I2C Drivers, Part I

The detach_state file and power directory is created by the kernel driver core and is used for power management. It is not created by the lm75 driver. The functions of the other files in this directory are described below.

If we ask the lm75 driver for the current value of temp_max, we receive the following:

$ cat /sys/devices/legacy/i2c-0/0-0048/temp_max
1000

To get that value, the lm75 driver asked the tiny_i2c_adap driver to read some addresses on the I2C bus. This request is shown in the syslog:

$ dmesg
i2c_adapter i2c-0: tiny_access was called with the following parameters:
i2c_adapter i2c-0: addr = 0048
i2c_adapter i2c-0: flags = 0000
i2c_adapter i2c-0: read_write = read
i2c_adapter i2c-0: command = 0
i2c_adapter i2c-0: size = I2C_SMBUS_WORD_DATA
i2c_adapter i2c-0: tiny_access was called with the following parameters:
i2c_adapter i2c-0: addr = 0048
i2c_adapter i2c-0: flags = 0000
i2c_adapter i2c-0: read_write = read
i2c_adapter i2c-0: command = 3
i2c_adapter i2c-0: size = I2C_SMBUS_WORD_DATA
i2c_adapter i2c-0: tiny_access was called with the following parameters:
i2c_adapter i2c-0: addr = 0048
i2c_adapter i2c-0: flags = 0000
i2c_adapter i2c-0: read_write = read
i2c_adapter i2c-0: command = 2
i2c_adapter i2c-0: size = I2C_SMBUS_WORD_DATA

The log file shows that the tiny_access function was called three times. The first command wanted to read a word of data from register 0 out of the device with the address 0048. The second and third reads asked for register 3 and register 2 from the same device. The commands match up with the following code from the drivers/i2c/chips/lm75.c file in the lm75_update_client function:

data->temp_input = lm75_read_value(client,
                                   LM75_REG_TEMP);
data->temp_max = lm75_read_value(client,
                                LM75_REG_TEMP_OS);
data->temp_hyst = lm75_read_value(client,
                              LM75_REG_TEMP_HYST);

The lm75_read_value function in that same file contains the following code:

/* All registers are word-sized, except for the
   configuration register. LM75 uses a high-byte
   first convention, which is exactly opposite to
   the usual practice. */
static int lm75_read_value(struct i2c_client
                           *client, u8 reg)
{
    if (reg == LM75_REG_CONF)
        return i2c_smbus_read_byte_data(client,
                                        reg);
    else
        return swap_bytes(
               i2c_smbus_read_word_data(client,
                                        reg));
}

Therefore, when the lm75 driver wants to read the value of the max temperature, it calls the lm75_read_value function with the register number, which then calls the I2C core function i2c_smbus_read_word_data. That I2C core function looks up on which I2C bus the client device is, and then it calls the I2C algorithm associated with that specific I2C bus to do the data transfer. This is the method, then, by which our tiny_i2c_adap driver is asked to complete the transfer.

If this same sysfs file is written to, the lm75 driver asks the tiny_i2c_adap driver to write some data to a specific address on the I2C bus in the same way the read was requested. This request also is shown in the syslog:

$ echo 300 > /sys/devices/legacy/i2c-0/0-0048/temp_max
$ dmesg
i2c_adapter i2c-0: tiny_access was called with the following parameters:
i2c_adapter i2c-0: addr = 0048
i2c_adapter i2c-0: flags = 0000
i2c_adapter i2c-0: read_write = write
i2c_adapter i2c-0: command = 3
i2c_adapter i2c-0: size = I2C_SMBUS_WORD_DATA
i2c_adapter i2c-0: data = 8000

This month we covered the basics of the I2C driver subsystem and explained how to write a simple I2C bus and I2C algorithm driver that work with any existing I2C client driver. The complete driver, dmn-09-i2c-adap.c, is available from the Linux Journal FTP site at ftp.linuxjournal.com/pub/lj/listings/issue116/7136.tgz. In Part II, we will cover how to write an I2C chip driver.