[PATCH] hwmon: (scpi) Add slope and offset to SCP sensor readings

[linux@roeck-us.net (Guenter Roeck)]

On 03/01/2017 05:20 AM, Carlo Caione wrote:
> From: Carlo Caione <carlo@endlessm.com>
>
> The temperature provided by the SCP sensors not always is expressed in
> millicelsius, whereas this is required by the thermal framework. This is
> for example the case for the Amlogic devices, where the SCP sensor
> readings are expressed in degree (and not milli degree) Celsius.
>
Are you saying that SCPI does not specify or provide the units to be used
when reading values, and thus effectively just reports a more or less
random number ?

> To convert the sensor readings, the thermal framework provides the
> "coefficients" property, used by the thermal sensor device to adjust
> their reading. This adjustment in currently not considered by the SCPI
> hwmon driver. Fix this introducing slope and offset for the SCP sensor
> readings.
>
> Signed-off-by: Carlo Caione <carlo@endlessm.com>
> ---
>  drivers/hwmon/scpi-hwmon.c | 18 +++++++++++-------
>  1 file changed, 11 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/hwmon/scpi-hwmon.c b/drivers/hwmon/scpi-hwmon.c
> index 094f948f99ff..cdc05c60ba67 100644
> --- a/drivers/hwmon/scpi-hwmon.c
> +++ b/drivers/hwmon/scpi-hwmon.c
> @@ -33,6 +33,7 @@ struct sensor_data {
>  struct scpi_thermal_zone {
>  	int sensor_id;
>  	struct scpi_sensors *scpi_sensors;
> +	struct thermal_zone_device *z;
>  };
>
>  struct scpi_sensors {
> @@ -51,13 +52,17 @@ static int scpi_read_temp(void *dev, int *temp)
>  	struct scpi_ops *scpi_ops = scpi_sensors->scpi_ops;
>  	struct sensor_data *sensor = &scpi_sensors->data[zone->sensor_id];
>  	u64 value;
> +	int slope, offset;
>  	int ret;
>
>  	ret = scpi_ops->sensor_get_value(sensor->info.sensor_id, &value);
>  	if (ret)
>  		return ret;
>
> -	*temp = value;
> +	slope = thermal_zone_get_slope(zone->z);
> +	offset = thermal_zone_get_offset(zone->z);

This is conceptually wrong. The functions return -ENODEV if thermal is disabled.
While a negative slope does not make sense, a negative offset does.

The code in the thermal subsystem does not clarify well how slope and offset
are supposed to be used. Since coming from the thermal subsystem, one would think
that the thermal subsystem would apply any corrections if they are supposed
to be software correction values, but that does not appear to be the case,
leaving it up to drivers to use or not use the provided values.
This is kind of odd; the values are thermal subsystem attributes/properties,
and one would think that they are to be used there unless they are supposed to be
written into hardware. Given that, I'd rather wait for the API to be clarified
instead of jumping into using it.

There are secondary problems with this approach; other drivers such as lm90 which
support the thermal subsystem have their own means to specify and use the offset
(since it needs to be programmed into the chip registers).

Also, this would only solve the problem for temperatures and does not address
the generic problem (ie voltage and current values).

> +
> +	*temp = value * slope + offset;
>  	return 0;
>  }
>
> @@ -216,7 +221,6 @@ static int scpi_hwmon_probe(struct platform_device *pdev)
>  	INIT_LIST_HEAD(&scpi_sensors->thermal_zones);
>  	for (i = 0; i < nr_sensors; i++) {
>  		struct sensor_data *sensor = &scpi_sensors->data[i];
> -		struct thermal_zone_device *z;
>  		struct scpi_thermal_zone *zone;
>
>  		if (sensor->info.class != TEMPERATURE)
> @@ -228,17 +232,17 @@ static int scpi_hwmon_probe(struct platform_device *pdev)
>
>  		zone->sensor_id = i;
>  		zone->scpi_sensors = scpi_sensors;
> -		z = devm_thermal_zone_of_sensor_register(dev,
> -							 sensor->info.sensor_id,
> -							 zone,
> -							 &scpi_sensor_ops);
> +		zone->z = devm_thermal_zone_of_sensor_register(dev,
> +							       sensor->info.sensor_id,
> +							       zone,
> +							       &scpi_sensor_ops);
>  		/*
>  		 * The call to thermal_zone_of_sensor_register returns
>  		 * an error for sensors that are not associated with
>  		 * any thermal zones or if the thermal subsystem is
>  		 * not configured.
>  		 */
> -		if (IS_ERR(z)) {
> +		if (IS_ERR(zone->z)) {
>  			devm_kfree(dev, zone);
>  			continue;
>  		}
>