I2C Drivers, Part II

That call creates the sysfs files for the device:

/sys/devices/pci0000:00/0000:00:06.0/i2c-0/0-0009
|-- detach_state
|-- name
|-- power
|   `-- state
|-- temp_input
|-- temp_max
`-- temp_min

The file name is created by the I2C core, and the files detach_state and power/state are created by the driver core.

But, let's go back to the DEVICE_ATTR macro. That macro wants to know the name of the file to be created, the mode of the file to be created, the name of the function to be called when the file is read from and the name of the function to be called when the file is written to. For the file temp_max, this declaration was:

static DEVICE_ATTR(temp_max, S_IWUSR | S_IRUGO,
                   show_temp_max, set_temp_max);

The function called when the file is read from is show_temp_max. This is defined, as are many sysfs files, with another macro that creates a function:

#define show(value) \
static ssize_t \
show_##value(struct device *dev, char *buf)        \
{                                                  \
    struct i2c_client *client = to_i2c_client(dev);\
    struct chip_data *data =                       \
        i2c_get_clientdata(client);                \
                                                   \
    chip_update_client(client);                    \
    return sprintf(buf, "%d\n", data->value);      \
}
show(temp_max);
show(temp_hyst);
show(temp_input);

The reason this function is created with a macro is that it is quite simple to create other sysfs files that do almost the same thing, with different names and that read from different variables, without having to duplicate code. This single macro creates three different functions to read from three different variables from the struct chip_data structure.

In this function, the struct device * is converted into a struct i2c_client *. Then the private struct chip_data * is obtained from the struct i2c_client *. After that the chip data is updated with a call to chip_update_client. From there, the variable that has been asked for is printed into a buffer and returned to the driver core, which then returns it to the user:

$ cat /sys/devices/pci0000:00/0000:00:06.0/i2c-0/0-0009/temp_input
1

The chip_update_client increments all values by one every time it is called:

static void
chip_update_client(struct i2c_client *client)
{
    struct chip_data *data =
        i2c_get_clientdata(client);

    down(&data->update_lock);
    dev_dbg(&client->dev, "%s\n", __FUNCTION__);
    ++data->temp_input;
    ++data->temp_max;
    ++data->temp_hyst;
    data->last_updated = jiffies;
    data->valid = 1;
    up(&data->update_lock);
}

So, all subsequent requests for this value are different:

$ cat /sys/devices/pci0000:00/0000:00:06.0/i2c-0/0-0009/temp_input
2
$ cat /sys/devices/pci0000:00/0000:00:06.0/i2c-0/0-0009/temp_input
3

The set_temp_max function also is created from a macro to allow variables to be written to:

#define set(value, reg)	\
static ssize_t                                     \
set_##value(struct device *dev,                    \
            const char *buf, size_t count)         \
{                                                  \
    struct i2c_client *client = to_i2c_client(dev);\
    struct chip_data *data =                       \
        i2c_get_clientdata(client);                \
    int temp = simple_strtoul(buf, NULL, 10);      \
                                                   \
    down(&data->update_lock);                      \
    data->value = temp;                            \
    up(&data->update_lock);                        \
    return count;                                  \
}
set(temp_max, REG_TEMP_OS);
set(temp_hyst, REG_TEMP_HYST);

Just like the show functions, this function converts the struct device * to a struct i2c_client *, and then the private struct chip_data * is found. The data the user provides then is turned into a number with a call to simple_strtoul and is saved into the proper variable:

$ cat /sys/devices/pci0000:00/0000:00:06.0/i2c-0/0-0009/temp_max
1
$ echo 41 > /sys/devices/pci0000:00/0000:00:06.0/i2c-0/0-0009/temp_max
$ cat /sys/devices/pci0000:00/0000:00:06.0/i2c-0/0-0009/temp_max
42

When the I2C chip device is removed from the system, either by the I2C bus driver being unloaded or by the I2C chip driver being unloaded, the I2C core calls the detatch_client function specified in the struct i2c_driver structure. This usually is a simple function, as can be seen in the example driver's implementation:

static int chip_detach_client(struct i2c_client *client)
{
    struct chip_data *data = i2c_get_clientdata(client);
    int err;

    err = i2c_detach_client(client);
    if (err) {
        dev_err(&client->dev,
                "Client deregistration failed, "
                "client not detached.\n");
        return err;
    }
    kfree(client);
    kfree(data);
    return 0;
}

As the i2c_attach_client function was called to register the struct i2c_client structure with the I2C core, the i2c_detach_client function must be called to unregister it. If that function succeeds, the memory the driver has allocated for the I2C device then needs to be freed before returning from the function.

This example driver does not specifically remove the sysfs files from the sysfs core. This step is done automatically in the driver core within the i2c_detach_client function. But if the author would like, the file can be removed manually by a call to device_remove_file.