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Subject[PATCH v3 3/3] clk: fractional-divider: switch to rational best approximation
Date
This patch converts the code to use rational best approximation algorithm which
is more precise.

Suggested-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
---
drivers/clk/Kconfig | 1 +
drivers/clk/clk-fractional-divider.c | 38 +++++++++++++++++++++---------------
2 files changed, 23 insertions(+), 16 deletions(-)

diff --git a/drivers/clk/Kconfig b/drivers/clk/Kconfig
index 0b474a0..3a1b1dc 100644
--- a/drivers/clk/Kconfig
+++ b/drivers/clk/Kconfig
@@ -14,6 +14,7 @@ config COMMON_CLK
select HAVE_CLK_PREPARE
select CLKDEV_LOOKUP
select SRCU
+ select RATIONAL
---help---
The common clock framework is a single definition of struct
clk, useful across many platforms, as well as an
diff --git a/drivers/clk/clk-fractional-divider.c b/drivers/clk/clk-fractional-divider.c
index 23a56a0..f060be8 100644
--- a/drivers/clk/clk-fractional-divider.c
+++ b/drivers/clk/clk-fractional-divider.c
@@ -7,13 +7,14 @@
*
* Adjustable fractional divider clock implementation.
* Output rate = (m / n) * parent_rate.
+ * Uses rational best approximation algorithm.
*/

#include <linux/clk-provider.h>
#include <linux/module.h>
#include <linux/device.h>
#include <linux/slab.h>
-#include <linux/gcd.h>
+#include <linux/rational.h>

#define to_clk_fd(_hw) container_of(_hw, struct clk_fractional_divider, hw)

@@ -47,23 +48,29 @@ static unsigned long clk_fd_recalc_rate(struct clk_hw *hw,
}

static long clk_fd_round_rate(struct clk_hw *hw, unsigned long rate,
- unsigned long *prate)
+ unsigned long *parent_rate)
{
struct clk_fractional_divider *fd = to_clk_fd(hw);
- unsigned maxn = clk_div_mask(fd->nwidth) + 1;
- unsigned div;
+ unsigned long scale;
+ unsigned long m, n;
+ u64 ret;

- if (!rate || rate >= *prate)
- return *prate;
+ if (!rate || rate >= *parent_rate)
+ return *parent_rate;

- div = gcd(*prate, rate);
+ /* Get rate closer to *parent_rate */
+ scale = fls_long(*parent_rate / rate - 1);
+ if (scale > fd->nwidth)
+ rate <<= scale - fd->nwidth;

- while ((*prate / div) > maxn) {
- div <<= 1;
- rate <<= 1;
- }
+ rational_best_approximation(rate, *parent_rate,
+ clk_div_mask(fd->mwidth), clk_div_mask(fd->nwidth),
+ &m, &n);

- return rate;
+ ret = (u64)*parent_rate * m;
+ do_div(ret, n);
+
+ return ret;
}

static void clk_fd_update(struct clk_fractional_divider *fd,
@@ -82,12 +89,11 @@ static int clk_fd_set_rate(struct clk_hw *hw, unsigned long rate,
{
struct clk_fractional_divider *fd = to_clk_fd(hw);
unsigned long flags;
- unsigned long div;
unsigned long m, n;

- div = gcd(parent_rate, rate);
- m = rate / div;
- n = parent_rate / div;
+ rational_best_approximation(rate, parent_rate,
+ clk_div_mask(fd->mwidth), clk_div_mask(fd->nwidth),
+ &m, &n);

if (fd->lock) {
spin_lock_irqsave(fd->lock, flags);
--
2.1.4


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