RE: [PATCH] clk: imx: composite-8m: Add imx8m_divider_determine_rate

[Peng Fan]

> Subject: Re: [PATCH] clk: imx: composite-8m: Add
> imx8m_divider_determine_rate
> 
> On Thu, May 11, 2023 at 10:03 PM Adam Ford <aford173@xxxxxxxxx>
> wrote:
> >
> > On Sat, May 6, 2023 at 2:53 PM Adam Ford <aford173@xxxxxxxxx> wrote:
> > >
> > > Currently, certain clocks are derrived as a divider from their
> > > parent clock.  For some clocks, even when CLK_SET_RATE_PARENT is
> > > set, the parent clock is not properly set which can lead to some
> > > relatively inaccurate clock values.
> > >
> > > Unlike imx/clk-composite-93 and imx/clk-divider-gate, it cannot rely
> > > on calling a standard determine_rate function, because the 8m
> > > composite clocks have a pre-divider and post-divider. Because of
> > > this, a custom determine_rate function is necessary to determine the
> > > maximum clock division which is equivalent to pre-divider * the
> > > post-divider.
> > >
> > > With this added, the system can attempt to adjust the parent rate
> > > when the proper flags are set which can lead to a more precise clock
> > > value.
> > >
> > > On the imx8mplus, no clock changes are present.
> > > On the Mini and Nano, this can help achieve more accurate lcdif
> > > clocks. When trying to get a pixel clock of 31.500MHz on an imx8m
> > > Nano, the clocks divided the 594MHz down, but left the parent rate
> > > untouched which caused a calulation error.
> > >
> > > Before:
> > > video_pll              594000000
> > >   video_pll_bypass     594000000
> > >     video_pll_out      594000000
> > >       disp_pixel       31263158
> > >         disp_pixel_clk 31263158
> > >
> > > Variance = -236842 Hz
> > >
> > > After this patch:
> > > video_pll               31500000
> > >   video_pll_bypass      31500000
> > >     video_pll_out       31500000
> > >       disp_pixel        31500000
> > >         disp_pixel_clk  31500000
> > >
> > > Variance = 0 Hz
> > >
> > > All other clocks rates and parent were the same.
> > > Similar results on imx8mm were found.
> > >
> >
> > Peng / Abel,
> >
> > I was curious if either of you might have time to review this attempt
> > at enabling determine_rate on the 8m's.  I tested this on the 8mm,
> > 8mn, and 8mp, and found no regressions.
> 
> Gentle nudge.
> 
> It's been several weeks since the initial post and the DSI driver is now
> available for Mini and Nano, so having this in Mini and Nano will really help
> it sync various video sources.

Sorry, overlooked this patch. Will take a look.

Regards,
Peng.

> 
> thanks,
> 
> adam
> 
> >
> > adam
> > > Fixes: 690dccc4a0bf ("Revert "clk: imx: composite-8m: Add support to
> > > determine_rate"")
> > > Signed-off-by: Adam Ford <aford173@xxxxxxxxx>
> > > ---
> > > V2:  Fix build warning found by build bot and fix prediv_value
> > >      and div_value because the values stored are the divisor - 1,
> > >      so we need to add 1 to the values to be correct.
> > >
> > > diff --git a/drivers/clk/imx/clk-composite-8m.c
> > > b/drivers/clk/imx/clk-composite-8m.c
> > > index cbf0d7955a00..7a6e3ce97133 100644
> > > --- a/drivers/clk/imx/clk-composite-8m.c
> > > +++ b/drivers/clk/imx/clk-composite-8m.c
> > > @@ -119,10 +119,41 @@ static int
> imx8m_clk_composite_divider_set_rate(struct clk_hw *hw,
> > >         return ret;
> > >  }
> > >
> > > +static int imx8m_divider_determine_rate(struct clk_hw *hw,
> > > +                                     struct clk_rate_request *req)
> > > +{
> > > +       struct clk_divider *divider = to_clk_divider(hw);
> > > +       int prediv_value;
> > > +       int div_value;
> > > +
> > > +       /* if read only, just return current value */
> > > +       if (divider->flags & CLK_DIVIDER_READ_ONLY) {
> > > +               u32 val;
> > > +
> > > +               val = readl(divider->reg);
> > > +               prediv_value = val >> divider->shift;
> > > +               prediv_value &= clk_div_mask(divider->width);
> > > +               prediv_value++;
> > > +
> > > +               div_value = val >> PCG_DIV_SHIFT;
> > > +               div_value &= clk_div_mask(PCG_DIV_WIDTH);
> > > +               div_value++;
> > > +
> > > +               return divider_ro_determine_rate(hw, req, divider->table,
> > > +                                                PCG_PREDIV_WIDTH + PCG_DIV_WIDTH,
> > > +                                                divider->flags, prediv_value * div_value);
> > > +       }
> > > +
> > > +       return divider_determine_rate(hw, req, divider->table,
> > > +                                     PCG_PREDIV_WIDTH + PCG_DIV_WIDTH,
> > > +                                     divider->flags); }
> > > +
> > >  static const struct clk_ops imx8m_clk_composite_divider_ops = {
> > >         .recalc_rate = imx8m_clk_composite_divider_recalc_rate,
> > >         .round_rate = imx8m_clk_composite_divider_round_rate,
> > >         .set_rate = imx8m_clk_composite_divider_set_rate,
> > > +       .determine_rate = imx8m_divider_determine_rate,
> > >  };
> > >
> > >  static u8 imx8m_clk_composite_mux_get_parent(struct clk_hw *hw)
> > > --
> > > 2.39.2
> > >