Linux i2c子系统驱动probe

I2C 子系统

I2C 子系统使用的概率非常大，我之前有做过手机的经验，手机跑的安卓系统，内核是Linux，手机的很多器件都是用I2C通信的，我经历过从板级设备到dts设备树的阶段，知道I2C在整个系统的举足轻重，正常的TP，Camera,sonser等等都是使用I2C进行控制的。

吹牛逼这么多，就是让大家知道理解I2C子系统的重要性，不过这篇文章就写一个小细节，I2C驱动的probe是如何被触发的，如果你不知道其中的原理，可能在写驱动的时候不能成功执行probe也是有可能的。

static const struct i2c_device_id goodix_ts_id[] = {	{ GTP_I2C_NAME, 0 },	{ }	
};	static struct of_device_id goodix_ts_dt_ids[] = {	{ .compatible = "goodix,gt9xx" },	{ }	
};	static struct i2c_driver goodix_ts_driver = {	.probe      = goodix_ts_probe,	.remove     = goodix_ts_remove,	.id_table   = goodix_ts_id,	.driver = {	.name     = GTP_I2C_NAME,	.owner    = THIS_MODULE,	.of_match_table = of_match_ptr(goodix_ts_dt_ids),	},	
};	/*******************************************************    	
Function:	Driver Install function.	
Input:	None.	
Output:	Executive Outcomes. 0---succeed.	
********************************************************/	
static int goodix_ts_init(void)	
{	s32 ret;	/*	......	*/	ret = i2c_add_driver(&goodix_ts_driver);	return ret; 	
}

i2c_add_driver 驱动和设备匹配

i2c_add_driver()

i2c_register_driver

driver_register

driver_find

bus_add_driver

driver_attach

bus_for_each_dev

next_device

__driver_attach

driver_match_device

i2c_device_match

acpi_driver_match_device

i2c_match_id

of_driver_match_device

of_match_device of_match_node

__of_match_node

__of_device_is_compatible

这个要说明的一个点是，我提出来一下，可能大家看代码的时候就不会那么困惑了，Linux 下的指针那么多，你每次如果调用都要追根溯源，那可能需要花费非常多的时间。

总线上的device和driver进行匹配的时候会调用 bus 对应的 match函数，对于i2c bus而言就是i2c_match，如果是platform_bus 那么就会回调到platform_match里面去执行。

static const struct i2c_device_id *i2c_match_id(const struct i2c_device_id *id,	const struct i2c_client *client)	
{	while (id->name[0]) {	if (strcmp(client->name, id->name) == 0)	return id;	id++;	}	return NULL;	
}	static int i2c_device_match(struct device *dev, struct device_driver *drv)	
{	struct i2c_client    *client = i2c_verify_client(dev);	struct i2c_driver    *driver;	if (!client)	return 0;	/* Attempt an OF style match */	if (of_driver_match_device(dev, drv))	return 1;	/* Then ACPI style match */	if (acpi_driver_match_device(dev, drv))	return 1;	driver = to_i2c_driver(drv);	/* match on an id table if there is one */	if (driver->id_table)	return i2c_match_id(driver->id_table, client) != NULL;	return 0;	
}

里面有三种 match 的函数，最后才会调用 i2c_match_id 这个函数，这个也是低版本内核还没有使用dts的时候使用的方式，也就是匹配dev和driver的name。

看下面新的 compatible 匹配方式

/**	* __of_device_is_compatible() - Check if the node matches given constraints	* @device: pointer to node	* @compat: required compatible string, NULL or "" for any match	* @type: required device_type value, NULL or "" for any match	* @name: required node name, NULL or "" for any match	*	* Checks if the given @compat, @type and @name strings match the	* properties of the given @device. A constraints can be skipped by	* passing NULL or an empty string as the constraint.	*	* Returns 0 for no match, and a positive integer on match. The return	* value is a relative score with larger values indicating better	* matches. The score is weighted for the most specific compatible value	* to get the highest score. Matching type is next, followed by matching	* name. Practically speaking, this results in the following priority	* order for matches:	*	* 1. specific compatible && type && name	* 2. specific compatible && type	* 3. specific compatible && name	* 4. specific compatible	* 5. general compatible && type && name	* 6. general compatible && type	* 7. general compatible && name	* 8. general compatible	* 9. type && name	* 10. type	* 11. name	*/	
static int __of_device_is_compatible(const struct device_node *device,	const char *compat, const char *type, const char *name)	
{	struct property *prop;	const char *cp;	int index = 0, score = 0;	/* Compatible match has highest priority */	if (compat && compat[0]) {	/*获取dts里面该节点的值*/	prop = __of_find_property(device, "compatible", NULL);	for (cp = of_prop_next_string(prop, NULL); cp;	cp = of_prop_next_string(prop, cp), index++) {	/*字符串比较*/	if (of_compat_cmp(cp, compat, strlen(compat)) == 0) {	score = INT_MAX/2 - (index << 2);	break;	}	}	/*返回成功*/	if (!score)	return 0;	}	/* Matching type is better than matching name */	if (type && type[0]) {	if (!device->type || of_node_cmp(type, device->type))	return 0;	score += 2;	}	/* Matching name is a bit better than not */	if (name && name[0]) {	if (!device->name || of_node_cmp(name, device->name))	return 0;	score++;	}	return score;	
}

代码里面我们看到是同时比较 name ，type，compatible 这三个属性的，但是我们使用dts进行设置的时候，name和type的属性很多时候都是空的。

&i2c1 {	status = "okay";	/*	......	*/	gt9xx: gt9xx@14 {	compatible = "goodix,gt9xx";	reg = <0x14>;	touch-gpio = <&gpio1 0 IRQ_TYPE_EDGE_RISING>;	reset-gpio = <&gpio0 11 GPIO_ACTIVE_HIGH>;	max-x = <800>;	max-y = <1280>;	tp-size = <89>;	configfile-num = <1>;	status = "okay";	tp-supply =  <&vcc3v0_tp>;	};	
};

i2c probe被探测执行的流程

i2c_add_driver()

i2c_register_driver

driver_register driver_find

kset_find_obj

kobject_put

to_driver

bus_add_driver

driver_attach

bus_for_each_dev

next_device

__driver_attach

driver_match_device

driver_probe_device

really_probe

i2c_device_probe

i2c_match_id

你以为上面设置就好了吗？我们看到

static struct i2c_driver goodix_ts_driver = {	.probe      = goodix_ts_probe,	.remove     = goodix_ts_remove,	.id_table   = goodix_ts_id,	.driver = {	.name     = GTP_I2C_NAME,	.owner    = THIS_MODULE,	.of_match_table = of_match_ptr(goodix_ts_dt_ids),	},	
};

里面有一个 id_tabel和一个 of_match_table 两个东西，既然probe探测只需要 of_match_tabel就可以了，是不是可以去掉id_tabel了呢？

这感觉是一个遗留问题，在i2c_probe函数里面有一个判断，不知道历史原因还是为何，不能做到完全兼容，看代码如下

static int i2c_device_probe(struct device *dev)	
{	/* ...... */	driver = to_i2c_driver(dev->driver);	/* 判断id_table为空就退出 */	if (!driver->probe || !driver->id_table)	return -ENODEV;	/* ...... */	
}

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