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

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Date:   Fri, 15 Jul 2022 20:41:06 +0200
From:   Michael Walle <michael@...le.cc>
To:     Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Kavyasree Kotagiri <kavyasree.kotagiri@...rochip.com>,
        Claudiu.Beznea@...rochip.com
Cc:     devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM: dts: lan966x: fix sys_clk frequency

[+ Claudiu as he seems to pick patches for at91, too]

Am 2022-06-22 13:51, schrieb Michael Walle:
> Am 2022-04-28 10:49, schrieb Michael Walle:
>> Am 2022-03-26 20:40, schrieb Michael Walle:
>>> 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>
>> 
>> Ping :)
>> 
>> Btw. this is also true for the new B0 silicon. I just verified it
>> with the CLK_MON output.
> 
> Ping #2.
> 
> Could this please be picked up because most drivers use this property
> to calculate output frequencies and so on, e.g. the PWM driver.

Ping #3. Now it even got a Reviewed-by.

-michael