Messages in this thread | | | Date | Tue, 30 May 2017 21:04:41 +0200 (CEST) | From | Stefan Wahren <> | Subject | Re: [PATCH 2/2] clk: bcm2835: Minimise clock jitter for PCM clock |
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Hi Phil,
> Phil Elwell <phil@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 <phil@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
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