4. I2C 使用

4.1. 简介

ITX-3568Q开发板上有 6 个片上 I2C 控制器,各个 I2C 的使用情况如下表:

Port Pin name Device
I2C0 GPIO0_B1/I2C0_SCL
GPIO0_B2/I2C0_SDA
RK809
I2C1 GPIO0_B3/I2C1_SCL
GPIO0_B4/I2C1_SDA
TP/HYM8563
I2C2_M0 GPIO0_B5/I2C2_SCL_M0
GPIO0_B6/I2C2_SDA_M0
复用为其他功能
I2C2_M1 GPIO4_B5/I2C2_SCL_M1
GPIO4_B4/I2C2_SDA_M1
VGA
I2C3_M0 GPIO1_A1/I2C3_SCL_M0
GPIO1_A0/I2C3_SDA_M0
复用为其他功能
I2C3_M1 GPIO3_B5/I2C3_SCL_M1
GPIO3_B6/I2C3_SDA_M1
复用为其他功能
I2C4_M0 GPIO4_B3/I2C4_SCL_M0
GPIO4_B2/I2C4_SDA_M0
camera
I2C4_M1 GPIO2_B2/I2C4_SCL_M1
GPIO2_B1/I2C4_SDA_M1
复用为其他功能
I2C5_M0 GPIO3_B3/I2C5_SCL_M0
GPIO3_B4/I2C5_SDA_M0
复用为其他功能
I2C5_M1 GPIO4_C7/I2C5_SCL_M1
GPIO4_D0/I2C5_SDA_M1
复用为其他功能

本文主要描述如何在该开发板上配置 I2C。

配置 I2C 可分为两大步骤:

  • 定义和注册 I2C 设备

  • 定义和注册 I2C 驱动

下面以配置 GSL3680 为例。

4.2. 定义和注册 I2C 设备

在注册 I2C 设备时,需要结构体 i2c_client 来描述 I2C 设备。然而在标准 Linux 中,用户只需要提供相应的 I2C 设备信息,Linux 就会根据所提供的信息构造 i2c_client 结构体。

用户所提供的 I2C 设备信息以节点的形式写到 DTS 文件中,如下所示:

kernel/arch/arm64/boot/dts/rockchip/rk3568-firefly-itx-3568q-lvds101_HSX101H40C.dts

&i2c1 {
    status = "okay";
    clock-frequency = <100000>;
    //i2c-scl-rising-time-ns = <800>;
    //i2c-scl-falling-time-ns = <100>;
    gslx680: gslx680@41 {
        status = "okay";
        compatible = "gslX680";
        reg = <0x41>;
        screen_max_x = <800>;
        screen_max_y = <1280>;
        touch-gpio = <&gpio0 RK_PC7 IRQ_TYPE_LEVEL_LOW>;
        reset-gpio = <&pca9555 PCA_IO1_0 GPIO_ACTIVE_HIGH>;
        flip-x = <1>;
        flip-y = <0>;
        swap-xy = <0>;
        gsl,fw = <1>;
    };
};
...

4.3. 定义和注册 I2C 驱动

4.3.1. 定义 I2C 驱动

在定义 I2C 驱动之前,用户首先要定义变量 of_device_idi2c_device_id

of_device_id 用于在驱动中调用 DTS 文件中定义的设备信息,其定义如下所示:

 static struct of_device_id gsl_ts_ids[] = {
   {.compatible = "gslX680"},
   {}
 };

定义变量 i2c_device_id

 static const struct i2c_device_id gsl_ts_id[] = {
    {GSLX680_I2C_NAME, 0},
    {}
 };
 MODULE_DEVICE_TABLE(i2c, gsl_ts_id);

i2c_driver 如下所示:

 static struct i2c_driver gsl_ts_driver = {
     .driver = { .name = GSLX680_I2C_NAME,
     .owner = THIS_MODULE,
     .of_match_table = of_match_ptr(gsl_ts_ids),
     },
 #ifndef CONFIG_HAS_EARLYSUSPEND
    //.suspend  = gsl_ts_suspend,
    //.resume   = gsl_ts_resume,
 #endif
      .probe      = gsl_ts_probe,
      .remove     = gsl_ts_remove,
      .id_table   = gsl_ts_id,
 };

