为什么linux中不支持重复启动的i2c操作？

why repeated start based i2c operation are not supported in linux?

事实上,它们得到了支持.

如果您正在寻找一种在用户空间中执行重复启动条件的方法,您可能需要使用I2C_RDWR请求执行ioctl(),如here所述(参见原始问题中的最后一个代码段)和here(相关代码).

下面描述了在内核空间中执行重复启动的方法.

在Linux内核中,对于组合(写入/读取)消息,默认情况下会执行repeated start condition的I2C读取操作.

以下是如何执行组合I2C传输的示例：

/**
 * Read set of registers via I2C using "repeated start" condition.
 *
 * Two I2C messages are being sent by this function:
 *   1. I2C write operation (write register address) with no STOP bit in the end
 *   2. I2C read operation
 *
 * @client: I2C client structure
 * @reg: register address (subaddress)
 * @len: bytes count to read
 * @buf: buffer which will contain read data
 *
 * Returns 0 on success or negative value on error.
 */
static int i2c_read_regs(struct i2c_client *client,u8 reg,u8 len,u8 *buf)
{
    int ret;
    struct i2c_msg msg[2] = {
        {
            .addr = client->addr,.len = 1,.buf = &reg,},{
            .addr = client->addr,.flags = I2C_M_RD,.len = len,.buf = buf,}
    };

    ret = i2c_transfer(client->adapter,msg,2);
    if (ret < 0) {
        dev_err(&client->dev,"I2C read failedn");
        return ret;
    }

    return 0;
}

要只读取1个字节(单个寄存器值),您可以使用下一个辅助函数：

/**
 * Read one register via I2C using "repeated start" condition.
 *
 * @client: I2C client structure
 * @reg: register address (subaddress)
 * @val: variable to store read value
 *
 * Returns 0 on success or negative value on error.
 */
static int i2c_read_reg(struct i2c_client *client,u8 *val)
{
    return i2c_read_regs(client,reg,1,val);
}

以下是i2c_read_regs(client,val)调用的说明：

>设备地址是客户端 – > addr
>注册地址是reg
> 1表示我们要读取1个字节的数据(图片上的粉红色矩形)
>读取数据将驻留在val

注意：如果您的I2C控制器(或其驱动程序)不支持在组合消息中重复启动,您仍然可以使用I2C的bit-bang实现,即i2c-gpio驱动程序.

如果没有任何作用,你可以尝试下一个作为最后的手段.出于某种原因,我不记得了,为了重复开始工作,我需要将I2C_M_NOSTART添加到第一条消息的.flags中,如下所示：

struct i2c_msg msg[2] = {
    {
        .addr = client->addr,.flags = I2C_M_NOSTART,{
        .addr = client->addr,}
};

如Documentation / i2c / i2c-protocol中所述：

If you set the I2C_M_NOSTART variable for the first partial message,
we do not generate Addr,but we do generate the startbit S.

参考文献：

[1] I2C on STLinux