.c|   |-- pca963x.c|   `-- tsl2550.c|-- i2c-boardinfo.c|-- i2c-core.c|-- i2c-core.h`-- i2c-dev.c3 directories, 86 files

其中i2c-core.c是I2C核心部分，提供接口函数，i2c_adapter和i2c_driver在模块初始化时分别调用i2c_add_numbered_adapter()和i2c_add_driver()将其注册到I2C Core中。

i2c-dev.c是I2C设备部分，实现字符设备访问接口，对硬件的具体访问是通过i2c_adapter来实现的。它在初始化时，需要向I2C Core注册一个i2c_driver。我认为这部分仅仅是让用户层访问I2C总线的，如果内核态访问I2C总线的话，则没有必要理会它；但是我们需要参考它来实现我们自己的i2c_driver，并且需要将这个i2c_driver注册到I2C Core。

busses目录下有许多个文件，为各种平台的I2C总线驱动，如s2c2410、pxa、mpc等等，跟硬件spec息息相关。一个总线驱动需要两个模块，用结构体i2c_adapter和i2c_algorithm来描述。

algos目录是对bus中的i2c_algorithm的补充，特别的i2c_algorithm可在这里实现。chips是I2C Slave芯片驱动代码，如I2C控制接口的eeprom、LED段码显示芯片等。

更详细的有关I2C驱动架构介绍的文章见：http://blog.chinaunix.net/space.php?uid=11134731&do=blog&id=33193

下面是正题，附上示范代码，碰到相关场合，直接将其复制过去使用即可。

#define AZURE_I2C_ADDR   0x1a#define AZURE_I2C_BUS_NO 0static struct i2c_client *azure_i2c;static int azure_i2c_probe(struct i2c_client *i2c,const struct i2c_device_id *id){azure_i2c = i2c;i2c->addr = AZURE_I2C_ADDR;return 0;}static int azure_i2c_remove(struct i2c_client *client){return 0;}static const struct i2c_device_id azure_i2c_id[] = {{ "azure", 0 },{ }};MODULE_DEVICE_TABLE(i2c, azure_i2c_id);static struct i2c_driver azure_i2c_driver = {.driver  = {.name  = "AZURE I2C",.owner = THIS_MODULE,},.probe    = azure_i2c_probe,.remove   = azure_i2c_remove,.id_table = azure_i2c_id,};static int azure_add_i2c_device(){struct i2c_board_info info;struct i2c_adapter *adapter;struct i2c_client *client;int ret;memset(&info, 0, sizeof(struct i2c_board_info));info.addr = AZURE_I2C_ADDR;strlcpy(info.type, "azure", I2C_NAME_SIZE);adapter = i2c_get_adapter(AZURE_I2C_BUS_NO);if (!adapter) {printk(KERN_ERR "can't get i2c adapter %d/n", AZURE_I2C_BUS_NO);goto err_driver;}client = i2c_new_device(adapter, &info);i2c_put_adapter(adapter);if (!client) {printk(KERN_ERR "can't add i2c device at 0x%x/n", (unsigned int)info.addr);goto err_driver;}ret = i2c_add_driver(&azure_i2c_driver);if (ret != 0) {printk(KERN_ERR "can't add i2c driver/n");return ret;}return 0;err_driver:i2c_del_driver(&azure_i2c_driver);return -ENODEV;}

然后在模块初始化的时候调用azure_add_i2c_device()即将一个i2c_driver注册到I2C Core，之后便可调用i2c_master_recv()和i2c_master_send()进行I2C总线读写操作。i2c_master_send()/i2c_master_recv()第一个参数client由azure_i2c传入。

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以上并不是聪明的做法。如果有N个模块要用到I2C，那么在N个模块就要添加基本一样的代码。更好的解决方法是为内核调用I2C写一个独立的模块，EXPORT_SYMBOL读写接口函数，在其他模块里就只需要调用这些读写接口函数就可以了。如下是模块代码：

