ping?
Any opinions?
Thanks,
Sören
On Tue, Jan 29, 2013 at 05:25:44PM -0800, Soren Brinkmann wrote:
> Use the DIV_ROUND_CLOSEST macro to calculate divider values and minimize
> rounding errors.
>
> Cc: linux-arm-ker...@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Soren Brinkmann <soren.brinkm...@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
>
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majord...@vger.kernel.org
> More majordomo info at http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at http://www.tux.org/lkml/
>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
