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Date:   Wed, 06 Mar 2019 23:14:09 +0530
From:   Sibi Sankar <sibis@...eaurora.org>
To:     myungjoo.ham@...sung.com
Cc:     Kyungmin Park <kyungmin.park@...sung.com>,
        Chanwoo Choi <cw00.choi@...sung.com>, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-msm-owner@...r.kernel.org,
        skannan@...eaurora.org, linux-kernel-owner@...r.kernel.org
Subject: Re: [PATCH v4] PM / devfreq: Restart previous governor if new
 governor fails to start

On 2019-03-05 12:48, MyungJoo Ham wrote:
>> Hey MyungJoo, Kyungmin
>> Did you get a chance to think about how you
>> want this fix implemented?
>> 
>> On 2019-02-19 10:42, Sibi Sankar wrote:
>>> Hey MyungJoo,
>>> 
>>> On 12/14/18 7:15 AM, MyungJoo Ham wrote:
>>>>> From: Saravana Kannan <skannan@...eaurora.org>
>>>>> 
>>>>> If the new governor fails to start, switch back to old governor so
>>>>> that the
>>>>> devfreq state is not left in some weird limbo.
>>>>> 
>>>>> Signed-off-by: Sibi Sankar <sibis@...eaurora.org>
>>>>> Signed-off-by: Saravana Kannan <skannan@...eaurora.org>
>>>>> Reviewed-by: Chanwoo Choi <cw00.choi@...sung.com>
>>>> 
>>>> Hello,
>>>> 
>>>> In overall, the idea and the implementation looks good.
>>>> 
>>>> However, I have a question:
>>>> 
>>>> What if the following line fails?
>>>> 
>>>> +		df->governor->event_handler(df, DEVFREQ_GOV_START,
>>>> +					    NULL);
>>>> 
>>>> Don't we still need something to handle for such events?
>>> 
>>> The original discussion went as follows:
>>> governor_store is expected to be used only on cases
>>> where devfreq_add_device() succeeded i.e prev->governor
>>> is expected to be present and DEVFREQ_GOV_START is
>>> expected to succeed. Hence falling back to the previous
>>> governor seems like a sensible idea.
>>> 
>>> This would also prevent DEVFREQ_GOV_STOP from being called
>>> on a governor were DEVFREQ_GOV_START had failed which is
>>> ideal.
>>> 
>>> That being said DEVFREQ_GOV_START can still fail for the
>>> prev-governor due to some change in state of the system.
>>> Do you want to handle this case by clearing the state of
>>> governor rather than switching to previous governor?
>>> 
> 
> If moving back to previous governor fails after
> failing for "next" governor, we may assume it's fatal
> and stop the device; we can simply return errors.
> 
> In such a case, df->governor may need to be NULL as well.

Thanks. Will make the necessary changes in the
next re-spin.

> 
> 
> Cheers,
> MyungJoo

-- 
-- Sibi Sankar --
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project.

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