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Date:   Wed, 05 Dec 2018 09:09:36 +0900
From:   Chanwoo Choi <cw00.choi@samsung.com>
Organization: Samsung Electronics
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To:     Lukasz Luba <l.luba@partner.samsung.com>,
        linux-arm-kernel@lists.infradead.org,
        linux-samsung-soc@vger.kernel.org, linux-kernel@vger.kernel.org,
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        robh+dt@kernel.org, mark.rutland@arm.com, kgene@kernel.org,
        krzk@kernel.org, m.szyprowski@samsung.com, b.zolnierkie@samsung.com
Subject: Re: [PATCH v2 2/5] devfreq: add support for suspend/resume of a
 devfreq device
In-reply-to: <7662d626-5c3e-aec5-05e4-c6cd2e56ef49@partner.samsung.com>
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Hi Lukasz,

On 2018년 12월 04일 18:53, Lukasz Luba wrote:
> Hi Chanwoo,
> 
> On 12/4/18 7:10 AM, Chanwoo Choi wrote:
>> Hi Lukasz,
>>
>> I add the comment about 'suspend_count'.
>>
>> On 2018년 12월 04일 14:43, Chanwoo Choi wrote:
>>> Hi,
>>>
>>> On 2018년 12월 04일 14:36, Chanwoo Choi wrote:
>>>> Hi Lukasz,
>>>>
>>>> Looks good to me. But, I add the some comments.
>>>> If you will fix it, feel free to add my tag:
>>>> Reviewed-by: Chanwoo choi <cw00.choi@samsung.com>
>>>
>>> Sorry. Fix typo 'choi' to 'Choi' as following.
>>> Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
>>>
>>>>
>>>> On 2018년 12월 03일 23:31, Lukasz Luba wrote:
>>>>> The patch prepares devfreq device for handling suspend/resume
>>>>> functionality.  The new fields will store needed information during this
>>>>
>>>> nitpick. Remove unneeded space. There are two spaces between '.' and 'The new'.
>>>>
>>>>> process.  Devfreq framework handles opp-suspend DT entry and there is no
>>>>
>>>> ditto.
>>>>
>>>>> need of modyfications in the drivers code.  It uses atomic variables to
>>>>
>>>> ditto.
>>>>
>>>>> make sure no race condition affects the process.
>>>>>
>>>>> The patch is based on earlier work by Tobias Jakobi.
>>>>
>>>> Please remove it from each patch description.
>>>>
>>>>>
>>>>> Suggested-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
>>>>> Suggested-by: Chanwoo Choi <cw00.choi@samsung.com>
>>>>> Signed-off-by: Lukasz Luba <l.luba@partner.samsung.com>
>>>>> ---
>>>>>   drivers/devfreq/devfreq.c | 51 +++++++++++++++++++++++++++++++++++++++--------
>>>>>   include/linux/devfreq.h   |  7 +++++++
>>>>>   2 files changed, 50 insertions(+), 8 deletions(-)
>>>>>
>>>>> diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
>>>>> index a9fd61b..36bed24 100644
>>>>> --- a/drivers/devfreq/devfreq.c
>>>>> +++ b/drivers/devfreq/devfreq.c
>>>>> @@ -316,6 +316,10 @@ static int devfreq_set_target(struct devfreq *devfreq, unsigned long new_freq,
>>>>>   			"Couldn't update frequency transition information.\n");
>>>>>   
>>>>>   	devfreq->previous_freq = new_freq;
>>>>> +
>>>>> +	if (devfreq->suspend_freq)
>>>>> +		devfreq->resume_freq = cur_freq;
>>>>> +
>>>>>   	return err;
>>>>>   }
>>>>>   
>>>>> @@ -667,6 +671,9 @@ struct devfreq *devfreq_add_device(struct device *dev,
>>>>>   	}
>>>>>   	devfreq->max_freq = devfreq->scaling_max_freq;
>>>>>   
>>>>> +	devfreq->suspend_freq = dev_pm_opp_get_suspend_opp_freq(dev);
>>>>> +	atomic_set(&devfreq->suspend_count, 0);
>>>>> +
>>>>>   	dev_set_name(&devfreq->dev, "devfreq%d",
>>>>>   				atomic_inc_return(&devfreq_no));
>>>>>   	err = device_register(&devfreq->dev);
>>>>> @@ -867,14 +874,28 @@ EXPORT_SYMBOL(devm_devfreq_remove_device);
>>>>>    */
>>>>>   int devfreq_suspend_device(struct devfreq *devfreq)
>>>>>   {
>>>>> +	int ret;
>>>>> +
>>>>>   	if (!devfreq)
>>>>>   		return -EINVAL;
>>>>>   
>>>>> -	if (!devfreq->governor)
>>>>> -		return 0;
>>>>> +	if (devfreq->governor) {
>>>>> +		ret = devfreq->governor->event_handler(devfreq,
>>>>> +					DEVFREQ_GOV_SUSPEND, NULL);
>>>>> +		if (ret)
>>>>> +			return ret;
>>>>> +	}
>>>>> +
>>>>> +	if (devfreq->suspend_freq) {
>>>>> +		if (atomic_inc_return(&devfreq->suspend_count) > 1)
>>>>> +			return 0;
>>>>> +
>>>>> +		ret = devfreq_set_target(devfreq, devfreq->suspend_freq, 0);
>>>>> +		if (ret)
>>>>> +			return ret;
>>>>> +	}
>>
>> In this patch, if some users call 'devfreq_suspend_device' twice,
>> 'devfreq->governor->event_handler(devfreq, DEVFREQ_GOV_SUSPEND, NULL)'
>> is called twice but devfreq_set_target() is called only one.
>> I knew that it is no problem for operation.
>>
>> But,
>> I think that you better to use 'suspend_count' as the reference count
>> of devfreq_suspend/resume_device(). But, if you use 'suspend_count'
>> in order to check whether this devfreq is suspended or not,
>> we can reduce the unneeded redundant call when calling it twice.
>>
>> clock and regulator used the 'reference count' method in order to
>> remove the redundant call.
> 
> I think I've got the point. I will move the atomic check just
> after the !devfreq check. Something like the code bellow is what you
> would like to see?
> ---8<-----
> if (!devfreq)
> 	return -EINVAL;
> if (atomic_inc_return(&devfreq->suspend_count) > 1)
> 	return0;

