linux-kernel - Re: [PATCH V5 11/18] clk: tegra210: Add support for Tegra210 clocks

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Date:   Thu, 18 Jul 2019 12:24:54 -0700
From:   Sowjanya Komatineni <skomatineni@...dia.com>
To:     Peter De Schrijver <pdeschrijver@...dia.com>,
        Dmitry Osipenko <digetx@...il.com>
CC:     Joseph Lo <josephl@...dia.com>, <thierry.reding@...il.com>,
        <jonathanh@...dia.com>, <tglx@...utronix.de>,
        <jason@...edaemon.net>, <marc.zyngier@....com>,
        <linus.walleij@...aro.org>, <stefan@...er.ch>,
        <mark.rutland@....com>, <pgaikwad@...dia.com>, <sboyd@...nel.org>,
        <linux-clk@...r.kernel.org>, <linux-gpio@...r.kernel.org>,
        <jckuo@...dia.com>, <talho@...dia.com>,
        <linux-tegra@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <mperttunen@...dia.com>, <spatra@...dia.com>, <robh+dt@...nel.org>,
        <devicetree@...r.kernel.org>
Subject: Re: [PATCH V5 11/18] clk: tegra210: Add support for Tegra210 clocks


On 7/18/19 12:18 PM, Peter De Schrijver wrote:
> On Tue, Jul 16, 2019 at 09:43:16PM +0300, Dmitry Osipenko wrote:
>>> CPU parents are PLL_X, PLL_P, and dfll. PLL_X always runs at higher rate
>>> so switching to PLL_P during CPUFreq probe prior to dfll clock enable
>>> should be safe.
>> AFAIK, PLLX could run at ~200MHz. There is also a divided output of PLLP
>> which CCLKG supports, the PLLP_OUT4.
>>
>> Probably, realistically, CPU is always running off a fast PLLX during
>> boot, but I'm wondering what may happen on KEXEC. I guess ideally
>> CPUFreq driver should also have a 'shutdown' callback to teardown DFLL
>> on a reboot, but likely that there are other clock-related problems as
>> well that may break KEXEC and thus it is not very important at the moment.
>>
> If you turn off the DFLL, you have to be aware that the voltage margins
> for DFLL use are lower than for PLL use. So you either need to be sure
> to switch to a frequency below fmax @ Vmin or you program the boot
> voltage and then you can use PLLX as setup by the bootloader. For OVR
> regulators you can't program a voltage without the DFLL, so you have to
> tristate the PWM output which will give you a hardwired boot voltage.
>
> Peter.

Yes, we switch CPU to PLLP and then disable DFLL during suspend.