注:变量 id_table 指示该驱动所支持的设备。

4.3.2. 注册 I2C 驱动

使用 i2c_add_driver 函数注册 I2C 驱动。

i2c_add_driver(&gsl_ts_driver);

在调用 i2c_add_driver 注册 I2C 驱动时,会遍历 I2C 设备,如果该驱动支持所遍历到的设备,则会调用该驱动的 probe 函数。

4.3.3. 通过 I2C 收发数据

在注册好 I2C 驱动后,即可进行 I2C 通讯。

  • 向从机发送信息:

 int i2c_master_send(const struct i2c_client *client, const char *buf, int count)
 {
     int ret;
     struct i2c_adapter *adap = client->adapter;
     struct i2c_msg msg;
     msg.addr = client->addr;
     msg.flags = client->flags & I2C_M_TEN;
     msg.len = count;
     msg.buf = (char *)buf;
     ret = i2c_transfer(adap, &msg, 1);
     /*
      * If everything went ok (i.e. 1 msg transmitted), return #bytes
      * transmitted, else error code.
      */
     return (ret == 1) ? count : ret;
 }
  • 向从机读取信息:

 int i2c_master_recv(const struct i2c_client *client, char *buf, int count)
 {
     struct i2c_adapter *adap = client->adapter;
     struct i2c_msg msg;
     int ret;
     msg.addr = client->addr;
     msg.flags = client->flags & I2C_M_TEN;
     msg.flags |= I2C_M_RD;
     msg.len = count;
     msg.buf = buf;
     ret = i2c_transfer(adap, &msg, 1);
     /*
      * If everything went ok (i.e. 1 msg received), return #bytes received,
      * else error code.
      */
     return (ret == 1) ? count : ret;
 }
 EXPORT_SYMBOL(i2c_master_recv);

4.4. FAQs

4.4.1. Q1: 通信失败,出现这种 log: “timeout, ipd: 0x00, state: 1” 该如何调试?

A1: 请检查硬件上拉是否给电。

4.4.2. Q2: 调用 i2c_transfer 返回值为 -6?

A2: 返回值为 -6 表示为 NACK 错误,即对方设备无应答响应,这种情况一般为外设的问题,常见的有以下几种情况:

  • I2C 地址错误,解决方法是测量 I2C 波形,确认是否 I2C 设备地址错误;

  • I2C slave 设备不处于正常工作状态,比如未给电,错误的上电时序等;

  • 时序不符合 I2C slave 设备所要求也会产生 Nack 信号。

4.4.3. Q3: 当外设对于读时序要求中间是 stop 信号不是 repeat start 信号的时候,该如何处理?

A3: 这时需要调用两次 i2c_transfer, I2C read 拆分成两次,修改如下:

static int i2c_read_bytes(struct i2c_client *client, u8 cmd, u8 *data, u8 data_len) {
    struct i2c_msg msgs[2];
	int ret;
	u8 *buffer;
	buffer = kzalloc(data_len, GFP_KERNEL);
	if (!buffer)
		return -ENOMEM;;
	msgs[0].addr = client->addr;
	msgs[0].flags = client->flags;
	msgs[0].len = 1;
	msgs[0].buf = &cmd;
	ret = i2c_transfer(client->adapter, msgs, 1);
	if (ret < 0) {
		dev_err(&client->adapter->dev, "i2c read failed\n");
        kfree(buffer);
			return ret;
    }
    msgs[1].addr = client->addr;
    msgs[1].flags = client->flags | I2C_M_RD;
    msgs[1].len = data_len;
    msgs[1].buf = buffer;
    ret = i2c_transfer(client->adapter, &msgs[1], 1);
    if (ret < 0)
		dev_err(&client->adapter->dev, "i2c read failed\n");
    else
        memcpy(data, buffer, data_len);
    kfree(buffer);
    return ret;
}