Hi Niklas,
On Tue, Dec 13, 2016 at 10:53 AM, Niklas Söderlund
<[email protected]> wrote:
> On 2016-12-13 09:19:53 +0100, Geert Uytterhoeven wrote:
>> On Mon, Dec 12, 2016 at 3:18 PM, Niklas Söderlund
>> <[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
>>
>> What about using 128 instead?
>> Fixed point is much easier with shifts
>> (the compiler will turn multiplications in shifts where appropriate).
>
> I tried using binary scaled instead of decimal scaled fixed point but
> noticed that the diff I got compared to the original formula which used
> decimal scaled (factor 1000) where larger so I picked decimal scaling.
Interesting...
>> > +#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))
>>
>> DIV_ROUND_CLOSEST()
>
> Don't DIV_ROUND_CLOSEST() require the divisor to be a positive integer?
Right.
> tj_2 = SCALE_DIV(SCALE_MUL(SCALE_INT(ptat[1] - ptat[2]), SCALE_INT(137)),
> SCALE_INT(ptat[0] - ptat[2])) - SCALE_INT(41);
>
> In this case if ptat[0] < (ptat[2] + 41) the divisor is negative so I
> think DIV_ROUND_CLOSEST() can't be used, or am I misunderstanding?
Then we need our own macro that can handle that. Or enhance
DIV_ROUND_CLOSEST().
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
