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

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Message-ID: <5f9364c0-4ef5-0883-583d-42da8f016a80@gmail.com>
Date:   Thu, 24 May 2018 15:28:00 +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 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.

> 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.