linux-kernel - [PATCH 3/3] clk: divider: fix calculation of initial best divider when rounding to closest

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Date:	Sat, 21 Feb 2015 11:40:25 +0100
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Mike Turquette <mturquette@...aro.org>,
	Stephen Boyd <sboyd@...eaurora.org>
Cc:	kernel@...gutronix.de, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Sören Brinkmann <soren.brinkmann@...inx.com>
Subject: [PATCH 3/3] clk: divider: fix calculation of initial best divider when rounding to closest

Similar to the reasoning for the previous commit

	DIV_ROUND_CLOSEST(parent_rate, rate)

might not be the best integer divisor to get a good approximation for
rate from parent_rate (given the metric for CLK_DIVIDER_ROUND_CLOSEST).

For example assume a parent rate of 1000 Hz and a target rate of 700.
Using DIV_ROUND_CLOSEST the suggested divisor gets calculated to 1
resulting in a target rate of 1000 with a delta of 300 to the desired
rate. With choosing 2 as divisor however the resulting rate is 500 which
is nearer to 700.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
---
 drivers/clk/clk-divider.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/drivers/clk/clk-divider.c b/drivers/clk/clk-divider.c
index 66a6211b8834..202a17e9a38b 100644
--- a/drivers/clk/clk-divider.c
+++ b/drivers/clk/clk-divider.c
@@ -201,17 +201,18 @@ static int _div_round_up(struct clk_divider *divider,
 static int _div_round_closest(struct clk_divider *divider,
 		unsigned long parent_rate, unsigned long rate)
 {
-	int up, down, div;
+	int up, down;
 	unsigned long up_rate, down_rate;
 
-	up = down = div = DIV_ROUND_CLOSEST(parent_rate, rate);
+	up = DIV_ROUND_UP(parent_rate, rate);
+	down = parent_rate / rate;
 
 	if (divider->flags & CLK_DIVIDER_POWER_OF_TWO) {
-		up = __roundup_pow_of_two(div);
-		down = __rounddown_pow_of_two(div);
+		up = __roundup_pow_of_two(up);
+		down = __rounddown_pow_of_two(down);
 	} else if (divider->table) {
-		up = _round_up_table(divider->table, div);
-		down = _round_down_table(divider->table, div);
+		up = _round_up_table(divider->table, up);
+		down = _round_down_table(divider->table, down);
 	}
 
 	up_rate = DIV_ROUND_UP(parent_rate, up);
-- 
2.1.4

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