On Mon, Dec 12, 2016 at 10:45 PM, Wolfram Sang <[email protected]> wrote:
>> +/*
>> + * Linear approximation for temperature
>> + *
>> + * [reg] = [temp] * a + b => [temp] = ([reg] - b) / a
>> + *
>> + * The constants a and b are calculated using two triplets of int values
>> PTAT
>> + * and THCODE. PTAT and THCODE can either be read from hardware or use hard
>> + * coded values from driver. The formula to calculate a and b are taken from
>> + * BSP and sparsely documented and understood.
>> + *
>> + * Examining the linear formula and the formula used to calculate constants
>> a
>> + * and b while knowing that the span for PTAT and THCODE values are between
>> + * 0x000 and 0xfff the largest integer possible is 0xfff * 0xfff ==
>> 0xffe001.
>> + * Integer also needs to be signed so that leaves 7 bits for decimal
>> + * fixed point scaling, which amounts to a decimal scaling factor of 100.
>> + */
>> +
>> +#define SCALE_FACTOR 100
>> +#define SCALE_INT(_x) ((_x) * SCALE_FACTOR)
>> +#define SCALE_MUL(_a, _b) (((_a)*(_b)) / SCALE_FACTOR)
>> +#define SCALE_DIV(_a, _b) (((_a)*SCALE_FACTOR)/(_b))
>> +#define SCALE_TO_MCELSIUS(_x) ((_x) * 10)
>
> Spaces around operators everywhere, please.
>
> I wonder about SCALE_MUL; isn't that more like "unscaling" because _a
> and _b are already scaled?
No, it's a standard fixed point multiplication, where you have to compensate
for the double scaling due to the multiplication.
Perhaps the macros should be called e.g. FIXPT_MUL() and FIXPT_DIV()?
> And since _b is always a constant, couldn't we simply drop this macro
> and simply do _a * _b (with _a being scaled already and _b not)?
Yes, for multiplication by an integer (not a fixed point number), you can
just do the multiplication. Which also avoids having to care about rounding.
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- [email protected]
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
