引言
单片机的IIC编程中，如果我们直接一点，只需要控制IIC硬件GPIO脚，然后根据IIC协议模拟各种电平时序实现与IIC设备的通信。但是这种编程方法，移植性较差(假如新加了一种IIC设备，同样的代码，又要重新复制一份)。这种做法完全不适应Linux的通用性的设计理念，对于Linux来讲：同样的事情我只做一遍，向外提供接口，不管你是什么IIC设备挂载那条IIC总线上，都可以用。因此，这就需要Linux在代码架构上有非常严谨的模块化设计。
架构设计
在Linux设计中，将I2C代码框架分为三个部分：I2C总线、I2C核心、I2C驱动。
- I2C核心（i2c-core）： 主要定义i2c驱动所用到的通用API，高内聚的代码会放到i2c-core.c。
- I2C总线驱动（i2c adapter）： 根据平台定制的i2c驱动，其中包含i2c传输的算法设计。主要工作负责生成i2c_client，注册适配器，以及i2c_client与i2c_driver的匹配。
-
I2C设备驱动（i2c client driver）： 驱动I2C设备的代码。I2C设备驱动定义了外设的交互方式，对与不同的I2C外设需要不同的设备驱动。I2C设备驱动对上和用户应用程序打交道，对下和I2C核心对接。
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本篇主要对IIC总线驱动的总结。
i2c总线结构体
Linux在分层中，必不可少的将每一层模块封装成一个结构体，然后将结构体作为一个与外接交互的桥梁。I2C总线驱动也一样被抽象成结构体：
/*
* i2c_adapter is the structure used to identify a physical i2c bus along
* with the access algorithms necessary to access it.
*/
struct i2c_adapter {
struct module *owner;
unsigned int class; /* classes to allow probing for */
const struct i2c_algorithm *algo; /* the algorithm to access the bus */
void *algo_data;
/* data fields that are valid for all devices */
const struct i2c_lock_operations *lock_ops;
struct rt_mutex bus_lock;
struct rt_mutex mux_lock;
int timeout; /* in jiffies */
int retries;
struct device dev; /* the adapter device */
int nr;
char name[48];
struct completion dev_released;
struct mutex userspace_clients_lock;
struct list_head userspace_clients;
struct i2c_bus_recovery_info *bus_recovery_info;
const struct i2c_adapter_quirks *quirks;
};
驱动代码
总线驱动采用platform虚拟总线架构，其中包括一些platform常规注册流程，主要关注probe中的代码。
//linux/drivers/i2c/busses/i2c-imx.c
……
struct imx_i2c_struct {
struct i2c_adapter adapter;
struct clk *clk;
void __iomem *base;
wait_queue_head_t queue;
unsigned long i2csr;
unsigned int disable_delay;
int stopped;
unsigned int ifdr; /* IMX_I2C_IFDR */
unsigned int cur_clk;
unsigned int bitrate;
const struct imx_i2c_hwdata *hwdata;
struct i2c_bus_recovery_info rinfo;
struct pinctrl *pinctrl;
struct pinctrl_state *pinctrl_pins_default;
struct pinctrl_state *pinctrl_pins_gpio;
struct imx_i2c_dma *dma;
};
……
static struct i2c_algorithm i2c_imx_algo = {
.master_xfer = i2c_imx_xfer,
.functionality = i2c_imx_func,
};
static int i2c_imx_probe(struct platform_device *pdev)
{
const struct of_device_id *of_id = of_match_device(i2c_imx_dt_ids,
&pdev->dev);
struct imx_i2c_struct *i2c_imx;
struct resource *res;
struct imxi2c_platform_data *pdata = dev_get_platdata(&pdev->dev);
void __iomem *base;
int irq, ret;
dma_addr_t phy_addr;
irq = platform_get_irq(pdev, 0);
res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
base = devm_ioremap_resource(&pdev->dev, res);
phy_addr = (dma_addr_t)res->start;
i2c_imx = devm_kzalloc(&pdev->dev, sizeof(*i2c_imx), GFP_KERNEL);
if (!i2c_imx)
return -ENOMEM;
if (of_id)
i2c_imx->hwdata = of_id->data;
else
i2c_imx->hwdata = (struct imx_i2c_hwdata *)
platform_get_device_id(pdev)->driver_data;
/* Setup i2c_imx driver structure */
strlcpy(i2c_imx->adapter.name, pdev->name, sizeof(i2c_imx->adapter.name));
i2c_imx->adapter.owner = THIS_MODULE;
i2c_imx->adapter.algo = &i2c_imx_algo;
i2c_imx->adapter.dev.parent = &pdev->dev;
i2c_imx->adapter.nr = pdev->id;
i2c_imx->adapter.dev.of_node = pdev->dev.of_node;
i2c_imx->base = base;
/* Get I2C clock */
i2c_imx->clk = devm_clk_get(&pdev->dev, NULL);
ret = clk_prepare_enable(i2c_imx->clk);
/* Request IRQ */
ret = devm_request_irq(&pdev->dev, irq, i2c_imx_isr,
IRQF_NO_SUSPEND, pdev->name, i2c_imx);
/* Init queue */
init_waitqueue_head(&i2c_imx->queue);
/* Set up adapter data */
i2c_set_adapdata(&i2c_imx->adapter, i2c_imx);
/* Set up platform driver data */
platform_set_drvdata(pdev, i2c_imx);
ret = pm_runtime_get_sync(&pdev->dev);
/* Set up clock divider */
i2c_imx->bitrate = IMX_I2C_BIT_RATE;
ret = of_property_read_u32(pdev->dev.of_node,
