I2C驱动(十一) -- gpio模拟的i2c总线驱动i2c-gpio.c分析

相关文章

I2C驱动(一) – I2C协议
I2C驱动(二) – SMBus协议
I2C驱动(三) – 驱动中的几个重要结构
I2C驱动(四) – I2C-Tools介绍
I2C驱动(五) – 通用驱动i2c-dev.c分析
I2C驱动(六) – I2C驱动程序模型
I2C驱动(七) – 编写I2C设备驱动之i2c_driver
I2C驱动(八) – 编写I2C设备驱动之i2c_client
I2C驱动(九) – i2c_adapter控制器驱动框架编写
I2C驱动(十) – i2c_adapter控制器驱动完善与上机实验

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参考资料

• i2c_spec.pdf
• Linux文档
  • Linux-4.9.88\Documentation\devicetree\bindings\i2c\i2c-gpio.txt
• Linux驱动源码
  • Linux-4.9.88\drivers\i2c\busses\i2c-gpio.c

一、平台-总线-设备驱动模型

i2c-gpio.c也是基于万能框架：平台-总线-设备模型来写的。platform_device部分来自设备树，platform_driver就是i2c-gpio.c驱动。下面分析两边的代码。

二、设备树分析

设备树节点如下：

i2c_gpio: i2c-gpio {compatible = "i2c-gpio";#address-cells = <1>;#size-cells = <0>;pinctrl-names = "default";pinctrl-0 = <&pinctrl_i2c_gpio>;gpios = <&gpio5 1 GPIO_ACTIVE_HIGH /* SDA */&gpio5 0 GPIO_ACTIVE_HIGH /* SCL */>;i2c-gpio,delay-us = <5>;	/* ~100 kHz */status = "okay";ds1339: rtc@68 {compatible = "dallas,ds1339";reg = <0x68>;status = "disabled";};};

• compatible 属性用于和i2c-gpio.c程序中.of_match_table结构中的compatible 进行比较。
• #address-cells 和 #size-cells 属性用来指定 reg属性的地址和大小用多少个32位数据表示。reg属性在子节点中用了表示i2c设备地址。
• pinctrl-names 和 pinctrl-0属性表示使用pinctrl将引脚配置成gpio模式。
• gpios 属性用于指定gpio引脚
• i2c-gpio,delay-us属性表示时钟频率
• status 属性节点使能状态
• ds1339: rtc@68表示的是这个i2c总线下挂的设备，地址是0x68。

三、驱动程序分析

3.1 i2c-gpio驱动层次

从入口函数开始，入口函数注册了一个platform_driver结构。

tatic int __init i2c_gpio_init(void)
{
...ret = platform_driver_register(&i2c_gpio_driver);
...
}

platform_driver结构包含了of_match_table数组，probe函数。

static struct platform_driver i2c_gpio_driver = {.driver		= {.name	= "i2c-gpio",.of_match_table	= of_match_ptr(i2c_gpio_dt_ids), //和设备树比较},.probe		= i2c_gpio_probe,  //匹配成功调用.remove		= i2c_gpio_remove, //做一些和probe相反的工作
};

of_match_table数组中的compatible 和设备树匹配成功，调用probe函数。

static const struct of_device_id i2c_gpio_dt_ids[] = {{ .compatible = "i2c-gpio", }, //与设备树的compatible 比较{ /* sentinel */ }
};

来看probe函数：

• of_i2c_gpio_get_pins从设备树获取gpio引脚。
• adap = &priv->adap;分配的i2c_adapter。
• of_i2c_gpio_get_props从设备树获取属性值，用来设置硬件参数和i2c_adapter结构。
• i2c_bit_add_numbered_bus注册i2c_adapter，这个是重点，这里面会有算法部分设置，下面继续分析。

