linux-kernel - Re: [PATCH 1/3] PM / devfreq: Actually support providing freq

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Message-ID: <20180424052916.GD2052@tuxbook-pro>
Date:   Mon, 23 Apr 2018 22:29:16 -0700
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Chanwoo Choi <cw00.choi@...sung.com>
Cc:     MyungJoo Ham <myungjoo.ham@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Vinayak Holikatti <vinholikatti@...il.com>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        linux-scsi@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        Vivek Gautam <vivek.gautam@...eaurora.org>
Subject: Re: [PATCH 1/3] PM / devfreq: Actually support providing freq_table

On Mon 23 Apr 19:48 PDT 2018, Chanwoo Choi wrote:

> Hi,
> 
> On 2018??? 04??? 24??? 09:20, Bjorn Andersson wrote:
> > The code in devfreq_add_device() handles the case where a freq_table is
> > passed by the client, but then requests min and max frequences from
> > the, in this case absent, opp tables.
> > 
> > Read the min and max frequencies from the frequency table, which has
> > been built from the opp table if one exists, instead of querying the
> > opp table.
> > 
> > Signed-off-by: Bjorn Andersson <bjorn.andersson@...aro.org>
> > ---
> > 
> > An alternative approach is to clarify in the devfreq code that it's not
> > possible to pass a freq_table and then in patch 3 create an opp table for the
> > device in runtime; although the error handling of this becomes non-trivial.
> > 
> > Transitioning the UFSHCD to use opp tables directly is hindered by the fact
> > that the Qualcomm UFS hardware has two different clocks that needs to be
> > running at different rates, so we would need a way to describe the two rates in
> > the opp table. (And would force us to change the DT binding)
> > 
> >  drivers/devfreq/devfreq.c | 22 ++++------------------
> >  1 file changed, 4 insertions(+), 18 deletions(-)
> > 
> > diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
> > index fe2af6aa88fc..086ced50a13d 100644
> > --- a/drivers/devfreq/devfreq.c
> > +++ b/drivers/devfreq/devfreq.c
> > @@ -74,30 +74,16 @@ static struct devfreq *find_device_devfreq(struct device *dev)
> >  
> >  static unsigned long find_available_min_freq(struct devfreq *devfreq)
> >  {
> > -	struct dev_pm_opp *opp;
> > -	unsigned long min_freq = 0;
> > -
> > -	opp = dev_pm_opp_find_freq_ceil(devfreq->dev.parent, &min_freq);
> > -	if (IS_ERR(opp))
> > -		min_freq = 0;
> > -	else
> > -		dev_pm_opp_put(opp);
> > +	struct devfreq_dev_profile *profile = devfreq->profile;
> >  
> > -	return min_freq;
> > +	return profile->freq_table[0];
> 
> It is wrong. The thermal framework support the devfreq-cooling device
> which uses the dev_pm_opp_enable/disable().
> 

Okay, that makes sense. So rather than registering a custom freq_table I
should register the min and max frequency using dev_pm_opp_add().

> In order to find the correct available min frequency,
> the devfreq have to use the OPP function instead of using the first entry
> of the freq_table array.
> 

Based on this there seems to be room for cleaning out the freq_table
from devfreq, to reduce the confusion. I will review this further.

Thanks,
Bjorn