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authorElaine Zhang <zhangqing@rock-chips.com>2017-08-01 18:22:24 +0200
committerHeiko Stuebner <heiko@sntech.de>2017-08-08 17:45:42 +0200
commit5d890c2df900db0197d46ec75383d7633ef41c82 (patch)
tree2e9e50736f9d15cddd389b1f968655651d9a15db /drivers/clk
parentec52e462564b9c5bfdf1f79638c537c7103e1d2b (diff)
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>
Diffstat (limited to 'drivers/clk')
-rw-r--r--drivers/clk/rockchip/clk.c36
1 files changed, 36 insertions, 0 deletions
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,