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Date:   Tue, 23 May 2023 10:32:57 +0800
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Subject: Re: [PATCH] clk: imx: composite-8m: Add imx8m_divider_determine_rate
To:     Adam Ford <aford173@gmail.com>, linux-clk@vger.kernel.org
Cc:     aford@beaconembedded.com, Abel Vesa <abelvesa@kernel.org>,
        Peng Fan <peng.fan@nxp.com>,
        Michael Turquette <mturquette@baylibre.com>,
        Stephen Boyd <sboyd@kernel.org>,
        Shawn Guo <shawnguo@kernel.org>,
        Sascha Hauer <s.hauer@pengutronix.de>,
        Pengutronix Kernel Team <kernel@pengutronix.de>,
        Fabio Estevam <festevam@gmail.com>,
        NXP Linux Team <linux-imx@nxp.com>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@lists.infradead.org>,
        open list <linux-kernel@vger.kernel.org>
References: <20230506195325.876871-1-aford173@gmail.com>
From:   Peng Fan <peng.fan@oss.nxp.com>
In-Reply-To: <20230506195325.876871-1-aford173@gmail.com>
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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.

If CLK_SET_RATE_PARENT not set, would there be any issues for
other clocks?

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@gmail.com>
> ---
> 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
>