linux-kernel - Re: [PATCH v1 2/2] cpufreq: tegra20: Use PLL

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Date:   Fri, 25 May 2018 10:14:48 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Dmitry Osipenko <digetx@...il.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Peter De Schrijver <pdeschrijver@...dia.com>,
        linux-tegra@...r.kernel.org, Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 2/2] cpufreq: tegra20: Use PLL_C as intermediate clock source

On Thu, May 24, 2018 at 2:28 PM, Dmitry Osipenko <digetx@...il.com> wrote:
> On 24.05.2018 11:01, Rafael J. Wysocki wrote:
>> On Thu, May 24, 2018 at 7:37 AM, Dmitry Osipenko <digetx@...il.com> wrote:
>>> On 24.05.2018 07:30, Viresh Kumar wrote:
>>>> On 23-05-18, 19:00, Dmitry Osipenko wrote:
>>>>> PLL_C is running at 600MHz which is significantly higher than the 216MHz
>>>>> of the PLL_P and it is known that PLL_C is always-ON because AHB BUS is
>>>>> running on that PLL. Let's use PLL_C as intermediate clock source, making
>>>>> CPU snappier a tad during of the frequency transition.
>>>>>
>>>>> Signed-off-by: Dmitry Osipenko <digetx@...il.com>
>>>>> ---
>>>>>  drivers/cpufreq/tegra20-cpufreq.c | 25 +++++++++++++++++++++----
>>>>>  1 file changed, 21 insertions(+), 4 deletions(-)
>>>>>
>>>>> diff --git a/drivers/cpufreq/tegra20-cpufreq.c b/drivers/cpufreq/tegra20-cpufreq.c
>>>>> index 3ad6bded6efc..4bf5ba7da40b 100644
>>>>> --- a/drivers/cpufreq/tegra20-cpufreq.c
>>>>> +++ b/drivers/cpufreq/tegra20-cpufreq.c
>>>>> @@ -25,12 +25,13 @@
>>>>>  #include <linux/types.h>
>>>>>
>>>>>  #define PLL_P_FREQ  216000
>>>>> +#define PLL_C_FREQ  600000
>>>>>
>>>>>  static struct cpufreq_frequency_table freq_table[] = {
>>>>>      { .frequency = 216000 },
>>>>>      { .frequency = 312000 },
>>>>>      { .frequency = 456000 },
>>>>> -    { .frequency = 608000 },
>>>>> +    { .frequency = 600000 },
>>>>>      { .frequency = 760000 },
>>>>>      { .frequency = 816000 },
>>>>>      { .frequency = 912000 },
>>>>> @@ -44,6 +45,7 @@ struct tegra20_cpufreq {
>>>>>      struct clk *cpu_clk;
>>>>>      struct clk *pll_x_clk;
>>>>>      struct clk *pll_p_clk;
>>>>> +    struct clk *pll_c_clk;
>>>>>      bool pll_x_prepared;
>>>>>  };
>>>>>
>>>>> @@ -58,7 +60,10 @@ static unsigned int tegra_get_intermediate(struct cpufreq_policy *policy,
>>>>>      if (index == 0 || policy->cur == PLL_P_FREQ)
>>>>>              return 0;
>>>>>
>>>>> -    return PLL_P_FREQ;
>>>>> +    if (index == 3 || policy->cur == PLL_C_FREQ)
>>>>> +            return 0;
>>>>
>>>> So we can choose between two different intermediate frequencies ? And
>>>> I didn't like the way magic number 3 is used here. Its prone to errors
>>>> and we better use a macro or something else here.
>>>>
>>>> Like instead of doing index == 3, what about freq_table[index].freq ==
>>>> PLL_C_FREQ ? Same for the previous patch as well.
>>>
>>> The frequency is determined by the parent clock of CCLK (CPU clock), we can
>>> choose between different parents for the CCLK. PLL_C as PLL_P and PLL_X are
>>> among the available parents for the CCLK to choose from and there some others.
>>>
>>> I don't mind to use freq_table[index].freq, though I'd like to keep compiled
>>> assembly minimal where possible. Hence the freq_table should be made constant to
>>> tell compiler that it doesn't need to emit data fetches for the table values and
>>> could embed the constants into the code where appropriate.
>>>
>>> Could we constify the "struct cpufreq_frequency_table" within the cpufreq core?
>>> Seems nothing prevents this (I already tried to constify - there are no
>>> obstacles), unless some cpufreq driver would try to modify
>>> policy->freq_table->... within the cpufreq callback implementation.
>>
>> Some drivers generate frequency tables out of external data
>> unavailable at compile time, like ACPI tables.
>
> Instead of making the table constant itself (with its values), seems we can just
> make the policy->freq_table pointer constant. I'll try to make a patch for that,
> adjusting the pointers in cpufreq core and the drivers. This works for the
> acpi-cpufreq at least.

Honestly, messing up with the whole subsystem in order to avoid an
explicit pointer case doesn't sound right to me.

>
>> But if you know it for the fact that the core doesn't modify the
>> frequency table, you could pass a constant table from the driver to
>> it, can't you?
>>
>
> Yes, but that will require to explicitly silencing the compiler warning about
> const -> non-const pointer conversion (if you're meaning this pointer
> conversion), which generally should be avoided.

Why?