Re: [PATCH 4/4] drm/sun4i: dotclock: Round to closest clock rate

[Maxime Ripard]

On Mon, Sep 19, 2016 at 11:36:18PM +0800, Chen-Yu Tsai wrote:
> 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...

Yes, sorry, I didn't remember rounded was actually the rounded parent
rate.

Thanks!
Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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