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

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

On Wed, May 08, 2019 at 11:34:50AM -0700, Florian Fainelli wrote:
> On 5/8/19 11:32 AM, Guenter Roeck wrote:
> > 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.
> 
> Yes silly me, my test program was flawed, thanks for pointing out that.
> You are okay with returning E2BIG when we overflow?

Yes.

Thanks,
Guenter