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

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Date:   Fri, 25 May 2018 11:41:23 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     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 25.05.2018 11:36, Rafael J. Wysocki wrote:
> 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.

No worries.

As I wrote in the reply to the other patch, the Tegra30 support is now on the
way. These changes will collide a tad with the support integration, so I'll
return to re-considering the changes made in this patchset after Tegra30 support
will land. Thank you very much for your reviews and suggestions, I'll take them
into account in the next iteration.