linux-kernel - [PATCH v1 1/1] clk: fractional-divider: eliminate while-loop

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Date:	Thu, 19 Mar 2015 20:54:20 +0200
From:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:	linux-kernel@...r.kernel.org, Stephen Boyd <sboyd@...eaurora.org>,
	Heikki Krogerus <heikki.krogerus@...ux.intel.com>
Cc:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: [PATCH v1 1/1] clk: fractional-divider: eliminate while-loop

We may do a calculation of the rounded rate based on (*prate / div) value in
case it is not 0. It's as much higher as power of two of nearest value to
(*prate / div / maxn). Thus, the patch replaces while-loop by direct
calculations.

While here, fix off-by-one error. maxn is the maximum value that can be hold by
a register which means all ones in it.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
---
 drivers/clk/clk-fractional-divider.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/drivers/clk/clk-fractional-divider.c b/drivers/clk/clk-fractional-divider.c
index 6aa72d9..4c0541b 100644
--- a/drivers/clk/clk-fractional-divider.c
+++ b/drivers/clk/clk-fractional-divider.c
@@ -14,6 +14,7 @@
 #include <linux/device.h>
 #include <linux/slab.h>
 #include <linux/gcd.h>
+#include <linux/log2.h>
 
 #define to_clk_fd(_hw) container_of(_hw, struct clk_fractional_divider, hw)
 
@@ -49,18 +50,16 @@ static long clk_fd_round_rate(struct clk_hw *hw, unsigned long rate,
 			      unsigned long *prate)
 {
 	struct clk_fractional_divider *fd = to_clk_fd(hw);
-	unsigned maxn = (fd->nmask >> fd->nshift) + 1;
-	unsigned div;
+	unsigned maxn = fd->nmask >> fd->nshift;
+	unsigned long div, n;
 
 	if (!rate || rate >= *prate)
 		return *prate;
 
 	div = gcd(*prate, rate);
-
-	while ((*prate / div) > maxn) {
-		div <<= 1;
-		rate <<= 1;
-	}
+	n = *prate / div;
+	if (n > maxn)
+		return rate * roundup_pow_of_two(n / maxn);
 
 	return rate;
 }
-- 
2.1.4

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