Subject: Re: [PATCH v2 2/5] devfreq: add support for suspend/resume of a devfreq device

From: Chanwoo Choi <[email-masked]>
Date: Tue, 04 Dec 2018 15:10:31 +0900

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 <[email-masked]>
> 
> Sorry. Fix typo 'choi' to 'Choi' as following.
> Reviewed-by: Chanwoo Choi <[email-masked]>
> 
>>
>> 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 <[email-masked]>
>>> Suggested-by: Chanwoo Choi <[email-masked]>
>>> Signed-off-by: Lukasz Luba <[email-masked]>
>>> ---
>>>  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.

>>>  
>>> -	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.

>>>  
>>> -	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

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