On Wed, Jan 25, 2017 at 11:53 AM, Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> On 01/25/2017 01:51 AM, Andrea Merello wrote:
>>
>> On Tue, Jan 24, 2017 at 9:18 PM, Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
>>>
>>> On Tue, Jan 24, 2017 at 04:02:20PM +0100, Andrea Merello wrote:
>>>>
>>>> This patch adds a HWMON driver for ST Microelectronics STTS751
>>>> temperature sensors.
>>>>
>>>> It does support manual-triggered conversions as well as automatic
>>>> conversions. The latter is used when the "event" or "therm" function
>>>> is present (declaring the physical wire is attached in the DT).
>>>>
>>>> Thanks-to: LABBE Corentin [for suggestions]
>>>> Signed-off-by: Andrea Merello <andrea.merello@xxxxxxxxx>
>>>> Cc: LABBE Corentin <clabbe.montjoie@xxxxxxxxx>
>>>> Cc: Guenter Roeck <linux@xxxxxxxxxxxx>
>>>> Cc: Jean Delvare <jdelvare@xxxxxxxx>
>>>> ---
>>>
>>>
>
> [ ... ]
>
>>>> +
>>>> +static s32 stts751_to_hw(int val)
>>>> +{
>>>> + s32 hw_val;
>>>> +
>>>> + if (val < 0)
>>>> + hw_val = (val - 62) / 125 * 32;
>>>> + else
>>>> + hw_val = (val + 62) / 125 * 32;
>>>> +
>>>
>>>
>>> How about "return DIV_ROUND_CLOSEST(val, 125) * 32;" ?
>>
>>
>> I admit that I stale this from lm90 without caring too much.. Now,
>> looking at it, I realized that we could also improve the calculation
>> precision by performing the multiplication before the division:
>>
>> DIV_ROUND_CLOSEST(val * 32, 125);
>>
>> (I think doing in this way makes rounding almost useless. Maybe it has
>> a slight effect.)
>>
>
> The purpose of doing the multiplication afterwards is to make sure
> that the lower 5 bits are 0 (or, rather, to shift the result 5 bits
> to the left). Doing it first defeats that purpose, and would actually
> mess up the rounding (because the lower bits will be ignored by the
> hardware). For example, writing 0xff would yield 0xe0, while what
> you would probably really want is to write 0x1e0 in that situation.
Ah, right. Sorry, while looking at the calculations I lost focus on
the specific context (hw details).
> Guenter
>
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