Re: [PATCH] shift and clocksource

[Sascha Hauer <s.hauer@pengutronix.de>]

On Tue, Jun 21, 2011 at 12:08:00AM +0200, Carlo Caione wrote:
> Added clocks_calc_mult_shift()
> 
> Signed-off-by: Carlo Caione <carlo.caione@gmail.com>
> ---
> common/clock.c |   55 +++++++++++++++++++++++++++++++++++++++++++++++++++++++

You should also add this to include/clock.h. Otherwise the patch looks
good.

Sascha

> 1 files changed, 55 insertions(+), 0 deletions(-)
> 
> diff --git a/common/clock.c b/common/clock.c
> index 15df0ab..79c06c8 100644
> --- a/common/clock.c
> +++ b/common/clock.c
> @@ -57,6 +57,61 @@ uint64_t get_time_ns(void)
> EXPORT_SYMBOL(get_time_ns);
> 
> /**
> + * clocks_calc_mult_shift - calculate mult/shift factors for scaled math of clocks
> + * @mult:       pointer to mult variable
> + * @shift:      pointer to shift variable
> + * @from:       frequency to convert from
> + * @to:         frequency to convert to
> + * @maxsec:     guaranteed runtime conversion range in seconds
> + *
> + * The function evaluates the shift/mult pair for the scaled math
> + * operations of clocksources and clockevents.
> + *
> + * @to and @from are frequency values in HZ. For clock sources @to is
> + * NSEC_PER_SEC == 1GHz and @from is the counter frequency. For clock
> + * event @to is the counter frequency and @from is NSEC_PER_SEC.
> + *
> + * The @maxsec conversion range argument controls the time frame in
> + * seconds which must be covered by the runtime conversion with the
> + * calculated mult and shift factors. This guarantees that no 64bit
> + * overflow happens when the input value of the conversion is
> + * multiplied with the calculated mult factor. Larger ranges may
> + * reduce the conversion accuracy by chosing smaller mult and shift
> + * factors.
> + */
> +
> +void clocks_calc_mult_shift(uint32_t *mult, uint32_t *shift, uint32_t from, uint32_t to, uint32_t maxsec)
> +{
> +        uint64_t tmp;
> +        uint32_t sft, sftacc = 32;
> +
> +        /*
> +         * Calculate the shift factor which is limiting the conversion
> +         * range:
> +         */
> +        tmp = ((uint64_t)maxsec * from) >> 32;
> +        while (tmp) {
> +                tmp >>=1;
> +                sftacc--;
> +        }
> +
> +        /*
> +         * Find the conversion shift/mult pair which has the best
> +         * accuracy and fits the maxsec conversion range:
> +         */
> +        for (sft = 32; sft > 0; sft--) {
> +                tmp = (uint64_t) to << sft;
> +                tmp += from / 2;
> +                do_div(tmp, from);
> +                if ((tmp >> sftacc) == 0)
> +                        break;
> +        }
> +        *mult = tmp;
> +        *shift = sft;
> +}
> +
> +
> +/**
>  * clocksource_hz2mult - calculates mult from hz and shift
>  * @hz:                 Clocksource frequency in Hz
>  * @shift_constant:     Clocksource shift factor
> -- 
> 1.7.5.2
> 
> 

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