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Subject: Re: [PATCH v1] clk: tegra20: Add 216 MHz entry for PLL_X
To: Peter De Schrijver <pdeschrijver@nvidia.com>
Cc: Prashant Gaikwad <pgaikwad@nvidia.com>,
        Michael Turquette <mturquette@baylibre.com>,
        Stephen Boyd <sboyd@codeaurora.org>,
        Thierry Reding <thierry.reding@gmail.com>,
        Jonathan Hunter <jonathanh@nvidia.com>, linux-clk@vger.kernel.org,
        linux-tegra@vger.kernel.org, linux-kernel@vger.kernel.org
References: <adc15215591018b4a35a7d64e065f81bb09e214e.1513018125.git.digetx@gmail.com>
 <20171212100200.GW32106@tbergstrom-lnx.Nvidia.com>
 <508cb22d-42cb-d169-dbea-0073d7a4e034@gmail.com>
 <20171212151749.GA29158@tbergstrom-lnx.Nvidia.com>
From: Dmitry Osipenko <digetx@gmail.com>
Message-ID: <1469d692-077a-93c7-8722-1bf86e53f481@gmail.com>
Date: Wed, 13 Dec 2017 00:37:28 +0300
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On 12.12.2017 18:17, Peter De Schrijver wrote:
> On Tue, Dec 12, 2017 at 03:08:08PM +0300, Dmitry Osipenko wrote:
>> On 12.12.2017 13:02, Peter De Schrijver wrote:
>>> On Mon, Dec 11, 2017 at 09:50:09PM +0300, Dmitry Osipenko wrote:
>>>> The cpufreq driver uses 216 MHz as the lowest CPU clock frequency, but
>>>> clock driver doesn't provide that rate, so the requested clock is rounded
>>>> up to 312 MHz. Let's add entry for 216 MHz to match with cpufreq.
>>>>
>>>
>>> This seems odd. If there's no table entry, _calc_rate should kick in and
>>> calculate the parameters for 216MHz. Any idea why this is not happening?
>>
>> Actually, it is happening. Please ignore this patch.
>>
>> If PLL's rate could be calculated, why do we need the predefined tables?
> 
> The algorithm to calculate the PLL parameters is rather crude. It will
> favour undershooting the rate rather than overshooting. This is fine for
> DVFS usecases when you want to avoid a too high clock rate, but not good
> for eg display or memory, where as close as match as possible is needed.
Okay, thank you for the clarification.