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

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Message-ID: <1d2cbab375b50c0be31780f2d8d7a088@walle.cc>
Date:   Wed, 22 Jun 2022 13:51:21 +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>
Cc:     devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM: dts: lan966x: fix sys_clk frequency

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.

-michael