linux-kernel - Re: [PATCH v4] PM / devfreq: Restart previous governor if new governor fails to start

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Date:   Tue, 19 Feb 2019 10:42:01 +0530
From:   Sibi Sankar <sibis@...eaurora.org>
To:     myungjoo.ham@...sung.com, Kyungmin Park <kyungmin.park@...sung.com>
Cc:     Chanwoo Choi <cw00.choi@...sung.com>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-msm-owner@...r.kernel.org" 
        <linux-arm-msm-owner@...r.kernel.org>,
        "skannan@...eaurora.org" <skannan@...eaurora.org>
Subject: Re: [PATCH v4] PM / devfreq: Restart previous governor if new
 governor fails to start

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?

> 
> Cheers,
> MyungJoo
> 
> 

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