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

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Date:   Tue, 7 May 2019 11:26:31 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        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 Tue, May 07, 2019 at 10:44:00AM -0700, Florian Fainelli wrote:
> On 5/7/19 6:55 AM, Guenter Roeck wrote:
> > 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
> 
> Is not removing the comment going to upset gcc when using
> -Wimplicit-fallthrough?
> 

There is no implicit fallthrough, and the comment would have to be
ahead of the previous case statement. Such as:

	case VOLTAGE:
		scale++;
		/* fall through */
	case CURRENT:
		scale++;
		break;
	...

Two case statements together don't count as fall through.

Guenter

> > 
> >> +        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 ?
> 
> You raise a valid point, not that could happen today because if the
> sensor type has a value we don't recognize, we have not registered it,
> so we would not even try to read rom it, but let's be future proof.
> 
> > 
> >> +    }
> >> +
> >> +    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.
> 
> Sounds reasonable. Thanks for your feedback!
> -- 
> Florian