linux-kernel - Re: [PATCH] PM / devfreq: Remove redundant frequency adjustment from governors

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Message-ID: <20180517154705.GN19594@google.com>
Date:   Thu, 17 May 2018 08:47:05 -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>,
        Douglas Anderson <dianders@...omium.org>
Subject: Re: [PATCH] PM / devfreq: Remove redundant frequency adjustment from
 governors

Hi,

On Thu, May 17, 2018 at 10:44:08AM +0900, Chanwoo Choi wrote:
> Hi,
> 
> On 2018년 05월 17일 06:10, Matthias Kaehlcke wrote:
> > The performance, powersave, simpleondemand and userspace governors
> > determine a target frequency and then adjust it according to the
> > df->min/max_freq limits that might have been set by user space. This
> > adjustment is redundant, it is done in update_devfreq() for any
> > governor, right after governor->get_target_freq().
> > 
> > Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
> > ---
> >  drivers/devfreq/governor_performance.c    | 10 ++--------
> >  drivers/devfreq/governor_powersave.c      |  5 -----
> >  drivers/devfreq/governor_simpleondemand.c |  7 +------
> >  drivers/devfreq/governor_userspace.c      | 16 ++++------------
> >  4 files changed, 7 insertions(+), 31 deletions(-)
> > 
> > diff --git a/drivers/devfreq/governor_performance.c b/drivers/devfreq/governor_performance.c
> > index 4d23ecfbd948..31ee30622c00 100644
> > --- a/drivers/devfreq/governor_performance.c
> > +++ b/drivers/devfreq/governor_performance.c
> > @@ -16,14 +16,8 @@
> >  static int devfreq_performance_func(struct devfreq *df,
> >  				    unsigned long *freq)
> >  {
> > -	/*
> > -	 * target callback should be able to get floor value as
> > -	 * said in devfreq.h
> > -	 */
> > -	if (!df->max_freq)
> > -		*freq = UINT_MAX;
> > -	else
> > -		*freq = df->max_freq;
> > +	*freq = UINT_MAX;
> > +
> 
> It is difficult to understand why use UINT_MAX instead of df->max_freq.
> 
> Instead, after merged the commit ab8f58ad72c4 ("PM / devfreq: Set min/max_freq
> when adding the devfreq device"), df->max/min_freq have the specific frequency
> value always. So, we can change it as following without UINT_MAX. 
> 
> 	*freq = df->max_freq;

There are two reasons why I don't like to return df->max_freq:

1. update_devfreq() already handles the user limits (which is what
min/max_freq actually are), no need to spread parts of this
additionally over all governors.

2. I plan to introduce the concept of a devfreq policy [1], which
would introduce another pair of frequencies, df->policy.min/max, and
min/max_freq would become df->policy.user.min/max. The governors would
then return df->policy.user.min/max, which isn't really incorrect
since update_devfreq() takes care of adjusting the value with
df->policy.min/max if needed, but it also isn't very clear. And we
almost certainly shouldn't additionally handle df->policy.min/max in
the governors.

I agree though that just returning UINT_MAX isn't very clear either,
even though that's what some governors are doing currently when
df->min/max_freq is not set (which can still occur, since user space
is free to set the value to 0).

I think there are two better options than returning df->min/max_freq:

a) create constants DEVFREQ_MIN/MAX_FREQ and return them, this clearly
states the intent.

b) return df->scaling_min/max_freq, which is the min/max frequency
that is actually available on the device side, depending on the
enabled OPPs.

A slightly related question: Is it actually intended to keep
supporting a value of 0 for df->min/max_freq to keep backwards
compatibility, or should the related code be removed?

Thanks

Matthias

[1] https://patchwork.kernel.org/patch/10401999/ (first draft, without
df->policy.min/max)