#include <linux/kernel.h>#include <linux/module.h>#include <linux/init.h>#include <linux/errno.h>#include <linux/init.h>#include <linux/list.h>#include <linux/i2c.h>#include <linux/smp_lock.h>#define I2C_API_FAKE_ADDR 0x7f#define I2C_MINORS      256int  i2c_api_attach(struct i2c_adapter *adapter);int  i2c_api_detach(struct i2c_adapter *adapter);struct i2c_api {struct list_head list;struct i2c_client *client;};static LIST_HEAD(i2c_api_list);static DEFINE_SPINLOCK(i2c_api_list_lock);static const unsigned short normal_addr[] = { I2C_API_FAKE_ADDR, I2C_CLIENT_END };static const unsigned short ignore[]      = { I2C_CLIENT_END };static struct i2c_client_address_data addr_data = {.normal_i2c = normal_addr,.probe      = ignore,.ignore     = ignore,.forces     = NULL,};static const struct i2c_device_id id[] = {{"I2C-API", 0},{} };      MODULE_DEVICE_TABLE(i2c, id);static struct i2c_driver i2c_api_driver = {.id_table       = id,.attach_adapter = i2c_api_attach,.detach_adapter= i2c_api_detach, .command        = NULL,.driver         = {.name  = "I2C-API",.owner = THIS_MODULE,},.address_data   = &addr_data,};static struct i2c_api *get_i2c_api(int bus_id){struct i2c_api *i2c_api;spin_lock(&i2c_api_list_lock);list_for_each_entry(i2c_api, &i2c_api_list, list) {if (i2c_api->client->adapter->nr == bus_id)goto found;}i2c_api = NULL;found:spin_unlock(&i2c_api_list_lock);return i2c_api;}static struct i2c_api *add_i2c_api(struct i2c_client *client){struct i2c_api *i2c_api;if (client->adapter->nr >= I2C_MINORS) {printk(KERN_ERR "i2c_api: Out of device minors (%d)/n",client->adapter->nr);return NULL;}i2c_api = kzalloc(sizeof(*i2c_api), GFP_KERNEL);if (!i2c_api)return NULL;i2c_api->client = client;spin_lock(&i2c_api_list_lock);list_add_tail(&i2c_api->list, &i2c_api_list);spin_unlock(&i2c_api_list_lock);return i2c_api;}static void del_i2c_api(struct i2c_api *i2c_api){spin_lock(&i2c_api_list_lock);list_del(&i2c_api->list);spin_unlock(&i2c_api_list_lock);kfree(i2c_api);}static int i2c_api_do_xfer(int bus_id, char chip_addr, char sub_addr, int mode, char *buf, unsigned int size){/** you could define more transfer mode here, implement it following. */#define I2C_API_XFER_MODE_SEND            0x0 /* standard send */#define I2C_API_XFER_MODE_RECV            0x1 /* standard receive */#define I2C_API_XFER_MODE_SEND_NO_SUBADDR 0x2 /* send without sub-address */#define I2C_API_XFER_MODE_RECV_NO_SUBADDR 0x3 /* receive without sub-address */int ret = 0;char *tmp;struct i2c_api *i2c_api = get_i2c_api(bus_id);if (!i2c_api)return -ENODEV;i2c_api->client->addr = chip_addr;switch (mode) {case I2C_API_XFER_MODE_SEND:tmp = kmalloc(size + 1,GFP_KERNEL);if (tmp == NULL)return -ENOMEM;tmp[0] = sub_addr;memcpy(&tmp[1], buf, size);ret = i2c_master_send(i2c_api->client, tmp, size + 1);ret = (ret == size + 1) ? size : ret;break;case I2C_API_XFER_MODE_RECV:ret = i2c_master_send(i2c_api->client, &sub_addr, 1);if (ret < 0)return ret;ret = i2c_master_recv(i2c_api->client, buf, size);break;case I2C_API_XFER_MODE_SEND_NO_SUBADDR:ret = i2c_master_send(i2c_api->client, buf, size);break;case I2C_API_XFER_MODE_RECV_NO_SUBADDR:ret = i2c_master_recv(i2c_api->client, buf, size);break;default:return -EINVAL;}return ret;}int i2c_api_do_send(int bus_id, char chip_addr, char sub_addr, char *buf, unsigned int size){return i2c_api_do_xfer(bus_id, chip_addr, sub_addr, I2C_API_XFER_MODE_SEND, buf, size);}int i2c_api_do_recv(int bus_id, char chip_addr, char sub_addr, char *buf, unsigned int size){return i2c_api_do_xfer(bus_id, chip_addr, sub_addr, I2C_API_XFER_MODE_RECV, buf, size);}int i2c_api_attach(struct i2c_adapter *adap){struct i2c_board_info info;struct i2c_client *client;memset(&info, 0, sizeof(struct i2c_board_info));strlcpy(info.type, "i2c_api", I2C_NAME_SIZE);info.addr = I2C_API_FAKE_ADDR;client = i2c_new_device(adap, &info);if (client)add_i2c_api(client);printk(KERN_INFO "i2c_api_attach adap[%d]/n", adap->nr);return 0;}int i2c_api_detach(struct i2c_adapter *adap){struct i2c_api *i2c_api;i2c_api = get_i2c_api(adap->nr);if (i2c_api)del_i2c_api(i2c_api);return 0;}static int __init i2c_api_init(void){int ret = i2c_add_driver(&i2c_api_driver);if (ret) {printk(KERN_ERR "[%s] Driver registration failed, module not inserted./n", __func__);return ret;}return 0 ;}static void __exit i2c_api_exit(void){i2c_del_driver(&i2c_api_driver);}MODULE_AUTHOR("Loon, <[e-mail]>");MODULE_DESCRIPTION("I2C i2c_api Driver");MODULE_LICENSE("GPL");module_init(i2c_api_init);module_exit(i2c_api_exit);EXPORT_SYMBOL_GPL(i2c_api_do_send);EXPORT_SYMBOL_GPL(i2c_api_do_recv);

内核版本是2.6.32，i2c_driver结构体成员attach_adapter和detach_adapter可能以后有增删，届时需要对模块里的i2c_api_attach()和i2c_api_detach()做调整。

加载该模块后，其他模块即可调用i2c_api_do_recv()/i2c_api_do_send()读写I2C总线。