[PATCH v3] clk: fractional-divider: fix up the fractional clk's jitter

[Elaine Zhang]

Fractional dividers may have special requirements concerning numerator
and denominator selection that differ from just getting the best
approximation.

For example on Rockchip socs the denominator must be at least 20 times
larger than the numerator to generate precise clock frequencies.

Therefore add the ability to provide custom approximation functions,
approx = rockchip_fractional_special_approx;
but approx = NULL in the default case.

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.

Signed-off-by: Elaine Zhang <zhangqing at rock-chips.com>
---
 drivers/clk/clk-fractional-divider.c |  4 ++++
 drivers/clk/rockchip/clk.c           | 20 ++++++++++++++++++++
 include/linux/clk-provider.h         |  3 +++
 3 files changed, 27 insertions(+)

diff --git a/drivers/clk/clk-fractional-divider.c b/drivers/clk/clk-fractional-divider.c
index aab904618eb6..ab2bc229ef2f 100644
--- a/drivers/clk/clk-fractional-divider.c
+++ b/drivers/clk/clk-fractional-divider.c
@@ -60,6 +60,9 @@ static long clk_fd_round_rate(struct clk_hw *hw, unsigned long rate,
 	if (!rate || rate >= *parent_rate)
 		return *parent_rate;
 
+	if (fd->approx)
+		fd->approx(hw, rate, 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
@@ -145,6 +148,7 @@ struct clk_hw *clk_hw_register_fractional_divider(struct device *dev,
 	fd->nmask = GENMASK(nwidth - 1, 0) << nshift;
 	fd->flags = clk_divider_flags;
 	fd->lock = lock;
+	fd->approx = NULL;
 	fd->hw.init = &init;
 
 	hw = &fd->hw;
diff --git a/drivers/clk/rockchip/clk.c b/drivers/clk/rockchip/clk.c
index fe1d393cf678..8b21c5b19107 100644
--- a/drivers/clk/rockchip/clk.c
+++ b/drivers/clk/rockchip/clk.c
@@ -164,6 +164,25 @@ 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_special_approx(struct clk_hw *hw,
+					unsigned long rate,
+					unsigned long *parent_rate)
+{
+	struct clk_hw *p_parent;
+	unsigned long p_rate, p_parent_rate;
+
+	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;
+	}
+}
+
 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 +229,7 @@ static struct clk *rockchip_clk_register_frac_branch(
 	div->nwidth = 16;
 	div->nmask = GENMASK(div->nwidth - 1, 0) << div->nshift;
 	div->lock = lock;
+	div->approx = rockchip_fractional_special_approx;
 	div_ops = &clk_fractional_divider_ops;
 
 	clk = clk_register_composite(NULL, name, parent_names, num_parents,
diff --git a/include/linux/clk-provider.h b/include/linux/clk-provider.h
index a428aec36ace..807262375292 100644
--- a/include/linux/clk-provider.h
+++ b/include/linux/clk-provider.h
@@ -564,6 +564,9 @@ struct clk_fractional_divider {
 	u8		nwidth;
 	u32		nmask;
 	u8		flags;
+	void            (*approx)(struct clk_hw *hw,
+				  unsigned long rate,
+				  unsigned long *parent_rate);
 	spinlock_t	*lock;
 };
 
-- 
1.9.1