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

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Updating the hysteresis value when updating the critical temperature limit
was following the rule of 'least surprise'. However, it had the undesirable
side effect of changing the hysteresis for all other attributes, which
defeats the purpose of least surprise. In addition, it could result in
invalid hysteresis values if the resulting hysteresis was too large. In such
cases the resulting hysteresis ended up changed anyway, which again defeats
the purpose. So drop that code and document the new behavior.

Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
This commit is contained in:
Guenter Roeck 2014-04-20 08:07:42 -07:00
parent 9f9edcd4c3
commit 50bf46509f
2 changed files with 18 additions and 8 deletions
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20
Documentation/hwmon/lm77
View File

@ -18,5 +18,21 @@ sensor incorporates a band-gap type temperature sensor,
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.

6
drivers/hwmon/lm77.c
View File

@ -216,13 +216,11 @@ static ssize_t set_temp_crit_hyst(struct device *dev,
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;
@ -231,13 +229,9 @@ static ssize_t set_temp_crit(struct device *dev, struct device_attribute *attr,
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;
}