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SubjectRe: [PATCH] hwmon: (pmbus) Use 64bit math for DIRECT format values
On Wed, Nov 22, 2017 at 2:28 PM, Guenter Roeck <linux@roeck-us.net> wrote:
> On Wed, Nov 22, 2017 at 02:08:43PM -0800, Robert Lippert wrote:
>> Power values in the 100s of watt range can easily blow past
>> 32bit math limits when processing everything in microwatts.
>>
>> Use 64bit math instead to avoid these issues on common 32bit ARM
>> BMC platforms.
>>
>> Signed-off-by: Robert Lippert <rlippert@google.com>
>> ---
>> drivers/hwmon/pmbus/pmbus_core.c | 21 ++++++++++++---------
>> 1 file changed, 12 insertions(+), 9 deletions(-)
>>
>> diff --git a/drivers/hwmon/pmbus/pmbus_core.c b/drivers/hwmon/pmbus/pmbus_core.c
>> index 52a58b8b6e1b..f4efea9f282e 100644
>> --- a/drivers/hwmon/pmbus/pmbus_core.c
>> +++ b/drivers/hwmon/pmbus/pmbus_core.c
>> @@ -499,8 +499,8 @@ static long pmbus_reg2data_linear(struct pmbus_data *data,
>> static long pmbus_reg2data_direct(struct pmbus_data *data,
>> struct pmbus_sensor *sensor)
>> {
>> - long val = (s16) sensor->data;
>> - long m, b, R;
>> + s64 val = (s16) sensor->data;
>> + s64 m, b, R;
>>
>> m = data->info->m[sensor->class];
>> b = data->info->b[sensor->class];
>> @@ -528,11 +528,13 @@ static long pmbus_reg2data_direct(struct pmbus_data *data,
>> R--;
>> }
>> while (R < 0) {
>> - val = DIV_ROUND_CLOSEST(val, 10);
>> + do_div(val, 10);
>
> Any reason to not use DIV_ROUND_CLOSEST_ULL ?

That macro doesn't quite work for signed division. v2 changes this to
use the signed 64bit division functions and I emulated the "round
closest" by just adding 5 before dividing.

>
>> R++;
>> }
>>
>> - return (val - b) / m;
>> + val = val - b;
>> + do_div(val, m);
>> + return val;
>
> Can this overflow ?

Added clamp() to the return values in v2 to prevent overflow when
returning a narrower type.

>
>> }
>>
>> /*
>> @@ -656,7 +658,8 @@ static u16 pmbus_data2reg_linear(struct pmbus_data *data,
>> static u16 pmbus_data2reg_direct(struct pmbus_data *data,
>> struct pmbus_sensor *sensor, long val)
>> {
>> - long m, b, R;
>> + s64 m, b, R;
>> + s64 val64 = val;
>>
>> m = data->info->m[sensor->class];
>> b = data->info->b[sensor->class];
>> @@ -673,18 +676,18 @@ static u16 pmbus_data2reg_direct(struct pmbus_data *data,
>> R -= 3; /* Adjust R and b for data in milli-units */
>> b *= 1000;
>> }
>> - val = val * m + b;
>> + val64 = val64 * m + b;
>>
>> while (R > 0) {
>> - val *= 10;
>> + val64 *= 10;
>> R--;
>> }
>> while (R < 0) {
>> - val = DIV_ROUND_CLOSEST(val, 10);
>> + do_div(val64, 10);
>
> Same here.
>
>> R++;
>> }
>>
>> - return val;
>> + return (u16) val64;
>
> Can this overflow ?
>
>> }
>>
>> static u16 pmbus_data2reg_vid(struct pmbus_data *data,
>> --
>> 2.15.0.448.gf294e3d99a-goog
>>

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