linux-kernel - Re: [PATCH] devfreq: Suspend all devices on system shutdown

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Message-ID: <5621c652-6786-6bbb-ffbd-71567d8d882e@samsung.com>
Date:   Thu, 7 Mar 2019 10:27:02 +0900
From:   Chanwoo Choi <cw00.choi@...sung.com>
To:     Pavel Machek <pavel@....cz>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-samsung-soc@...r.kernel.org,
        MyungJoo Ham <myungjoo.ham@...sung.com>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Lukasz Luba <l.luba@...tner.samsung.com>,
        Markus Reichl <m.reichl@...etechno.de>
Subject: Re: [PATCH] devfreq: Suspend all devices on system shutdown

Hi Pavel,

On 19. 3. 7. 오전 8:10, Pavel Machek wrote:
> On Fri 2019-01-25 14:54:03, Marek Szyprowski wrote:
>> This way devfreq core ensures that all its devices will be set to safe
>> operation points before reboot operation. There are board on which some
>> aggressive power saving operation points are behind the capabilities of
>> the bootloader to properly reset the hardware and boot the board. This
>> way one can avoid board crash early after reboot.
>>
>> Similar pattern is used in CPUfreq subsystem.
> 
> This looks somehow dangerous to me. I guess this will break someone's
> shutdown, and on battery-powered devices, that's quite bad thing to
> do.

This patch executes the devfreq_suspend() for all devfreq devices.
The devfreq_suspend() does the following:
	1. (mandatory) stop the polling of governor.
	2. (optional)  If devfreq device specifies the 'opp-suspend'
	property in DT, the devfreq changes the frequency by using
	the devfreq->suspend_freq.

If the user of devfreq device doesn't want to change the frequency
during the system shutdown, just don't add the 'opp-suspend' property
in DT. And the devfreq_suspend() will just stop the polling for governor.

Even if on battery-powered devices, I think it is not problem
to stop the polling of governor during the system shutdown.

In my case, I don't know what is dangerous.
Could you explain it more detailed.

> 
> Could we explicitely do it only for devices that need it?
> 								Pavel
> 								
> 


-- 
Best Regards,
Chanwoo Choi
Samsung Electronics