linux-kernel - RE: [PATCH] ARM: dts: lan966x: fix sys

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Message-ID: <SA2PR11MB4874148E216CF66072104AC292BB9@SA2PR11MB4874.namprd11.prod.outlook.com>
Date:   Wed, 29 Jun 2022 11:40:14 +0000
From:   <Kavyasree.Kotagiri@...rochip.com>
To:     <michael@...le.cc>, <robh+dt@...nel.org>, <krzk+dt@...nel.org>,
        <Nicolas.Ferre@...rochip.com>
CC:     <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] ARM: dts: lan966x: fix sys_clk frequency

> The sys_clk frequency is 165.625MHz. The register reference of the
> Generic Clock controller lists the CPU clock as 600MHz, the DDR clock as
> 300MHz and the SYS clock as 162.5MHz. This is wrong. It was first
> noticed during the fan driver development and it was measured and
> verified via the CLK_MON output of the SoC which can be configured to
> output sys_clk/64.
> 
> The core PLL settings (which drives the SYS clock) seems to be as
> follows:
>   DIVF = 52
>   DIVQ = 3
>   DIVR = 1
> 
> With a refernce clock of 25MHz, this means we have a post divider clock
>   Fpfd = Fref / (DIVR + 1) = 25MHz / (1 + 1) = 12.5MHz
> 
> The resulting VCO frequency is then
>   Fvco = Fpfd * (DIVF + 1) * 2 = 12.5MHz * (52 + 1) * 2 = 1325MHz
> 
> And the output frequency is
>   Fout = Fvco / 2^DIVQ = 1325MHz / 2^3 = 165.625Mhz
> 
> This all adds up to the constrains of the PLL:
>     10MHz <= Fpfd <= 200MHz
>     20MHz <= Fout <= 1000MHz
>   1000MHz <= Fvco <= 2000MHz
> 
> Fixes: 290deaa10c50 ("ARM: dts: add DT for lan966 SoC and 2-port board
> pcb8291")
> Signed-off-by: Michael Walle <michael@...le.cc>

Reviewed-by: Kavyasree Kotagiri <kavyasree.kotagiri@...rochip.com>

> ---
>  arch/arm/boot/dts/lan966x.dtsi | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm/boot/dts/lan966x.dtsi
> b/arch/arm/boot/dts/lan966x.dtsi
> index 14c9cb3c0f3b..03045ec4aca4 100644
> --- a/arch/arm/boot/dts/lan966x.dtsi
> +++ b/arch/arm/boot/dts/lan966x.dtsi
> @@ -38,7 +38,7 @@ clocks {
>                 sys_clk: sys_clk {
>                         compatible = "fixed-clock";
>                         #clock-cells = <0>;
> -                       clock-frequency = <162500000>;
> +                       clock-frequency = <165625000>;
>                 };
> 
>                 cpu_clk: cpu_clk {
> --
> 2.30.2