clk: divider: fix rate calculation for fractional rates

clk-divider.c does not calculate the rates consistently at the moment.

As an example, on OMAP3 we have a clock divider with a source clock of
864000000 Hz. With dividers 6, 7 and 8 the theoretical rates are:

6: 144000000
7: 123428571.428571...
8: 108000000

Calling clk_round_rate() with the rate in the first column will give the
rate in the second column:

144000000 -> 144000000
143999999 -> 123428571
123428572 -> 123428571
123428571 -> 108000000

Note how clk_round_rate() returns 123428571 for rates from 123428572 to
143999999, which is mathematically correct, but when clk_round_rate() is
called with 123428571, the returned value is surprisingly 108000000.

This means that the following code works a bit oddly:

rate = clk_round_rate(clk, 123428572);
clk_set_rate(clk, rate);

As clk_set_rate() also does clock rate rounding, the result is that the
clock is set to the rate of 108000000, not 123428571 returned by the
clk_round_rate.

This patch changes the clk-divider.c to use DIV_ROUND_UP when
calculating the rate. This gives the following behavior which fixes the
inconsistency:

144000000 -> 144000000
143999999 -> 123428572
123428572 -> 123428572
123428571 -> 108000000

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>

diff --git a/drivers/clk/clk-divider.c b/drivers/clk/clk-divider.c
index 5543b7d..ec22112 100644 (file)
--- a/drivers/clk/clk-divider.c
+++ b/drivers/clk/clk-divider.c
@@ -24,7 +24,7 @@
  * Traits of this clock:
  * prepare - clk_prepare only ensures that parents are prepared
  * enable - clk_enable only ensures that parents are enabled
- * rate - rate is adjustable.  clk->rate = parent->rate / divisor
+ * rate - rate is adjustable.  clk->rate = DIV_ROUND_UP(parent->rate / divisor)
  * parent - fixed parent.  No clk_set_parent support
  */
 
@@ -115,7 +115,7 @@ static unsigned long clk_divider_recalc_rate(struct clk_hw *hw,
                return parent_rate;
        }
 
-       return parent_rate / div;
+       return DIV_ROUND_UP(parent_rate, div);
 }
 
 /*
@@ -185,7 +185,7 @@ static int clk_divider_bestdiv(struct clk_hw *hw, unsigned long rate,
                }
                parent_rate = __clk_round_rate(__clk_get_parent(hw->clk),
                                MULT_ROUND_UP(rate, i));
-               now = parent_rate / i;
+               now = DIV_ROUND_UP(parent_rate, i);
                if (now <= rate && now > best) {
                        bestdiv = i;
                        best = now;
@@ -207,7 +207,7 @@ static long clk_divider_round_rate(struct clk_hw *hw, unsigned long rate,
        int div;
        div = clk_divider_bestdiv(hw, rate, prate);
 
-       return *prate / div;
+       return DIV_ROUND_UP(*prate, div);
 }
 
 static int clk_divider_set_rate(struct clk_hw *hw, unsigned long rate,
@@ -218,7 +218,7 @@ static int clk_divider_set_rate(struct clk_hw *hw, unsigned long rate,
        unsigned long flags = 0;
        u32 val;
 
-       div = parent_rate / rate;
+       div = DIV_ROUND_UP(parent_rate, rate);
        value = _get_val(divider, div);
 
        if (value > div_mask(divider))