"clock-frequency", &i2c_imx->bitrate);
if (ret < 0 && pdata && pdata->bitrate)
i2c_imx->bitrate = pdata->bitrate;
/* Set up chip registers to defaults */
imx_i2c_write_reg(i2c_imx->hwdata->i2cr_ien_opcode ^ I2CR_IEN,
i2c_imx, IMX_I2C_I2CR);
imx_i2c_write_reg(i2c_imx->hwdata->i2sr_clr_opcode, i2c_imx, IMX_I2C_I2SR);
/* Init optional bus recovery function */
ret = i2c_imx_init_recovery_info(i2c_imx, pdev);
/* Give it another chance if pinctrl used is not ready yet */
/* Add I2C adapter */
ret = i2c_add_numbered_adapter(&i2c_imx->adapter);
/* Init DMA config if supported */
i2c_imx_dma_request(i2c_imx, phy_addr);
return 0; /* Return OK */
}
……
static struct platform_driver i2c_imx_driver = {
.probe = i2c_imx_probe,
.remove = i2c_imx_remove,
.driver = {
.name = DRIVER_NAME,
.pm = I2C_IMX_PM_OPS,
.of_match_table = i2c_imx_dt_ids,
},
.id_table = imx_i2c_devtype,
};
……
static int __init i2c_adap_imx_init(void)
{
return platform_driver_register(&i2c_imx_driver);
}
subsys_initcall(i2c_adap_imx_init);
static void __exit i2c_adap_imx_exit(void)
{
platform_driver_unregister(&i2c_imx_driver);
}
module_exit(i2c_adap_imx_exit);
以上代码有删减
内核实现分析
在进入probe中，先填充i2c_imx成员adapter结构体：
i2c_imx->adapter.owner = THIS_MODULE;
i2c_imx->adapter.algo = &i2c_imx_algo; //i2c数据传输接口
i2c_imx->adapter.dev.parent = &pdev->dev;
i2c_imx->adapter.nr = pdev->id; //适配器编号
i2c_imx->adapter.dev.of_node = pdev->dev.of_node;
使用i2c_set_adapdata(&i2c_imx->adapter, i2c_imx)将i2c_imx数据存入到adapter内部私有数据中，然后利用i2c-core中APIi2c_add_numbered_adapter(&i2c_imx->adapter)将适配器adapter注册到内核中，在此过程中还会生成i2c_client。
总线驱动注册流程图如下：
--- drivers --- i2c --- i2c_core.c --- i2c_add_numbered_adapter( --- if (adap->nr == -1)
| | struct i2c_adapter *adap) | i2c_add_adapter(adap);
| | |- __i2c_add_numbered_adapter(adap);
| |- __i2c_add_numbered_adapter( --- id = idr_alloc(&i2c_adapter_idr,adap,adap->nr,adap->nr+1,GFP_KERNEL)
| | struct i2c_adapter *adap) |- i2c_register_adapter(adap)
| |- i2c_register_adapter( --- INIT_LIST_HEAD(&adap->userspace_clients)
| | struct i2c_adapter *adap) |- dev_set_name(&adap->dev, "i2c-%d", adap->nr)
| | |- adap->dev.bus = &i2c_bus_type
| | |- adap->dev.type = &i2c_adapter_type
| | |- device_register(&adap->dev)
| | |- bus_for_each_drv(&i2c_bus_type, NULL, adap, __process_new_adapter);
| |- __process_new_adapter( --- i2c_do_add_adapter(to_i2c_driver(d), data)
| | struct device_driver *d,
| | void *data)
| |- i2c_do_add_adapter( --- i2c_detect(adap, driver)
| | struct i2c_driver *driver, |- 这里是废弃的driver->attach_adapter方法
| | struct i2c_adapter *adap)
| |- i2c_detect( --- address_list = driver->address_list
| | struct i2c_adapter *adapter, |- if (!driver->detect || !address_list)
| | struct i2c_driver *driver) | return 0;
| | |- temp_client = kzalloc(sizeof(struct i2c_client), GFP_KERNEL)
| | |- 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);
| | | err不是错误码，则break;
| | | }
| | |- kfree(temp_client)
| |- i2c_detect_address( --- struct i2c_board_info info
| | struct i2c_client *temp_client, |- adapter = temp_client->adapter
| | struct i2c_driver *driver) |- addr = temp_client->addr
| | |- info.addr = addr
| | |- driver->detect(temp_client, &info)
| | |- if (info.type[0] == '\0')
| | | 报错;
| | | else {
| | | struct i2c_client *client;
| | | client = i2c_new_device(adapter, &info);
| | | list_add_tail(&client->detected, &driver->clients);
| | | }
|
|- base --- bus.c --- bus_for_each_drv( --- while ((drv = next_driver(&i)) && !error)
struct bus_type *bus, |- error = fn(drv, data);
struct device_driver *start,
void *data,
int (*fn)(struct device_driver *, void *))
总结
- i2c总线驱动，由系统或者厂家实现，开机就会自动注册。
- 总线驱动会被抽象成adapter结构体，代码中实例其结构体成员，利用i2c-core的API将此结构体注册到内核。
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