linux-kernel - Re: [PATCH] clk: imx: composite-8m: Add imx8m_divider_determine

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Message-ID: <CAOMZO5AwixYybR1am=V4m1AnTkum+0Wn0xegU_y6Po9Y--G0mA@mail.gmail.com>
Date:   Tue, 6 Jun 2023 15:45:15 -0300
From:   Fabio Estevam <festevam@...il.com>
To:     Adam Ford <aford173@...il.com>
Cc:     linux-clk@...r.kernel.org, aford@...conembedded.com,
        Abel Vesa <abelvesa@...nel.org>, Peng Fan <peng.fan@....com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        NXP Linux Team <linux-imx@....com>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] clk: imx: composite-8m: Add imx8m_divider_determine_rate

On Sat, May 6, 2023 at 4:53 PM Adam Ford <aford173@...il.com> 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.
>
> Fixes: 690dccc4a0bf ("Revert "clk: imx: composite-8m: Add support to determine_rate"")
> Signed-off-by: Adam Ford <aford173@...il.com>

This works fine on my imx8mm-evk, so:

Tested-by: Fabio Estevam <festevam@...il.com>