linux-kernel - Re: [PATCH 2/2] clk: bcm2835: Minimise clock jitter for PCM clock

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Message-ID: <1412052002.45738.1496171081779@email.1und1.de>
Date:   Tue, 30 May 2017 21:04:41 +0200 (CEST)
From:   Stefan Wahren <stefan.wahren@...e.com>
To:     Phil Elwell <phil@...pberrypi.org>, linux-clk@...r.kernel.org,
        Eric Anholt <eric@...olt.net>, linux-kernel@...r.kernel.org,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...eaurora.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        linux-rpi-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/2] clk: bcm2835: Minimise clock jitter for PCM clock

Hi Phil,

> Phil Elwell <phil@...pberrypi.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@...pberrypi.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