clk: rockchip: add special approximation to fix up fractional clk's jitter

>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@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

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,