Hi Phil,
> Phil Elwell <p...@raspberrypi.org> hat am 30. Mai 2017 um 18:28 geschrieben:
>
>
> Fractional clock dividers generate accurate average frequencies but
> with jitter, particularly when the integer divisor is small.
>
> Introduce a new metric of clock accuracy to penalise clocks with a good
> average but worse jitter compared to clocks with an average which is no
> better but with lower jitter. The metric is the ideal rate minus the
> worse deviation from that ideal using the nearest integer divisors.
>
> Use this metric for parent selection for clocks requiring low jitter
> (currently just PCM).
>
> Signed-off-by: Phil Elwell <p...@raspberrypi.org>
> ---
> drivers/clk/bcm/clk-bcm2835.c | 39 ++++++++++++++++++++++++++++++++++-----
> 1 file changed, 34 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/clk/bcm/clk-bcm2835.c b/drivers/clk/bcm/clk-bcm2835.c
> index facc346..e0ce5e7 100644
> --- a/drivers/clk/bcm/clk-bcm2835.c
> +++ b/drivers/clk/bcm/clk-bcm2835.c
> @@ -530,6 +530,7 @@ struct bcm2835_clock_data {
>
> bool is_vpu_clock;
> bool is_mash_clock;
> + bool low_jitter;
>
> u32 tcnt_mux;
> };
> @@ -1124,7 +1125,8 @@ static unsigned long
> bcm2835_clock_choose_div_and_prate(struct clk_hw *hw,
> int parent_idx,
> unsigned long rate,
> u32 *div,
> - unsigned long *prate)
> + unsigned long *prate,
> + unsigned long *avgrate)
> {
> struct bcm2835_clock *clock = bcm2835_clock_from_hw(hw);
> struct bcm2835_cprman *cprman = clock->cprman;
> @@ -1136,11 +1138,33 @@ static unsigned long
> bcm2835_clock_choose_div_and_prate(struct clk_hw *hw,
> parent = clk_hw_get_parent_by_index(hw, parent_idx);
>
> if (!(BIT(parent_idx) & data->set_rate_parent)) {
> + unsigned long tmp_rate;
> +
> *prate = clk_hw_get_rate(parent);
> *div = bcm2835_clock_choose_div(hw, rate, *prate, true);
>
> - return bcm2835_clock_rate_from_divisor(clock, *prate,
> - *div);
> + tmp_rate = bcm2835_clock_rate_from_divisor(clock, *prate, *div);
> + *avgrate = tmp_rate;
> +
> + if (data->low_jitter && (*div & CM_DIV_FRAC_MASK)) {
> + unsigned long high, low;
> + u32 idiv = *div & ~CM_DIV_FRAC_MASK;
> +
> + high = bcm2835_clock_rate_from_divisor(clock, *prate,
> + idiv);
> + idiv += CM_DIV_FRAC_MASK + 1;
> + low = bcm2835_clock_rate_from_divisor(clock, *prate,
> + idiv);
What about int_div or intdiv instead of idiv as variable name?
> +
> + /* Return a value which is the maximum deviation
> + * below the ideal rate, for use as a metric.
> + */
Please fix coding style of the comment.
Thanks
Stefan
