On Mon, Sep 19, 2016 at 3:16 AM, Maxime Ripard
<maxime.ripard@xxxxxxxxxxxxxxxxxx> wrote:
> Hi,
>
> On Thu, Sep 15, 2016 at 11:14:02PM +0800, Chen-Yu Tsai wrote:
>> With display pixel clocks we want to have the closest possible clock
>> rate, to minimize timing and refresh rate skews. Whether the actual
>> clock rate is higher or lower than the requested rate is less important.
>>
>> Also check candidates against the requested rate, rather than the
>> ideal parent rate, the varying dividers also influence the difference
>> between the requested rate and the rounded rate.
>>
>> Signed-off-by: Chen-Yu Tsai <wens@xxxxxxxx>
>> ---
>> drivers/gpu/drm/sun4i/sun4i_dotclock.c | 3 ++-
>> 1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/sun4i/sun4i_dotclock.c b/drivers/gpu/drm/sun4i/sun4i_dotclock.c
>> index 3eb99784f371..d401156490f3 100644
>> --- a/drivers/gpu/drm/sun4i/sun4i_dotclock.c
>> +++ b/drivers/gpu/drm/sun4i/sun4i_dotclock.c
>> @@ -90,7 +90,8 @@ static long sun4i_dclk_round_rate(struct clk_hw *hw, unsigned long rate,
>> goto out;
>> }
>>
>> - if ((rounded < ideal) && (rounded > best_parent)) {
>> + if (abs(rate - rounded / i) <
>> + abs(rate - best_parent / best_div)) {
>
> I'm not sure what you're trying to do here. Why is the divider involved?
Say you want the dotclock at X, so you try Y = 6 ~ 127 for the divider.
Now you're asking the CCF to round (X*Y).
In the original code, you were comparing the result of rounding (X * Y).
if ((rounded < ideal) && (rounded > best_parent)) {
best_parent = rounded;
best_div = i;
}
where ideal = X * Y (i in the code). Given the divider increases in
the loop, you are actually not closing in on the best divider, but the
highest divider that doesn't give a higher rate than the ideal rate.
Including the divider makes it compare the actual dot clock frequency
if a given divider was used.
Does this makes sense? Explaining this kind of makes my head spin...
Regards
ChenYu
>
> Maxime
>
> --
> Maxime Ripard, Free Electrons
> Embedded Linux and Kernel engineering
> http://free-electrons.com
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