Messages in this thread Patch in this message | | | From | Soren Brinkmann <> | Subject | [PATCH] clk: divider: Use DIV_ROUND_CLOSEST | Date | Tue, 29 Jan 2013 17:25:44 -0800 |
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Use the DIV_ROUND_CLOSEST macro to calculate divider values and minimize rounding errors.
Cc: linux-arm-kernel@lists.infradead.org Cc: linux-kernel@vger.kernel.org Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com> --- Hi,
I just came across this behavior: I'm using the clk-divider as cpuclock which can be scaled through cpufreq. During boot I create an opp table which in this case holds the frequencies [MHz] 666, 333 and 222. To make sure those are actually valid frequencies I call clk_round_rate(). I added a debug line in clk-divider.c:clk_divider_bestdiv() before the return in line 163 giving me: prate:1333333320, rate:333333330, div:4 for adding the 333 MHz operating point.
In the running system this gives me: zynq:/sys/devices/system/cpu/cpu0/cpufreq # cat scaling_available_frequencies 222222 333333 666666 zynq:/sys/devices/system/cpu/cpu0/cpufreq # cat scaling_cur_freq 666666
So far, so good. But now, let's scale the frequency: zynq:/sys/devices/system/cpu/cpu0/cpufreq # echo 333333 > scaling_setspeed zynq:/sys/devices/system/cpu/cpu0/cpufreq # cat scaling_cur_freq 266666
And the corresponding debug line: prate:1333333320, rate:333333000, div:5
So, with DIV_ROUND_UP an actual divider of 4.00000396 becomes 5, resulting in a huge error.
Regards, Sören
drivers/clk/clk-divider.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/drivers/clk/clk-divider.c b/drivers/clk/clk-divider.c index a9204c6..32e1b6a 100644 --- a/drivers/clk/clk-divider.c +++ b/drivers/clk/clk-divider.c @@ -157,7 +157,7 @@ static int clk_divider_bestdiv(struct clk_hw *hw, unsigned long rate, if (!(__clk_get_flags(hw->clk) & CLK_SET_RATE_PARENT)) { parent_rate = *best_parent_rate; - bestdiv = DIV_ROUND_UP(parent_rate, rate); + bestdiv = DIV_ROUND_CLOSEST(parent_rate, rate); bestdiv = bestdiv == 0 ? 1 : bestdiv; bestdiv = bestdiv > maxdiv ? maxdiv : bestdiv; return bestdiv; @@ -207,7 +207,7 @@ 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_CLOSEST(parent_rate, rate); value = _get_val(divider, div); if (value > div_mask(divider)) -- 1.8.1.2
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