[PATCH v2] clk: fractional-divider: fix up the fractional clk's jitter

[Elaine Zhang <zhangqing@rock-chips.com>]

add clk_fractional_divider_special_ops for rockchip specific requirements,
fractional divider must set that denominator is 20 times larger than
numerator to generate precise clock frequency.
Otherwise the CLK jitter is very big, poor quality of the clock signal.

RK document 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.igned-off-by: Elaine Zhang <zhangqing@rock-chips.com>

Signed-off-by: Elaine Zhang <zhangqing@rock-chips.com>
---
 drivers/clk/clk-fractional-divider.c | 32 ++++++++++++++++++++++++++++++++
 drivers/clk/rockchip/clk.c           |  2 +-
 include/linux/clk-provider.h         |  1 +
 3 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/drivers/clk/clk-fractional-divider.c b/drivers/clk/clk-fractional-divider.c
index aab904618eb6..3107b33327f9 100644
--- a/drivers/clk/clk-fractional-divider.c
+++ b/drivers/clk/clk-fractional-divider.c
@@ -158,6 +158,38 @@ struct clk_hw *clk_hw_register_fractional_divider(struct device *dev,
 }
 EXPORT_SYMBOL_GPL(clk_hw_register_fractional_divider);
 
+static long clk_fd_round_rate_special(struct clk_hw *hw, unsigned long rate,
+				      unsigned long *parent_rate)
+{
+	struct clk_hw *p_parent;
+	unsigned long p_rate, p_parent_rate;
+
+	if (!rate || rate >= *parent_rate)
+		return *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.
+	 * fractional divider must set that denominator is 20 times larger than
+	 * numerator to generate precise clock frequency.
+	 */
+	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;
+	}
+	return clk_fd_round_rate(hw, rate, parent_rate);
+}
+
+const struct clk_ops clk_fractional_divider_special_ops = {
+	.recalc_rate = clk_fd_recalc_rate,
+	.round_rate = clk_fd_round_rate_special,
+	.set_rate = clk_fd_set_rate,
+};
+EXPORT_SYMBOL_GPL(clk_fractional_divider_special_ops);
+
 struct clk *clk_register_fractional_divider(struct device *dev,
 		const char *name, const char *parent_name, unsigned long flags,
 		void __iomem *reg, u8 mshift, u8 mwidth, u8 nshift, u8 nwidth,
diff --git a/drivers/clk/rockchip/clk.c b/drivers/clk/rockchip/clk.c
index fe1d393cf678..9eac9a579d18 100644
--- a/drivers/clk/rockchip/clk.c
+++ b/drivers/clk/rockchip/clk.c
@@ -210,7 +210,7 @@ static struct clk *rockchip_clk_register_frac_branch(
 	div->nwidth = 16;
 	div->nmask = GENMASK(div->nwidth - 1, 0) << div->nshift;
 	div->lock = lock;
-	div_ops = &clk_fractional_divider_ops;
+	div_ops = &clk_fractional_divider_special_ops;
 
 	clk = clk_register_composite(NULL, name, parent_names, num_parents,
 				     NULL, NULL,
diff --git a/include/linux/clk-provider.h b/include/linux/clk-provider.h
index a428aec36ace..3f5f36973df6 100644
--- a/include/linux/clk-provider.h
+++ b/include/linux/clk-provider.h
@@ -570,6 +570,7 @@ struct clk_fractional_divider {
 #define to_clk_fd(_hw) container_of(_hw, struct clk_fractional_divider, hw)
 
 extern const struct clk_ops clk_fractional_divider_ops;
+extern const struct clk_ops clk_fractional_divider_special_ops;
 struct clk *clk_register_fractional_divider(struct device *dev,
 		const char *name, const char *parent_name, unsigned long flags,
 		void __iomem *reg, u8 mshift, u8 mwidth, u8 nshift, u8 nwidth,
-- 
1.9.1