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

[Adam Ford]

On Mon, May 22, 2023 at 10:23 PM Adam Ford <aford173@xxxxxxxxx> wrote:
>
> On Mon, May 22, 2023 at 9:33 PM Peng Fan <peng.fan@xxxxxxxxxxx> wrote:
> >
> >
> >
> > On 5/7/2023 3:53 AM, Adam Ford wrote:
> > > Caution: This is an external email. Please take care when clicking links or opening attachments. When in doubt, report the message using the 'Report this email' button
> > >
> > >
> > > 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.
> >
> > Not all clocks has pre/post div both.

Peng,

Any suggestions on how to identify this?  looking at the code that
sets the clock rates, it seemed to me like the code was mostly common
code and it looked like the pre and post div's were basically the
same.  CAn you point me to an example where they're identified with
different values?
> >
> > If CLK_SET_RATE_PARENT not set, would there be any issues for
> > other clocks?
>
> I did a dump of the clk_summary for Mini, Nano and Plus, and I found
> no changes to any clock other than the video_pll, and most of the
> clocks do not have CLK_SET_RATE_PARENT set, so from what I could tell
> it seemed harmless.

What do you need from me to be able to move this forward?

thanks

adam
>
> >
> > Regards,
> > Peng.
> >
> > >
> > > 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.
> > >
> > > 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
> > >