clk: divider: fix calculation of maximal parent rate for a given divider

[ Upstream commit da321133b53caf7889ed3ca1dabe4cc368db2604 ]

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@pengutronix.de>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Michael Turquette <mturquette@linaro.org>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>

1 files changed, 1 insertions, 7 deletions

diff --git a/drivers/clk/clk-divider.c b/drivers/clk/clk-divider.c
index 0c0a865c1266..a52154caf526 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)
 {
@@ -304,7 +298,7 @@ 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;