Re: [PATCHv2] PM / devfreq: Add dev_pm_qos support

[Chanwoo Choi <cw00.choi@samsung.com>]

On 19. 8. 14. 오전 10:06, Chanwoo Choi wrote:
> Hi,
> 
> On 19. 8. 13. 오후 8:27, Leonard Crestez wrote:
>> On 13.08.2019 09:10, Chanwoo Choi wrote:
>>> In case of cpufreq, cpufreq.c replace the body of store_min_freq()
>>> and store_max_freq() by using struct dev_pm_qos_request instancce
>>> with dev_pm_qos_update_request().
>>>
>>> If you use the new way with dev_pm_qos_update_request() for
>>> min_freq_store() and max_freq_store(), it doesn't need to
>>> get the final frequency from three candidate frequencies.
>>
>> Yes, I saw that but didn't implement the equivalent for devfreq because 
>> it's not clear what there is to gain.
> 
> I think that it is clear. Just use the dev_pm_qos_request interface
> for both user input through sysfs and device input with qos request.
> Already PM_QOS has the feature to get the final freuency among
> the multiple request. When use the dev_pm_qos request, the devfreq
> doesn't need to compare between user input and device input with qos.
> It make devfreq core more clear and simple
> 
>>
>> Since dev_pm_qos is measured in khz it means that min_freq/max_req on 
>> sysfq would lose 3 significant digits, however those digits are probably 
>> useless anyway.
> 
> I think that it doesn't matter. This patch already considers the this issue
> by using '* 1000'. We can get either KHz or MHz with additional operation.
> I think that it is not problem.
> 
>>
>>> In result, We only consider the following two candidate frequencies
>>> as following:
>>> 1. "devfreq->scaling_min_freq" will contain the requirement
>>>     from devfreq thermal throttling by OPP interface.
>>
>> It's a bit awkward that the OPPs enable/disable API is not obviously 
>> specific to "thermal".
> 
> driver/thermal/devfreq_cooling.c uses the OPP interface to enable/disable
> the OPP entries according to the thermal throttling policy. 
> 
>>
>>> 2. "devfreq->min_freq" will contain the requirements
>>>     from both user input through sysfs and the dev_pm_qos_request.
>> According to a comment on a previous version it would be useful to have 
>> a separate files for "constraint min/max freq" and "user min/max freq":
>>
>>      https://patchwork.kernel.org/patch/11078475/#22805379
> 
> Actually, I think that I want to use the only dev_pm_qos_request
> for all external request like devfreq cooling of thermal,
> user input through sysfs and device request with dev_pm_qos_request.
> 
> Already, dev_pm_qos_request is designed to consider the multiple requirements.
> We don't need to use the various method (OPP interface, sysfs input, dev_pm_qos)
> because make it more simple and easy.
> 
> I think that after finished the review of this patch, I will do refactor the devfreq_cooling.c
> by using the dev_pm_qos_request. Or, if there are some volunteeer,

Sorry, I would withdraw the this opinion about replacing
the OPP enable/disable interface with the dev_pm_qos_request
because even if devfreq-cooling.c needs the 'dev' instance
to use the dev_pm_qos_request method, it is not clear until now.
It needs how to get the device instance of devfreq on device-tree.

Keep discussing without it. 

> 
>>
>> Combining min/max requests from dev_pm_qos and sysfs would make this 
>> harder to implement. I guess user_min_freq could be implemented by 
> 
> I think that it is not difficult. Just make the different dev_pm_qos_request
> instances. When qos has the multiple request from one more dev_pm_qos_request,
> just get the value by using dev_pm_qos_read_value().
> - a dev_pm_qos_request for user input
> - b dev_pm_qos_request for device request on other device driver
> 
> 
>> reading back the dev_pm_qos request but there would be no way to 
>> implement a qos_min_freq entry.
> 
> I don't understand. Just devfreq show the min freq from dev_pm_qos_request
> which contains the all requirement of minimum frequency.
> 
> If there are no any request to dev_pm_qos_request, we can just
> get the minimum frequency from dev_pm_qos_request because 
> the devfreq device would call the dev_pm_qos_update_request(..., min_freq)
> on the probe time.
> 
> If I know the wrong information, please let me know.
> 
> 
>>
>>>> +static int devfreq_qos_min_notifier_call(struct notifier_block *nb, unsigned long val, void *ptr)
>>>
>>> Please keep the under 80 line if there are no any special reason.
>>
>> OK, will check.
>>
>>
> 
> 


-- 
Best Regards,
Chanwoo Choi
Samsung Electronics