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

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Date:   Tue, 7 May 2019 06:55:41 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Florian Fainelli <f.fainelli@...il.com>,
        linux-kernel@...r.kernel.org
Cc:     bcm-kernel-feedback-list@...adcom.com,
        Sudeep Holla <sudeep.holla@....com>,
        Jean Delvare <jdelvare@...e.com>,
        linux-arm-kernel@...ts.infradead.org,
        "open list:HARDWARE MONITORING" <linux-hwmon@...r.kernel.org>
Subject: Re: [PATCH 2/2] hwmon: scmi: Scale values to target desired HWMON
 units

Hi Florian,

On 5/6/19 3:41 PM, 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@...il.com>
> ---
>   drivers/hwmon/scmi-hwmon.c | 55 +++++++++++++++++++++++++++++++-------
>   1 file changed, 46 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/hwmon/scmi-hwmon.c b/drivers/hwmon/scmi-hwmon.c
> index a80183a488c5..e9913509cb88 100644
> --- a/drivers/hwmon/scmi-hwmon.c
> +++ b/drivers/hwmon/scmi-hwmon.c
> @@ -18,6 +18,51 @@ struct scmi_sensors {
>   	const struct scmi_sensor_info **info[hwmon_max];
>   };
>   
> +static enum hwmon_sensor_types scmi_types[] = {
> +	[TEMPERATURE_C] = hwmon_temp,
> +	[VOLTAGE] = hwmon_in,
> +	[CURRENT] = hwmon_curr,
> +	[POWER] = hwmon_power,
> +	[ENERGY] = hwmon_energy,
> +};
> +
> +static u64 scmi_hwmon_scale(const struct scmi_sensor_info *sensor, u64 value)
> +{
> +	u64 scaled_value = value;

I don't think that variable is necessary.

> +	s8 desired_scale;

Just scale ? Also, you could assign scale here directly, and subtract
the offset below. Then "n" would not be necessary.
Such as
	s8 scale = sensor->scale;	// assuming scale is s8
	...
	case CURRENT:
		scale += 3;
	...

That would also be less confusing, since it would avoid the double
negation.

> +	int n, p;

> +
> +	switch (sensor->type) {
> +	case TEMPERATURE_C:
> +	case VOLTAGE:
> +	case CURRENT:
> +		/* fall through */
Unnecessary comment

> +		desired_scale = -3;
> +		break;
> +	case POWER:
> +	case ENERGY:
> +		/* fall through */
Unnecessary comment.

> +		desired_scale = -6;
> +		break;
> +	default:
> +		return scaled_value;

Here we presumably want a scale of 0. However, if the scale passed
from SCMI is, say, -5 or +5, we return the original (unadjusted)
value. Seems to me we would still want to adjust the value to match
hwmon expectations. Am I missing something ?

> +	}
> +
> +	n = (s8)sensor->scale - desired_scale;
> +        if (n == 0)

Indentation seems off here.

> +                return scaled_value;
> +
> +	for (p = 0; p < abs(n); p++) {
> +		/* Need to scale up from sensor to HWMON */
> +		if (n > 0)
> +			scaled_value *= 10;
> +		else
> +			do_div(scaled_value, 10);
> +	}

Something like

	factor = pow10(abs(scale));
	if (scale > 0)
		value *= factor;
	else
		do_div(value, factor);

would avoid the repeated abs() and do_div(). Unfortunately there is
no pow10() helper, so you would have to write that. Still, I think
that would be much more efficient.

Thanks,
Guenter

> +
> +        return scaled_value;
> +}
> +
>   static int scmi_hwmon_read(struct device *dev, enum hwmon_sensor_types type,
>   			   u32 attr, int channel, long *val)
>   {
> @@ -30,7 +75,7 @@ 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)
> -		*val = value;
> +		*val = scmi_hwmon_scale(sensor, value);
>   
>   	return ret;
>   }
> @@ -91,14 +136,6 @@ static int scmi_hwmon_add_chan_info(struct hwmon_channel_info *scmi_hwmon_chan,
>   	return 0;
>   }
>   
> -static enum hwmon_sensor_types scmi_types[] = {
> -	[TEMPERATURE_C] = hwmon_temp,
> -	[VOLTAGE] = hwmon_in,
> -	[CURRENT] = hwmon_curr,
> -	[POWER] = hwmon_power,
> -	[ENERGY] = hwmon_energy,
> -};
> -
>   static u32 hwmon_attributes[] = {
>   	[hwmon_chip] = HWMON_C_REGISTER_TZ,
>   	[hwmon_temp] = HWMON_T_INPUT | HWMON_T_LABEL,
>