linux-kernel - Re: [PATCH v1] cpufreq: CPPC: Fix unused-function warning

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Message-ID: <2b2df076-f8fe-10f7-9bc4-3e1f3c1639d0@arm.com>
Date:   Mon, 30 May 2022 11:42:49 +0200
From:   Pierre Gondois <pierre.gondois@....com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     linux-kernel@...r.kernel.org, Ionela.Voinescu@....com,
        Dietmar.Eggemann@....com,
        Shaokun Zhang <zhangshaokun@...ilicon.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>, linux-pm@...r.kernel.org
Subject: Re: [PATCH v1] cpufreq: CPPC: Fix unused-function warning



On 5/30/22 11:07, Viresh Kumar wrote:
> On 30-05-22, 10:44, Pierre Gondois wrote:
>>
>>
>> On 5/30/22 10:20, Viresh Kumar wrote:
>>> On 30-05-22, 10:12, Pierre Gondois wrote:
>>>> Building the cppc_cpufreq driver with for arm64 with
>>>> CONFIG_ENERGY_MODEL=n triggers the following warnings:
>>>>    drivers/cpufreq/cppc_cpufreq.c:550:12: error: ‘cppc_get_cpu_cost’ defined but not used
>>>> [-Werror=unused-function]
>>>>      550 | static int cppc_get_cpu_cost(struct device *cpu_dev, unsigned long KHz,
>>>>          |            ^~~~~~~~~~~~~~~~~
>>>>    drivers/cpufreq/cppc_cpufreq.c:481:12: error: ‘cppc_get_cpu_power’ defined but not used
>>>> [-Werror=unused-function]
>>>>      481 | static int cppc_get_cpu_power(struct device *cpu_dev,
>>>>          |            ^~~~~~~~~~~~~~~~~~
>>>>
>>>> Fixes: 740fcdc2c20e ("cpufreq: CPPC: Register EM based on efficiency class information")
>>>> Reported-by: Shaokun Zhang <zhangshaokun@...ilicon.com>
>>>> Signed-off-by: Pierre Gondois <pierre.gondois@....com>
>>>> ---
>>>>    drivers/cpufreq/cppc_cpufreq.c | 6 +++---
>>>>    1 file changed, 3 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
>>>> index d092c9bb4ba3..ecd0d3ee48c5 100644
>>>> --- a/drivers/cpufreq/cppc_cpufreq.c
>>>> +++ b/drivers/cpufreq/cppc_cpufreq.c
>>>> @@ -478,7 +478,7 @@ static inline unsigned long compute_cost(int cpu, int step)
>>>>    			step * CPPC_EM_COST_STEP;
>>>>    }
>>>> -static int cppc_get_cpu_power(struct device *cpu_dev,
>>>> +static __maybe_unused int cppc_get_cpu_power(struct device *cpu_dev,
>>>>    		unsigned long *power, unsigned long *KHz)
>>>>    {
>>>>    	unsigned long perf_step, perf_prev, perf, perf_check;
>>>> @@ -547,8 +547,8 @@ static int cppc_get_cpu_power(struct device *cpu_dev,
>>>>    	return 0;
>>>>    }
>>>> -static int cppc_get_cpu_cost(struct device *cpu_dev, unsigned long KHz,
>>>> -		unsigned long *cost)
>>>> +static __maybe_unused int cppc_get_cpu_cost(struct device *cpu_dev,
>>>> +		unsigned long KHz, unsigned long *cost)
>>>>    {
>>>>    	unsigned long perf_step, perf_prev;
>>>>    	struct cppc_perf_caps *perf_caps;
>>>
>>> Should we actually run cppc_cpufreq_register_em() for
>>> !CONFIG_ENERGY_MODEL ? Why?
>>>
>>
>> Hello Viresh,
>> It seems that when CONFIG_ENERGY_MODEL=n, the compiler is already
>> considering cppc_cpufreq_register_em() as an empty function.
>>
>> Indeed, CONFIG_ENERGY_MODEL=n makes em_dev_register_perf_domain()
>> an empty function, so cppc_cpufreq_register_em() is only made of
>> variable definitions. This compiler optimization also explains
>> why cppc_get_cpu_power() and cppc_get_cpu_cost() trigger the
>> -Wunused-function warning.
>>
>> Putting cppc_cpufreq_register_em() inside an
>> #ifdef CONFIG_ENERGY_MODEL
>> guard seems also valid to me. To avoid too many empty definitions
>> of cppc_cpufreq_register_em(), I guess it should be inside an
>> #if defined(CONFIG_ARM64) && defined(CONFIG_ENERGY_MODEL)
>> guard instead.
>> Please let me know what you prefer.
> 
> In that case we shouldn't do:
> 
> cppc_cpufreq_driver.register_em = cppc_cpufreq_register_em;
> 
> as well, as that is extra work for the cpufreq core, which won't be
> used at all.
> 
> So instead of __maybe_unused, lets put all dependent stuff within
> CONFIG_ENERGY_MODEL ?
> 
Ok yes. Just to be sure and except if disagreed, I will use the
following structure:
#if CONFIG_ARM64
#else
#endif

#if defined(CONFIG_ARM64) && defined(CONFIG_ENERGY_MODEL)
int populate_efficiency_class();
#else
int populate_efficiency_class();
#endif

to avoid having multiple empty definitions of
populate_efficiency_class() (for eg.) that we would have with:
#if CONFIG_ARM64
#if CONFIG_ENERGY_MODEL
int populate_efficiency_class();
#else // CONFIG_ENERGY_MODEL
int populate_efficiency_class();
#endif // CONFIG_ENERGY_MODEL
#else // CONFIG_ARM64
int populate_efficiency_class();
#endif // CONFIG_ARM64