hwmon: (lm77) Do not preserve hysteresis when updating critical temp limit

10-bit ADC, and a digital comparator with user-programmable upper

and lower limit values.

Limits can be set through the Overtemperature Shutdown register and

Hysteresis register.

The LM77 implements 3 limits: low (temp1_min), high (temp1_max) and

critical (temp1_crit.) It also implements an hysteresis mechanism which

applies to all 3 limits. The relative difference is stored in a single

register on the chip, which means that the relative difference between

the limit and its hysteresis is always the same for all 3 limits.

This implementation detail implies the following:

* When setting a limit, its hysteresis will automatically follow, the

  difference staying unchanged. For example, if the old critical limit

  was 80 degrees C, and the hysteresis was 75 degrees C, and you change

  the critical limit to 90 degrees C, then the hysteresis will

  automatically change to 85 degrees C.

* All 3 hysteresis can't be set independently. We decided to make

  temp1_crit_hyst writable, while temp1_min_hyst and temp1_max_hyst are

  read-only. Setting temp1_crit_hyst writes the difference between

  temp1_crit_hyst and temp1_crit into the chip, and the same relative

  hysteresis applies automatically to the low and high limits.

* The limits should be set before the hysteresis.

	return count;

}

/* preserve hysteresis when setting T_crit */

static ssize_t set_temp_crit(struct device *dev, struct device_attribute *attr,

			     const char *buf, size_t count)

{

	struct i2c_client *client = to_i2c_client(dev);

	struct lm77_data *data = i2c_get_clientdata(client);

	int oldcrithyst;

	unsigned long val;

	int err;

		return err;

	mutex_lock(&data->update_lock);

	oldcrithyst = data->temp_crit - data->temp_hyst;

	data->temp_crit = val;

	data->temp_hyst = data->temp_crit - oldcrithyst;

	lm77_write_value(client, LM77_REG_TEMP_CRIT,

			 LM77_TEMP_TO_REG(data->temp_crit));

	lm77_write_value(client, LM77_REG_TEMP_HYST,

			 LM77_TEMP_TO_REG(data->temp_hyst));

	mutex_unlock(&data->update_lock);

	return count;

}