linux-kernel - Re: [PATCH 1/3] PM / devfreq: Actually support providing freq

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Message-id: <5ADEDC1A.2000101@samsung.com>
Date:   Tue, 24 Apr 2018 16:26:18 +0900
From:   Chanwoo Choi <cw00.choi@...sung.com>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     MyungJoo Ham <myungjoo.ham@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Vinayak Holikatti <vinholikatti@...il.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        linux-scsi@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        Vivek Gautam <vivek.gautam@...eaurora.org>
Subject: Re: [PATCH 1/3] PM / devfreq: Actually support providing freq_table

Hi,

On 2018년 04월 24일 14:29, Bjorn Andersson wrote:
> On Mon 23 Apr 19:48 PDT 2018, Chanwoo Choi wrote:
> 
>> Hi,
>>
>> On 2018??? 04??? 24??? 09:20, Bjorn Andersson wrote:
>>> The code in devfreq_add_device() handles the case where a freq_table is
>>> passed by the client, but then requests min and max frequences from
>>> the, in this case absent, opp tables.
>>>
>>> Read the min and max frequencies from the frequency table, which has
>>> been built from the opp table if one exists, instead of querying the
>>> opp table.
>>>
>>> Signed-off-by: Bjorn Andersson <bjorn.andersson@...aro.org>
>>> ---
>>>
>>> An alternative approach is to clarify in the devfreq code that it's not
>>> possible to pass a freq_table and then in patch 3 create an opp table for the
>>> device in runtime; although the error handling of this becomes non-trivial.
>>>
>>> Transitioning the UFSHCD to use opp tables directly is hindered by the fact
>>> that the Qualcomm UFS hardware has two different clocks that needs to be
>>> running at different rates, so we would need a way to describe the two rates in
>>> the opp table. (And would force us to change the DT binding)
>>>
>>>  drivers/devfreq/devfreq.c | 22 ++++------------------
>>>  1 file changed, 4 insertions(+), 18 deletions(-)
>>>
>>> diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
>>> index fe2af6aa88fc..086ced50a13d 100644
>>> --- a/drivers/devfreq/devfreq.c
>>> +++ b/drivers/devfreq/devfreq.c
>>> @@ -74,30 +74,16 @@ static struct devfreq *find_device_devfreq(struct device *dev)
>>>  
>>>  static unsigned long find_available_min_freq(struct devfreq *devfreq)
>>>  {
>>> -	struct dev_pm_opp *opp;
>>> -	unsigned long min_freq = 0;
>>> -
>>> -	opp = dev_pm_opp_find_freq_ceil(devfreq->dev.parent, &min_freq);
>>> -	if (IS_ERR(opp))
>>> -		min_freq = 0;
>>> -	else
>>> -		dev_pm_opp_put(opp);
>>> +	struct devfreq_dev_profile *profile = devfreq->profile;
>>>  
>>> -	return min_freq;
>>> +	return profile->freq_table[0];
>>
>> It is wrong. The thermal framework support the devfreq-cooling device
>> which uses the dev_pm_opp_enable/disable().
>>
> 
> Okay, that makes sense. So rather than registering a custom freq_table I
> should register the min and max frequency using dev_pm_opp_add().

Thanks.

> 
>> In order to find the correct available min frequency,
>> the devfreq have to use the OPP function instead of using the first entry
>> of the freq_table array.
>>
> 
> Based on this there seems to be room for cleaning out the freq_table
> from devfreq, to reduce the confusion. I will review this further.

Actually, devfreq must need to have the freq_table[] array. But, freq_table[]
array should be handled in the devfreq core. Now, the devfreq device drivers can
touch the freq_table. I think it is not good.

There is a reason why we have to maintain the freq_table[] as the internal variable.
OPP doesn't provide the OPP API which get the all registered frequencies.
If devfreq-cooling device disables the specific frequency by using dev_pm_oppdisable(),
the user of OPP interface can not get the disabled frequency list.
So, I maintain the freq_table even if using the OPP interface.

And, devfreq-cooling device uses the freq_table directly because released MALi driver
from ARM initializes the freq_table list directly.

I have no any objection for refactoring. Just I'm sharing the issue and current status.

> 
> Thanks,
> Bjorn
> 
> 
> 


-- 
Best Regards,
Chanwoo Choi
Samsung Electronics