static int i2c_gpio_probe(struct platform_device *pdev)
{
...unsigned int sda_pin, scl_pin; //sda 和 scl引脚int ret;/* First get the GPIO pins; if it fails, we'll defer the probe. */if (pdev->dev.of_node) {/* 从设备树中获取        sda 和 scl */ret = of_i2c_gpio_get_pins(pdev->dev.of_node,&sda_pin, &scl_pin);if (ret)return ret;} else {if (!dev_get_platdata(&pdev->dev))return -ENXIO;pdata = dev_get_platdata(&pdev->dev);sda_pin = pdata->sda_pin;scl_pin = pdata->scl_pin;}/*devm_gpio_request 可以自动处理清理工作 */ret = devm_gpio_request(&pdev->dev, sda_pin, "sda");if (ret) {if (ret == -EINVAL)ret = -EPROBE_DEFER;	/* Try again later */return ret;}ret = devm_gpio_request(&pdev->dev, scl_pin, "scl");if (ret) {if (ret == -EINVAL)ret = -EPROBE_DEFER;	/* Try again later */return ret;}priv = devm_kzalloc(&pdev->dev, sizeof(*priv), GFP_KERNEL);if (!priv)return -ENOMEM;/* 分配了i2c_adapter */	adap = &priv->adap;bit_data = &priv->bit_data;pdata = &priv->pdata;if (pdev->dev.of_node) {pdata->sda_pin = sda_pin;pdata->scl_pin = scl_pin;/*从设备树获取属性*/of_i2c_gpio_get_props(pdev->dev.of_node, pdata);} else {memcpy(pdata, dev_get_platdata(&pdev->dev), sizeof(*pdata));}/* 根据获取的设备树属性值设置开漏情况 */if (pdata->sda_is_open_drain) {gpio_direction_output(pdata->sda_pin, 1);bit_data->setsda = i2c_gpio_setsda_val;} else {gpio_direction_input(pdata->sda_pin);bit_data->setsda = i2c_gpio_setsda_dir;}if (pdata->scl_is_open_drain || pdata->scl_is_output_only) {gpio_direction_output(pdata->scl_pin, 1);bit_data->setscl = i2c_gpio_setscl_val;} else {gpio_direction_input(pdata->scl_pin);bit_data->setscl = i2c_gpio_setscl_dir;}/* 根据获取的设备树属性值设置时间参数 */if (!pdata->scl_is_output_only)bit_data->getscl = i2c_gpio_getscl;bit_data->getsda = i2c_gpio_getsda;if (pdata->udelay)bit_data->udelay = pdata->udelay;else if (pdata->scl_is_output_only)bit_data->udelay = 50;			/* 10 kHz */elsebit_data->udelay = 5;			/* 100 kHz */if (pdata->timeout)bit_data->timeout = pdata->timeout;elsebit_data->timeout = HZ / 10;		/* 100 ms */bit_data->data = pdata;/* 根据设备树解析的值设置i2c_adapter */adap->owner = THIS_MODULE;if (pdev->dev.of_node)strlcpy(adap->name, dev_name(&pdev->dev), sizeof(adap->name));elsesnprintf(adap->name, sizeof(adap->name), "i2c-gpio%d", pdev->id);adap->algo_data = bit_data;adap->class = I2C_CLASS_HWMON | I2C_CLASS_SPD;adap->dev.parent = &pdev->dev;adap->dev.of_node = pdev->dev.of_node;adap->nr = pdev->id;/* 注册i2c_adapter */ret = i2c_bit_add_numbered_bus(adap);
...
}

进入i2c_bit_add_numbered_bus看看，这个函数在drivers\i2c\algos\i2c-algo-bit.c中定义，他调用__i2c_bit_add_bus，__i2c_bit_add_bus里面设置了adap->algo = &i2c_bit_algo;，这就是i2c核心算法结构，下面继续看i2c_bit_algo。

int i2c_bit_add_numbered_bus(struct i2c_adapter *adap)
{return __i2c_bit_add_bus(adap, i2c_add_numbered_adapter);
}static int __i2c_bit_add_bus(struct i2c_adapter *adap,int (*add_adapter)(struct i2c_adapter *))
{
.../* 核心算法 */adap->algo = &i2c_bit_algo;
...
}

i2c_bit_algo结构中bit_xfer就是gpio模拟i2c_adapter的传输函数。

const struct i2c_algorithm i2c_bit_algo = {.master_xfer	= bit_xfer,.functionality	= bit_func,
};

