linux-kernel - Re: [PATCH V4 07/14] cpufreq: mediatek: Add .get function

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Date:   Tue, 26 Apr 2022 19:13:26 +0800
From:   Rex-BC Chen <rex-bc.chen@...iatek.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
CC:     <rafael@...nel.org>, <robh+dt@...nel.org>, <krzk+dt@...nel.org>,
        <matthias.bgg@...il.com>, <jia-wei.chang@...iatek.com>,
        <roger.lu@...iatek.com>, <hsinyi@...gle.com>,
        <khilman@...libre.com>, <angelogioacchino.delregno@...labora.com>,
        <linux-pm@...r.kernel.org>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-mediatek@...ts.infradead.org>,
        <Project_Global_Chrome_Upstream_Group@...iatek.com>
Subject: Re: [PATCH V4 07/14] cpufreq: mediatek: Add .get function

On Mon, 2022-04-25 at 15:30 +0530, Viresh Kumar wrote:
> On 25-04-22, 17:34, Rex-BC Chen wrote:
> > We found that the pulses of cpu voltage could be observed when
> > frequency is fixed (scaling_max_freq == scaling_min_freq) if using
> > cpufreq_generic_get as '.get' callback in MT8186.
> > cpufreq framework will constantly (~ 1 sec) call 'update' if the
> > policy
> 
> Which function gets called here in that case ? I would expect
> cpufreq_driver_target() to not make a call to MTK driver in that
> case, after it
> finds that new and old frequency are same (it will check the
> corresponding freq
> from cpufreq table).
> 
> > frequency is NOT equal to hardware frequency in
> > cpufreq_verify_current_freq.
> > The problem is that there might be a tiny difference between the
> > policy
> > frequency and the hardware frequency even they are very close.
> > e.g. policy frequency is 500,000,000 Hz however, hardware frequency
> > is
> > 499,999,726 Hz for MT8186 opp15.
> > 
> > To prevent the voltage pulses, we currently use the software cached
> > values as you pointed out.
> > I wonder is it possible to add a tolerence for checking difference
> > between policy frequency and hardware frequency in cpufreq
> > framework so
> > that we can use cpufreq_generic_get as callback without pulse
> > issue.
> > Or any suggestion would be appreciated.
> 
> 

Hello Viresh,

We have a non-upstream driver which tries to get frequency by
'cpufreq_get'.
When we use that non-upstream driver, 'cpufreq_verify_current_freq'
will be further invoked by 'cpufreq_get' and it would cause voltage
pulse issue as I described previously.
Therefore, we apply the solution in this series.

Recently, we found that using 'cpufreq_generic_get' directly in our
non-upstream driver can do the same thing without pulse issue.
It can meet your request as well.

So here, for cpufreq, I think it is proper to drop this patch and I
will do it in the next version.

Thanks for your review.

BRs,
Rex