linux-kernel - [PATCH 1/3] clk: divider: fix calculation of maximal parent rate for a given divider

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Date:	Sat, 21 Feb 2015 11:40:23 +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 1/3] clk: divider: fix calculation of maximal parent rate for a given divider

The rate provided at the output of a clk-divider is calculated as:

	DIV_ROUND_UP(parent_rate, div)

since commit b11d282dbea2 (clk: divider: fix rate calculation for
fractional rates). So to yield a rate not bigger than r parent_rate
must be <= r * div.

The effect of choosing a parent rate that is too big as was done before
this patch results in wrongly ruling out good dividers.

Note that this is not a complete fix as __clk_round_rate might return a
value >= its 2nd parameter. Also for dividers with
CLK_DIVIDER_ROUND_CLOSEST set the calculation is not accurate. But this
fixes the test case by Sascha Hauer that uses a chain of three dividers
under a fixed clock.

Fixes: b11d282dbea2 (clk: divider: fix rate calculation for fractional rates)
Suggested-by: Sascha Hauer <s.hauer@...gutronix.de>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
---
 drivers/clk/clk-divider.c | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/drivers/clk/clk-divider.c b/drivers/clk/clk-divider.c
index 531d5a48e69f..fbd9e8270791 100644
--- a/drivers/clk/clk-divider.c
+++ b/drivers/clk/clk-divider.c
@@ -129,12 +129,6 @@ static unsigned long clk_divider_recalc_rate(struct clk_hw *hw,
 	return DIV_ROUND_UP(parent_rate, div);
 }
 
-/*
- * The reverse of DIV_ROUND_UP: The maximum number which
- * divided by m is r
- */
-#define MULT_ROUND_UP(r, m) ((r) * (m) + (m) - 1)
-
 static bool _is_valid_table_div(const struct clk_div_table *table,
 							 unsigned int div)
 {
@@ -300,7 +294,7 @@ static int clk_divider_bestdiv(struct clk_hw *hw, unsigned long rate,
 			return i;
 		}
 		parent_rate = __clk_round_rate(__clk_get_parent(hw->clk),
-				MULT_ROUND_UP(rate, i));
+					       rate * i);
 		now = DIV_ROUND_UP(parent_rate, i);
 		if (_is_best_div(divider, rate, now, best)) {
 			bestdiv = i;
-- 
2.1.4

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