Messages in this thread | | | From | Eric Anholt <> | Subject | Re: [PATCH v2 2/2] clk: bcm2835: Minimise clock jitter for PCM clock | Date | Wed, 31 May 2017 14:36:05 -0700 |
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Phil Elwell <phil@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@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@anholt.net> [unhandled content-type:application/pgp-signature] | |