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

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

	DIV_ROUND_UP(parent_rate, div)

since commit b11d282dbe (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: b11d282dbe (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>
This commit is contained in:
Uwe Kleine-König 2015-02-21 11:40:23 +01:00 committed by Michael Turquette
parent 2f7bf4af5c
commit da321133b5
1 changed files with 1 additions and 7 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
drivers/clk/clk-divider.c
View File

@ -144,12 +144,6 @@ static unsigned long clk_divider_recalc_rate(struct clk_hw *hw,
divider->flags);
}
/*
* 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)
{
@ -313,7 +307,7 @@ 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(rate, now, best, flags)) {
bestdiv = i;