clk: divider: fix selection of divider when rounding to closest

[ Upstream commit 26bac95aa88c2b1747808c0b885abe7814c0165d ]

It's an invalid approach to assume that among two divider values
the one nearer the exact divider is the better one.

Assume a parent rate of 1000 Hz, a divider with CLK_DIVIDER_POWER_OF_TWO
and a target rate of 89 Hz. The exact divider is ~ 11.236 so 8 and 16
are the candidates to choose from yielding rates 125 Hz and 62.5 Hz
respectivly. While 8 is nearer to 11.236 than 16 is, the latter is still
the better divider as 62.5 is nearer to 89 than 125 is.

Fixes: 774b514390b1 (clk: divider: Add round to closest divider)
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Michael Turquette <mturquette@linaro.org>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>

diff --git a/drivers/clk/clk-divider.c b/drivers/clk/clk-divider.c
index c0a842b..0c0a865 100644 (file)
--- a/drivers/clk/clk-divider.c
+++ b/drivers/clk/clk-divider.c
@@ -208,6 +208,7 @@ static int _div_round_closest(struct clk_divider *divider,
                unsigned long parent_rate, unsigned long rate)
 {
        int up, down, div;
+       unsigned long up_rate, down_rate;
 
        up = down = div = DIV_ROUND_CLOSEST(parent_rate, rate);
 
@@ -219,7 +220,10 @@ static int _div_round_closest(struct clk_divider *divider,
                down = _round_down_table(divider->table, div);
        }
 
-       return (up - div) <= (div - down) ? up : down;
+       up_rate = DIV_ROUND_UP(parent_rate, up);
+       down_rate = DIV_ROUND_UP(parent_rate, down);
+
+       return (rate - up_rate) <= (down_rate - rate) ? up : down;
 }
 
 static int _div_round(struct clk_divider *divider, unsigned long parent_rate,