linux-kernel - Re: [Patch v3 3/6] cpufreq: qcom-cpufreq-hw: Add dcvs interrupt support

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Date:   Fri, 9 Jul 2021 11:37:19 -0400
From:   Thara Gopinath <thara.gopinath@...aro.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     agross@...nel.org, bjorn.andersson@...aro.org, rui.zhang@...el.com,
        daniel.lezcano@...aro.org, rjw@...ysocki.net, robh+dt@...nel.org,
        tdas@...eaurora.org, mka@...omium.org,
        linux-arm-msm@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: [Patch v3 3/6] cpufreq: qcom-cpufreq-hw: Add dcvs interrupt
 support



On 7/9/21 2:46 AM, Viresh Kumar wrote:
> On 08-07-21, 08:06, Thara Gopinath wrote:
>>   static int qcom_cpufreq_hw_cpu_init(struct cpufreq_policy *policy)
>>   {
>>   	struct platform_device *pdev = cpufreq_get_driver_data();
>> @@ -370,6 +480,10 @@ static int qcom_cpufreq_hw_cpu_init(struct cpufreq_policy *policy)
>>   			dev_warn(cpu_dev, "failed to enable boost: %d\n", ret);
>>   	}
>>   
>> +	ret = qcom_cpufreq_hw_lmh_init(policy, index);
> 
> You missed unregistering EM here (which is also missing from exit,
> which you need to fix first in a separate patch).

Hi!

So how exactly do you do this? I checked other users of the api and I do 
not see any free. I would say if needed, it should be a separate patch 
and outside of this series.

> 
>> +	if (ret)
>> +		goto error;
>> +
>>   	return 0;
>>   error:
>>   	kfree(data);
>> @@ -389,6 +503,10 @@ static int qcom_cpufreq_hw_cpu_exit(struct cpufreq_policy *policy)
>>   
>>   	dev_pm_opp_remove_all_dynamic(cpu_dev);
>>   	dev_pm_opp_of_cpumask_remove_table(policy->related_cpus);
>> +	if (data->lmh_dcvs_irq > 0) {
>> +		devm_free_irq(cpu_dev, data->lmh_dcvs_irq, data);
> 
> Why using devm variants here and while requesting the irq ?
> 
>> +		cancel_delayed_work_sync(&data->lmh_dcvs_poll_work);
>> +	}
> 
> Please move this to qcom_cpufreq_hw_lmh_exit() or something.

Ok.

> 
> Now with sequence of disabling interrupt, etc, I see a potential
> problem.
> 
> CPU0                                    CPU1
> 
> qcom_cpufreq_hw_cpu_exit()
> -> devm_free_irq();
>                                          qcom_lmh_dcvs_poll()
>                                          -> qcom_lmh_dcvs_notify()
>                                            -> enable_irq()
> 
> -> cancel_delayed_work_sync();
> 
> 
> What will happen if enable_irq() gets called after freeing the irq ?
> Not sure, but it looks like you will hit this then from manage.c:
> 
> WARN(!desc->irq_data.chip, KERN_ERR "enable_irq before
>                                       setup/request_irq: irq %u\n", irq))
> 
> ?
> 
> You got a chicken n egg problem :)

Yes indeed! But also it is a very rare chicken and egg problem.
The scenario here is that the cpus are busy and running load causing a 
thermal overrun and lmh is engaged. At the same time for this issue to 
be hit the cpu is trying to exit/disable cpufreq. Calling 
cancel_delayed_work_sync first could solve this issue, right ? 
cancel_delayed_work_sync guarantees the work not to be pending even if
it requeues itself on return. So once the delayed work is cancelled, the 
interrupts can be safely disabled. Thoughts ?


> 

-- 
Warm Regards
Thara (She/Her/Hers)