linux-kernel - Re: [PATCH v1 01/17] clk: tegra: Add custom CCLK implementation

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Date:   Tue, 29 Oct 2019 02:48:31 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Peter De Schrijver <pdeschrijver@...dia.com>,
        Stephen Boyd <sboyd@...nel.org>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Prashant Gaikwad <pgaikwad@...dia.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Peter Geis <pgwipeout@...il.com>,
        Nicolas Chauvet <kwizart@...il.com>,
        Marcel Ziswiler <marcel.ziswiler@...adex.com>,
        linux-pm@...r.kernel.org, linux-tegra@...r.kernel.org,
        devicetree@...r.kernel.org, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 01/17] clk: tegra: Add custom CCLK implementation

28.10.2019 17:57, Peter De Schrijver пишет:
> On Wed, Oct 16, 2019 at 12:16:02AM +0300, Dmitry Osipenko wrote:
>> CCLK stands for "CPU Clock", CPU core is running off CCLK. CCLK supports
>> multiple parents and it has internal clock divider which uses clock
>> skipping technique, meaning that CPU's voltage should correspond to the
>> parent clock rate and not CCLK. PLLX is the main CCLK parent that provides
>> clock rates above 1GHz and it has special property such that the CCLK's
>> internal divider is set into bypass mode when PLLX is set as a parent for
>> CCLK.
>>
>> This patch forks generic Super Clock into CCLK implementation which takes
>> into account all CCLK specifics. The proper CCLK implementation is needed
>> by the upcoming Tegra20 CPUFreq driver update that will allow to utilize
>> the generic cpufreq-dt driver by moving intermediate clock handling into
>> the clock driver. Note that technically this all could be squashed into
>> clk-super, but result will be messier.
>>
>> Note that currently all CCLKLP bits are left in the clk-super.c and only
>> CCLKG is supported by clk-tegra-super-cclk. It shouldn't be difficult
>> to move the CCLKLP bits, but CCLKLP is not used by anything in kernel
>> and thus better not to touch it for now.
> 
> ..
> 
>> +	super->reg = reg;
>> +	super->lock = lock;
>> +	super->width = 4;
>> +	super->flags = clk_super_flags;
>> +	super->frac_div.reg = reg + 4;
>> +	super->frac_div.shift = 16;
>> +	super->frac_div.width = 8;
>> +	super->frac_div.frac_width = 1;
>> +	super->frac_div.lock = lock;
>> +	super->frac_div.flags = TEGRA_DIVIDER_SUPER;
>> +	super->div_ops = &tegra_clk_frac_div_ops;
>> +
> 
> This is not right. The super clock divider is not a divider, it's a
> pulse skipper.

For the reference: on #tegra Peter explained to me in a more details
what was meant here. Turned out that T30+ has a real CCLK divider and we
won't use the pulse skipper for T20 nor for T30+, I'll update clk
patches accordingly in the next revision.