[PATCH 2/2] hwmon: (scpi) Fix the scale of SCP sensor readings

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

On Thu, Mar 02, 2017 at 01:15:00PM +0100, Carlo Caione wrote:
> From: Carlo Caione <carlo@endlessm.com>
> 
> The implementation details for SCPI seems to suggest that the sensor
> readings must be reported by SCP using a well defined scale
> (millidegree Celsius for temperature, millivolts for voltage,
> milliamperes for current, microwatts for power and microjoules for
> energy).
> 
> This is also important for the interaction with other subsystems: for
> example both the thermal sub-system and the hwmon sysfs interface expect
> the temperature expressed in millidegree Celsius.
> 
> Unfortunately since this behaviour is dependent on the firmware
> implementation there are cases where the sensor readings are reported
> using a different scale. For example in the Amlogic SoCs the
> temperature is reported in degree and not millidegree Celsius.
> 
> To take into account this discrepancy and fixup the values reported by
> SCP a new DT property `scpi,sensors-scale' is introduced and used in
> this patch by the scpi-hwmon driver to convert the sensor readings to
> the expected scale.
> 
> Signed-off-by: Carlo Caione <carlo@endlessm.com>
> ---
>  drivers/hwmon/scpi-hwmon.c | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/drivers/hwmon/scpi-hwmon.c b/drivers/hwmon/scpi-hwmon.c
> index 094f948f99ff..64a51e06c8df 100644
> --- a/drivers/hwmon/scpi-hwmon.c
> +++ b/drivers/hwmon/scpi-hwmon.c
> @@ -23,6 +23,7 @@
>  #include <linux/thermal.h>
>  
>  struct sensor_data {
> +	unsigned int scale;
>  	struct scpi_sensor_info info;
>  	struct device_attribute dev_attr_input;
>  	struct device_attribute dev_attr_label;
> @@ -44,6 +45,14 @@ struct scpi_sensors {
>  	const struct attribute_group *groups[2];
>  };
>  
> +static const unsigned int scpi_scale[] = {
> +	1000,		/* Temperature (millidegrees Celsius)	*/
> +	1000,		/* Voltage     (millivolts)		*/
> +	1000,		/* Current     (milliamperes)		*/
> +	1000000,	/* Power       (microwatts)		*/
> +	1000000,	/* Energy      (microjoules)		*/
> +};
> +
>  static int scpi_read_temp(void *dev, int *temp)
>  {
>  	struct scpi_thermal_zone *zone = dev;
> @@ -57,6 +66,11 @@ static int scpi_read_temp(void *dev, int *temp)
>  	if (ret)
>  		return ret;
>  
> +	if (scpi_scale[sensor->info.class] != sensor->scale) {
> +		value *= scpi_scale[sensor->info.class];
> +		do_div(value, sensor->scale);

Pretty elegant, I like that solution.

> +	}
> +
>  	*temp = value;
>  	return 0;
>  }
> @@ -77,6 +91,11 @@ scpi_show_sensor(struct device *dev, struct device_attribute *attr, char *buf)
>  	if (ret)
>  		return ret;
>  
> +	if (scpi_scale[sensor->info.class] != sensor->scale) {
> +		value *= scpi_scale[sensor->info.class];
> +		do_div(value, sensor->scale);
> +	}
> +
>  	return sprintf(buf, "%llu\n", value);
>  }
>  
> @@ -97,6 +116,7 @@ static struct thermal_zone_of_device_ops scpi_sensor_ops = {
>  static int scpi_hwmon_probe(struct platform_device *pdev)
>  {
>  	u16 nr_sensors, i;
> +	u32 scale[] = { 1000, 1000, 1000, 1000000, 1000000 };
>  	int num_temp = 0, num_volt = 0, num_current = 0, num_power = 0;
>  	int num_energy = 0;
>  	struct scpi_ops *scpi_ops;
> @@ -131,6 +151,9 @@ static int scpi_hwmon_probe(struct platform_device *pdev)
>  
>  	scpi_sensors->scpi_ops = scpi_ops;
>  
> +	of_property_read_u32_array(dev->of_node, "scpi,sensors-scale",
> +				   scale, ARRAY_SIZE(scale));
> +
We'll need some validation here. Otherwise a provided scale of 0 would
crash the system.

Guenter

>  	for (i = 0, idx = 0; i < nr_sensors; i++) {
>  		struct sensor_data *sensor = &scpi_sensors->data[idx];
>  
> @@ -178,6 +201,8 @@ static int scpi_hwmon_probe(struct platform_device *pdev)
>  			continue;
>  		}
>  
> +		sensor->scale = scale[sensor->info.class];
> +
>  		sensor->dev_attr_input.attr.mode = S_IRUGO;
>  		sensor->dev_attr_input.show = scpi_show_sensor;
>  		sensor->dev_attr_input.attr.name = sensor->input;
> -- 
> 2.12.0
> 
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