linux-kernel - Re: [PATCH V3 2/4] cpufreq: add support for intermediate (stable) frequencies

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Message-ID: <CAKohpok+WgikxY-YSXfF5DgfMdSqWd-Mvh=Y9B5CtjEB119Jmw@mail.gmail.com>
Date:	Wed, 21 May 2014 09:44:53 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Doug Anderson <dianders@...omium.org>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Arvind Chauhan <arvind.chauhan@....com>,
	Stephen Warren <swarren@...dia.com>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Stephen Warren <swarren@...dotorg.org>,
	Russell King <linux@....linux.org.uk>,
	Thomas Abraham <thomas.abraham@...aro.org>,
	Peter De Schrijver <pdeschrijver@...dia.com>
Subject: Re: [PATCH V3 2/4] cpufreq: add support for intermediate (stable) frequencies

Doug,

On 20 May 2014 22:18, Doug Anderson <dianders@...omium.org> wrote:

> Is it worth documenting that if we implement target_intermediate()
> that target_index() must not fail?  That means that any failure-prone
> things (like setting a regulator) should happen in target_index().

You meant target_intermediate() is the last line, right ?

Yeah, we can add that..

>>  2. Frequency Table Helpers
>> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
>> index 9bf12a2..f38f2f2 100644
>> --- a/drivers/cpufreq/cpufreq.c
>> +++ b/drivers/cpufreq/cpufreq.c
>> @@ -1819,27 +1819,50 @@ EXPORT_SYMBOL(cpufreq_unregister_notifier);
>>  static int __target_index(struct cpufreq_policy *policy,
>>                           struct cpufreq_frequency_table *freq_table, int index)
>>  {
>> -       struct cpufreq_freqs freqs;
>> +       struct cpufreq_freqs freqs = {.old = policy->cur, .flags = 0};
>>         int retval = -EINVAL;
>>         bool notify;
>>
>>         notify = !(cpufreq_driver->flags & CPUFREQ_ASYNC_NOTIFICATION);
>> +       if (!notify)
>> +               goto skip_notify;
>
> I'm personally not a fan of using goto here.  All you're trying to do
> is avoiding a level of indentation?  If it really matters that much
> then create a sub function.  IMHO goto should generally be reserved
> for error handling.

Yeah, I was trying that :) .. Another routine wouldn't be right here
as the POST_NOTIFICATION will be handled in this routine only.

I will try again to see if I can write some better code here, but wouldn't
promise that :)

>> -       if (notify) {
>> -               freqs.old = policy->cur;
>> -               freqs.new = freq_table[index].frequency;
>> -               freqs.flags = 0;
>> +       /* Handle switching to intermediate frequency */
>> +       if (cpufreq_driver->get_intermediate) {
>> +               freqs.new = cpufreq_driver->get_intermediate(policy, index);
>
> Would it be worth it to change this to?
>
> intermediate = 0
> if (cpufreq_driver->get_intermediate)
>   intermediate = cpufreq_driver->get_intermediate();
> if (intermediate)
>
> ...the idea being that a driver may use an intermediate frequency for
> some transitions but not for all.  For instance: on tegra if you
> happen to change to the exact clock frequency of the intermediate PLL
> it just stays there.  There's no need for two notifications in that
> case.  There may be other systems that can optimize some transitions
> to totally skip the intermediate stage (maybe you've got an
> non-glitching divider somewhere so you can optimize a transition from
> 1.4GHz to 700MHz to go w/ no intermediate jump).

Hmm, will try to fix that as well. Looks like a valid point.

>> -               pr_debug("%s: cpu: %d, oldfreq: %u, new freq: %u\n",
>> +               pr_debug("%s: cpu: %d, switching to intermediate freq: oldfreq: %u, intermediate freq: %u\n",
>>                                 __func__, policy->cpu, freqs.old, freqs.new);
>>
>>                 cpufreq_freq_transition_begin(policy, &freqs);
>> +               retval = cpufreq_driver->target_intermediate(policy, freqs.new);
>
> It feels like you want to pass in "index" here too, just in case.  A
> driver may need to make decisions about other clocks based on the
> eventual final frequency.  They could cache it themselves from the
> get_intermediate() call, but that seems ugly.

I had index here initially, but then it looked like they may need to perform
get_intermediate() again from this routine and so sending the intermediate
freq is probably better. So, probably just wait for some drivers which may
need index here ?

>> @@ -2361,7 +2384,8 @@ int cpufreq_register_driver(struct cpufreq_driver *driver_data)
>>             !(driver_data->setpolicy || driver_data->target_index ||
>>                     driver_data->target) ||
>>              (driver_data->setpolicy && (driver_data->target_index ||
>> -                   driver_data->target)))
>> +                   driver_data->target)) ||
>> +            (!!driver_data->get_intermediate ^ !!driver_data->target_intermediate))
>
> I'm OK with the !! trick, but using ^ here seems more confusing.  Why
> not use "!="?
>   (!!driver_data->get_intermediate != !!driver_data->target_intermediate))

Will work as well :)
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