[PATCH 2/2] clk: rockchip: add special approximation to fix up fractional clk's jitter

[Elaine Zhang]

On 08/02/2017 12:22 AM, Heiko Stuebner wrote:
> From: Elaine Zhang <zhangqing at rock-chips.com>
>
>>From Rockchips fractional divider description:
>    3.1.9  Fractional divider usage
>    To get specific frequency, clocks of I2S, SPDIF, UARTcan be generated by
>    fractional divider. Generally you must set that denominator is 20 times
>    larger than numerator to generate precise clock frequency. So the
>    fractional divider applies only to generate low frequency clock like
>    I2S, UART.
>
> Therefore add a special approximation function that handles this
> special requirement.
>
> Signed-off-by: Elaine Zhang <zhangqing at rock-chips.com>
> ---
>   drivers/clk/rockchip/clk.c | 36 ++++++++++++++++++++++++++++++++++++
>   1 file changed, 36 insertions(+)
>
> diff --git a/drivers/clk/rockchip/clk.c b/drivers/clk/rockchip/clk.c
> index fe1d393cf678..b6db79a00602 100644
> --- a/drivers/clk/rockchip/clk.c
> +++ b/drivers/clk/rockchip/clk.c
> @@ -29,6 +29,7 @@
>   #include <linux/mfd/syscon.h>
>   #include <linux/regmap.h>
>   #include <linux/reboot.h>
> +#include <linux/rational.h>
>   #include "clk.h"
>
>   /**
> @@ -164,6 +165,40 @@ static int rockchip_clk_frac_notifier_cb(struct notifier_block *nb,
>   	return notifier_from_errno(ret);
>   }
>
> +/**
> + * fractional divider must set that denominator is 20 times larger than
> + * numerator to generate precise clock frequency.
> + */
> +void rockchip_fractional_approximation(struct clk_hw *hw,
> +		unsigned long rate, unsigned long *parent_rate,
> +		unsigned long *m, unsigned long *n)
> +{
> +	struct clk_fractional_divider *fd = to_clk_fd(hw);
> +	unsigned long p_rate, p_parent_rate;
> +	struct clk_hw *p_parent;
> +	unsigned long scale;
> +
> +	p_rate = clk_hw_get_rate(clk_hw_get_parent(hw));
> +	if ((rate * 20 > p_rate) && (p_rate % rate != 0)) {
> +		p_parent = clk_hw_get_parent(clk_hw_get_parent(hw));
> +		p_parent_rate = clk_hw_get_rate(p_parent);
> +		*parent_rate = p_parent_rate;
> +	}
> +
> +	/*
> +	 * Get rate closer to *parent_rate to guarantee there is no overflow
> +	 * for m and n. In the result it will be the nearest rate left shifted
> +	 * by (scale - fd->nwidth) bits.
> +	 */
> +	scale = fls_long(*parent_rate / rate - 1);
> +	if (scale > fd->nwidth)
> +		rate <<= scale - fd->nwidth;
> +
> +	rational_best_approximation(rate, *parent_rate,
> +			GENMASK(fd->mwidth - 1, 0), GENMASK(fd->nwidth - 1, 0),
> +			m, n);
> +}
> +
>   static struct clk *rockchip_clk_register_frac_branch(
>   		struct rockchip_clk_provider *ctx, const char *name,
>   		const char *const *parent_names, u8 num_parents,
> @@ -210,6 +245,7 @@ static struct clk *rockchip_clk_register_frac_branch(
>   	div->nwidth = 16;
>   	div->nmask = GENMASK(div->nwidth - 1, 0) << div->nshift;
>   	div->lock = lock;
> +	div->approximation = rockchip_fractional_approximation;
>   	div_ops = &clk_fractional_divider_ops;
>
>   	clk = clk_register_composite(NULL, name, parent_names, num_parents,
>
Tested-by: Elaine Zhang <zhangqing at rock-chips.com>