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Subject: Re: [PATCH v2 2/5] devfreq: add support for suspend/resume of a
 devfreq device
To:     Chanwoo Choi <cw00.choi@samsung.com>,
        linux-arm-kernel@lists.infradead.org,
        linux-samsung-soc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-pm@vger.kernel.org, devicetree@vger.kernel.org
Cc:     tjakobi@math.uni-bielefeld.de, myungjoo.ham@samsung.com,
        kyungmin.park@samsung.com, rjw@rjwysocki.net, len.brown@intel.com,
        pavel@ucw.cz, gregkh@linuxfoundation.org, keescook@chromium.org,
        anton@enomsg.org, ccross@android.com, tony.luck@intel.com,
        robh+dt@kernel.org, mark.rutland@arm.com, kgene@kernel.org,
        krzk@kernel.org, m.szyprowski@samsung.com, b.zolnierkie@samsung.com
From:   Lukasz Luba <l.luba@partner.samsung.com>
Message-ID: <7662d626-5c3e-aec5-05e4-c6cd2e56ef49@partner.samsung.com>
Date:   Tue, 4 Dec 2018 10:53:25 +0100
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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;

---->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;
---->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;
>>>>
>>>
>>>
>>
>>
> 
>