linux-kernel - Re: [RFC v2 01/11] OPP: Don't overwrite rounded clk rate

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Message-ID: <20190617041721.5xdr3kl4xxe6gy4m@vireshk-i7>
Date:   Mon, 17 Jun 2019 09:47:21 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Rajendra Nayak <rnayak@...eaurora.org>
Cc:     swboyd@...omium.org, vincent.guittot@...aro.org,
        mturquette@...libre.com, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-serial@...r.kernel.org, linux-spi@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-scsi@...r.kernel.org,
        ulf.hansson@...aro.org, dianders@...omium.org, rafael@...nel.org
Subject: Re: [RFC v2 01/11] OPP: Don't overwrite rounded clk rate

On 17-06-19, 09:37, Rajendra Nayak wrote:
> 
> 
> On 6/17/2019 9:20 AM, Viresh Kumar wrote:
> > On 14-06-19, 10:57, Viresh Kumar wrote:
> > > Hmm, so this patch won't break anything and I am inclined to apply it again :)
> > > 
> > > Does anyone see any other issues with it, which I might be missing ?
> > 
> > I have updated the commit log a bit more to clarify on things, please let me
> > know if it looks okay.
> > 
> >      opp: Don't overwrite rounded clk rate
> >      The OPP table normally contains 'fmax' values corresponding to the
> >      voltage or performance levels of each OPP, but we don't necessarily want
> >      all the devices to run at fmax all the time. Running at fmax makes sense
> >      for devices like CPU/GPU, which have a finite amount of work to do and
> >      since a specific amount of energy is consumed at an OPP, its better to
> >      run at the highest possible frequency for that voltage value.
> >      On the other hand, we have IO devices which need to run at specific
> >      frequencies only for their proper functioning, instead of maximum
> >      possible frequency.
> >      The OPP core currently roundup to the next possible OPP for a frequency
> >      and select the fmax value. To support the IO devices by the OPP core,
> >      lets do the roundup to fetch the voltage or performance state values,
> >      but not use the OPP frequency value. Rather use the value returned by
> >      clk_round_rate().
> >      The current user, cpufreq, of dev_pm_opp_set_rate() already does the
> >      rounding to the next OPP before calling this routine and it won't
> >      have any side affects because of this change.
> 
> Looks good to me. Should this also be documented someplace that dev_pm_opp_set_rate()
> would not be able to distinguish between its users trying to scale CPU/GPU's vs IO
> devices, so its the callers responsibility to round it accordingly before calling the
> API?

diff --git a/drivers/opp/core.c b/drivers/opp/core.c
index 0fbc77f05048..bae94bfa1e96 100644
--- a/drivers/opp/core.c
+++ b/drivers/opp/core.c
@@ -751,8 +751,11 @@ static int _set_required_opps(struct device *dev,
  * @dev:        device for which we do this operation
  * @target_freq: frequency to achieve
  *
- * This configures the power-supplies and clock source to the levels specified
- * by the OPP corresponding to the target_freq.
+ * This configures the power-supplies to the levels specified by the OPP
+ * corresponding to the target_freq, and programs the clock to a value <=
+ * target_freq, as rounded by clk_round_rate(). Device wanting to run at fmax
+ * provided by the opp, should have already rounded to the target OPP's
+ * frequency.
  */

-- 
viresh