Linux I2C驱动--用户态驱动简单示例

1. Linux内核支持I2C通用设备驱动（用户态驱动：由应用层实现对硬件的控制可以称之为用户态驱动），实现文件位于drivers/i2c/i2c-dev.c，设备文件为/dev/i2c-0

2. I2C通用设备驱动以字符设备注册进内核的

static const struct file_operations i2cdev_fops = {
    .owner        = THIS_MODULE,
    .llseek        = no_llseek,
    .read        = i2cdev_read,
    .write        = i2cdev_write,
    .unlocked_ioctl    = i2cdev_ioctl,
    .open        = i2cdev_open,
    .release    = i2cdev_release,
};

res = register_chrdev(I2C_MAJOR, "i2c", &i2cdev_fops);

3. 对设备文件进行读写时，可以调用read、write或者ioctl等方法，他们都是通过调用函数i2c_transfer来实现的

int i2c_transfer(struct i2c_adapter * adap, struct i2c_msg *msgs, int num)
{
    int ret;

    /* REVISIT the fault reporting model here is weak:
     *
     *  - When we get an error after receiving N bytes from a slave,
     *    there is no way to report "N".
     *
     *  - When we get a NAK after transmitting N bytes to a slave,
     *    there is no way to report "N" ... or to let the master
     *    continue executing the rest of this combined message, if
     *    that‘s the appropriate response.
     *
     *  - When for example "num" is two and we successfully complete
     *    the first message but get an error part way through the
     *    second, it‘s unclear whether that should be reported as
     *    one (discarding status on the second message) or errno
     *    (discarding status on the first one).
     */

    if (adap->algo->master_xfer) {
#ifdef DEBUG
        for (ret = 0; ret < num; ret++) {
            dev_dbg(&adap->dev, "master_xfer[%d] %c, addr=0x%02x, "
                "len=%d%s\n", ret, (msgs[ret].flags & I2C_M_RD)
                ? ‘R‘ : ‘W‘, msgs[ret].addr, msgs[ret].len,
                (msgs[ret].flags & I2C_M_RECV_LEN) ? "+" : "");
        }
#endif

        if (in_atomic() || irqs_disabled()) {
            ret = mutex_trylock(&adap->bus_lock);
            if (!ret)
                /* I2C activity is ongoing. */
                return -EAGAIN;
        } else {
            mutex_lock_nested(&adap->bus_lock, adap->level);
        }

        ret = adap->algo->master_xfer(adap,msgs,num);
        mutex_unlock(&adap->bus_lock);

        return ret;
    } else {
        dev_dbg(&adap->dev, "I2C level transfers not supported\n");
        return -EOPNOTSUPP;
    }
}

4. i2c_transfer通过代码可以看出，i2c_transfer 通过调用相应的 adapter 的 master_xfer 方法实现的，而 master_xfer 主要是根据 struct i2c_msg 类型的msgs来进行处理的。

struct i2c_msg {
    __u16 addr;    /* slave address            */
    __u16 flags;
#define I2C_M_TEN        0x0010    /* this is a ten bit chip address */
#define I2C_M_RD        0x0001    /* read data, from slave to master */
#define I2C_M_NOSTART        0x4000    /* if I2C_FUNC_PROTOCOL_MANGLING */
#define I2C_M_REV_DIR_ADDR    0x2000    /* if I2C_FUNC_PROTOCOL_MANGLING */
#define I2C_M_IGNORE_NAK    0x1000    /* if I2C_FUNC_PROTOCOL_MANGLING */
#define I2C_M_NO_RD_ACK        0x0800    /* if I2C_FUNC_PROTOCOL_MANGLING */
#define I2C_M_RECV_LEN        0x0400    /* length will be first received byte */
    __u16 len;        /* msg length                */
    __u8 *buf;        /* pointer to msg data            */
};

5. I2C用户态驱动简单示例

原文地址：https://www.cnblogs.com/wulei0630/p/9607606.html