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

[Florian Fainelli <f.fainelli@gmail.com>]

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?
-- 
Florian