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

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Message-ID: <CAJZ5v0hcvSFfWVCgVU8ApvODhyZqXduaGoBmmh345aGe5MN59g@mail.gmail.com>
Date:   Fri, 25 May 2018 10:36:29 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Dmitry Osipenko <digetx@...il.com>,
        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 Fri, May 25, 2018 at 10:14 AM, Rafael J. Wysocki <rafael@...nel.org> wrote:
> 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.

Actually, on a second thought I agree that it is better to do it as
you suggested: make the policy->freq_table pointer constant
everywhere.

Sorry for the noise.