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SubjectRe: [PATCH v2] hwmon: (pmbus) Use 64bit math for DIRECT format values
On Mon, Nov 27, 2017 at 2:11 PM, Guenter Roeck <linux@roeck-us.net> wrote:
> On Mon, Nov 27, 2017 at 01:39:14PM -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 | 23 ++++++++++++++---------
>> 1 file changed, 14 insertions(+), 9 deletions(-)
>>
>> diff --git a/drivers/hwmon/pmbus/pmbus_core.c b/drivers/hwmon/pmbus/pmbus_core.c
>> index 52a58b8b6e1b..ff701502974a 100644
>> --- a/drivers/hwmon/pmbus/pmbus_core.c
>> +++ b/drivers/hwmon/pmbus/pmbus_core.c
>> @@ -21,6 +21,7 @@
>>
>> #include <linux/debugfs.h>
>> #include <linux/kernel.h>
>> +#include <linux/math64.h>
>> #include <linux/module.h>
>> #include <linux/init.h>
>> #include <linux/err.h>
>> @@ -499,8 +500,9 @@ 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 b, R;
>
> Do b and R have to be s64 ? Seems excessive.

b has a range of -32767 to 32768 and is multiplied in the code below
by 1,000,000 for a power sensor so I think needs to be 64bit to avoid
overflow.
R has limited range so changed in v3 to s32 type.

>
>> + s32 m;
>>
>> m = data->info->m[sensor->class];
>> b = data->info->b[sensor->class];
>> @@ -528,11 +530,12 @@ static long pmbus_reg2data_direct(struct pmbus_data *data,
>> R--;
>> }
>> while (R < 0) {
>> - val = DIV_ROUND_CLOSEST(val, 10);
>> + val = div_s64(val + 5LL, 10L); // round closest

(also fixed up in v3 this C++ style comment that slipped in...)

>> R++;
>> }
>>
>> - return (val - b) / m;
>> + val = div_s64(val - b, m);
>> + return clamp_val(val, LONG_MIN, LONG_MAX);
>> }
>>
>> /*
>> @@ -656,7 +659,9 @@ 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 b, R;
>
> Same question as above - do those have to be s64 ?
>
>> + s32 m;
>> + s64 val64 = val;
>>
>> m = data->info->m[sensor->class];
>> b = data->info->b[sensor->class];
>> @@ -673,18 +678,18 @@ 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);
>> + val64 = div_s64(val64 + 5LL, 10L); // round closest
>> R++;
>> }
>>
>> - return val;
>> + return (u16) clamp_val(val64, S16_MIN, S16_MAX);
>> }
>>
>> static u16 pmbus_data2reg_vid(struct pmbus_data *data,
>> --
>> 2.15.0.417.g466bffb3ac-goog
>>
>> --
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