从上面的分析，可以知道I2C-GPIO的驱动层次如下：

3.2 bit_xfer传输函数分析

传输函数是根据i2c协议来完成的，i2c_start发起一个start信号，接着根据i2c_msg 来判断读写，readbytes读一个字节，sendbytes写一个字节。结束后i2c_stop发出停止信号。

static int bit_xfer(struct i2c_adapter *i2c_adap,struct i2c_msg msgs[], int num)
{struct i2c_msg *pmsg;struct i2c_algo_bit_data *adap = i2c_adap->algo_data;int i, ret;unsigned short nak_ok;if (adap->pre_xfer) {ret = adap->pre_xfer(i2c_adap);if (ret < 0)return ret;}bit_dbg(3, &i2c_adap->dev, "emitting start condition\n");i2c_start(adap); //发出Start信号for (i = 0; i < num; i++) {pmsg = &msgs[i]; //循环取出i2c_msg nak_ok = pmsg->flags & I2C_M_IGNORE_NAK;if (!(pmsg->flags & I2C_M_NOSTART)) {if (i) {bit_dbg(3, &i2c_adap->dev, "emitting ""repeated start condition\n");i2c_repstart(adap);}ret = bit_doAddress(i2c_adap, pmsg);if ((ret != 0) && !nak_ok) {bit_dbg(1, &i2c_adap->dev, "NAK from ""device addr 0x%02x msg #%d\n",msgs[i].addr, i);goto bailout;}}if (pmsg->flags & I2C_M_RD) {/* read bytes into buffer（读一个字节）*/ ret = readbytes(i2c_adap, pmsg);if (ret >= 1)bit_dbg(2, &i2c_adap->dev, "read %d byte%s\n",ret, ret == 1 ? "" : "s");if (ret < pmsg->len) {if (ret >= 0)ret = -EIO;goto bailout;}} else {/* write bytes from buffer （写一个字节）*/ret = sendbytes(i2c_adap, pmsg);if (ret >= 1)bit_dbg(2, &i2c_adap->dev, "wrote %d byte%s\n",ret, ret == 1 ? "" : "s");if (ret < pmsg->len) {if (ret >= 0)ret = -EIO;goto bailout;}}}ret = i;bailout:bit_dbg(3, &i2c_adap->dev, "emitting stop condition\n");i2c_stop(adap);if (adap->post_xfer)adap->post_xfer(i2c_adap);return ret;
}

字节读写再细分到位操作，在i2c_outb函数中实现。

static int i2c_outb(struct i2c_adapter *i2c_adap, unsigned char c)
{int i;int sb;int ack;struct i2c_algo_bit_data *adap = i2c_adap->algo_data;/* assert: scl is low */for (i = 7; i >= 0; i--) {sb = (c >> i) & 1;setsda(adap, sb);udelay((adap->udelay + 1) / 2);if (sclhi(adap) < 0) { /* timed out */bit_dbg(1, &i2c_adap->dev, "i2c_outb: 0x%02x, ""timeout at bit #%d\n", (int)c, i);return -ETIMEDOUT;}/* FIXME do arbitration here:* if (sb && !getsda(adap)) -> ouch! Get out of here.** Report a unique code, so higher level code can retry* the whole (combined) message and *NOT* issue STOP.*/scllo(adap);}sdahi(adap);if (sclhi(adap) < 0) { /* timeout */bit_dbg(1, &i2c_adap->dev, "i2c_outb: 0x%02x, ""timeout at ack\n", (int)c);return -ETIMEDOUT;}/* read ack: SDA should be pulled down by slave, or it may* NAK (usually to report problems with the data we wrote).*/ack = !getsda(adap);    /* ack: sda is pulled low -> success */bit_dbg(2, &i2c_adap->dev, "i2c_outb: 0x%02x %s\n", (int)c,ack ? "A" : "NA");scllo(adap);return ack;/* assert: scl is low (sda undef) */
}

四、怎么使用i2c-gpio

设置设备树，在里面添加一个节点即可，示例代码看上面"设备树分析"部分，也可以参考 Linux-4.9.88\Documentation\devicetree\bindings\i2c\i2c-gpio.txt。

五、总结

本文分析了gpio模拟的i2c_adapter驱动程序i2c-gpio.c。