linux-kernel - Re: [RFC PATCH] PM / devfreq: Add policy notifier

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Message-ID: <20180518162525.GA29467@google.com>
Date:   Fri, 18 May 2018 09:25:25 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Chanwoo Choi <cw00.choi@...sung.com>
Cc:     MyungJoo Ham <myungjoo.ham@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Brian Norris <briannorris@...omium.org>
Subject: Re: [RFC PATCH] PM / devfreq: Add policy notifier

On Fri, May 18, 2018 at 08:26:30AM +0900, Chanwoo Choi wrote:
> Hi,
> 
> On 2018년 05월 18일 08:07, Matthias Kaehlcke wrote:
> > Hi,
> > 
> > On Thu, May 17, 2018 at 11:01:34AM +0900, Chanwoo Choi wrote:
> >> Hi,
> >>
> >> Could you give some use-case of DEVFREQ_POLICY_NOTIFIER
> >> or send use-case patch with this patch?
> > 
> > This is a WIP patch that makes use of the DEVFREQ_POLICY_NOTIFIER:
> > 
> > https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/1065122
> > 
> >> I already knew the CPUFREQ_POLICY_NOTIFIER.
> >> But, until now, there are no any requirements of DEVFREQ_POLICY_NOTIFIER.
> >> If there are no any use-case, it is not necessary codes.
> > 
> > Sure, I intend to land the above driver upstream if devfreq can
> > provide the necessary interfaces.
> 
> I recommend that you should send the patch with the use-case patch.

One of the reasons this patch was sent as an RFC was to get an idea
whether it would be feasible to add support for
DEVFREQ_POLICY_NOTIFIER before spending much time implementing a
driver that relies on it, and then possibly hear that it's not going
to fly.

However since you didn't voice general concerns about the idea I made
progress with the driver in the last days. I'll polish it a bit more
and send it in a series with related devfreq patches.

> >> On 2018년 05월 16일 06:24, Matthias Kaehlcke wrote:
> >>> diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
> >>> index fe2af6aa88fc..a7294c056f65 100644
> >>> --- a/drivers/devfreq/devfreq.c
> >>> +++ b/drivers/devfreq/devfreq.c
> >>> @@ -273,6 +273,9 @@ int update_devfreq(struct devfreq *devfreq)
> >>>  	if (err)
> >>>  		return err;
> >>>  
> >>> +	srcu_notifier_call_chain(&devfreq->policy_notifier_list,
> >>> +			DEVFREQ_ADJUST, &freq);
> >>
> >> It is not proper to used 'freq' as the passed data.
> >> In current step,'freq' is not adjusted and is not final decided
> >> frequency.
> > 
> > Right, the next revision will pass a struct devfreq_policy instead,
> > where the notifiers can adjust the min/max values, similar to what
> > cpufreq does.
> 
> Actually, I don't know the devfreq_policy(?). As I already commented,
> it is not proper to discuss it because there is no any real code and patches.
> It is difficult to understand for me.