Phil Elwell <p...@raspberrypi.org> writes: > 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.
"worst" the second time
> 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 | 40 +++++++++++++++++++++++++++++++++++-----
> 1 file changed, 35 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/clk/bcm/clk-bcm2835.c b/drivers/clk/bcm/clk-bcm2835.c
> index 81ecd4c..c7ee951 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 +1138,34 @@ 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 int_div = *div & ~CM_DIV_FRAC_MASK;
> +
> + high = bcm2835_clock_rate_from_divisor(clock, *prate,
> + int_div);
> + int_div += CM_DIV_FRAC_MASK + 1;
> + low = bcm2835_clock_rate_from_divisor(clock, *prate,
> + int_div);
> +
> + /*
> + * Return a value which is the maximum deviation
> + * below the ideal rate, for use as a metric.
> + */
> + if ((tmp_rate - low) < (high - tmp_rate))
> + tmp_rate = low;
> + else
> + tmp_rate -= high - tmp_rate;
Simplification suggestion: Remove tmp_rate variable, just assign to
rate_from_divisor result to *avgrate. At the end of the low_jitter
block, just "return *avgrate - max(*avgrate - low, high - *avgrate)".
With that, feel free to add:
Reviewed-by: Eric Anholt <e...@anholt.net>
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