Re: [PATCH v4 2/2] hwmon: scmi: Scale values to target desired HWMON units

[Guenter Roeck <linux@roeck-us.net>]

Hi Florian,

On Wed, May 08, 2019 at 10:00:35AM -0700, Florian Fainelli wrote:
> If the SCMI firmware implementation is reporting values in a scale that
> is different from the HWMON units, we need to scale up or down the value
> according to how far appart they are.
> 
> Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
> ---
>  drivers/hwmon/scmi-hwmon.c | 46 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 46 insertions(+)
> 
> diff --git a/drivers/hwmon/scmi-hwmon.c b/drivers/hwmon/scmi-hwmon.c
> index a80183a488c5..4399372e2131 100644
> --- a/drivers/hwmon/scmi-hwmon.c
> +++ b/drivers/hwmon/scmi-hwmon.c
> @@ -7,6 +7,7 @@
>   */
>  
>  #include <linux/hwmon.h>
> +#include <linux/limits.h>
>  #include <linux/module.h>
>  #include <linux/scmi_protocol.h>
>  #include <linux/slab.h>
> @@ -18,6 +19,47 @@ struct scmi_sensors {
>  	const struct scmi_sensor_info **info[hwmon_max];
>  };
>  
> +static inline u64 __pow10(u8 x)
> +{
> +	u64 r = 1;
> +
> +	while (x--)
> +		r *= 10;
> +
> +	return r;
> +}
> +
> +static int scmi_hwmon_scale(const struct scmi_sensor_info *sensor, u64 *value)
> +{
> +	s8 scale = sensor->scale;
> +	u64 f;
> +
> +	switch (sensor->type) {
> +	case TEMPERATURE_C:
> +	case VOLTAGE:
> +	case CURRENT:
> +		scale += 3;
> +		break;
> +	case POWER:
> +	case ENERGY:
> +		scale += 6;
> +		break;
> +	default:
> +		break;
> +	}
> +
> +	f = __pow10(abs(scale));
> +	if (f == U64_MAX)
> +		return -E2BIG;

Unfortunately that is not how integer overflows work.

A test program with increasing values of scale reports:

0: 1
...
18: 1000000000000000000
19: 10000000000000000000
20: 7766279631452241920
21: 3875820019684212736
22: 1864712049423024128
23: 200376420520689664
24: 2003764205206896640
...
61: 11529215046068469760
62: 4611686018427387904
63: 9223372036854775808
64: 0
...

You'll have to check for abs(scale) > 19 if you want to report overflows.

Guenter

> +
> +	if (scale > 0)
> +		*value *= f;
> +	else
> +		*value = div64_u64(*value, f);
> +
> +        return 0;
> +}
> +
>  static int scmi_hwmon_read(struct device *dev, enum hwmon_sensor_types type,
>  			   u32 attr, int channel, long *val)
>  {
> @@ -29,6 +71,10 @@ static int scmi_hwmon_read(struct device *dev, enum hwmon_sensor_types type,
>  
>  	sensor = *(scmi_sensors->info[type] + channel);
>  	ret = h->sensor_ops->reading_get(h, sensor->id, false, &value);
> +	if (ret)
> +		return ret;
> +
> +	ret = scmi_hwmon_scale(sensor, value);
>  	if (!ret)
>  		*val = value;
>  
> -- 
> 2.17.1
>