Linux下I2C驅動分析（一）
阿新 • • 發佈：2019-01-23
最近在做一個基於全志A33晶片的android移植時發現嵌入式裝置很多都用到了I2C匯流排通訊，比如說攝像頭，G-sensor，觸控式螢幕等，為此我覺得很好的理解I2C裝置驅動在今後的嵌入式開發中是非常有好處的，而目前我也是處於學習階段，便將這些學習的過程給記錄下來，如果有存在的問題，還希望不吝指正。
我曾經用51微控制器的IO口模擬I2C匯流排寫過驅動，實現24C02存取資料還是非常簡單的，100多行程式碼就能解決，但是Linux中的I2C框架卻極為複雜，主要是在Linux下裝置驅動都採用了分層的思想，在裝置模型的框架下，實現一個驅動，對匯流排，驅動，裝置的描述都將增大整個驅動的程式碼，然而這樣的好處就是很方便的進行移植，當我們想要實現某一個硬體裝置的驅動時，只需要做好最底層函式的實現就可以了，下面我以MMA7660（Freescale G-sensor）的驅動為例，分析一下I2C的驅動的掛載和繫結。
首先是
module_init(mma7660_init); //模組入口
module_exit(mma7660_exit); //模組出口
其實上述函式和驅動框架相關的就一句話ret = i2c_add_driver(&mma7660_driver)，其餘部分和全志的引數獲取方式有關，而這一句話表示向I2C匯流排註冊一個驅動，根據巨集static int __init mma7660_init(void) //為了簡潔已經去掉所有debug資訊 { int ret = -1; if (input_fetch_sysconfig_para(&(gsensor_info.input_type))) { //從配置檔案中獲取是否存在G-sensor裝置引數 return -1; } else twi_id = gsensor_info.twi_id; ret = i2c_add_driver(&mma7660_driver); //向I2C匯流排註冊一個MMA7660的驅動(此處的I2C匯流排指的是Linux驅動中裝置模型的匯流排，這是一個虛擬的平臺匯流排，而不是實際的I2C匯流排) if (ret < 0) { return -ENODEV; } return ret; }
#define i2c_add_driver(driver) \
i2c_register_driver(THIS_MODULE, driver)很明顯向匯流排註冊一個驅動最終呼叫了driver_register註冊，如果註冊成功則res = 0 之後變判斷suspend以及resume的存在性，最後呼叫i2c_for_each_dev函式，其實這個函式的目的就是遍歷裝置連結串列，我們可以用source insight向下檢索可以知道最終呼叫了bus_for_each_dev函式而這個函式有如下內容，具體函式就不貼出來了。int i2c_register_driver(struct module *owner, struct i2c_driver *driver) { int res; if (unlikely(WARN_ON(!i2c_bus_type.p))) return -EAGAIN; driver->driver.owner = owner; driver->driver.bus = &i2c_bus_type; res = driver_register(&driver->driver); if (res) return res; if (driver->suspend) pr_warn("i2c-core: driver [%s] using legacy suspend method\n", driver->driver.name); if (driver->resume) pr_warn("i2c-core: driver [%s] using legacy resume method\n", driver->driver.name); INIT_LIST_HEAD(&driver->clients); i2c_for_each_dev(driver, __process_new_driver); return 0; }
while ((dev = next_device(&i)) && !error)
error = fn(dev, data);static int __process_new_driver(struct device *dev, void *data)
{
if (dev->type != &i2c_adapter_type)
return 0;
return i2c_do_add_adapter(data, to_i2c_adapter(dev));
}static int i2c_detect(struct i2c_adapter *adapter, struct i2c_driver *driver)
{
const unsigned short *address_list;
struct i2c_client *temp_client;
int i, err = 0;
int adap_id = i2c_adapter_id(adapter);
address_list = driver->address_list;
if (!driver->detect || !address_list)
return 0;
if (!(adapter->class & driver->class))
return 0;
temp_client = kzalloc(sizeof(struct i2c_client), GFP_KERNEL);
if (!temp_client)
return -ENOMEM;
temp_client->adapter = adapter;
for (i = 0; address_list[i] != I2C_CLIENT_END; i += 1) {
temp_client->addr = address_list[i];
err = i2c_detect_address(temp_client, driver);
if (unlikely(err))
break;
}
kfree(temp_client);
return err;
}
memset(&info, 0, sizeof(struct i2c_board_info));
info.addr = addr;
err = driver->detect(temp_client, &info);
if (err) {
return err == -ENODEV ? 0 : err;
}
if (info.type[0] == '\0') {
dev_err(&adapter->dev, "%s detection function provided "
"no name for 0x%x\n", driver->driver.name,
addr);
} else {
struct i2c_client *client;
client = i2c_new_device(adapter, &info);
if (client)
list_add_tail(&client->detected, &driver->clients);
else
dev_err(&adapter->dev, "Failed creating %s at 0x%02x\n",
info.type, info.addr);
}
static int gsensor_detect(struct i2c_client *client, struct i2c_board_info *info)
{
struct i2c_adapter *adapter = client->adapter;
int ret;
if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE_DATA))
return -ENODEV;
if(twi_id == adapter->nr){
pr_info("%s: addr= %x\n",__func__,client->addr);
ret = gsensor_i2c_test(client);
if(!ret){
pr_info("%s:I2C connection might be something wrong or maybe the other gsensor equipment! \n",__func__);
return -ENODEV;
}else{
pr_info("I2C connection sucess!\n");
strlcpy(info->type, SENSOR_NAME, I2C_NAME_SIZE);
return 0;
}
}else{
return -ENODEV;
}
}