[PATCH v2 2/2] clk: bcm2835: Minimise clock jitter for PCM clock

[Eric Anholt]

Phil Elwell <phil at 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 <phil at 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 <eric at anholt.net>
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