[PATCH] clk: divider: do not propagate rate change request when unnecessary

[Mike Turquette]

Quoting Shawn Guo (2013-06-02 07:20:55)
> If the current rate of parent clock is sufficient to provide child a
> requested rate with a proper divider setting, the rate change request
> should not be propagated.  Instead, changing the divider setting is good
> enough to get child clock run at the requested rate.
> 
> On an imx6q clock configuration illustrated below,
> 
>   ahb --> ipg --> ipg_per
>   132M    66M     66M
> 
> calling clk_set_rate(ipg_per, 22M) with the current
> clk_divider_bestdiv() implementation will result in the rate change up
> to ahb level like the following, because of the unnecessary/incorrect
> rate change propagation.
> 
>   ahb --> ipg --> ipg_per
>   66M     22M     22M
> 
> Fix the problem by trying to see if the requested rate can be achieved
> by simply changing the divider value, and in that case return the
> divider immediately from function clk_divider_bestdiv() as the best
> one, so that all those unnecessary rate change propagation can be saved.
> 
> Reported-by: Anson Huang <b20788 at freescale.com>
> Signed-off-by: Shawn Guo <shawn.guo at linaro.org>

I've taken this into clk-next.  I'm keeping an eye out for any
regressions this change might cause.  Any help testing it would be
appreciated.

Regards,
Mike

> ---
>  drivers/clk/clk-divider.c |   10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/clk/clk-divider.c b/drivers/clk/clk-divider.c
> index 6d96741..6024e60 100644
> --- a/drivers/clk/clk-divider.c
> +++ b/drivers/clk/clk-divider.c
> @@ -150,6 +150,7 @@ static int clk_divider_bestdiv(struct clk_hw *hw, unsigned long rate,
>         struct clk_divider *divider = to_clk_divider(hw);
>         int i, bestdiv = 0;
>         unsigned long parent_rate, best = 0, now, maxdiv;
> +       unsigned long parent_rate_saved = *best_parent_rate;
>  
>         if (!rate)
>                 rate = 1;
> @@ -173,6 +174,15 @@ static int clk_divider_bestdiv(struct clk_hw *hw, unsigned long rate,
>         for (i = 1; i <= maxdiv; i++) {
>                 if (!_is_valid_div(divider, i))
>                         continue;
> +               if (rate * i == parent_rate_saved) {
> +                       /*
> +                        * It's the most ideal case if the requested rate can be
> +                        * divided from parent clock without needing to change
> +                        * parent rate, so return the divider immediately.
> +                        */
> +                       *best_parent_rate = parent_rate_saved;
> +                       return i;
> +               }
>                 parent_rate = __clk_round_rate(__clk_get_parent(hw->clk),
>                                 MULT_ROUND_UP(rate, i));
>                 now = parent_rate / i;
> -- 
> 1.7.9.5