Looks good to me. I agree.

> 
> ---->8-------
>>
>>
>>>>>   
>>>>> -	return devfreq->governor->event_handler(devfreq,
>>>>> -				DEVFREQ_GOV_SUSPEND, NULL);
>>>>> +	return 0;
>>>>>   }
>>>>>   EXPORT_SYMBOL(devfreq_suspend_device);
>>>>>   
>>>>> @@ -888,14 +909,28 @@ EXPORT_SYMBOL(devfreq_suspend_device);
>>>>>    */
>>>>>   int devfreq_resume_device(struct devfreq *devfreq)
>>>>>   {
>>>>> +	int ret;
>>>>> +
>>>>>   	if (!devfreq)
>>>>>   		return -EINVAL;
>>>>>   
>>>>> -	if (!devfreq->governor)
>>>>> -		return 0;
>>>>> +	if (devfreq->resume_freq) {
>>>>> +		if (atomic_dec_return(&devfreq->suspend_count) >= 1)
>>>>> +			return 0;
>>
>> ditto.
> Same approach here:
> ---8<-----
> if (!devfreq)
> 	return -EINVAL;
> if (atomic_dec_return(&devfreq->suspend_count) >= 1)
> 	return 0;

Looks good to me.

> ---->8-------
> 
> Regards,
> Lukasz
>>
>>>>>   
>>>>> -	return devfreq->governor->event_handler(devfreq,
>>>>> -				DEVFREQ_GOV_RESUME, NULL);
>>>>> +		ret = devfreq_set_target(devfreq, devfreq->resume_freq, 0);
>>>>> +		if (ret)
>>>>> +			return ret;
>>>>> +	}
>>>>> +
>>>>> +	if (devfreq->governor) {
>>>>> +		ret = devfreq->governor->event_handler(devfreq,
>>>>> +					DEVFREQ_GOV_RESUME, NULL);
>>>>> +		if (ret)
>>>>> +			return ret;
>>>>> +	}
>>>>> +
>>>>> +	return 0;
>>>>>   }
>>>>>   EXPORT_SYMBOL(devfreq_resume_device);
>>>>>   
>>>>> diff --git a/include/linux/devfreq.h b/include/linux/devfreq.h
>>>>> index e4963b0..d985199 100644
>>>>> --- a/include/linux/devfreq.h
>>>>> +++ b/include/linux/devfreq.h
>>>>> @@ -131,6 +131,9 @@ struct devfreq_dev_profile {
>>>>>    * @scaling_min_freq:	Limit minimum frequency requested by OPP interface
>>>>>    * @scaling_max_freq:	Limit maximum frequency requested by OPP interface
>>>>>    * @stop_polling:	 devfreq polling status of a device.
>>>>> + * @suspend_freq:	 frequency of a device set during suspend phase.
>>>>> + * @resume_freq:	 frequency of a device set in resume phase.
>>>>> + * @suspend_count:	 suspend requests counter for a device.
>>>>>    * @total_trans:	Number of devfreq transitions
>>>>>    * @trans_table:	Statistics of devfreq transitions
>>>>>    * @time_in_state:	Statistics of devfreq states
>>>>> @@ -167,6 +170,10 @@ struct devfreq {
>>>>>   	unsigned long scaling_max_freq;
>>>>>   	bool stop_polling;
>>>>>   
>>>>> +	unsigned long suspend_freq;
>>>>> +	unsigned long resume_freq;
>>>>> +	atomic_t suspend_count;
>>>>> +
>>>>>   	/* information for device frequency transition */
>>>>>   	unsigned int total_trans;
>>>>>   	unsigned int *trans_table;
>>>>>
>>>>
>>>>
>>>
>>>
>>
>>
> 
> 


-- 
Best Regards,
Chanwoo Choi
Samsung Electronics